VISUAL TESTJNG I 433
TABLE 2. Causes of acquired color vision TABLE 3. Classification of color vision deficiencies
deficiencies and percent of males affected
Blue-yellow deficiency by Color Vision Percent
MalesAffected
Central serous retinopathy (accompanied
luminosity loss in red) Hereditary deficiencies 92
6 or 7
Chorioretinitis Trichromatism (three colors: red,
Diabetic retinopathy green, blue) rare
Dominant hereditary optic atrophy very rare
Glaucoma Normal vision
Hypertensive retinopathy Anomalous (defective)
Juvenile macular degeneration
Methyl alcohol poisoning Dichromatism (two colors)*
Myopic retinal degeneration
Papilledema Protanopia (red lacking)
Pigmentary degeneration of the retina Deuteranopia (green lacking)
Tritanopia (blue lacking)
(including retinitis pigmentosa) Tetratanopia (yellow lacking)
Retinal detachment
Retinal vascular occlusion Acquired deficiencies data not available
Senile macular degeneration Tritan (blue-yellow) data not available
Protan-deutan (red-yellow)
Red-green deficiency * DEFICIENCY MOST OFTEN REFERENCED WHEN DISCUSSING COLOR
Hereditary juvenile macular degeneration
[Starqardts and Bests disease) BLINDNESS
Juvenile macular degeneration
Lebers optic atrophy Color Vision Classifications
Lesions of the optic nerve and pathway
Optic neuritis (including retrobulbar neuritis) Two functions that determine an individual's sensation
Papillitis range are color perception and color discrimination. When a
Tobacco or toxic amblyopia primary color is mistaken for another primary color, this is
an error in perception. An error in discrimination is an error
The most common color deficiencies are hereditary and of lesser magnitude involving a mistake in hue selection.9
occur in the red-green range. About 0.5 percent of the During a vision examination, these two functions are tested
affected individuals are female, in the red-green range. independently.
Women constitute about 50 percent of those affected in the
blue-yellow range. Most such deficiencies occur in both A color vision examination performed with an anomalo-
eyes and in rare instances in only one eye. About 0.001 per- scope allows the mixing of red and green lights to match a
cent of the affected groups in the hereditary portion have yellow light standard. Yellow and blue lights may be mixed
their deficiency in the blue-green range. Individuals in the to match a white light. An individual with normal vision
red-green group may make misinterpretations of disconti- requires red, blue and green light to mix and match colors of
nuities in shades of red, brown, olive and gold. the entire color spectrum. A color deficient person may
require fewer than the three lights to satisfy the color sensa-
Acquired color deficiency is a greater problem to good tion.l? Table 3 indicates the type of deficiencies and the per-
color vision testing. The acquired deficiencies may affect cent of the male population known to be affected.11
only one eye and a change from acceptable color vision to a
recognizable problem may be very gradual. Various medical For the practical purpose of classifying personnel
conditions can cause such a change to occur. Table 2 lists affected by hereditary color deficiencies, the Naval Subma-
conditions that produce color vision deficiencies in particu- rine Medical Research Laboratory has developed classifica-
lar color ranges. Most acquired color vision problems vary in tions shown in Table 4. About 50 percent of color deficient
severity and may be associated with ocular pathology. If the people can be categorized in accordance with this table.
disease continues for an extended period of time without Class I covers 30 percent of the color deficient population
treatment, the deficiencies may become erratic in intensity and Class III accounts for 20 percent. Individuals in Class I
and may vary from the red-green or blue-yellow ranges. can judge colors used as standards for signaling, communica-
Aging can also affect color vision.8 tion and identification as fast and as accurately as zero class
persons can. The limitation of Class I people is when good
color discrimination is necessary. Persons in Class III may be
used in other areas such as radio repair, chemistry, medicine
and surgery, electrical manufacturing or general painting.
434 I NONDESTRUCTIVE TESTING OVERVIEW
TABLE 4. Naval Submarine Medical Research examinations. The illuminance of full spectrum fluores-
Laboratory color vision classification system cent lighting should be no less than 200 lx (20 ftc). The
rating of the light source is known as the color tempera
Class Description ture. A low color temperature lamp such as an incandes-
cent lamp makes it easier for persons with borderline
0 normal color deficiencies to guess the colors correctly. A color
mild anomalous trichromat temperature of 6,700 K is preferred. Too high a color tem-
unclassified anomalous trichromat perature increases the number of reading errors. To elim-
(included mild and moderate classes) inate glare, the light source should be 45 degrees to the
surface while the patient is perpendicular to it. The read-
Ill moderate anomalous trichromat ing distance should be about 400 to 600 mm (16 to 24 in.)
IV severely color deficient (includes severe or arm's length. To perform such an examination, two
minutes should be allotted to arrange all fifteen caps in
anomalous trichromats, dichromats their appropriate positions.
monochromats)
In summary, color deficiency can be acquired or inher-
Class II encompasses staff members, managers or clerical ited. Some color deficiencies may be treated, alleviated or
help, whose need for color resolution is not critical. Individu- minimized. Pseudoisochromatic plates in conjunction with
als in Class IV must be restricted from occupations where the progressive hue color caps provide an adequate test for
color differentiation of any magnitude is required. most industrial visual inspectors. Full spectrum lighting
(6,700 K) is necessary for accurate test results.
As with vision acuity examinations, there are many differ-
ent examinations for color vision.12 Color visionis often tested It should be added that, because the visible spectrum is
with pseudoisochromatic plates or cards on which the detec- made up of colors of varying wavelengths and the black and
tion of certain figures depends on red-green discrimination. white colors consist of various combinations of colors, defi-
Unfortunately, most common vision acuity examinations ciencies in any part of the color spectrum has an impact on
were designed to identify hereditary red-green deficiencies certain black and white inspection methods, including
and ignore blue-yellow deficiencies.13
X-ray film review.
As in the normal vision acuity examinations, lighting It is recommended that all nondestructive testing per-
requirements and time must be controlled for color vision
sonnel have their color vision tested annually, while taking
their vision acuity examination.
FIGURE 6. Electromagnetic spectrum and enlargement of ultraviolet region
VIOLET 1WVISIBLE RED
COSMIC RAYS RADIO WAVES
SCHUMANN ~ ....__SHORTWAVE--> I+ LONG WAVE ..... J:i:j
I+ >ISUN TAN 0> VISIBLE
X-RAYS ~ FAR ULTRAVIOLET
0.1 100 200 300 400
WAVELENGTH
(nanometers)
VISUAL TESTING I 435
Fluorescent Materials destroyed by improper use of any light source. Consult the
most recent safety documents and the manufacturer's litera
Fluorescence is a complex phenomenon that occurs in ture before working near any artificial light or radiation
gases, liquids and solids. For the purpose of visual nonde- source.
structive testing, fluorescence is used in conjunction with
long wave ultraviolet radiation as an excitation source (see Developments in optical testing technology have created
Fig. 6). a need for better understanding of the potential health haz-
ards caused by high intensity light sources or by artificial
Visible light rays are made up of billions of photons that light sources of any intensity in the work area. The human
carry energy and this is what we see when a light bulb is eye operates optimally in an environment illuminated
energized - the photons have carried energy from the bulb directly or indirectly by sunlight, with characteristic spectral
to the eye. distribution and range of intensities that are very different
from those of most artificial sources. The eye can handle
Photons have different energies or wavelengths which only a limited range of night vision tasks.
we distinguish as different colors. Red light photons are less
energetic than blue light photons. Invisible ultraviolet pho- Over time, there has accumulated evidence that photo-
tons are more energetic than the most energetic violet light chemical changes occur in eyes under the influence of nor-
that our eyes can see. mal daylight illumination - short term and long term visual
impairment and exacerbation of retinal disease have been
Studies show that the intensity of fluorescence in many observed and it is important to understand why this occurs.
situations is directly proportional to the intensity of the Periodic fluctuations of visible and ultraviolet radiation
ultraviolet radiation that excites it. Fluorescence is the occur with the regular diurnal light-dark cycles and with the
absorption of light at one wavelength and reemission of this lengthening and shortening of the cycle as a result of sea-
light at another wavelength. The whole absorption and sonal changes. These fluctuations are known to affect all
emission process occurs in about a nanosecond and because biological systems critically. The majority of such light-dark
it keeps happening as long as there are ultraviolet radiation effects is based on circadian cycles and controlled by the
photons to absorb, a glow is observed to begin and end with pineal system, which can be affected directly by the trans-
the turning on and off of the ultraviolet radiation. Care must mission of light to the pineal gland or indirectly by effects on
be taken when using short wave or wide bandwidth ultravio- the optic nerve pathway.
let sources. A safe, general operating principle is to always
hold the lamp so the light is directed away from you and to FIGURE 7. Human eye response at 1,070 Ix
wear protection. (100 ftc)
Long wave ultraviolet is generally considered safer. 100
However, individuals should use adequate protection if they
are photosensitive or subjected to long exposure times. ~> 80
zvi 60
Commercially available fluorescent dyes span the visible o.:.c0... 40
spectrum. Because the human eye is still the most com- llJ 20
monly used sensing device, most nondestructive testing V)
applications are designed to fluoresce as close as possible to
the eye's peak response. Figure 7 shows the spectral LI.J ~
response of the human eye, with the colors at the ends of C!::: llJ
the spectrum (red, blue and violet) appearing much dimmer >-LI.J Q
than those in the center (orange, yellow and green).
~
Nondestructive testing methods using fluorescence will
continue to improve with the development of new dyes or LI.J
new solvents to increase brightness or eye response matching. C!:::
Safety for Visual and Optical Tests ~,4000 ~ 500 6:J 600 700
50
This information is presented solely for educational pur- ~ I:3lJ.J ""lJ.J 1§1~1 I0
poses and should not be consulted in place of current safety
regulations. Note that units of measure have been con- CD l'.J i>:d- 0~ ""lJ.J
verted to this book's format and are not those commonly
used in all industries. Human vision can be disrupted or t::;
:J
WAVELENGTH
(nanometers)
436 I NONDESTRUCTIVE TESTING OVERVIEW
Also of concern are the results of work that has been resulting scotomas (damaged unresponsive areas) can some-
done demonstrating that light affects immunological reac- times be ignored by the accident victim.
tions in vitro and in vivo by influencing the antigenicity of
molecules, antibody function and the reactivity of lympho- The tissue surrounding the absorption site can much
cytes. more readily conduct away heat for small image sizes than it
can for large image sizes. In fact, retinal injury thresholds
Given the variety of visual tasks and illumination that (see Fig. 8) for less than 0.1 to 10 s exposure show a high
confronts the visual inspector, it is important to consider dependence on the image size (0.01 to 0.1 W·mm-2 for a
whether failures in performance might be a result of exces- 1,000 µm image up to about 0.01 kW,mm-2 for a 20 µm
sive exposure to light or other radiation or even a result of image. To put the scale into perspective, the sun produces a
insufficient light sources. A myth exists that 20/20 foveal
vision, in the absence of color blindness, is all that is neces- 160 urn diameter image on the retina.14J5
sary for optimal vision. In fact, this is not so: there may be
visual field loss in and beyond the fovea centralis for many High Luminance Visible Light Sources
reasons; the inspector may have poor stereoscopic vision;
visual ability may be impaired by glare or reflection; or The normal reaction to a high luminance light source is
actual vision may be affected by medical or psychological to blink and to direct the eyes away from the source. The
conditions. probability of overexposure to noncoherent light sources is
higher than the probability of exposure to lasers, yet
Laser Hazards extended (high luminance) sources are used in a more
Loss of vision resulting from retinal burns following FIGURE 8. Typical retinal burn thresholds14•15
observation of the sun has been described throughout his-
tory. Now there is a common technological equivalent to J04 LASER ( I W INTO EYE)
this problem with laser light sources. In addition to the
development of lasers, further improvement in other high J03 */ LASER (I mW)
radiance light sources (a result of smaller, more efficient 20 kW XENON SHORT ARC SEARCHLIGHT
reflectors and more compact, brighter sources) has pre-
sented the potential for chorioretinal injury. It is thought 102
that chorioretinal burns from artificial sources in industrial
situations have been very much less frequent than similar 101 // RETINAL BURN
burns from the sun.
~uLz.LJ ffll>// (THRESHOLD FOR RABBIT
Because of the publicity of the health hazard caused by
exposure to laser radiation, awareness of such hazards is 0 I O s EXPOSURE)
probably much greater than the general awareness of the
hazard from high intensity extended visible sources which ~ 10-1 ~ ::b~ / M/\XIMUM PERMISSIBLE
may be as great or greater. Generally, lasers are used in spe- §:z<Q:::(::'. :iN1 JQ-2 EXPOSURE FOR 0. 16 s
cialized environments by technicians familiar with the haz- ~~ I 0-3 ~/:1·.' SUN
ards and trained to avoid exposure by the use of protective J0-4
eyewear and clothing. Laser standards of manufacture and Q:::'.- 10-5 ELECTRIC WELDING ARC OR CARBON ARC
use have been well developed and probably have con- J0-6
tributed more than anything else to a heightened awareness 0 I 0-7 M TUNGSTEN FROSTED ' M/\XIMUM
of safe laser operation. The American National Standards Lc.LoJ J0--8
Institute (ANSI) has published a "Safe Use of Lasers" stan- FILAMENT INCANDESCENT PERMISSIBLE
dard, ANSI 2136.1-1993, that should answer most of the Q:::'.
readers' specific concerns. LAMP EXPOSURE FOR
0
Laser hazard controls are common sense procedures cVo1 FLUORESCENT CONTINUOUS
designed to (1) restrict personnel from entering the beam , ,/ LAMP
path and (2) limit the primary and reflected beams from <( SOURCES
occupied areas. Should an individual be exposed to exces-
sive laser light, the probability of damage to the retina is ---~;~ / +
high because of the high energy pulse capabilities of some OUTDOOR DAYLIGHT
lasers. However, the probability of visual impairment is rela- ~ c:::==>
tively low because of the small area of damage on the retina.
Once the initial flash blindness and pain have subsided, the CANDLE P. TELEVISION
I 0-9 INTERIOR (DAY)
I 0-10
I 0-11 10· o.s- 1° 2° 4° 10°
10-12 I II I I I
10 SOURCE ANGLE
102 J03 J04
TYPICAL RETINALIMAGE SIZE
(micrometers)
VISUAL TESTING I 437
casual and possibly more hazardous way. In the nondestruc- halide discharge lamps, fluorescent lamps and flash lamps
tive testing industry, extended sources are used as general among others. Because of the high ultraviolet attenuation
illumination and in many specialized applications. U nfortu- afforded by many visually transparent materials, an empiri-
nately, there are comparatively few guidelines for the safe cal approach is sometimes taken for the problem of light
use of extended sources of visible light. sources associated with ultraviolet: the source is enclosed
and provided with ultraviolet absorbing glass or plastic
Infrared Hazards lenses. If injurious effects continue to develop, the thickness
of the protective lens is increased.
Infrared radiation comprises that invisible radiation
beyond the red end of the visible spectrum up to about The photochemical effects of ultraviolet radiation on the
1 mm wavelength. Infrared is absorbed by many substances skin and eye are still not completely understood. Records of
and its principal biological effect is known as hyperthermia, ultraviolet radiation's relative spectral effectiveness for elic-
heating that can be lethal to cells. Usually, the response to
intense infrared radiation is pain and the natural reaction is iting a particular biological effect (referred to by photobiol-
to move away from the source so that bums do not develop. ogists as action spectra) are generally available. Ultraviolet
irradiance may be measured at a point of interest with a
Ultraviolet Hazards portable radiometer and compared with the ultraviolet radi-
ation hazard criteria.
Before development of the laser, the principal hazard in
the use of intense light sources was the potential eye and skin Recent work in the field has shown that exposure of the
injury from ultraviolet radiation. Ultraviolet radiation is invis- eyes and skin to any ultraviolet source, including near
ible radiation beyond the violet end of the visible spectrum ultraviolet light should be limited to a maximum of 3
with wavelengths down to about 185 nm. It is strongly mj.cm+ effective irradiance, in which the effective irradi-
absorbed by the cornea and the lens of the eye. Ultraviolet ance is derived from the actual spectral irradiance
radiation at wavelengths shorter than 185 nm is absorbed by weighted against the hazard sensitivity spectrum. Values of
air, is often called vacuum ultraviolet and is rarely of concern the hazard sensitivity spectrum are provided both by the
to the visual inspector. Many useful high intensity arc sources American Conference of Governmental Industrial Hygienists
and some lasers may emit associated, potentially hazardous, (ACGIH) and the International Radiation Protection Associ-
levels of ultraviolet radiation. With appropriate precautions, ation for wavelengths from 180 to 400 nm.16 This general
such sources can serve very useful visual testing functions. requirement is subject to an additional exposure requirement
for the unprotected eye which is that the maximum permissi-
Studies have clarified the spectral radiant exposure doses ble exposure is limited to 1 J.cm-2 regardless of wavelength. It
and relative spectral effectiveness of ultraviolet radiation
required to elicit an adverse biological response. These is the minimum calculated exposure time that is applicable to
responses include keratoconjunctivitis (known as welder's any situation, taking into account that none of the above
flash), possible generation of cataracts and erythema or red- applies to exposures of photosensitive people or those simul-
dening of the skin. Longer wavelength ultraviolet radiation taneously exposed to photosensitizing agents.
can lead to fluorescence of the eye's lens and ocular media,
eyestrain and headache. These conditions lead, in tum, to For purposes of determining exposure levels, it is impor-
low task performance resulting from the fatigue associated tant to note that most inexpensive, portable radiometers are
with increased effort. Chronic exposure to ultraviolet radia- not equally responsive at all wavelengths throughout the
tion accelerates skin aging and possibly increases the risk of ultraviolet spectrum and are usually only calibrated at one
developing certain forms of skin cancer. wavelength with no guarantees at any other wavelength.
Such radiometers have been designed for a particular appli-
It should also be mentioned that some individuals are cation using a particular lamp.
hypersensitive to ultraviolet radiation and may develop a
reaction following, what would be for the average healthy A common example in the nondestructive testing indus-
human, suberythemal exposures. However, it is extremely try is the so-called black light radiometer used in fluorescent
unusual for these symptoms of exceptional photosensitivity liquid penetrant and magnetic particle applications. These
to be elicited solely by the limited emission spectrum of an meters are usually calibrated at 365 nm, the predominant
industrial light source. An inspector is typically aware of ultraviolet output of the filtered 100 W medium pressure
such sensitivity because of earlier exposures to sunlight. mercury vapor lamp commonly used in the industry. Use of
the meter at any other wavelength in the ultraviolet spec-
In industry, the visual inspector may encounter many trum may lead to significant errors. To minimize problems
sources of visible and invisible radiation: incandescent in assessing the hazard presented by industrial lighting, it is
lamps, compact arc sources (solar simulators), quartz halo- important to use a radiometer that has been calibrated with
gen lamps, metal vapor (sodium and mercury) and metal an ultraviolet spectral distribution as close as possible to the
lamp of interest.
If the inspector is concerned about the safety of a given
situation, ultraviolet absorbing eye protection and facewear
438 I NONDESTRUCTIVE TESTING OVERVIEW
is readily available from several sources. An additional ben- The retinal pigment epithelium is optically the most dense
efit of such protection is that it prevents the annoyance of absorbent layer (because of high concentrations of melanin
lens fluorescence and provides the wearer considerable pro- granules) and the greatest temperature changes arise in this
tection from all ultraviolet radiation. In certain applications, layer.
tinted lenses can also provide enhanced visibility of the test
object. For short (O.l to 100 s) accidental exposures to the sun
or artificial radiation sources, the mechanism of injury is
Photosensitizers generally thought to be hyperthermia resulting in protein
denaturation and enzyme inactivation. Because the large,
While ultraviolet radiation from most of the high inten- complex organic molecules absorbing the radiant energy
sity visible light sources may be the principal concern, the have broad spectral absorption bands, the hazard potential
potential for chorioretinal injury from visible radiation for chorioretinal injury is not expected to depend on the
should not be overlooked. coherence or monochromaticity of the source. Injury from a
laser or a nonlaser radiation source should not differ if
Over the past few decades, a large number of commonly image size, exposure time and wavelength are the same.
used drugs, food additives, soaps and cosmetics have been
identified as phototoxic or photoallergenic agents even at Because different regions of the retina play different
the longer wavelengths of the visible spectrum. Colored roles in vision, the functional loss of all or part of one of
drugs and food additives are possible photosensitizers for these regions varies in significance. The greatest vision acu-
organs below the skin because longer wavelength visible ity exists only for central (foveal) vision, so that the loss of
radiations penetrate deeply into the body. this retinal area dramatically reduces visual capabilities. In
comparison, the loss of an area of similar size located in the
Damage to the Retina peripheral retina: could be subjectively unnoticed.
It is possible to multiply the spectral absorption data of the The human retina is normally subjected to irradiances
human retina by the spectral transmission data of the eye's below 1 W-m-2, except for occasional momentary exposures
optical media at all wavelengths to arrive at an estimate of to the sun, arc lamps, quartz halogen lamps, normal incan-
the relative absorbed spectral dose in the retina and the descent lamps, flash lamps and similar radiant sources. The
underlying choroid for a given spectral radiant exposure of natural aversion or pain response to bright lights normally
the cornea. The computation should provide a relative spec- limits exposure to no more than 0.15 to 0.2 s. In some
tral effectiveness curve for chorioretinal bums. In practice, instances, individuals can suppress this response with little
the evaluation of potential chorioretinal bum hazards may difficulty and stare at bright sources, as commonly occurs
be complicated or straightforward, depending on the maxi- during solar eclipses.
mum luminance and spectral distribution of the source; pos-
sible retinal image sizes; the image quality; pupil size; Fortunately, few arc sources are sufficiently large and
spectral scattering and absorption by the cornea, aqueous sufficiently bright enough to be a retinal burn hazard under
humor, the lens and the vitreous humor; and absorption and normal viewing conditions. Only when an arc or hot fila-
scattering in the various retinal layers. Qualitatively, the ment is greatly magnified (in an optical projection system,
ocular media transmission rises steeply from somewhat less for example) can hazardous irradiance be imaged on a suffi-
than 400 nm and does not fall off again until about 900 nm ciently large area of the retina to cause a bum. Visual
in the near infrared after which a peak at about 1,100 nm is inspectors do not normally step into a projected beam at
exhibited. These values finally fall off to virtually zero at close range or view a welding arc with binoculars or a tele-
about 1,400 nm thus defining the potential hazardous wave-
length range. scope.
Nearly all conceivable accident situations require a haz-
Thermal Factor
ardous exposure to be delivered within the period of a blink
Visible and near infrared radiation up to about 1,400 nm reflex. If an arc is struck while an inspector is located at a
(associated with most optical sources) is transmitted very close viewing range, it is possible that a retinal burn
through the eye's ocular media and absorbed in significant could occur. At lower exposures, an inspector experiences a
doses principally in the retina. These radiations pass short term depression in photopic (daylight) sensitivity and
through the neural layers of the retina. A small amount is a marked, longer term loss of scotopic (dark adapted) vision.
absorbed by the visual pigments in the rods and cones, to That is why it is so important for visual inspectors in critical
initiate the visual response, and the remaining energy is fluorescent penetrant and magnetic particle test environ-
absorbed in the retinal pigment epithelium and choroid. ments to undergo dark adaptation before actually attempt-
ing to find discontinuities. Not only does the pupil have to
adapt to the reduced visible level in a booth but the actual
retinal receptors must attain maximum sensitivity. This
effect may take half an hour or more, depending on the pre-
ceding state of the eye's adaptation.
VISUAL TESTING I 439
Blue Hazard Eye Protection Filters
The so-called blue hazard function has been used in con- Because continuous visible light sources elicit a normal
junction with the thermal factor to calculate exposure dura- aversion or pain response that can protect the eye and skin
tions that do not damage the retina. from injury, visual comfort has often been used as an
approximate hazard index and eye protection and other haz-
The blue hazard is based on the demonstration that the ard controls have been provided on this basis.
retina can be damaged by blue light at intensities that do not
elevate retinal temperatures sufficiently to cause a thermal Eye protection filters for various workers were devel-
hazard. It has been found that blue light can produce 10 to oped empirically but now are standardized as shades and
100 times more retinal damage (permanent decrease in specified for particular applications.
spectral sensitivity in this spectral range) than longer visible
wavelengths. Note that there are some common situations Other protective techniques include use of high ambient
in which both thermal and blue hazards may be present. light levels and specialized filters to further attenuate
intense spectral lines. Laser eye protection is designed to
Visual Safety Recommendations have an adequate optical density at the laser wavelengths
along with the greatest visual transmission at all other wave-
The American Conference of Governmental Industrial lengths.
Hygienists (ACGIH) has proposed two threshold limit val-
ues (TLVs)for noncoherent visible light, one covering dam- Always bear in mind that hazard criteria must not be con
age to the retina by a thermal mechanism and one covering sidered to represent fine lines between safe and hazardous
retinal damage by a photochemical mechanism. Threshold exposure conditions. To be properly applied, interpretation
limit values for visible light, established by the American of hazard criteria must be based on practical knowledge of
Conference of Governmental Industrial Hygienists, are potential exposure conditions and the user, whether a profes-
intended only to prevent excessive occupational exposure sional inspector or a general consumer. Accuracy of hazard
and are limited to exposure durations of 8 h or less. They are criteria is limited by biological uncertainties including diet,
not intended to cover photosensitive individuals. 14 · genetic photosensitivity and the large safety factors required
to be built into the recommendations.
440 I NONDESTRUCTIVE TESTING OVERVIEW
PART 3
BASIC VISUAL AIDS
Environmental Factors test object. Glare from permanent lighting fixtures is more
difficult to control.
An important environmental factor affecting visual tests
is lighting. Often, emphasis is placed on equipment vari- Ceiling fixtures should be mounted as far above the line
ables such as borescope view angle or degree of magnifica- of sight as possible and must be shielded to eliminate light at
tion. But if the lighting is incorrect, no magnification is an angle greater than 45 degrees to the field of vision. Task
going to improve the image. Other working conditions are lighting should be shielded to at least 25 degrees from hori-
also important including factors causing operator discom- zontal. Such shielding must allow a sufficient amount of
fort and fatigue. light to reach the test area.
Cleanliness Lighting for Visual Tests
The act of seeing depends on the amount of light reach- The amount of light required for a visual test is depen-
ing the eye. In visual tests, the amount of light may be dent on several factors, including the type of test, the impor-
affected by distance, reflectance, brightness, contrast or the tance of speed or accuracy, reflections from backgrounds
cleanliness, texture, size and shape of the test object. and inspector variables. Physiologicalprocesses, psychologi-
cal state, experience, health and fatigue all contribute to the
Cleanliness is a basic requirement for a good visual test accuracy of a visual inspection.
- it is impossible to gather visual data through layers of
opaque dirt unless cleanliness itself is being examined. In The reflections and shadows from walls, ceiling, furni-
addition to obstructing vision, dirt on the test surface can ture and equipment must also be considered. Some
mask actual discontinuities with false indications. Cleaning reflectance from the environment must occur or the room
typically may be done by mechanical or chemical means or will be too dark to be practical. Recommended reflectance
both. Cleaning avoids the hazards of undetected discontinu- values are: ceiling, 80 to 90 percent; walls, 40 to 60 percent;
ities and improves customer product satisfaction. floors, not less than 20 percent; desks, benches and equip-
ment, 25 to 45 percent.
Texture and Reflectance
For visual and other nondestructive testing applications,
Vision is dependent on reflected light entering the eye. a ratio of 3: 1 between the test object and darker background
The easiest way to ensure adequate lighting is by placing the is recommended. A 1 :3 ratio is recommended for a test
light source and eye as close to the test surface as the focal object and lighter surroundings.
distance allows. Similarly, a magnifier should be held as
close to the eye as possible, ensuring that the maximum Certain psychological factors can also affect a visual
amount of light from the target area reaches the eye. inspector's performance. Wall colors and patterns have been
shown to have a measurable effect on attitude and this is
Reflectance and surface texture are related characteris- especially important when visually inspecting critical or
tics. It is important for lighting to enhance a target area, but small components. In general, a visual inspector's optimum
glare should not be allowed to mask the test surface. A attitude is relaxed but not bored, alert but not restless. To
highly reflective surface or a roughly textured surface may complement the illumination needed for visual testing, all
require special lighting to illuminate without masking. Sup- colors in a room should be light tones. Otherwise, up to 50
plementary lighting must be shielded to prevent glare from percent of the available light can be absorbed by dark walls
interfering with the inspector's view. and flooring. A strong contrast of pattern or color can cause
restlessness and eventually fatigue. Cool (blue) colors are
Reflected or direct glare can be a major problem that is recommended for work areas with high noise levels and
not easily corrected. Glare can be minimized by decreasing heavy physical exertion.
the amount of light reaching the eye. This is done by
increasing the angle between the glare source and line of Light Intensities
vision by increasing the background light in the area sur-
rounding the glare source or by dimming the light source. To perform a visual test, there must be a source of natu-
Such solutions are simple to implement for direct glare ral or artificial light adequate in both intensity and spectral
from a supplemental light or the reflected glare from a small distribution. Even under optimum conditions the human
eye can be stimulated by only a small part of the electro-
magnetic spectrum. The limits of this visible portion are ill
VISUAL TESTING I 441
defined, depending on the amount of energy available, its TABLE 5. Distances for minimum 500 Ix (50 ftc)
illumination
wavelength and the health of the eye. For most practical
Light Source Maximum
purposes, the visible spectrum may be considered to be Source-to-Object Distance
between about 380 nm at the beginning of the violet and millimeters(inches)
770 nm at the end of the red. However, with especially 20 cell flashlight 250 (10)
60 W incandescent bulb
intense sources and with a completely dark adapted eye, the 75 W incandescent bulb 250 fl OJ
1 00 W incandescent bulb 380 fl 5)
shorter wavelength boundary may be extended down to
460 (18)
350 nm or shorter, with a corresponding reduction in the
glasses is an inconvenience when using a borescope - it is
longest wavelength perceived. Similarly, with an especially difficult to place the eye at the ideal distance from the eye-
piece and the view is distorted by external glare and reflec-
intense longer wavelength source and an eye adapted to a tions. Rubber eyeshields on borescopes are designed to shut
out external light but are not as effective when glasses are
higher level of light, the longer wavelength boundary may worn. For these reasons, it is critical that the inspector be
able to adjust the instrument without wearing glasses to
extend up to 900 nm. These ranges together are only a small compensate for variations in vision acuity.
part of the electromagnetic spectrum. Effects of the Test Object
Brightness is an important factor in visual test environ- The test object determines the specifications for (1) the
instrument used during the visual test and (2) the required
ments. The brightness of a test surface depends on its illumination. Objective distance, object size, discontinuity
size, reflectivity, entry port size, object depth and direction
reflectivity and the intensity of the incident light. Excessive of view are all critical aspects of the test object that affect
the visual test.
or insufficient brightness interferes with the ability to see
Objective distance (see Fig. 9) is important in determin-
clearly and so obstructs critical observation and judgment. ing the illumination source, as well as the required objective
focal distance for the maximum power and magnification.
For this reason, light intensity must be tightly controlled.
A minimum intensity of 160 lx (15 ftc) of illumination FIGURE 9. Objective distance (arrows) for
direct and side viewing borescopes
should be used for general visual testing. A minimum of 500 lx
(50 ftc) should be used for critical or finely detailed tests. FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTEDWITH
PERMISSION.
According to the Illuminating Engineering Society,
visual testing requires light at 1,100 to 3,200 lx (100 to
300 ftc) for critical work.P A commercially available light
meter can be used to determine if the working environment
meets this standard.
To ensure compliance with the minimum intensity
requirement, a known light source held within a specified
maximum distance must be used. Alternatively, a light mea-
suring device such as a photocell or phototube must be
used. Examples of known light sources are shown in Table 5.
The human eye is an important component for perform-
ing visual nondestructive tests. However, there are situations
where the eye is not sensitive enough or cannot access the
test site. In these cases mechanical and optical devices can be
used to supplement the eye to achieve a complete visual test.
Visual tests comprise five basic elements: the inspector,
the test object, an optical instrument, illumination and a
recording method. Each of these elements interacts with
the others and affects the test results.
Training and vision acuity are the two most important
factors affecting the visual inspector. According to the
American Society of Mechanical Engineers' Boiler and
Pressure Vessel Code, Section XI, visual inspectors must be
qualified through formal training programs for certification
to ensure competency.
Levels of vision acuity are determined by eye examina-
tion. Approximately50 percent of Americans over the age of
twenty need corrective eyeglasses. In early stages of eye-
sight deficiency, many people are unaware of their condition
- some simply do not want to wear glasses.
It is important that borescopes be designed to allow
diopter adjustments on the eyepiece. Frequently, wearing
442 I NONDESTRUCTIVE TESTING OVERVIEW
Object size, combined with distance, determines what FIGURE 1 2. Reflectivity helps determine levels
lens angle or field of view is required to observe an entire of illumination
test surface (see Fig. 10).
DARK SURFACE
Discontinuity size determines the magnification and reso-
lution required for visualtesting. For example, greater resolu- h (gDIRECT VIEW BORESCOPE
tion is required to detect hairline cracks than to detect
undercut (see Fig. 11). ENT~PORT/ ~
Reflectivity is another factor affecting illumination. Dark FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH
surfaces such as those coated with carbon deposits require PERMISSION.
higher levels of illumination than light surfaces do (see
Fig. 12).
Entry port size determines the maximum diameter of the
instrument that can be used for the visual test (see Fig. 13).
Object depth affects focusing. If portions of the object
are in different planes, then the borescope must have suffi-
cient focus adjustment or depth of field to visualize these
different planes sharply (see Fig. 14).
FIGURE 1 0. Arrows indicate portion of object FIGURE 13. Entry port size (arrows) limits size
falling within field of view for side viewing of borescope
borescope
D . •ti •
/nlJ -------ODIRECT VIEW BORESCOPE
_J -b,--D-IR-ECT-V-IE-W-BO_R_E-SC_O_P_laLElJ --~-•-~-
ENTRY PORT
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH .t~0;; •
PERMISSION.
ENTRYPOITT/~
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH
PERMISSION.
FIGURE 11 . Discontinuity size affects resolution FIGURE 14. Object depth is critical factor
limits and magnification requirements affecting focus
D~DISCONTINUITY h D @DIRECT VIEW BORESCOPE .•
b,--D-IR_E_CT-VI-EW_B_O-RE-S-CO_tPi_E_«•._g
alLlJ • ti •
ii3 •
0 ... 0•
~ ~
f- f-
ENT~POITT/ ~ ~ 1-1
DEPTH
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH
PERMISSION. FROM ELECTRICPOWER RESEARCH INSTITUTE. REPRINTED WITH
PERMISSION.
VISUAL TESTING I 443
TABLE 6. Characteristics of typical magnifiers
Working Distance Resolving Power
Field of View
MagnifierType millimeters(inches) Power millimeters(inches) micrometers(inches x 1 0-31
Readerlens 90 x 40 (3.5 x 1.6) 1.5 x 100 (4) 50 (2)
Eyeglassloupe 60 (2.4) 2x 40 (1.6)
Doublet magnifier 60 (2.4) 90 (3.5) 25 (I)
Coddington magnifier 19 (0.75) 3.5 x 75 (3) 10 (0.4)
Triplet magnifier 22 (0.87) 7x 25 (I)
10 x 20 (0.75) 7.5 (0.3)
Direction of view determines positioning of the refer to enlargement in one dimension only. A two-dimen-
borescope, especially with rigid borescopes. Viewing direc- sional image magnified 2x, for example, doubles in width
tion also contributes to the required length of the borescope. and in height though its area quadruples.
Some of the factors affecting visual tests with borescopes The microscope is a typical magnifier. In its simplest
are in conflict and compromise is often needed. For exam- form, it is a single biconvex lens in a housing adjustable for
ple, a wide field of view reduces magnification but has focus. Many forms of illumination are available, including
greater depth offield (see Fig. 15). A narrow field of view bright field, dark field, oblique, polarized, phase contrast
produces higher magnification but results in shallow depth and interference.
of field. Interaction of these effects must be considered in
determining the optimum setup for detection and evalua- Conventional Magnifiers and Readers
tion of discontinuities in the test object.
The major considerations for choosing a magnifier are:
Magnifiers (1) power or magnification, (2) working distance, (3) field of
view, (4) chromatic correction and (5) binocular or monocu-
Magnification as an aid to vision ranges in magnifying lar vision.
power from 5x to 2,000x. Field coverage of conventional
magnifiers ranges from 90 mm (3.5 in.) down to 0.15 mm These magnifier attributes are interrelated. A high
(0.006 in.) wide. Resolving powers range from 0.05 mm power magnifier, for example, has a short working distance,
(0.002 in.) to 0.2 µm (8 x 10-6 in.). Powers of magnification a small field of view and cannot be used for binocular obser-
vation. A low power magnifier, such as a rectangular reader
FIGURE 1 5. Effects of viewing angle on other lens, has a long working distance, a large field of view and
test parameters: (a) narrow angle with high can be used for binocular vision. To attain chromatic correc-
magnification and shorter depth of field; tion (to eliminate color fringing), the high power lens must
(b) wide angle with low magnification and be complex. It typically contains a cemented doublet or
greater depth of field triplet of different optical glasses. By comparison, the low
power reader lens is sufficiently achromatic as a simple lens.
faJ
Table 6 shows the characteristics of a few typical magni-
fbJ fiers. These values are approximations because eye accom-
modation can cause each of the values to vary. Except for
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH the reader lens, all magnifiers are used with the eye fairly
PERMISSION. close to the magnifier, giving the largest field of view. The
reader lens is used binocularly and is normally held some
distance away from the eyes.
Because of its large diameter, the 3.5x doublet magnifier
has as large a field as the 2x loupe. The double convex lens
of the doublet magnifier with its central iris has a compara-
tively small field. The triplet is a three-element design hav-
ing excellent optical correction for field coverage and
reduction of color fringing. Its resolving power is the limit of
detection for fine structures. In comparison, the doublet
magnifier can barely differentiate two points 0.025 mm
(0.001 in.) apart.
444 I NONDESTRUCTIVETESTINGOVERVIEW
There are many variations of these characteristics. Com- The light is divided between the reference surface and
mercial magnifiers can be as high as 30x in power and there the standard surface. Flat and shiny surfaces reflect the fila-
are many special mountings for particular applications. ment image directly into the pupil of the eye so that these
parts look bright. Sloping or rough surfaces reflect the light
Surface Comparators away from the pupil and such areas appear dark This form
The surface comparator is a magnifier that provides a of illumination sharply delineates surface pattern character-
means for comparing a test surface against a standard surface istics. The resolving power is about 7.5 urn (3 x 10-4 in.).
finish. The observer views the two surfaces side by side, as
shown in Fig. 16. The surface comparator uses a small bat- The field of view is about 1 mm (0.04 in.) diameter.
tery powered light source, a semitransparent beam divider
and a lOx triplet. Measuring Magnifier
FIGURE 1 6. Surfacecomparato:r(a) two surfaces A measuring magnifier incorporates a measuring scale
magnifiedfor comparison;(b) test setup that is positioned against the test object to measure tiny
details on its flat surfaces (see Fig. 17). A transparent hous-
(aJ ing permits light to fall on the measured surface. Scales are
available for measurements in inches, millimeters and other
units (see Fig. 18). The magnifier uses a 7x triplet lens. The
resolving power is about 1 µm (4 x 10-5 in.). The diameter of
the field of view is about 25 mm ( 1 in.).
Illuminated Magnifiers
Illuminated magnifiers range from large circular reader
lenses, equipped with fluorescent lighting and an adjustable
stand, to a small battery powered lOx magnifier shaped like
a pencil. Some illuminated magnifiers can be obtained in
either a battery powered model or equipped for 115 V line
FIGURE 1 7. Measuring magnifierin transparent
sleeve mount
(bJ
BEAM
DIVIDER
LIGHT H
SOURCE
FROM BAUSCHAND LOMB OPTICALCOMPANY. REPRINTEDWITH FROM BAUSCHAND LOMB OPTICAL COMPANY. REPRINTEDWITH
PERMISSION. PERMISSION.
VISUAL TESTING I 445
operation. Such triplet magnifiers give about a 50 mm (2 in.) low power compound microscope is preferred. Two such
magnifiers are described below. Their resolving powers are
field of view. Resolving power is about 1.5 urn (6 x 10--5 in.).
about 7.5 urn (3 x I0-4 in.).
Low Power Microscopes Wide Field Tubes
When magnifications above lOx are required, the short The simplest form of compound microscope is a wide
working distance of the magnifier becomes a problem and a field tube, comprising an objective lens mounted in one end
of a tube and an eyepiece in the other. This design is typically
FIGURE 18. Typical measuring scalesand reticules (in inches) for measuringmagnifier
a3 11T69547T6sI s3T6 3 I I
32 32 32 32 32
11111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
4
10-1 in.
011111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111
8 , 9 10
.oo,-1- mm -1--.002
-1--.003
446 I NONDESTRUCTIVETESTING OVERVIEW
supplied either in a tripod sleeve mount or in a simplified overall discontinuity size are made directly off a photo-
microscope stand. Focusing is accomplished by a friction graph, the principal plane of focus must be at the widest
slide fit in a sleeve. The lOx wide field tube covers a field of part of the subject.
25 mm (1 in.) and has a working distance (the clearance
between the objective and test object) of about 80 mm Lighting
(3.25 in.). The 40x version has a field of about 6 mm (0.25 in.)
and a working distance of about 40 mm (1.6 in.). In general, orientation of the light source is an important
first consideration. Where possible, lighting should origi-
The image from such a simple microscope is inverted nate from the top of the subject. Lighting should originate
and reversed and is not convenient for hand manipulation of from one direction on most three-dimensional objects to
the test object during observation. Wide field tubes are fre- avoid ambiguity in relief - if more light is required, it
quently equipped with eyepiece scales to permit measure- should be slightly weaker and more diffuse than the main
ments in the test object plane. light source.
Wide Field Macroscope The photographer should exercise care to ensure that
the illumination is sufficient for purposes of the inspection.
The wide field macroscope is similar to a wide field tube, If surfaces of interest are obscured in shadow, another light
with the same magnification range (lOx to 40x) and the same source may be added. A single light source may create
mounting and focusing devices. Unlike the wide field tube, patches of glare, a problem that may be solved by side light-
the macroscope produces an image that is upright and not ing. Too much light, on the other hand, may reduce contrast
reversed, so that manipulation of the test object can be con- and make it difficult to see indications.
veniently done during observation. The prism system that
corrects the image also provides an inclined observation tube When photographing certain test objects (pipe welds, for
for more convenient prolonged viewing. The macroscope is example), unwanted reflections from the flash unit are a com-
often supplied with measuring scales for size determinations. mon problem. These can usually be eliminated by moving the
flash unit to direct specularly reflected light away from the
Photographic Techniques for lens. Another effective method of eliminating subject reflec-
Recording Visual Test Results tion is to bounce the flash off a white surface (see Fig. 20).
Depth of Field FIGURE19. Effectof apertureon depth of
field in still photography:(a) large aperture;
Depth of field may be defined as the range of distance (b) smallaperture
over which a camera gives satisfactory definition, when its
lens is in the best focus for a certain specific distance. It 0 ~~~-~~~fil:-:_--Jfa) , LENSDIAPHRAGMDEPTH OF
determines the overall sharpness of focus throughout a pho- p PRINCIPAL PLANE OF FOCUSFIELD
tograph. When a photograph is made, a single plane -r A A
through the subject is actually in focus and this is called the
principal plane offocus. Ina typical 35 mm camera, the lens LENS DIAPHRAGM
aperture (f-stop) controls the thickness of the principal _ ( P PRINCIPAL PLANE OF FOCUS
plane of focus or what is known as the image's depth of field O r~~~==lff ~:fb)
(Fig. 19). When making a photographic record of a visual EXPANDED
test, focusing is normally done with the lens diaphragm fully
- BB DEPTH OF
closed (highf number) for best image quality.
FIELD
Using a standard 35 mm camera and a 55 mm lens, the
best control over depth of field can usually be obtained by FROM ELECTRICPOWER RESEARCHINSTITUTE. REPRINTEDWITH
focusing one-third into the area of interest. This is done PERMISSION.
because the depth of field with a standard 55 mm lens
extends farther behind than in front of the principal plane of
focus. As magnification is increased (with longer lenses), the
depth of field extends farther in front of the principal plane.
Because most discontinuities are three-dimensional,
there is another factor to consider. Magnification is exact
only at the principal plane of focus. Where measurements of
VISUAL TESTING I 44 7
Film Choice TABLE7. InternationalOrganizationfor
Standardization(ISO) photographicfilm speeds
The size of a photographic negative is another important
consideration. Negative size directly affects the quality of Color Blackand White
enlargements - larger negatives produce better enlarge-
ments. Slow 25 32
32 64
The film speed is equally critical. Several factors influ- 80
ence the choice, including the amount of light available on
the subject and the size of the final photographic print. Medium 64 100
High speed film (high ISO number) requires less light but 80 125
can produce grain in the final print. This effect is increased 100 160
with increasing enlargement. 125
Slow speed films are used when fine detail is required. Fast 160 200
The disadvantage is that more light is required for proper 200 400
exposure with a slow speed film. 400 1,000
Film speed is rated according to International Organiza- 1,000
tion for Standardization (ISO) guidelines. Table 7 gives ISO
numbers for both black-and-white and color photographic
films.
Image Enhancement contains sufficient sensitivity to detect density differences of
0.05 to 0.01 percent (1,000 to 2,000 gray levels). The human
Visual images are a valuable tool in nondestructive test- eye can only resolve gray levels that differ by at least 2 per-
ing of many kinds. The primary advantage of a visual record cent (between 32 and 64 gray levels). A boundary or edge
is that it can be reviewed and evaluated more than once. condition can be distinguished by the eye only when two
adjoining areas of an image differ in density by 12 percent
Digital image processing can be a powerful tool in the or more.
interpretation of many types of visual images. Frequently,
such images contain more information than the human eye Enhancement systems digitize an image in order to pro-
can see because of the eye's limited ability to detect edges vide data in a format acceptable to standard computers. The
and gray level differences. For example, radiographic film original picture information can be generated from a variety
of sources: X-rays, gamma rays, ultrasonics and visible or
FIGURE20. Setup for use of bounceflash to infrared light. After the image information is transferred by
help reduce subjectreflection appropriate mathematical models, the resulting image
enhancement is then displayed for analysis by the user.
I /
The digital image from the computer is an array of over
// ---~'";.,:~r. . -v/ FLASH UNIT 256,000 elements. Each element of the array is called a pixel
WHITE CA ;/ and each pixel has a numerical value attached to it. The
I ,; /// /1'II higher the number associated with the pixel, the brighter is
its appearance. The enhanced image is a result of the trans-
: - - - - - - - J - - ._ - - - r fer of these numbers from the host computer through an
: '- SHIELD analog-to-digital processor and onto video tape.
I Digital Matrix
I A standard 8 x 8 matrix contains 64 gray levels. Moving
from left to right along a row of the matrix, each box is 1/64
gI brighter than the box preceding it. The boxes along each
I row are 1/8 brighter than the boxes in the row above. Each
I box contains pixels of identical value, but because of an opti-
cal phenomenon called the match bend effect, each box
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH appears to be lighter on the top and darker on the bottom.
PERMISSION.
It is more difficult to distinguish gray level differences
between the boxes on the top and bottom of the 8 x 8 matrix,
as the intensity of the image is very high or very low. Again, it
448 I NONDESTRUCTIVE TESTING OVERVIEW
is a property of human vision that the eye can distinguish can be digitally enhanced for easier identification of discon-
smaller gray level differences at medium intensity than at tinuities or features in an image.
high or low intensity. Once the information from a picture is
digitized, the relative gray level differences between any of Mathematical models of the photographic process can
the 256,000 elements can be increased to improve visibility. be used to correct exposures. Errors in focus and blurring of
video tapes can be corrected with digital techniques.
Digital computer enhancement can be used to correct a Selected portions of an image can be expanded by digital
variety of image problems. A limited contrast range can be magnification to create enlargements for viewing and inter-
mathematically expanded over the full visual range of the pretation. Existing enhancement techniques may be used
viewer, permitting observation of details missed by limited with archival images or new data produced from various
gray scale resolution of the human eye. Edges and contours sources.
VISUAL TESTING I 449
PART4
BORESCOPES
Fiber Optic Borescopes guide must be a coherent bundle: the individual fibers must
be precisely aligned so that they are in identical relative
The industrial fiber optic borescope is a flexible, layered positions at their terminations.
sheath protecting two fiber optic bundles, each comprising
thousands of glass fibers. One bundle serves as the image Image guide fibers range from 9 to 17 urn (3.5 x 10-4 to
guide and the other bundle helps illuminate the test object. 6. 7 x 10-4 in.) in diameter. Their size is one of the factors
affecting resolution, although the preciseness of alignment
Light travels only in straight lines but optical glass fibers is far more important.
bend light by internal reflection and so can carry light
around comers (see Fig. 21). Such fibers are 9 to 30 µm (4 x Note that a real image is formed on both highly polished
10-4 to 1.2 x 10-3 in.) in diameter or roughly one-tenth the faces of the image guide. Therefore, to focus a fiber optic
thickness of a human hair.
FIGURE 22. Light paths in fiber bundles:
A single fiber transmits very little light, but thousands of (aJ uncoated fibers allow light to travel laterally
fibers may be bundled for transmission of light and images. through bundle; (bJ coated fibers restrict lights
To prevent the light from diffusing, each fiber consists of a path to its original fiber
central core of high quality optical glass coated with a thin
layer of another glass with a different refractive index (aJ
(Fig. 22). This cladding acts as a mirror - all light entering
the end of the fiber is reflected internally as it travels B . .#'
(Fig. 21) and cannot escape by passing through the sides to
an adjacent fiber in the bundle. &:
Although the light is effectively trapped within. each ~
fiber, not all of it emerges from the opposite end. Some of
the light is absorbed by the fiber itself and the amount of CORE
absorption depends on the length of the fiber and its optical .1.+. --- ------:~COATING
quality. For example, plastic fiber can transmit light and is ]0~-------(bJ
less expensive to produce than optical glass but plastic is less
efficient in its transmission and unsuitable for use in fiber --------.--_--_--,----...-...-.-. .-.-.. -,-.-.-.-.-..--.,---,,-----,-;-l-f-l-'·-·-----
optic borescopes.
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH
Fiber Image Guides PERMISSION.
The fiber bundle used as an image guide (see Fig. 23) FIGURE 23. Optical fiber bundle used as image
carries the image formed by the objective lens at the distal guide
end or tip of the borescope back to the eyepiece. The image
FIBER BUNDLE
FIGURE 21 . Internal reflection of light in optic
fiber can be used to move light path in curve
{),--·' ·---3 ~
EYEPIECE OBJECTIVE
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH
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450 I NONDESTRUCTIVE TESTING OVERVIEW
borescope for different distances, the objective lens at the tip Rigid Borescopes
must be moved in or out, usually by remote control at the
eyepiece section. A separate diopter adjustment at the eye- The rigid borescope (see Fig. 25) was invented to inspect
piece is necessary to compensate for differences in eyesight. the bore of rifles and cannons. It was a thin telescope with a
small lamp at the top for illumination. Most rigid
Fiber Light Guides borescopes now use a fiber optic light guide system as an
illumination source.
Another fiber bundle carries light from an external high
intensity source to illuminate the test object. This is called The image is brought to the eyepiece by an optical train
the light guide bundle and is noncoherent (see Fig. 24). consisting of an objective lens, sometimes a prism, relay
These fibers are about 30 µm (1.2 x 10-:3 in.) in diameter lenses and an eyepiece lens. The image is not a real image
and the size of the bundle is determined by the diameter of but an aerial image: it is formed in the air between the
the scope. lenses. This means that it is possible to both provide diopter
correction for the observer and to control the objective
Fiber optic borescopes usually have a controllable bend- focus with a single adjustment to the focusing ring at the
ing section near the tip so that the inspector can direct the eyepiece.
borescope during testing and can scan an area inside the test
object. Fiber optic borescopes are made in a variety of diam- Focusing a Rigid Borescope
eters, some as small as .3.7 mm (0.1.5 in.), in lengths up to
10 m (30 ft), and with a choice of viewingdirections at the tip. The focus control in a rigid borescope greatly expands
the depth of field over nonfocusing or fixed focus designs.
FIGURE 24. Diagram of typical fiber optic At the same time, focusing can help compensate for the
borescope wide variations in eyesight among inspectors.
LENS IMAGE Figures 26 and 27 emphasize the importance of focus
EYEPIECE GUIDE adjustment for expanding the depth of field. Figure 26 was
LIGHT SOURCE LIGHT FIGURE 26. Borescope images for variety of
GUIDE distances with fixed focus (see Fig. 27): (a) at
~ 75 mm (3 in.J; (bJ at 200 mm (8 in.); (cJ at
EXIT 300 mm (12 in.)
PROJECTION LAMP
0(a)
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PERMISSION.
'
FIGURE 25. Typical lens system in rigid
borescope Ay(b)
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VISUAL TESTING I 451
taken at a variety of distances with fixed focus. Figure 27 · a borescope is usually 25 to 50 mm ( 1 to 2 in.) behind the
was taken at the same distances as in Fig. 26 but with a vari- lens window.
able focus, producing much sharper images. By sighting through the borescope, stick pins into the
board at the edge of the protractor to mark the center and
Need for Specifications both the left and right edges of the view field. This simple
procedure gives both the direction of view and the field of
Because rigid borescopes lack flexibilityand the ability to view (see Fig. 28).
scan areas, specifications regarding length, direction of view
and field of view become more critical for achieving a valid Miniborescope
visual test. For example, the direction of view should always
be specified in degrees rather than in letters or words such One variation of the rigid borescope is called the mini-
as north, up,forward or left. Tolerances should also be spec- borescope (see Fig. 29). In this design, the relay lens train is
ified. replaced with a single, solid fiber. The fiber diffuses ions in
Some manufacturers consider the eyepiece to be zero FIGURE 28. Field of view for rigid borescope
degrees and therefore a direct view rigid borescope is 180
degrees. Other manufacturers start with the borescope tip
as zero degrees and then count back toward the eyepiece,
making a direct-view O degrees.
Setup of a Rigid Borescope
To find the direction and field of view during visual test-
ing with a rigid borescope, place a protractor scale on a
board or worktable. Position the borescope carefully so it is
parallel to the zero line, with the lens directly over the center
mark on the protractor. Remember that the optical center of
FIGURE 27. Borescope images with variable FROM ELECTRICPOWER RESEARCHINSTITUTE. REPRINTEDWITH
focus (see Fig. 26): (a) 75 mm (3 in.); PERMISSION.
(bJ 200 mm (8 in.); (c) 300 mm (12 in.)
FIGURE 29. Miniborescope wide angle lens:
0(aJ (a) general shape; (b) lens detail
0(bJ (aJ
(cJ (bJ
FROM ELECTRICPOWER RESEARCHINSTITUTE. REPRINTEDWITH LIGHT GUIDE FOR ILLUMINATION
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452 I NONDESTRUCTIVETESTINGOVERVIEW
a parabola from the center to the periphery of the housing, Typical Industrial Borescope
giving a graded index of refraction. Light passes through the Applications
fiber and at specific intervals an image is formed.
Aviation Industry
The solid fiber is about 1 mm (0.04 in.) in diameter, mak-
ing it possible to produce high quality and thin rigid The use of borescopes for tests of airplane engines and
borescopes from 1.7 to 2.7 mm (0.07 to 0.11 in.) in diame- other components without disassembly has resulted in sub-
ter. The lens aperture is so small that the lens has an infinite stantial savings in costs and time. A borescope of 11 mm
depth offield (like a pinhole camera) and no focusing mech- (0.44 in.) diameter by 380 mm (15 in.) working length can
anism is needed. be used by maintenance and service departments for visual
testing of engines through spark plug openings, without dis-
Accessories mantling the engines. An excellent view of the cylinder wall,
piston head, valves and valve seats is possible and several
Many accessories are available for rigid borescopes. hundred hours oflabor are saved for each engine test. Spare
Instant cameras, 35 mm cameras, and video cameras can be engines in storage can also be inspected for corrosion of
added to provide a permanent record of a visual test. Closed cylinder wall surfaces.
circuit television displays, with or without video tape, are
com~on as well. Also available are attachments at the eye- Aircraft propeller blades are visually tested during man-
piece permitting dual viewing or right angle viewing for ufacture. The entire welded seam of a blade can be
increased accessibility. inspected internally for cracks and other discontinuities.
Propeller hubs, reverse pitch gearing mechanisms,
Special Purpose Borescopes hydraulic cylinders, landing gear mechanisms and electrical
components also can be inspected with borescopes. Aircraft
Angulated borescopes are available with forward oblique, wing spars and struts are inspected for evidence of fatigue
right angle or retrospective visual systems. These instru- cracks and rivets and wing sections cam be tested visually
ments usually consist of an objective section with provision for corrosion. Borescopes used for tests of internal wing
for attaching an eyepiece at right angles to the objective sec- tank surfaces and wing corrugations subject to corrosion
tion's axis. This permits inspection of shoulders or recesses have saved airlines large sums of money by reducing the
in areas not accessible with standard borescopes. time aircraft are out of service.
Calibrated borescopes are designed to meet specific test Automotive Industry
requirements. The external tubes of these instruments can
be calibrated to indicate the depth of insertion during a test. Borescopes are widely-used in the manufacturing and
Borescopes with calibrated reticles are used to determine maintenance divisions of the automotive industry. Engine
angles or sizes of objects in the field when held at a prede- cylinders can be examined through spark plug holes without
termined working distance. removing the cylinder head. The cylinder wall, valves and
piston head can be visually tested for excess wear, carbon
Panoramic borescopes are built with special optical sys- deposits and surface discontinuities. Crankcases and
tems to permit rapid panoramic scanning of internal cylin- crankshafts are examined through wall plug openings with-
drical surfaces of tubes or pipes. out removing the crankcase. Transmissions and differentials
are similarly inspected.
Wide field borescopes have rotating objective prisms to
provide fields of view up to 120 degrees. One application of Borescopes are also useful for locating discontinuities
wide field borescopes is the observation of models in wind such as cracks or blowholes in castings and forgings.
tunnels under difficult operating conditions. Machined components such as cross bored holes can be
examined for internal discontinuities. Borescopes are used
Ultraviolet borescopes are used during fluorescent mag- to inspect cylinders for internal surface finish after honing.
netic particle and fluorescent penetrant tests. These Tapped holes, shoulders or recesses also can be observed.
borescopes are equipped with ultraviolet lamps, filters and Inaccessible areas of hydraulic systems, small pumps,
special transformers to provide the necessary wavelengths. motors and mechanical or electrical assemblies can be visu-
ally tested without dismantling the engine.
Waterproof and vaporproof borescopes are used for
internal tests of liquid, gas or vapor environments. They are Machine Shops
completely sealed and impervious to water or other types of
liquid. Borescopes find applications in production machine
shops, tool and die departments and in ferrous, nonferrous
Water cooled or gas cooled borescopes are used for tests
of furnace cavities, jet engine test cells and for other high
temperature applications.
VISUAL TESTING I 453
and alloy foundries. In production machine operations, TABLE8. Comparisonof visiontypes and angles
borescopes of various sizes and angles of view are used to of obliquity
examine internal holes, cross bored holes, threads, internal
surface finishes and various inaccessible areas encountered Type of Vision Angle of Angular
in machine and mechanical assembly operations. Specific Obliquity Field
examples are visual tests of machine gun barrels, rifle bores, (degrees)
cannon bores, machine equipment and hydraulic cylinders. (degrees)
In tool and die shops, borescopes are used to examine Direct 0 45
internal finishes, threads, shoulders, recesses, dies, jigs, fix- Forward oblique 25 50
tures, fittings and the internal mating of mechanical parts. In Forward vision 45 45
foundries, borescopes are widely used for internal inspections Right angle 90 50
to locate discontinuities, cracks, porosity and blowholes. Retrospective 135 45
Borescopes are also used for tests of many types of defense Circumferential at O 45
materials, including the internal surface finish of rocket Circumferential at 90 15
heads, rocket head seats and guided missile components.
Borescope Optical Systems
Power Plants
Borescopes are precise optical devices containing a com-
In steam power plants, borescopes are used for visual plex system of prisms, achromatic lenses and plain lenses
tests of boiler tubes for pitting, corrosion, scaling or other that pass light to the observer with high efficiency. An inte-
discontinuities. Borescopes used for this type of work are gral light source is usually located at the objective end of the
usually made in 2 or 3 m (6 or 9 ft) sections. Each section is borescope to provide illumination for the test object.
designed so that it can be attached to the preceding section,
providing an instrument of any required length. Angles of Vision
Other borescopes are used to examine turbine blades, To meet a wide range of visual testing applications,
generators, motors, pumps, condensers, control panels and borescopes are available in various diameters and working
other electrical or mechanical components without disman- lengths to provide various angles of visionfor special require-
tling. In nuclear plants, borescopes offer the advantage that ments. The most common types of vision are: (1) right angle,
the inspector can be in a low radiation field while the distal, (2) forward oblique, (3) direct and (4) retrospective.
or sensor, end is in a high radiation field.
These types of vision are characterized by different
Chemical Industry angles of obliquity for the central ray of the visual field, with
respect to the forward direction of the borescope axis (see
Visual tests of high pressure distillation units are used Table 8).
to determine the internal condition of tubes or headers.
Evaporation tubes, fractionation units, reaction chambers, General Characteristics
cylinders, retorts, combustion chambers, heat exchangers,
pressure vessels, furnaces and many other types of chemi- Desirable properties of borescopic systems are large
cal process equipment are inspected with borescopes or field of vision, no image distortion, accurate transmission of
extension borescopes color values and adequate illumination.
Tank cars are inspected for internal rust, corrosion and The brightest images are obtained with borescopes of
the condition of outlet valves. Cylinders and drums can be large diameter and short length. As the length of the
examined for internal conditions such as corrosion, rust or borescope is increased, the image becomes less brilliant
other discontinuities. because of light losses from additional lenses required to
transmit the image. To minimize such losses, lenses are typ-
Petroleum Industry ically coated with antireflecting layers to provide maximum
light transmission.
Borescopes are used for visual tests of high pressure cat-
alytic cracking units, distillation equipment, fractionation Optical Components
units, hydrogenation equipment, pressure vessels, retorts,
pumps and similar process equipment. Use of the bore- The optical system of a borescope consists of an objec-
scope in the examination of such structures is doubly signif- tive, a middle lens system, correcting prisms and an ocular
icant. Not only does it allow the examination of inaccessible
areas without the lost time and expense incurred in disman-
tling, it avoids breakdown and the ensuing costly repair.
454 I NONDESTRUCTIVE TESTING OVERVIEW
section (see Fig. 30). The objective is an arrangement of Field of view, on the other hand, is relatively large, gen-
prisms and lenses mounted closely together. Its design erally on the order of 50 degrees of angular field. This cor-
determines the angle of vision, the field of view and the responds to a visual working field of about 25 mm ( 1 in.)
amount of light gathered by the system. diameter at 25 mm (1 in.) from the objective lens. At differ-
ent working distances, the diameter of the field of view
The middle lenses conserve the light entering the system varies almost directly with the working distance.
and conduct it through the borescope tube to the eye with a
minimum loss in transmission. Design of the middle lenses Magnification of a borescopes optical system is given by
has an important effect on the character of the image. For the relation:
this reason, the middle lenses are achromatic, each lens
being composed of two elements with specific curvatures M (Eq. 2)
and indexes of refraction. This design preserves sharpness
of the image and true color values. Where:
Depending on the length of the borescope, the image m1 = magnification of the objective;
may need reversal or inversion or both, at the ocular. This is m2 = magnification of the middle lenses; and
accomplished by a correcting prism within the ocular for m3 = magnification of the ocular.
borescopes of small diameter and by erecting lenses for
larger designs. The total magnification of borescopes varies with diameter
and length but generally ranges from about 2x to 8x in use.
Depth of Focus, Field of View and Magnification Note that the linear magnification of a given bore scope
changes with working distance and is about inversely pro-
The depth of focus for a borescopic system is inversely portional to the object distance. A borescope with 2x magni-
related to the numerical aperture N: fication at 25 mm (1 in.) working distance therefore will
magnify 4x at 13 mm (0.5 in.) distance.
N = n sin a (Eq. l)
Borescope Construction
Where:
A borescopic system usually consists of one or more
n = the refractive index of the object space; and borescopes having integral or attached illumination, addi-
a = the angle subtended by the half diameter of the tional sections or extensions, a battery handle, battery box or
transformer power supply and extra lamps, all designed to
entrance pupil of the optical system. fit in a portable case (see Fig. 31). The parts of a fixed length
The entrance pupil is that image of any of the lens apertures, FIGURE 31 . Components of typicalborescope
imaged in the object space, which subtends the smallest angle system (case not shown)
at the object plane. Because the numerical aperture of
borescope systems is usually very small compared with that of
a microscope, the corresponding depth of focus is exceed-
ingly large. This permits the use of fixed focus eyepieces in
many small and moderately sized instruments.
FIGURE 30. Sectionalview of typical borescope,
showingrelationshipof parts in its opticalsystem
OBJECTIVE I .. CONDUCTING coR~EGULATOR.· .
LENS
------ .....'(/ CORD
OCULAR ?.,JLAMP CAP
[ . CORD TIPS
>· / __ k:_:7 ~,
OBJECTIVE ~- - ROTATING f INDICATING
. LENS ~ =>:» itCONTACT . BUTTON
~ CONNE~~~ EYEPIECE
CORRECTING
PRISM
VISUAL TESTING I 455
borescope for right angle vision are shown in Fig. 32. Also may be critical to detect and remove burrs and similar irreg-
shown is a lamp at the objective end of the device. In this ularities that interfere with correct service. Drilled oil leads
configuration, insulated wires are located between the inner in castings can be visually inspected, immediately following
and outer tubes of the borescope and serve as electrical con- the drilling operation, for blowholes or other discontinuities
nections between the lamp and the contacts at the ocular that cause rejection of the component. Right angle
end. A contact ring permits rotation of the borescope borescopes can be equipped with fixtures to provide fast
through 360 degrees for scanning the object space without routine tests of parts in production. The device's portability
entangling the electrical cord. In other models, a fixed con- allows occasional tests to be made at any point in a machin-
tact post is provided for attachment to a battery or a trans- ing cycle ·
former, or the illumination is provided by fiber optic light
guides (see Fig. 24). Forward Oblique Borescopes
Borescopes with diameters under 37 mm ( 1.5 in.) are The forward oblique system is a design that permits the
usually made in sections, with focusing eyepieces, inter- mounting of a light source at the end of the borescope yet
changeable objectives and high power integral lamps. This also allows forward and oblique vision extending to an angle
kind of borescope typically consists of an eyepiece or ocular of about 55 degrees from the axis of the borescope.
section, a 1 or 2 m (3 or 6 ft) objective section, with 1, 2 or
3 m (3, 6 or 9 ft) extension sections. The extensions are A unique feature of this optical system is that, by rotating
threaded for fitting and ring contacts are incorporated in the the borescope, the working area of the visual field is greatly
junctions for electrical connections. Special optics can be enlarged.
added to increase magnification when the object is viewed
at a distance. Retrospective Borescope
Eyepiece extensions at right angles to the axis of the The retrospective borescope has an integral light source
borescope can be supplied, with provision to rotate the mounted slightly to the rear of the objective lens. For a bore
borescope with respect to the eyepiece extension, for scan- with an internal shoulder whose surfaces must be accurately
ning the object field. tooled, the retrospective borescope provides a unique
method of accurate visual inspection.
Right Angle Borescopes
Direct Vision Borescope
The right angle borescope is usually furnished with the
light source positioned ahead of the objective lens (see The direct vision instrument provides a view directly for-
Fig. 32). The optical system provides vision at right angles to ward with a typical visual area of about 19 mm (0.75 in.) at
the axis of the borescope and covers a working field of about 25 mm (1 in.) distance from the objective lens. The light
25 mm (1 in.) diameter at 25 mm (1 in.) from the objective carrier is removable so that the two parts can be passed suc-
lens. cessively through a small opening.
Applications of the right angle borescope are wide- PhotographicAdaptations
spread. The instrument permits testing of inaccessible cor-
ners and internal surfaces. It is available in a wide range of Many borescopes also include the ability to record with
lengths, in large diameters or for insertion into apertures as still photography, motion picture or video tape. For exam-
small as 2.3 mm (0.09 in.). It is the ideal instrument for ple, still pictures on 35 mm film can be taken with a
visual tests of rifle and pistol barrels, walls of cylindrical or borescope fitted with an adapter designed for the purpose.
recessed holes and similar components. A telescopic system with a movable prism built into the
adapter operates on the reflex principle, permitting obser-
Another application of the right angle borescope is vation of the visual field of the borescope up to the instant of
inspection of the internal entrance of cross holes, where it photographic exposure. High intensity light sources incor-
porated into the borescope provide illumination for 16 mm
FIGURE 32. Typical right angle borescope circular pictures on 35 mm film. Motion pictures are possi-
ble with a fiber optic light source or a rod illuminator that
LIGHT RIGHT ANGLE INDICATING BUTTON EYEPIECE eliminates electrical connections and the heat of a lamp
SOURCE OBJECTIVE LENS from the objective end of the borescope. This is especially
valuable where explosive vapors are present.
TELESCOPE TUBE ROTATING CONTACT ---
. --------·- -~
T ,___ WORKING LENGTH I
DIAMETER LAMP CHAMBER CAP CONTACT RINGS
456 I NONDESTRUCTIVE TESTING OVERVIEW
Photography of the interiors of large power plant fur- optical system and the camera from the furnace's high
naces during operation has been done since the 1940s temperatures. With this equipment, still and motion pic-
using a unit power periscope and camera. 17 The periscope ture studies have been made of the movement of the fuel
extends through the furnace wall and relays the optical bed and the action of the powdered fuel burner in fur-
image to the camera. A water cooled jacket protects the naces operating at full load.
VISUAL TESTING I 457
PART 5
VIDEO TECHNOLOGY
Photoelectric Devices Selenium was the earliest known photoconductor. In the
pure or combined form, it is still the basis for many photo-
Electronic aids to vision are based primarily on photo- conductive cells. More recently, many photoconductive
electric devices. These devices convert information in the materials have been discovered with efficiencies higher than
form of light into electrical signals that may then be ampli- selenium. Among them are lead sulfide, thallous sulfide,
fied or processed to perform some function that increases cadmium sulfide and cadmium selenide. Some of these
the ability of the observer to gather and interpret the test materials are now used in commercial cells.
data. The information may be in the visible region or in a
form invisible to the eye, such as ultraviolet or infrared radi- All photoconductive cells require an external source of
ation. current because their electrical resistance varies in response
to illumination. They are extremely sensitive but, because of
Classifications of Photoelectric Devices a lag inherent in photoconduction, they are slower in
response than photoemissive devices. Consequently, photo-
There are two broad classifications of photoelectric conductive cells are generally more useful in light measur-
devices: (1) detectors or measuring devices and (2) a two- ing devices and control equipment than in high speed
dimensional image of visual data over an area. The first cat- applications such as sound reproduction. Some photocon-
egory includes photoemissive cells and photomultipliers, ductive cells are useful at audio frequencies and the speed
photoconductive cells, photovoltaic cells and various devices of response of some photoconductors may be increased by
that measure radiant energy directly, such as bolometers bias lighting.
(thermal radiation detectors) and thermocouples. In the
second category are various image converter tubes, image Photovoltaic Devices
amplifier screens and television pickup tubes.
Photovoltaic devices are similar in some respects to pho-
Photoemissive Devices toconductive cells. They differ in one important way: photo-
voltaic cells are true energy converters rather than control
Photoemissive devices, whether simple phototubes or devices. Light falling on such cells produces a potential dif-
multiplier phototubes, are characterized by use of materials ference across the terminals and a continuous current can
that emit electrons under the influence of light. These elec- be drawn from them (the energy in the light is converted
trons are then drawn away from the emitting surface by an directly into electric energy). This is particularly useful in
electric field and used as the signal current, which may actu- light measuring devices such as photographic exposure
ate relays to be amplified electronically. meters because no external electrical source is required.
Photoconductive Cells or Construction
Photodiodes
Photovoltaic cells are made in sandwich form: a base
In the class of solid materials known as semiconductors, conductor coated with active material, over which is applied
the energy levels of electrons in atoms are so arranged that a transparent conductor. In most cells, the active material is
electrons can be excited into conduction bands by thermal either copper oxide on copper or selenium on iron. The lat-
or other means. Without such excitation, these materials ter is often preferred because of its high stability.
exhibit very little electrical conductivity. Photoconductors
are a class of semiconductors in which the absorption of The selenium barrier layer cell is a reasonably efficient
light energy excites some of the electrons into conduction converter. Under optimum conditions it delivers about 2
bands so that conductivity is increased under the influence percent of the light energy falling on it as electric energy.
of light. Recent work with silicon and other materials in solar batter-
ies has yielded efficiencies of 10 percent or greater. Power
output of several watts has been obtained in large area cells.
While the inherent response of the photovoltaic or bar-
rier layer cell is very fast, its high capacitance reduces its
response at higher frequencies.
458 I NONDESTRUCTIVE TESTING OVERVIEW
Uses of Photoelectric Detecting amplification of lOOx over the output of a conventional flu-
and Measuring Devices oroscope has been obtained.
The photoelectric cell or multiplier phototube is used in Television Systems
many ways in industrial nondestructive testing. One of the
most common uses is the measurement of radiant flux. In Television pickup systems make use of a scanning pro-
many respects, the phototube exceeds the capabilities of the cess to convert a two-dimensional spatial distribution of
human eye. It can detect not only radiation invisible to the light values into a time sequence. In this way, a three-
eye but can also accurately measure quantities of light with- dimensional system of information (two spatial dimensions
out reference to a standard, as required in a visual photome- and intensity) is converted to a two-dimensional signal (time
ter. The familiar photoelectric exposure meter is no more and amplitude). It is therefore possible to collect consider-
than a barrier layer photovoltaic cell and a sensitive meter. ably more information with a television system than with a
simple photocell circuit.
In addition to measuring light flux, photoelectric devices ·
permit measurements of reflectance and transmission of For most uses in visual testing instrumentation, the need
materials, comparisons of two or more sources of light and for a simple and inexpensive system has led to the develop-
(with the aid of filters or some type of spectrometer) colori- ment of the vidicon.l" The vidicon is a photoconductive
metric measurements. Phototubes find a natural application tube available in several sizes. The standard size is 25 mm
in spectrophotometry. Next to the measurement of radiant (1 in.) in diameter and 150 mm (6 in.) long. The sensitive
flux, perhaps the most widespread use of photoelectric area is about equal to a frame of 16 mm motion picture film.
devices is in monitoring and control applications . Street Because of the high quantum efficiency of the photocon-
lamps may be turned on by phototube circuits when day- ductive process, the vidicon compares favorably in sensitiv-
light decreases to a certain level. In power plants, photo- ity with the image orthicon. It is difficult to make a direct
tubes watch furnace flames. They monitor the amount of comparison because of differences such as amplitude
smoke emitted from the stack. They control door openers response characteristics (gamma) and lag effects but at ordi-
and safety interlocks on punch presses, inspect bottles after nary levels of illumination the factor is between 5 and 10
washing and detect foreign matter in filled soft drink bot- times in favor of the image orthicon.
tles. They sort products such as beans, peas and coffee and
actuate mechanisms for rejecting off-color products. The smooth gamma characteristic (= 0.7 power) of the
vidicon gives it an excellent halftone range. This, coupled
The electrical circuitry for these operations is usually with its high signal-to-noise ratio, enables it to reproduce a
simple and can be made extremely rugged and reliable. very high quality picture under good lighting conditions.
Photoconductive lag, which limits the response speed of all
Photoelectric Imaging Devices photoconductive devices, is present in the vidicon and may
limit its usefulness with rapidly moving objects at low light
Solid State Image Amplifier levels. The lag varies inversely with light level. At typical
light levels, the response speed is adequate for meeting
The solid state image amplifier18 is a sandwich of a pho- nearly all test requirements.
toconductive layer and an electroluminescent layer. The
electroluminescent layer is composed of a material that Many types of pickup equipment have been built around
emits light in response to an applied voltage. The photocon- the vidicon for industrial purposes. In general, there are two
ductor and the electroluminescent material are essentially basic designs: (1) a very compact and inexpensive type of
in series across a suitable alternating current voltage supply. self contained unit designed for use with a standard televi-
In darkness, the photoconductor is highly resistive and sion receiver as a monitor and (2) elaborate and more versa-
passes no current. When light falls on it, it becomes conduc- tile units complete with integral circuitry and monitoring
tive and current flows through the sandwich, causing the facilities, used where the requirements of performance are
luminescent material to emit light in the illuminated more strict.
regions. Light amplifications up to l,OOOx have been
obtained. There are many advantages of television for industrial
visual and optical testing. Television can be used in
A modification of this light amplifier is the amplifying restricted space or hazardous environments. Televised
fluoroscope. In this device, the photoconductive material information from several points can be brought to a central
responds directly to X-ray radiation and by the same princi- location for coordination. Conversely, the same information
ple as the light amplifier converts it to visible light. An may be distributed to a number of monitors in several loca-
tions simultaneously.
Television is used for visual inspection of shock
absorbers on automobiles by mounting a camera under-
neath the car and observing the spring action. Television
VISUAL TESTING I 459
observation of furnace flames has greatly aided the monitor- Video Borescope Components
ing of burner tests. For tests of radioactive materials, televi-
sion gives a safe, close view. Television may also be used in A video borescope has four main components (see
conjunction with the light microscope.i" particularly in biol- Fig. 33): (1) a probe, with a charge coupled device embed-
ogy and metallurgy. ded in the distal tip; (2) a video processor, to communicate
signals to the monitor; (3) a monitor, black and white or
In addition to the advantage of viewing convenience, vidi- color and (4) an alphanumeric keyboard, for entering identi-
cons can also be made sensitive to ultraviolet. This permits fication references onto the display or into a permanent
visual testing of materials under illumination invisible to the record.
eye but in a spectral region where many materials have dis-
tinct absorption characteristics.21 Likewise, vidicons can be Probe features vary to include lengths over 30 m (100 ft),
made sensitive to the near infrared region and to X-rays.22 diameters as small as 6 mm (0.25 in.), remote four way
steering control, right angle adapters for 90 degree viewing
With the wide variety of industrial television equipment and the ability to measure accurately what is seen in the
available commercially, it is relatively simple to select suit- borescope image.
able units to set up a visual information link. Various acces-
sories are available, including weatherproof housings and a Video borescopes are easily coupled with such acces-
wide variety of auxiliary monitors and switching units. sories as standard video recorders, telephone modems, elec-
tronic hardcopy devices or computer enhancement
Video Borescopes equipment.
The coupling of video and borescope technologies has Video Borescope Operation
solved some of the long standing problems experienced by
operators of conventional borescopes. In some cases, video Video borescopes transmit images in the following way.
equipment has simply been adapted to an existing bore- First, light is sent to the test area by fiber optic light guides
scope, transmitting images to a monitor as they appear in or by light emitting diodes. Fiber optic lighting can be used
the eyepiece. More sophisticated systems transmit images for either color or black and white imaging. Light emitting
to a monitor electronically by means of a tiny camera diodes are used only for black and white imaging, because
located at the distal tip of the borescope. This camera is typ-
ically a solid state silicon chip or light sensor known as a FIGURE 33. Components of video borescope
charge coupled device.
Development of Charge Coupled Device
Technology
The first chips were intrinsically sensitive only to light
intensity and not to light color, so they could be used only
for black and white imaging. Today, however, techniques
have been developed that achieve color imaging from them
as well.
One of these techniques uses color sequential lighting to
timeshare each pixel among the three additive primary col-
ors of light. Red, green and blue images are captured
sequentially and then resolved into a single image either at
the monitor or in a simple processor prior to the monitor.
This technique is completely external to the chip.
Another technique involves the placement of tiny color fil-
ters on each chip detector, yielding what the industry refers to
as the color chip. By grouping detectors of different color fil-
ters together and using them for a single pixel of information,
one can capture the three primary colors in each pixel simul-
taneously rather than sequentially. This technique simplifies
the image processing but cannot achieve the same resolution
without doubling the size of the chip.
460 I NONDESTRUCTIVE TESTING OVERVIEW
they cannot transmit multicolored light. While several col- By replacing fiber optic image guides with an electronic
ors are available, light emitting diodes transmitting red light signal, video borescopes solve such image quality problems
offer two advantages: red has the brightest output in the (see Fig. 35). Images are further enhanced because a video
light emitting diode family and charge coupled devices are borescope magnifies them and increases their resolution. As
more responsive to red than to any other color. a side benefit of replacing the breakable fiber optic image
guides, a video borescope's durability is increased and its
Once the light has reached the test area, a fixed focus service life is extended.
lens in the tip of the probe gathers reflected light and
directs it to the surface of the charge coupled device (see One of the standard borescope's most significant limita-
Fig. 34). On the chip, the pixels convert light into analog tions is that it requires frequent refocusing. Primaryfocusing
electrical signals. The signals travel down the length of the occurs when the lens focuses the image onto the fiber optic
probe through a series of amplifiers and filters. bundle at the tip of the probe. Secondary focusing takes
place at the eyepiece. When changing the viewing distance
Advantages of Video Borescopes even slightly, the operator must manually refocus the lens.
Standard borescopes can cause eyestrain and often A video borescope's depth of field (the range of distance
require the operator to adopt awkward positions to see in focus) is so much expanded that focusing becomes unnec-
through the eyepiece. Over time, fatigue and discomfort can essary. This feature is sometimes called automaticfocusing,
interfere with the inspector's ability to interpret images cor- although no mechanism is actively adjusted. The expanded
rectly, Video borescopes eliminate these problems by allow- depth of field is attributable to both the proximity of the
ing the inspector to sit comfortably in front of a monitor. lens to the charge coupled device and the small diameter of
the lens aperture. By eliminating the time consuming task of
Video borescopes also allow multiple views of the same refocusing, video borescopes make remote visual testing
image, making evaluations more reliable and facilitating more efficient and less fatiguing.
training. In fact, a given image can be transmitted simulta-
neously to any number of monitors at the site or, by modem Another major advantage of video borescopes is the abil-
or satellite, to remote locations. ity to use the shadow projection technique to calculate mag-
nification and measure objects and indications seen in the
Image quality can be low in certain borescopes - one video screen. Obviating optical comparators, this technique
complaint of some fiber optic borescopes is that they convey is practical in probes that use imaging chips in their distal or
an inherently fuzzy, honeycomb pattern caused by the tiny objective tips.
spaces between the optical fibers. Also, individual image
guide fibers degrade over time, causing density inconsisten- FIGURE 35. Interior of jet engine burner
cies in the image. In contrast, breakage of fibers in video showing seams and apertures: fa) through
borescopes has no effect on the transmitted image unless it fiber optic borescope; [b] through video
is so severe as to significantly decrease the amount of deliv- borescope
ered light. A third image quality problem is color inaccuracy
caused by the glass fibers' absorption of blue light, resulting fa)
in reddish images.
FIGURE 34. Distal tip of video borescope
LIGHT OUTLET fa)
LENS
WIRE CHARGE COUPLED
CARRYING DEVICE CHIP
IMAGE SIGNAL
VISUAL TESTING I 461
Finally, video taping with a standard borescope is an meet documentation requirements conveniently by attach-
awkward procedure but with a video borescope it becomes a ing a standard video recorder to a video borescope to docu-
simple and convenient way to document tests for future ref- ment processes of interest.
erence or training.
Finally, video borescopes are particularly advantageous
Some video borescopes offer other advantages, including when test time and proximity must be kept to a minimum,
illumination level adjustment, detail amplification in dark as with procedures involving exposure to radiation, heat or
areas, freeze frame capability and interchangeable probes. harmful chemicals. For example, nuclear power plants that
regularly inspect generator tube sheets for corrosion prod-
Disadvantages of Video Borescopes uct sludge deposits may discover that by switching to a
video borescope they can reduce test time significantly.
Video borescopes are typically less portable and more Less time will be spent retrieving debris because the oper-
costly than other designs. A system can weigh up to 25 kg ator can feed a recovery tool through the video probe's
(55 lb) and cannot be easily carried from site to site. The ini- operating channel rather than removing the probe and
tial cost of a video borescope can be greater than that of an using a recovery tool later. Given the limitation on the radi-
standard borescope, although the price difference decreases ation a technician can receive, increased efficiency means
or disappears when a closed circuit television camera is that video borescope tests are safer and can involve fewer
added to the borescope. workers.
Video Borescope Applications As a result of continuing miniaturization of electronic
components, video borescope systems will become more
Video borescopes lend themselves to any application portable as they decrease in size and weight. Another area
requiring remote visual testing, from aerospace and power with promise is the use of satellites for transmitting video
generation industries to engine manufacturing and marine borescope images. The technology is in place and, as indus-
operations. Video borescope technology is especially useful tries using nondestructive testing become more global,
for interpreting and confirming questionable indications from satellite communications will be used more often for long
other nondestructive testing techniques. For example, a distance consultation and training.
video borescope might be instrumental in identifying the
nature of a discontinuity revealed by an X-ray or ultrasonic Video borescope costs are not expected to drop because
test. the devices' specialized and customized uses eliminate the
possibility of mass production and the benefits of economies
Video borescopes are well suited to applications where of scale.
multiobserver viewing of inside surfaces is desirable. For
example, in the aerospace industries, inspection of an engine's Principles of Scanning
turbopump or a plane's wing cell are absolutely critical to safe
operation of the craft. The video borescopes ability to display The picture on a television screen is created by moving
an area evaluation to several viewers simultaneously, while an electron beam back and forth across the inside of the
minimizing fatigue, is of value in such test environments. The screen, varying the intensity of the beam as it scans (see
freeze frame ability is particularly useful for evaluation. Fig. 36). The electron scan initiates at the top left and
moves from left to right across the screen, forming the path
For applications requiring critical assessment of detail, shown in Fig. 36 between point 1 on the left and point A on
video borescopes provide high image quality in terms of mag- the right.
nification, resolution and color accuracy as well as accurate
measurement. Image quality and measurement are helpful After reading the right side, the electron beam is rapidly
when checking coatings and seals, analyzing chemical reac- returned from point A on the right to point 2 on the left. As
tions, identifying corrosion and pitting or locating weld area the electron beam returns to the left, no picture information
bum through. Steam plant operators, for example, need to is transmitted. The line 1-A is called a trace and the dashed
evaluate the inside surfaces of boiler tubes accurately. Using line A-2 is called the return trace. This process of electron
video borescopes, they can detect and identify chemical beam scanning is repeated throughout the generation of the
deposits and oxygen pits (depressions formed by oxygen picture.
attacking the metal) early enough to prevent the boiler tubes
from developing serious discontinuities. The electron beam continues to scan back and forth
across the screen even in the absence of a picture. In this
Video borescopes are also ideal for applications requir- situation, the beam traces a white rectangle on the screen
ing video tape documentation. For instance, pharmaceutical and this lighted rectangle is called a raster.
processing plants subject to exacting federal regulations can
After the electron beam reaches the bottom of the raster,
it is rapidly moved back to the top of the screen where it
462 I NONDESTRUCTIVE TESTING OVERVIEW
repeats the sequence. The image appearing on the screen is are converted into electrical impulses and this determines
formed by a number of complete rasters each second. the frame rate and the quality of the final image reproduced
at the receiver. For high resolution at the viewer, the camera
Taking into consideration brightness, transmission tube must separate the object image into as many picture
bandwidth and the effect of continuous motion, an opti- elements as possible. The higher the number of pixels, the
mum number of thirty rasters each second was chosen for greater the resolution for detail at the receiver.
standard television. These complete rasters are sometimes
referred to as frames. Because a frame frequency of thirty Television camera tubes are divided into two classifica-
pictures per second produces a flicker discernible to the tions, based on the way they produce an electrical image
eye, each picture is divided into two parts called fields. Two within the tube. The first method is called photoemission, in
fields are required to produce one complete picture or which electrons are emitted by a photosensitive surface
frame. The field frequency is sixtyfields per second and the when light reflected from the object is focused on the sur-
frame frequency is thirty frames per second. Each field face. Television tubes that use the photoemission method
contains one half of the total picture elements. are called image orthicon tubes.
The picture appearing on the television screen is divided The second method is called photoconduction. In this
into its two parts by a process called interlaced scanning. process, the conductivity of the photosensitive surface
The purpose of interlaced scanning is to eliminate flicker changes in relation to the intensity of the light reflected
and it is done by increasing the electron beam's downward from the scene focused onto the surface. Tubes using the
rate of travel so that every other line is sent. When the bot- photoconduction process are called vidicon tubes.
tom is reached, the beam is returned to the top and the
alternate lines are sent. The odd and even line scans are Cathode Ray Viewing Tube
each transmitted at 1/60 s, totaling 1/30 s per frame and
retaining the standard rate of 30 frames per second. The The two aspects of a cathode ray tube most important to
eye's persistence of vision allows the odd and even lines to visual interpretation are brightness and contrast. As the
appear as a single image without flicker (see Fig. 37). electron beam scans the back side of the fluorescent screen,
not all of the emitted light is useful. About 50 percent of the
TelevisionCamera Tubes light travels back into the tube, 20 percent is lost in the glass
of the tube screen by internal refraction, leaving only
The television camera tube is a critical component in the 30 percent to reach the observer.
closed circuit television system. Images received by the tube
FIGURE 36. Typical electron beam scan path as FIGURE 37. Complete electron beam scanning
seen on television screen path with two fields per frame using interlaced
scanning; beam path for second field, not
PATH OF shown, travels between scans of first field
SCANNING
RETURN TRACE ) START OF START OF
( ELECTRON BEAM SECOND FIELD
/ /FIRST FIELD
I - -- A
+-
2 -- L -- c I-
3 -- D
4 ~---- -E
5 -- -- F r
6 G I- -
H
+-
=--=-=--=-=~------------'
10 ____ - I END OF
II c:~:::::E~~:J,~ _ _ -
12 _1 FIRST FIELD
I 3 ....___ _ _ -- END OF SECOND FIELD /
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH
PERMISSION. PERMISSION.
VISUAL TESTING I 463
Image contrast is reduced by light returned to the screen Reflections at the Surface of the Screen Face
reflecting from some other point. There are four main
sources of this interference: (1) halitation, (2) reflections A portion of incident light is reflected when it reaches
from screen curvature, (3) reflections at the surface of the the outside surface of the glass (the glass/air interface).
screen face and (4) reflections from inside the tube. These rays are reflected back and forth between the inner
and outer surface of the glass with some of the light being
Halitation emitted and the balance being absorbed.
If the electron scanning beam is held in one spot, the visi- Reflections from Inside the Tube
ble spot on the screen is surrounded by rings of light. These
rings are caused by a phenomenon known as halitation (see Reflections from the inside surface of the tube can
Fig. 38). Ll.ght rays leaving the fluorescent crystalsat the inner decrease the field contrast of the image. Adding an
surface of the glass are refracted as they travel into the glass. extremely thin film of aluminum to the back of the fluores-
cent screen can virtually eliminate these reflections.
In Fig. 38, certain rays are reflected back into the glass
by the outside surface of the glass. Where these reflected Video Resolution
rays strike the fluorescent crystals, they produce visible
rings on the screen causing a hazy glow surrounding the Determining the Minimum Visible Discontinuity Size
beam spot. Halitation reduces the maximum detail contrast.
The resolution of a television system is the number of
Reflections Caused by Screen Curvature lines in the picture. As shown in Fig. 37, the electron beam
pr?duces a picture by drawing repeated lines of varying
Reflections caused by curvature of the screen (see bnghtness across the tube. In a video broadcast picture, a
Fig. 39) produce a loss of contrast. The interface between signal with 525 lines is used. About 480 lines actually form
glass and air provides an angle of incidence that refracts the the picture and the balance are used to return the beam
light striking it. Flatter surfaces retain more contrast. from the bottom to the top of the screen. There is also a
kind of resolution in the horizontal direction, because televi-
FIGURE 38. Diagram of reflected rays causing sion monitors are designed to have equivalent horizontal
halitation and vertical resolution. Closed circuit television systems
used for visual nondestructive tests may have resolution of
AIR SIDE GlASS FACE 500 lines, higher than consumer broadcast systems of about
200 lines.
,'
Usually, a video system cannot resolve detail smaller than
VACUUM SIDE one line. If the system has a 53 cm (21 in.) monitor and a
900 active line display, then the smallest detail that can be
STRIKING ELECTRON BEAM FLUORESCENT resolved is predicted as follows. First, the vertical dimension
SCREEN of the screen is determined - note that a nominal 53 cm
picture tube is 53 cm (21 in.) on the diagonal with a stan-
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH dard height-to-width ratio of 3:4 (forming the right triangle
PERMISSION. shown in Fig. 40). For every 5 units on the diagonal
(hypotenuse of the right triangle), there are 3 units on the
FIGURE 39. Reflections caused by screen vertical; for every 1 unit on the diagonal, there are 3/5 or 0.6
curvature units on the vertical. One might suppose that the smallest
detail that can be resolved with 1: 1 magnification and a
USEFUL DIAMETER 530 mm (21 in.) monitor then is (530 x 0.6) I 900 = 0.35 mm
or (21 x 0.6) I 900 = 0.014 in.
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH
PERMISSION. In practice, however, there are other variables to con-
sider. It is better to magnify by moving the camera closer to
the test object than merely to plug the video output into a
larger screen. Also, because of considerations such as orien-
tation and lighting, indications may be detected that are nar-
rower than a resolution calculation would predict. Moreover,
line resolution must be considered to be a characteristic of
464 I NONDESTRUCTIVE TESTING OVERVIEW
an entire system - the recording medium (video tape), the the camera itself. For scanning where no specific fixturing is
video camera and the monitor. The best way to quantify the involved, a zoom lens is very useful. The camera can be
resolution of a system is to use resolution charts. Such charts positioned with the lens in the wide angle mode and then
are expensive and may be replaced with visual acuity charts focused on the area of interest.
and test pieces with known anomalies.
Some underwater cameras have built-in panning sys-
In addition to the line spacing requirements, the fre- tems. Such units allow the lens to scan over a 180 degree
quency response of the system must be high enough to segment, looking forward and to each side of the camera -
allow small details to be electronically processed. The nec- a useful feature in restricted locations.
essary frequency response can be calculated from system
characteristics. If a camera with internal focusing is not available, it is
especially important to provide a stable operating platform.
If the vertical frame frequency of a high resolution tele- The camera mount should allow the camera to freely rotate
vision system is 30 frames per second and each frame con- so that accessible areas can be viewed from all angles. In
sists of 103 lines, the line frequency is 3 x 104 lines per nuclear reactor applications, the camera is mounted on the
second. To show a detail of the size figured above, the sys- refueling bridge and may be working at depths up to 30 m
tem uses 1.8 x 10-3 lines and requires a frequency response (100 ft). At these depths, water pressure increases at
of about 1.7 x 107 cycles per second. This is considerably roughly 10 kl'a-m-! (about 0.5 lbr·in;-2.ft---1).
more than can be expected of the best closed circuit televi-
sion equipment. A good system with a processing rate up to Proper performance of a television system depends on
5 x 106 cycles per second should be capable of displaying lighting. Many types of lights are available; Fig. 41 shows a
1.2 mm (0.05 in.) details. typical light source used with underwater television equip-
ment. Adjustment of light intensity with a rheostat is desirable.
These resolutions are computed without magnification.
If a lens with 2: 1 magnification is used, the requirements on FIGURE 41 . Typical underwaterlamp for visual
the electronic system are not so high and the size of the tests with television system
detail that can be resolved is reduced, as expected.
HANGING SHROUD
Effect of Magnification
The benefits of magnification come with a price - not
only does magnification increase the size of the image, it
also increases the effect of camera motion and noise. A cam-
era that has 1:1 magnification or greater is difficult to posi-
tion accurately. In order to minimize the effect of motion, it
is important to have as many adjustments as possible within
FIGURE 40. Relative unitsused to figurevertical PROTECTIVE B.AR
screen dimensionfrom diagonaldimension WATERPROOFDOME PORT
BULB
REFLECTOR
FOUR RELATIVE UNITS --I
T
~z
::)
u>i
~
"'llJ
"'ui
I
f--
_+ ¥
FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH FROM ELECTRICPOWER RESEARCH INSTITUTE. REPRINTED WITH
PERMISSION. PERMISSION.
VISUAL TESTING I 465
PART 6
REMOTE POSITIONING AND TRANSPORT
SYSTEMS
There are many types of remote positioning and transport With technology limitations, most of the equipment of
systems currently in use. Regardless of its specific form and the 1990s has been truly effective only in structured envi-
function, each system shares with the others a common ele- ronments. An environment can be structured in many ways.
ment: the controlled manipulation of a video camera for Mechanically structured environments can employ rails or
remote visual examination of system components. Integrating tracks. Autonomous submarines are being developed that
video camera technology with the latest in positioning and can calculate their position on the basis of known sonar buoy
transport systems results in the creation of new inspection locations. Vehicles that optically follow painted lines have
tools that can be used to safelycomplete the testing process in been operating in warehouses since the 1980s.
situations where radiation, heat or chemical environments
present serious health hazards to the visual inspector or Research has continued on systems of this type. How-
where physical configurations prevent inspector access. ever, unless the visual test requires simple pattern recogni-
tion, the visual data need a visual examiner to evaluate
The term positioning and transport systems as used in this them. This requires operator remote control over vehicle
text refers to any apparatus that puts a video camera and movement and opens the vehicle control logic loop, destroy-
lighting in proper spatial relationship to visually test a compo- ing the system's fully autonomous character. Because the
nent so that the camera can detect discontinuities. This defi- vehicle must perform semiautonomously at the inspection
nition is general to include as many configurations as possible. location, most automated vehicles are constructed as semi-
Equipment used in positioning and transport can be loosely autonomous from the onset, chiefly for economic reasons.
divided into three categories: fixed, automated and manual.
Semiautonomous
Fixed Systems
Semiautonomous positioning and transport systems oper-
Fixed systems, as the name suggests, hold cameras and ate on open loop control logic.These systemsrespond to input
lighting in fixed spatial orientation. A transport subsystem in from an outside source, typically an operator, rather than from
this system can carry components into the camera's inspec- acquired sensory input and internally processed feedback.
tion envelope and position them to detect discontinuities.
A good example of this type of device is a stepping pipe
Fixed systems range in complexity from simple wall crawler. The feet of a stepping pipe crawler extend to and
mounts to complex materials handling equipment. Fixed withdraw from the pipe wall as the vehicle body expands and
mounts can be used to provide monitoring capabilities on contracts, respectively. This design provides the vehicle with
components that either operate in a hazardous environment the ability to climb within vertical piping. Each step involves
or create such an environment as a result of operation. a series of cylinder pressurizations and depressurizations. An
operator oversees the overall motion (i.e., forward, reverse,
Sophisticated systems can manipulate the workpiece for stop, scan) and the computer determines the proper cylinder
inspection from multiple angles. sequencing to produce the motion. The operator-computer
relationship is necessary particularly in vertical piping where
Automated Systems the computer ensures leg engagement. This prevents an
operator from erroneously disengaging the fixture and let-
Fully Autonomous - Robotic ting it fall. In tum, the operator prevents the vehicle from
walking into thermowells or flow orifices.
A fully autonomous system will transport a camera to a
given location without operator intervention. These systems Manual Systems
have closed loop control logic and respond to the environment
in which they operate. In theory, a system such as this can Physical
transport a camera between two points negotiating all obsta-
cles encountered en route. Consequently, they require highly Physical positioning and transporting of cameras have
c leveloped sensors and sophisticated feedback processing. been the most common functions of manual systems in use
in the early 1990s. Physical positioning includes pushing
466 I NONDESTRUCTIVE TESTING OVERVIEW
cameras with rod or stiff cable and lowering cameras with monitor for real time viewing of the examination and a video
ropes, as well as many other simple yet effective methods. cassette recorder for a permanent record of the examination.
These systems usually provide adequate results and are eco- A wide array of video cameras suitable for visual testing are
nomical. available. These cameras usually employ tubes or semicon-
ductor chips.
In some applications the simple methods produce video
footage superior to that of more complex automated equip- The tube camera uses a vacuum tube similar to that
ment designed to replace them. For example, a flying rig is a found in a television to receive and convert video images into
three point suspension camera rig used in boiling water electrical signals. Tube type cameras commonly available
reactor vessel examination. It is simply a cleverly bent include vidicon and image orthicon video tubes. The primary
length of stainless tube suspended by ropes into the vessel. difference between the video tubes is the chemical composi-
This rig has been used for years and still outperforms the tion of the materials inside the tubes. Each type has its par-
latest technology in submarines and telescoping masts. ticular advantage but they all produce good video images.
Physically positioned cameras are heavily dependent on The chip camera employs a photosensitive semiconduc-
the skill and experience of the operator. Experience is essen- tor chip to receive and convert video images into electrical
tial in selecting the proper technique to be used for a given signals. Chip-type cameras commonly available use charge
task. Many inspections are performed by inserting video coupled device (CCD) chips, charge injection device (CID)
probes into pipes and headers. More often than not, inspec- chips, or metal oxide semiconductor (MOS) chips.
tors are disappointed with the results. Typically, an off-the-
shelf camera will have only a small field application without Each type of chip is constructed of different materials
modifications such as the use of sleds, carts, cable rigs or task and processes and converts video images in different ways.
specific lighting. Of these, the most important modification Like the various types of video tubes, the different chips
typically overlooked is lighting. The more versatile the light- have their characteristic advantages. The charge injection
ing capabilities, the more versatile the camera assembly. device chip can best tolerate radiation exposure whereas the
metal oxide semiconductor chip consumes the least power.
Mechanical
Typically, tube cameras can withstand higher radiation
Another common group of manually operated position- doses and higher temperatures than chip cameras. Chip
ing and transport systems are mechanical systems, normally cameras have proven successful for use in medium and low
providing better results or more versatility than physically radiation dose rate inspection work, and are typically more
positioned devices. The costs involved are often corre- reliable and less expensive than tube cameras.
spondingly greater. These range from simple designs such as
remote controlled cars and remotely operated booms to The inherent compactness of the photosensitive chips
more complex remotely controlled submarines and aircraft. enables chip cameras to be manufactured in much smaller
packages. This has made possible remote video inspection
Mechanical systems are typically created by the combi- of small diameter piping and other limited access compo-
nation of off-the-shelf cameras and transport vehicles cus- nents. As video borescopes, miniature video cameras are
tom designed per specific industry requirements. Most challenging fiber optic borescopes in performing internal
industrial research and development budgets are spent in inspection of small diameter piping. Video borescopes can
this category rather than for automated transport systems usually be inserted for a much greater distance than fiber
because mechanical control systems are the least expensive. optic borescopes and may give a better video image as well.
Unfortunately, highly specialized 1igs are limited in their
range of possible applications. All camera systems discussed here are available in color as
well as black and white versions. In some cases, black and
System Selection and Application white cameras offer higher resolution than color cameras.
For video footage recorded by a video cassette recorder
The criteria in developing the proper positioning and (VCR), picture resolution is no better than that of the
transport system for a particular task include proper selection
of both the video system (cameras and lighting) and the recorder, which is usually lower than resolution of nearly any
transport system. color or black and white cameras. For example, a 700 line
camera will only produce a 200 line video tape on a 200 line
Video Systems (Cameras and Lighting) video recorder.
All automated or robotic visual examination systems con- In reference to resolution, it is important to keep in mind
sist of a fixture or vehicle carrying a video camera and a the actual required resolution. High resolution equipment
remote lighting system. The system operator normally uses a will cost thousands of dollars more than commercial grade
equipment; however, most lower resolution equipment will
resolve a 25 µrn (0.001 in.) wire without incurring additional
cost. The camera and lighting should be selected for their
ability to detect discontinuities of interest and their ability to
function within the environment of the inspection area.
VISUAL TESTING I 467
Poor resolution is frequently a result of poor lighting. rolling vehicles in between. With no shortage of vehicles to
Lighting is often poor because, as the least sophisticated choose from, the difficulty arises in finding an appropriate
part of the system, it is given the least technical considera- transport vehicle for a given task.
tion. Operator attention to the range of lighting styles and
features available can help improve resolution. A significant The most effective method for the selection of transport
development in lighting systems has been an increase in systems is sistering: networking between similar industrial
their reliability and durability. Many can withstand thermal facilities to obtain the most current information regarding
and mechanical shock. vehicle application to specific tasks. The aim of this commu-
nication is to locate a vehicle that will most closely meet the
Transport Systems operator's immediate needs. Many companies employ per-
sonnel whose sole function is to network in this fashion and
There currently exists a great variety of transport vehi- maintain current files for a variety of applications. Vehicles
cles available, ranging from submersible vehicles to located by this method may not meet the operator's specific
remotely controlled aircraft with all manner of walking and needs; however, it is most likely that reasonable modifica-
tions can be made to the available vehicle to fit inspection
requirements.
468 I NONDESTRUCTIVE TESTING OVERVIEW
PART 7
MACHINE VISION TECHNOLOGY
Machine vision acquires, processes and analyzes an For example, a picture of a fast moving object taken with
image to reach conclusions automatically. A machine vision a video camera under continuous lighting results in a blurred
system consists of a light source, a video camera, a video dig- image. A clear image could be obtained with a strobe lamp.
itizer, a computer and an image display (see Fig. 42). The following text describes lighting techniques that are
commonly used in machine vision systems.
The light source illuminates the test object for the camera
to form a video image. The video digitizer converts the image Front Lighting
into digital form and the digital image is then stored in a two-
dimensional memory. The image is divided into rows and With front lighting, the light source and the image sensor
columns, which are subdivided into picture elements or pix- are placed on the same side of the test object (see Fig. 44).
els. Each pixel has an integer number which represents the Front lighting is the most convenient method of illumina-
brightness or darkness of the image at that point. This integer tion for machine vision systems. It is used when the features
value is called the gray level (see Fig. 43). of interest contrast sharply with the background - for
example, when scanning black characters on a white label.
Once an image is in the form of an array of gray level pix-
els, it is ready for computer processing. The computer first FIGURE 43. Digital image is made from
enhances the contrast of the image with a procedure known combinations of pixels with varying gray levels
as image enhancement. Following image enhancement, the
computer simplifies the image with image segmentation. [jROW PIXEL
The next step is known as feature extraction and finally, at
the classification stage, the computer identifies and groups COLUMN +T~ilT+wBL:A2CK:11 ::
objects in the image. 0
8-BIT GRAY LEVEL
Machine vision encompasses all the steps of image
acquisition, image conversion, image enhancement, image
segmentation, feature extraction and classification.
Lighting Techniques
The success or failure of a machine vision system is
largely dependent on the quality of the image acquired by
the system. And proper lighting techniques are essential for
obtaining high quality images.
FIGURE 44. Setup for front lighting with
machine vision
FIGURE 42. Basic machine vision system QSENSOR
COMPUTER DISPLAY
0 0\ I0UGHT SOURCE
/OBJECT
00 u:,nnJnn
II\ II\ BACKGROUND
LIGHT SOURCE
VISUAL TESTING I 469
Back Lighting Strobe Lighting
Strobe lighting is sometimes called flash lighting. The
With back lighting, the light source and the image sensor
are placed on opposite sides of the test object (see Fig. 45). electronic circuit for a strobe light is shown in Fig. 49. When
Back lighting is used when the silhouette of a feature is
important. FIGURE47. Formingline of light using
scanningmirror
Structured Lighting
Combining a light source with optical elements to form a
line of light is called structured lighting. There are two ways
to form a line of light: (1) place a semicylindrical lens in
front of the light source (see Fig. 46) and (2) use a scanning
mirror to deflect a laser beam (see Fig. 47). A line of light
may be used to measure the height or to detect the silhou-
ette of a test object (see Fig. 48).23
FIGURE45. Setup for back lightingwith
machinevision Q
SENSOR FIGURE48. Height and silhouettedetection
usingline of light
,~-~,/OBJECT
SENSOR
tttttttt
LIGHT BOX
CONVEYOR
FIGURE46. Formingline of light through
semicylindricallens
LENS LINE OF LIGHT
FIGURE49. Strobelighting circuit
CHOKE
HIGH +
VOLTAGE
CAPACITOR
T
470 I NONDESTRUCTIVE TESTING OVERVIEW
the circuit is triggered by a video camera, a flash of light illu- Image Tubes
minates the test object momentarily. Strobe lighting is used
to image moving objects or still objects with potential move- Image tubes are used to generate a train of electrical
ment. 24 pulses that represent light intensities present in an optical
image focused on the tube. The most widely used image
Ultraviolet Lighting tube is the vidicon.
Ultraviolet light causes fluorescent material to glow and The vidicon tube uses an electron beam to scan a photo-
is used in magnetic particle and liquid penetrant testing to conductive target known as the light sensor. A conductive
detect discontinuities.Pj" layer applied to the front of the photoconductor serves as
Optical Filtering FIGURE 51 . Characteristics of short pass filters
Image sensors used in machine vision systems detect the JOO
intensity of electromagnetic waves in the visible range. If
only a portion of the visible spectrum is of interest, a filter in 80
front of the sensor produces a higher quality image.
Qz ~- 65 CUT OFF WAVELENGTH--: I
Bandpass filters transmit a band of electromagnetic 50 CUT ON WAVELENGTH
waves and rejects the rest as shown in Fig. 50. Short pass fil- ~ [9 I
ters transmit electromagnetic waves below a cut off wave- 10 <1.2xCUTOFF: I
length as shown in Fig. 51. Long pass filters transmit ~V~zl -9au..5:. WAVELENGTH I
electromagnetic waves above a cut off wavelength as shown :
in Fig. 52. 1--- I
I
Neutral density filters attenuate the light level of the full I
spectrum incident on the image sensor when the light I
source itself is difficult to control.
:
Image Sensors
415T0475 I I
The two primary types of image sensors are image tubes
and solid state imaging devices, with the latter now becom- 450 TO I .000
ing much more generally used. Image tubes, however, can
typically withstand higher ionizing radiation doses and tem- WAVELENGTH
peratures than the solid state imaging devices. (nanometers)
FIGURE 52. Characteristics of long pass filters
100 --- - --- --2-~
FIGURE 50. Characteristics of bandpass filters z_ 80 PEAK T
50 AVERAGE T
~-9:0 0(rloJ1 10 - 50 PERCENT OF PEAK T / ! ""'
v 80 PERCENT OF PEAK T
PEAK WAVELENGTH V-Vll ~
~z .a.5.Vl u
BI·----------.:PEAK TRANSMISSION BLOCKED TO
(PERCENTAGE) WAVELENGTH 1--- CUT ON CUT OFF
WAVELENGTH WAVELENGTH
MAXIMUM
~-9:z~u.a.5. 85 PERCENT OF WAV)NGTH
Vl <,.,__
1--- . / ..;__
WAVELENGTH 2.200
{nanometers)
4 I 5 TO 1.000
WAVELENGTH
VISUAL TESTING I 4 71
the signal electrode. The signal electrode is operated at a Charge Coupled Devices
positive voltage with respect to the back of the photocon-
ductor which operates at the cathode voltage. The operation of a charge coupled device may be seen
by this example. When two out-of-phase clock strings are
The scanning beam initially charges the back of the pho- applied to the gate electrodes, the charge packets under-
toconductor to cathode potential. When a light pattern is neath the electrodes move from one storage element to the
focused on the photoconductor, its conductivity increases in next. Because each charge packet may be of different size,
the illuminated areas and the back of the photoconductor the line of elements becomes a simple analog shift register.
charges to more positive values. The scanning electron
beam deposits electrons on the positively charged areas, The transfer of charges from one element to the next is
resulting in current pulses that are read out as video signals. very efficient. The amount of charge in each packet stays
substantially the same, even after it has been passed through
In addition to the standard vidicon, variations such as sil- a thousand sequential elements. When the string of charge
icon target vidicons and intensifiers are useful for some packets is read through the output register line by line
applications. (Fig. 53), a video image is obtained.
Solid State Imaging Devices Charge Injected Devices
The principle of the solid state imaging device is based on The charge injected device resembles the charge cou-
the photoelectric effect and the fact that free electrons are pled device except that during sensing the charge is con-
created in a region of silicon illuminated by photons. The fined to the image site where it was generated (see Fig. 54).
number of free electrons is linearly proportional to the inci- The charges are read using an X, Y addressing technique
dent photons. If a silicon device is made with a repetitive pat- similar to that used in computer memory. Basically, the
tern of small but finite photo sensing sites, the number of stored charge is injected into the substrate and the resulting
electrons generated in each site (charge packet) is directly current is sensed as the video signal.
proportional to the incident light on that specific site. If the
pattern of incident intensity is an optical image of an object FIGURE54. Charge injecteddevice array
focused on the surface of the silicon array,the charge packets
generated in the array form an electronic image of the object. (>:: PHOTOSENSITIVE
ELEMENT
There are two basic classes of solid state imaging devices: 0
charge coupled devices (CCD) and charge injected devices
(CID). ~
zLlJ ·~~~+-'......---+-.-~-+r~-+--.-~-+-
FIGURE53. Charge coupleddevice array
ow
VERTICAL SYNC z
VIDEO OUT
~ •-+~~---~-+~~---~-+~~--
32 x 44 ELEMENT
BUCKET BRIGADE 6~~
PHOTOSENSITIVEARRAY
CHARGING TRANSFER VIDEO
HORIZONTAL '-t::=:=:=:=:=:=:=:=:=:=:=:=:=:::::'...J HOLDING ELEMENTS OUT
CLOCK HORIZONTAL REGISTER
472 I NONDESTRUCTIVETESTINGOVERVIEW
REFERENCES
l. Frisby, John P. Seeing: Illusion, Brain and Mind. 15. Kaufman, J.E. and J.F. Christensen. JES Lighting
New York, NY: Oxford University Press (1980). Handbook, fifth edition. New York, NY: Illuminat-
2. Masland, Richard H. The Functional Architecture ing Engineering Society (1972).
of the Retina. New York, NY: Scientific American 16. Ness, S. and C.E. Moss. "Current Concerns about
Books (December 1986).
Optical Radiation Safety in Fluorescent Magnetic
3. Bailey, William H. "Why Color Vision Tests?" Mate Particle and Penetrant Methods." Materials Evalu
rials Evaluation. Vol. 42, No. 13. Columbus, OH: ation. Vol. 54, No. 3. Columbus, OH (March 1996):
American Society for Nondestructive Testing p 364-367.
(1984): p 1,546-1,550. 17. Duncan, J.A. and A.A. Levin. "The Pyroscope."
Photo Technique. Vol. 2. New York, NY: McGraw-
4. Treisman, A. Features and Objects in Visual Pro Hill Book Company (1940): p 16.
cessing. New York, NY: Scientific American Books 18. Kazan, B. and F.H. Nicoll. "An Electroluminescent
(November 1986). Light-Amplifying Picture Panel." Proceedings of the
IRE. Vol. 43. New York, NY: Institute of Radio
5. Bailey, William H. "The Case for Eye Test Stan- Engineers (1955): p 1,888.
dardization." Materials Evaluation. Vol. 40, No. 8. 19. Weimer, P.K., S.V. Forgue and R.R. Goodrich. "The
Columbus, OH: American Society for Nondestruc- VidiconPhotoconductive Camera Tube." Electron
tive Testing (1982): p 826. ics. Vol. 23, No. 5. New York, NY: McGraw-Hill
Book Company (1950): p 70.
6. Heginbotham, W.B. "Machine Vision: 'I See,' Said 20. Flory, L.E. "The Television Microscope." Cold
the Robot." Engineering Research Association Jour Spring Harbor Symposia for Quantitative Biology.
nal. Melton Mowbray, United Kingdom: Engineer- Vol. 16. Cold Spring Harbor, NY: Cold Spring Har-
ing Research Association (October 1983): p 14-17. bor Laboratory (1951): p 505.
21. Zworykin, V.K., L.E. Flory and R.E. Shrader.
7. Montville, Vicky L. "Color Control from Start to "Ultraviolet Television Microscopy." Electronics.
Finish." Quality. Vol. 22, No. 3. Carol Stream, IL: Vol. 25, No. 9. New York, NY: McGraw-Hill Book
Hitchcock Publishing Company (March 1983): Company (1952): p 150.
p 36-39. 22. Cope, A.D. and A. Rose. "X-Ray Noise Observation
Using a Photoconductive Pickup Tube." Journal of
8. Bailey, William H. "Charts for Visual Testing of Applied Physics. Vol. 25. Woodbury, NY: American
Inspection Personnel." Materials Evaluation. Institute of Physics (1954): p 240.
Vol. 41, No. 7. Columbus, OH: American Society 23. Ward, M.R., L. Rossol, S.W. Holland and R. Dewar.
for Nondestructive Testing (1983): p 849. "CONSIGHT: A Practical Vision-Based Robot
Guidance System." Ninth International Symposium
9. Mossman, P.B. "Testing for Degrees of Color Blind- on Industrial Robots. Dearborn, MI: Society of
ness." Occupational Health and Safety. Washington, Manufacturing Engineers (1979).
DC: Occupational Safety and Health Administra- 24. Cheu, Y.F. "Automatic Crack Detection with
tion (August 1983): p 49-55. Computer Vision and Pattern Recognition of
Magnetic Particle Indications." Materials Evalua
10. Bailey, William H. "Surface (Temper Etch) Inspec- tion. Vol. 42, No. 12. Columbus, OH: American
tion and Inspectors." Materials Evaluation. Vol. 41, Society for Nondestructive Testing (November
No. 9. Columbus, OH: American Society for Non- 1984): p 1,506-1,510.
destructive Testing (1983): p 1,000. 25. Nondestructive Testing Handbook, second edition:
Vol. 2, Liquid Penetrant Tests. Columbus, OH:
11. Adams, A.J. "Color Vision Testing in Optometric American Society for Nondestructive Testing
Practice." Journal of the American Optometric (1982).
Association. Vol. 45, No. l. St. Louis, MO: Ameri- 26. Nondestructive Testing Handbook, second edition:
can Optometric Association (January 1974): Vol. 6, Magnetic Particle Testing. Columbus, OH:
p 35-42. American Society for Nondestructive Testing
(1989).
12. Miller, S.C. Private correspondence. St. Louis, MO:
American Optometric Association.
13. Frisby, John P. Seeing: Illusion, Brain and Mind.
New York, NY: Oxford University Press (1980).
14. Threshold Limit Valuesfor Chemical Substances and
Physical Agents in the Workroom Environment with
Intended Changesfor1980. Cincinatti, Ohio: Ameri-
can Conference of Governmental Industrial Hygien-
ists, Threshold Limit Values Committee (1980).
13SECTION
THERMOGRAPHY AND OTHER
SPECIALMETHODS 1
Ronald J. Botsko, NOT Systems, Incorporated, Huntington Beach, California (Part 2)
Thomas S. Jones, Industrial Quality, Incorporated, Gaithersburg, Maryland (Part 3)
474 I NONDESTRUCTIVE TESTING OVERVIEW
PART 1
THE SPECIAL NONDESTRUCTIVE TESTING
METHODS
Special method is a descriptive, nonscientific term that case of metal identification, how one material differs from
has been widely applied to any of several nondestructive other materials. These are the concerns of nondestructive
testing methods - such as Barkhausen noise analysis, strain characterization rather than discontinuity detection.
gaging and holography - that generally provide informa- Although the major methods do provide information of this
tion about a material's mechanical behavior and properties, sort, they were developed first for discontinuity detection.
that is, its temperature, its flexibility, its microstructure, its
reactions to stress and to chemicals. It is not accurate to suppose that special methods are
necessarily "emerging" in the sense of being new. Some,
One thing these methods have in common is that they are such as strain gaging, were well established in industry by
what is left over after the "major methods" of nondestructive the 1950s, as wereultrasonic and eddy current testing. What
testing have been listed: acoustic emission, electromagnetic, the special methods have in common is the way industry has
leak, magnetic particle, penetrant, radiographic, ultrasonic incorporated their technologies, often considering them
and visual testing. These eight major methods, plus thermal aspects of process monitoring rather than of quality control
and infrared testing and neutron radiographic testing, are or inspection.
represented by Level III personnel qualification examina-
tions offered by the American Society for Nondestructive Strain is measured by the optical methods, including
Testing (ASNT). Major methods are represented by commit- moire imaging, holography and all other forms of interferom
tees in ASNT and most are represented by subcommittees in etry, as well as by resistance strain gaging, photoelastic coat
sister societies including the American Society for Testing ings (see Fig. 1) and neutron diffraction testing. The laser
and Materials, the Society of Automotive Engineers, the based photoacoustic testing techniques measure microstrain
International Organization for Standardization (ISO) and as an interferometric means of measuring ultrasonic waves.
nondestructive testing societies of other countries including
France, Germany, Japan and the United Kingdom. Inferences about a component's integrity can be drawn
from its ability to retain and emit energy. This observation is
Some major methods - leak testing, acoustic emission the basis for infrared and thermal nondestructive testing.
testing, visual testing and thermal and infrared testing -
were formerly considered to be special methods. For pur- Alloy identification (see Fig. 2) is a branch of nonde-
poses of the Nondestructive Testing Handbook, second edi- structive testing but has sometimes been overlooked as
tion, a special method is one that is not yet represented by being such because it is not concerned with discontinuity
its own volume in the series. (It is a natural event in the con- detection per se. Although nondestructive alloy identifica-
tinuing upgrading of this series of books that some methods tion and sorting had been in use for more than 50 years,
in this second edition volume will appear in the third edi- metals characterization began to receive serious attention in
tion's major method volume - photoacoustics, for example, the 1980s. It became clear that alloy identification belonged
should be part of the next ultrasonic volume.) Table 1 lists with the other nondestructive methods. The divergence of a
the special methods discussed in this volume and suggests material from specified chemistry is a material characteristic
the kind of information each provides. that affects the service life of a component, in some cases
more surely than a crack might. Alloy identification is in fact
Special method, then, has been an arbitrary concept not one method but a family of methods, including elec-
invented for convenience. Nevertheless, special methods tronic, radiation, magnetic and chemical methods. Some
have some things in common that set them apart from major alloy identification methods involve minor surface degrada-
methods. Most significantly, the major methods were devel- tion through corrosion or abrasion.
oped first precisely because they were expected to find dis-
continuities. The special methods, in contrast, tend to The fact that different materials emit electromagnetic
provide measurements of material behavior: how a material radiation at different frequencies is the basis for the thermo-
bends, becomes warm, exhibits stress and strain and, in the graphic and spectroscopic methods of nondestructive test-
ing. The electromagnetic characteristics of materials can be
used by various techniques to draw inferences about stress.
THERMOGRAPHY AND OTHER SPECIAL METHODS I 475
Relationship between Material laminates, for example, causing exfoliation of skins in aero-
Property and Material Behavior space composite panels. The same mechanism also makes
coatings buckle and crack. This is only one area where the
Material discontinuities do not necessarily consist of measurement of displacement can help to detect or predict
apertures like porosity or cracks. The interface between two material failure.
dissimilar materials that is literally discontinuous may easily
become a place where fracture occurs. In response to pres- As suggested above, several special methods (photoelastic
sure or temperature, one material may contract and expand and resistance strain gaging, moire imaging, holography and
at different rates or to different degrees than a contacting others) involve the measurement of strain. Strain can lead to
material. This mechanism creates voids between layers in fatigue and eventual failure. Stress and strain are related to
each other in a way described by solid mechanics.2
FIGURE 1 . Photoelastic coating reveals
indication of strain in solid rocket motor: When materials are acted upon by externally applied
(a) pressure vessel subjected to combined axial forces they can do one or both of the following: (a) move in
compression loads and internal pressure; response to the forces or (b) change in shape. If a uniform
(b) severe stress concentration and resultant bar of cross sectional area A is subjected to equal and oppo-
yielding in joint area site pulling forces F at its ends then it is subject to an inter-
nal tensile stress defined by the ratio of the force F to the
faJ area A:
stress F (Eq. l)
A
Similarly, if the same bar is subject to pushing forces then
the bar is under compressive stress and numerically stress is
given by a similar expression.
FIGURE 2. X-ray fluorescence spectrometry
applied to verify alloy in pipe stem
fbJ
476 I NONDESTRUCTIVE TESTING OVERVIEW
TABLE 1. Special nondestructive testing methods
Purpose of Test
Method Alloy Discontinuity Material Strain and Stress
Identification Detection Characterization Measurement
Acoustic no yesa no no
no yes no no
Acoustic holography no no yes yes''
Acoustography no yes yes no
Acoustoultrasonictesting no yes yes no
Photoacoustic techniques no yesc no yesc
Tapping
Vibration analysis no no no yes"
yes yes" yes= no
Electrical and magnetic no no no yes"
no yes no no
Barkhausen noise no yes yes no
Eddy current no no no yes
Magnetooptic yes no yese no
Magnetic resonance no no yes yes"
Microwave testing yes no no no
Photoelastic coating yes no no no
Resistivity(potential drop) testing
Stress induced magnetic anisotropy no yes no yes
Thermoelectric no no no yes
Triboelectric no yes no yes
no no no yes
Optical no yes no yes
Holographic interferometry yes no no no
Laser speckle yes no no no
Moire inferferometry yes yes yes'" no
Moire imaging yes no no no
Point triangulation
yes no no no
Radiation no no no yes
no no no yes
Optical emission spectroscopy no no no yes
Sparktesting
Thermal and infrared testing
X-ray fluorescence spectroscopy
Other
Chemical spot testing
Grid measurement
Neutron diffraction
Resistance strain gaging
a. PRECISEINFORMATION ON ANOMALY SIZEAND POSITION.
b. STRESS MEASUREMENT.
c. VIBRATION ANALYSIS,AS PARTOF CONDITION MONITORING DIAGNOSTICS,CAN HELP DIAGNOSE MATERIALCONDITIONS BUT FURTHERINSPECTION
IS NEEDED TO CONFIRM PRESENCEOF ANOMALIES OR DEFORMATION.
d. EDDY CURRENTTESTING IS USED TO DETECTDISCONTINUITIESAS A MAJOR METHOD (ELECTROMAGNETICTESTING) BUT IS LISTEDHERE AS A SPECIAL
METHOD BECAUSEOF ITS USE IN ALLOY IDENTIFICATION.
e. FOR EXAMPLE, CASEHARDENING.
f. THERMALRESISTANCE,THERMAL CONDUCTIVITY,THERMAL DIFFUSIVITY,THERMAL CAPACITANCE,EMISSIVITY,REFLECTIVITYT, RANSMISSION
ABSORPTIVITY.
THERMOGRAPHY AND OTHER SPECIAL METHODS I 477
The term strain refers to the relative change in the to this point, the work done in producing the deformation
dimensions or shape of a body subject to stress. For the sim- may be recovered when the material returns to its original
ple bar previously considered, if the natural length is l and shape. Beyond the yield point the strain increases rapidly in
the applied axial forces cause an elongation or shortening response to increasing stress and the material will not return
respectively of t:..l then the strain (tensile or compressive) is to its original dimensions. The material is now permanently
given mathematically by: distorted and has taken up a permanent set. Still further
increase in stress causes a rapid increase in strain in this
strain st (Eq. 2) plastic deformation region terminating in a region in which
the strain can increase even with a reduction in stress. This
The case of a body in shear is a little more complicated rapidly terminates in the fracture point and the metal is
because of the deformation in more than one dimension, destroyed.
but the definition of strain is again a measure of the defor-
mation. If the fracture point is close to the elastic limit then the
material is considered to be brittle, and if a large plastic
When stress is plotted as a function of the strain, typi- deformation takes place before the fracture point then the
cally for a ductile metal there are four distinct behavior material is said to be ductile. If these dimensional changes
regions. Initially, the strain is proportional to the applied are constrained from partly or totally occurring in response
stress and the strain is reversible when the stress is to the externally applied forces then a residual stress is set
removed; this region is called the elastic behavior region. up in the material.
Another region of distinct behavior occurs beyond this pro-
portional elastic region up to the yield point (or elastic limit) The relationship of stress and strain to material integrity
in which again the deformation is completely reversible. Up is an important consideration in the design and interpreta-
tion of nondestructive tests, particularly using the special
methods.
478 I NONDESTRUCTIVETESTING OVERVIEW
PART 2
PRINCIPLES OF INFRARED
THERMOGRAPH.Y
Infrared and thermal methods for nondestructive testing forced convection or can rise, as a result of its lower density,
are based on the principle that heat flow in a material is by natural (free) convection.
altered by the presence of some types of anomalies. These
changes in heat flow cause localized temperature differ- Conduction
ences in the material surface. The imaging or study of such
thermal patterns is known as thermography. Heat flows through solids by means of conduction. How-
ever, for heat to flow, a temperature difference must exist
The terms infrared and thermal are used interchange- between two points in the solid. The mechanism of heat con-
ably in some contexts. Thermal refers to the physical phe- duction through a uniform material can be understood with
nomenon of heat, involving the movement of molecules. the aid of the simplified sketch shown in Fig. 3. If one side of
Infrared (below the color red) denotes radiation between material (slab) is continuously heated to some constant tem-
the visible and microwave regions of the electromagnetic perature, heat will flow uniformly through the material to
spectrum. The intensity and frequency/wavelength of the the cooler surface opposite. After a while, a constant temper-
radiation can be correlated closely with the heat of the radi- ature gradient will be set up between the two surfaces, with
ator. It follows that radiation sensors can be used to tell us temperature decreasing uniformly with distance (not time)
about the physical condition of the test object. This is the from the warmer to the cooler surface. Because the various
basis of the technology of thermography. temperatures through the thickness remain constant, this
heat flow condition is known as steady state:
Thermography can be practiced by various techniques.
One technique involves the direct application of tempera- w x (Eq. 3)
ture sensitive materials (usually coatings) to the test surface.
This approach relies upon thermal conduction to the tem- Where:
perature sensing medium. Note that although this technol-
ogy may be referred to as thermal, the term infrared does W = heat flow density;
not apply. Techniques monitoring the infrared radiation K = thermal conductivity;
emitted by the test surface were developed in the 1960s and T2 = temperature of warmer surface;
1970s and digitized in the 1980s. The temperature patterns T1 = temperature of cooler (opposite) surface; and
on the material surface produce corresponding radiation x = thickness of material.
patterns. Thus, heat flow by both conduction and radiation
may be observed and used to locate material discontinuities. The thermal conductivity given in this steady state heat
Heat flow is the key mechanism. flow equation is a constant that indicates the amount of heat
that can be transferred across a unit thickness of material for
Heat Transfer a unit time duration and a unit temperature differential.
Every material has a characteristic thermal conductivity,
Heat transfer (measured in watts per square meter with metals typically possessing high values and gases and
[W-m-2] or British thermal units per hour per square inch nonmetallic materials (such as insulators) typically possess-
[BTU-h-1-in.-2]) can occur by conduction, radiation, convec ing low values. Examination of the equation reveals that the
tion or a combination of these. Conduction occurs when rate of heat flow can be increased across a given material
warmer atomic particles collide with - and thus impart thickness either by increasing the temperature difference
some of their heat energy to - adjacent cooler (slower mov- between the surfaces or by selecting a material with a higher
ing) particles. This action is passed on from one atom (or thermal conductivity. Anomalies having thermal conductivi-
free electron) to the next in the direction of cooler regions. ties different from that of the basic material can be nonde-
Thus, heat always flows from a warmer to a cooler region. structively detected because of their effect on heat flow and
Heat transfer by radiation occurs through the emission of the resultant shift in the thermal gradient.
electromagnetic waves from the material surface. The term
convection denotes the transfer of heat by mass displace- Many important testing applications involve steady state
ment of a heated material, especially a gas or liquid. For heat flow. However, many nondestructive testing applica-
example, heated air can be blown to another region by tions involve an unsteady state, or transient heat flow. Tran-
sient heat flow occurs during the time it takes a material to
THERMOGRAPHY AND OTHER SPECIAL METHODS I 479
reach thermal equilibrium or steady state. Thus, the tem- Equation 4 indicates that temperature changes in non-
perature is changing at any given point or plane in the mate- linear fashion through material with respect to time, with
rial with respect to heating (or cooling) time. Transient heat the temperature gradient being steeper near the warmer
flow is much more complex than steady state, especially in surface. Heat will also penetrate in a similar nonlinear fash-
practical applications. ion. Figure 4 shows several time-to-temperature curves
(approximate sketches) that describe this transient behavior.
If a semi-infinite material is suddenly subjected to a Note that the steady state temperature-to-depth gradient is
change in thermal environment, transient heat flow will eventually reached.
occur. Consider the idealized (and hypothetical) case where
the temperature at only one surface of a material is instanta- A material's thermal diffusivity is the speed at which heat
neously increased to some constant value. According to ide- diffuses through it and is expressed as the rate of tempera-
alized transient heat flow theory, the temperature at any ture change with time. Thermal diffusivity a is a widely used
depth beneath the surface will change with time as: constant in transient heat flow. Each material has its own
characteristic value for a, combining the overall influence of
(Eq. 4) thermal conductivity, density and specific heat. In a practi-
cal sense, the rate of temperature change with time is more
Where: rapid in a material with a high thermal diffusivity (e.g., met-
als) and slower in a material with a lower diffusivity (e.g.,
i; gaussian error function for x/V(at); plastics). Thermal diffusivity determines how fast a material
will heat up or cool down. Because of the difference in ther-
Ta original temperature of material (and ambient mal diffusivity, momentary thermal patterns on the surface
medium); of a metal vanish much faster than those on nonmetals.
Td temperature at any depth d below the surface; For purposes of nondestructive testing, it is obvious that
T; new and constant surface temperature; discontinuities having thermal diffusivities different from
that of the material will cause changes in dynamic heat flow
t time; conditions. Under these circumstances, surface tempera-
x material thickness; and ture changes (indications) caused by an anomaly are time
a thermal diffusivity for material a = Kl(pCP) where dependent. This means that the surface indication will
K = thermal conductivity, p = density and change depending upon elapsed time and upon type, size,
CP = specific heat. shape, orientation and location (depth) of the discontinuity.
FIGURE 3. Steady state heat flow through FIGURE 4. Sketches of time-temperature
uniform solid; temperatures do not change relationships for various depths beneath surface
with time suddenly heated to constant temperature
(transient state)
HEAT FLOW- T1
(WARM) d < 0.05x
-----i-.,- "O
~I x --+:I tI APPROACH/NG STEADY STATE
I
l.!J
<0
d ~ O.Sx
al.:!:J:
:)
~
l.!J
~0....
ui d~x
I-
LEGEND TIME
THICKNESS x = MATERIAL THICKNESS
d = DEPTH BENEATH SURFACE
Tn= NEW SURFACE TEMPERATURE
T, = ORIGINAL TEMPERATURE
480 I NONDESTRUCTIVE TESTING OVERVIEW
Infrared Radiation with some modifications for longer wavelengths, lens materi-
als and sensors. When infrared radiation falls on a surface,
As mentioned earlier, the surface temperature patterns the absorbed part of the energy is converted into heat.
can also be remotely observed by sensing the radiation emit-
ted from the surface. All bodies above the temperature of The infrared radiation emitted by a heated solid body
absolute zero emit electromagnetic radiation by virtue of normally contains a continuous band of wavelengths over a
the motion of the constituent atoms.3•4 Electromagnetic specific range. The band of wavelengths results from the
radiation occurs when an electric charge is accelerated or chaotic motion and interaction of the constrained atomic
decelerated. The spectrum and intensity of the radiation particles in the solid. The radiation intensity (W-m-2 or
depend on the temperature and nature of the surface. BTU-h-1-in.-2) emitted by the solid depends upon the tem-
perature and nature of the surface. At lower temperatures
When a surface is heated, there is an increase in energy the radiation intensity is low and consists chiefly of longer
of the atomic particles leading to a corresponding increase wavelengths. At higher temperatures, the radiation intensity
in temperature and emitted energy. The chaotic thermal rapidly increases while the wavelength band shifts toward
agitation of atomic particles produces a form of radiant elec- shorter values. These behaviors can be described by several
tromagnetic energy known as infrared radiation (that is, fundamental radiation laws.
radiation of frequencies "below red"). The quantum energy
of infrared produces wave frequencies in the electromag- The wavelength independent rate of emission of radiant
netic spectrum between microwaves and visible light (see energy per unit area is governed by the Stefan-Boltzmann
Fig. 5). These frequencies involve wavelengths extending law:
just beyond the visible range, at about 750 nm to the
microwave region, which starts at about 1 mm. The infrared (Eq.5)
range is further broken down into near infrared, with wave-
lengths shorter than 10 urn, and far infrared, with longer Where:
wavelengths. Most infrared nondestructive testing takes
place in near infrared and slightly beyond it, say, up to W = intensity or rate of emission, radiant energy per
15µm. unit area (W·m-2);
Infrared radiation behaves like light at visible frequen- E emissivity (ratio of emittance of the surface relative
cies. It travels in straight lines, reflects, refracts, is absorbed, to a black body);
interferes, exhibits beam spreading, can be focused and
travels in a vacuum at the speed of light, 3 x 108 m-s"! B Stefan-Boltzmann constant= 5.7 x 10-8 W-m-2-K-4;
(1.86 x 105 mi-s"! ). The techniques of geometric optics apply, and
T absolute temperature (K).
FIGURE 5. The electromagnetic spectrum
FREQUENCY
(hertz)
103 105 J07 109 IO" 1013 101s 1017 I 019 1021 1023 102s
II III GAMMA RAYS
INFRARED ULTRAVIOLET
RADIO WAVES v X-RAYS
sI
I
B
MICROWAVES COSMIC
I
104 102 10° I o-2 J0-4 J0-6 I o-8 I 0-10 I 0-12 I 0-14 I 0-16
WAVELENGTH
meters
THERMOGRAPHY AND OTHER SPECIAL METHODS I 481
. The Stefan-Boltzmann Law relates the total radiation For practical infrared imaging, important criteria are the
spectrum of emitted energy and wavelength of maximum
intensity to the fourth power of absolute temperature and emittance (Planck's Law and Wiens Displacement Law,
respectively). Figure 6 shows a family of such energy distri-
emissivity of the material surface. For example, intensity bution curves. For the practical case of a body at a tempera-
ture of 27 °C (80 °F), the wavelength range of peak
(heat flow) from a copper block at 100 °C (212 °F) is
300 W-m-2 (95 BTU,ft-2-h-1). (Stefan-Boltzmann constant emission is in the 9 to 10 urn wavelength range. This turns
for photon emission = 1.52041 x 1015 photons-s+rrr+K'<)
out to be a very useful range.
The radiation intensity of the emittance at each particu- Various components of our atmosphere, notably water,
lar differential wavelength can be obtained from Planck's
Distribution Law, the distribution criterion for blackbody absorb a great deal of the emitted infrared energy. However,
radiation: there are two fairly transparent windows through the atmo-
sphere: one at 3 to 5 µm and the other at about 8 to 14 µm.
2rchc2 1 (Eq.6) Specific infrared detector elements have been developed that
__ A_5_ X ehc!HT _ 1 respond well to each of these wavelength bands. As seen in
Fig. 6, the intensity of infrared emittance within such a band
Where: can be used to indicate the temperature of the object surface.
W(A) = the rate of emission, radiant energy per unit Emissivity
area, as a function of wavelength;
Emissivity is a variable defined as a ratio of the total
A = the wavelength of the emitted radiation; energy radiated by a given surface at a given temperature to
h Planck's constant = 6.625 x 10-34 j-s: the total energy radiated by a blackbody at the same tem-
perature. A blackbodyis a hypothetical radiation source that
c speed of "light" = 2.998 x 108 m-s ": and yields the maximum radiation energy theoretically possible
k Boltzmann's constant= 1.380 x 10-23 J-K-1. at a given temperature. Also a blackbody will absorb all inci-
dent radiation falling upon it. Blackbodies have an emissiv-
Wavelength of maximum emittance is given by the single ity of 1.0 and all real materials have emissivities between O
temperature evaluation of Wien's Displacement Law: and 1.0.
b (Eq. 7) Figure 7 represents Planck's Distribution Law for black-
T bodies at various temperatures. Note that the wavelength
Where: FIGURE 7. Planck's Distribution Law for
blackbodies at various temperatures
wavelength A of maximum radiation intensity
(µm);
b Wien displacement constant= 2,897 um-K, and
T temperature.
FIGURE 6. Spectral radiant emittance
distribution at three background temperatures
~zvi •<( 5 (48)
EN' 4 (39)
LJ.J 3 (29)
::i ¢:: 2 (19)
I (JO)
to-f~-EN'u-J::
z~
:J
O~ E"o
~
0 2.0 4.0 6.0 8.0 I 0.0 12.0 14.0 16.0 18.0 10 100
WAVELENGTH WAVELENGTH
(micrometers) (micrometers)
LEGEND LEGEND
I. T = 50 °( ( 122 °F) I. 300 K (81 °F)
2. 195 K (-108 °F)
-c=2. T 25 (77 °FJ 3. 126 K (-233 °F)
3 T = IO 0( (SO °FJ
482 I NONDESTRUCTIVE TESTING OVERVIEW
envelope shifts toward the visible range for increasing black- Instrumentationand Techniques
body temperature, according to the Wien Displacement
Law. As shown by the Wien equation (Eq. 7), the wave- Active and PassiveTesting
length of maximum intensity is computed simply by dividing
2,897 by the temperature of the surface in degrees Kelvin. Infrared nondestructive testing is performed by either
active or passive techniques. Active techniques involve heat-
The effect of emissivity on the radiation curve is given in ing or cooling the material to generate the required heat
Fig. 8. As shown, it acts somewhat like a filter or valve. flow and thermal gradients. Transient heat flow usually is
Graybody materials have emissivity values that are less than used during active testing. Passive techniques involve appli-
that of a blackbody at all temperatures and wavelengths. cations where the material already contains its own internal
Some materials, called spectral radiators, have a spectral source of heat (such as an inservice heater element or the
emissivity that varies in a characteristic fashion over the human body).5 Thus, steady state conditions normally apply
range of emitted wavelengths. to passive tests.
Emissivity is a surface phenomenon depending on the Active tests are conducted by heating the material to
surface condition and composition. Smooth materials have observe the development of the transient state the_rmogram.
lower emissivities than rough materials. Freshly polished Heat of a given intensity and a given duration is applied by
metals have lower emissivities than oxidized or corroded hot gas jets, infrared lamps, electrical induction (of metals),
metal surfaces. Nonmetals usually have higher emissivities dielectric heating (of nonmetals), direct contact (conduction
than metals. Lampblack or certain metallic powders yield heating), or baking ("soaking") the test piece in an oven.
very high emissivities. Special blackbody cavities used for
calibrating radiation equipment produce the highest emis- The size of the heated spot can be a few millimeters
sivity available, within a fraction of a percent of that for a (about 0.1 in.) in diameter or much larger, depending on the
blackbody. thermographic detection scheme used. Small heat spots are
distributed across the surface and the thermographic pat-
Because the object of infrared testing is to measure sur- tern they leave is observed.
face temperature changes, emissivity can be an uncontrolled
variable. Variations of emissivity across the surface of a mate- Radiometry
rial can cause false indications. When the emissivity
decreases in a localized region, the radiation intensity Infrared testing measures surface temperatures by
decreases, falsely indicating a localized reduction in temper- means of devices called radiometers or infrared cameras. A
ature, and vice versa. Also, surfaces with low emissivity val- radiometer basically consists of optics that collect and focus
ues, such as polished metals, are more difficult to test than or image the received infrared radiation on a sensitive
high emissivity surfaces. detector. The detector converts the infrared radiation into
an electrical signal. Many radiometers are scanning or imag-
FIGURE 8. Emissivity effect on radiation from ing cameras that provide the operator with a thermographic
image of the test surface. A microprocessor processes the
surface (£ = emissivity} with hypothetical image and presents it on a television screen or digital output
display. Variations of the image intensity are related to the
intensity corresponding surface temperatures on the material under
test. Typical sensitivity of the instruments is 0.05 °C (0.1 °F)
E = I (BLACKBODY) or better. Figure 9 shows the components of a typical
infrared inspection system.
r: '\
I\ Pyrometry. The word pyrometry means "fire measure-
I\ ment." As the name implies, pyrometers are used for hot
~ I\ E = 0. 9 (GRAYBODYJ applications, such as the monitoring of furnace or foundry
zvi I conditions. A pyrometer is a kind of radiation thermometer,
giving readings for one point at a time, rather than imaging
llJ a scene the way an infrared video camera would. Many
~I- I pyrometers are digital devices with liquid crystal tempera-
z ture readouts that may be mounted in place. Hand held
0 units are also available.
0~ Video radiometry. Standard infrared video camera sys-
~ tems can resolve temperature variations of 0.05 °C (0.1 °F)
and display images with temperature gradients of 256 or
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