ASNT NDT Handbook Volume 9 visual Testing

stereoscope, a three-dimensional image of Monocular Photogrammetry
the object is reconstructed.
Monocular photogrammetry superimposes
Instead of looking directly at the a transparent layer on a photographic
object, it can be photographed from two image. The transparent material has a
points analogous to the location of the reticle, a calibrated scale for measurement.
two eyes. If these pictures are then The distance between the reticle and the
viewed, left picture with left eye and right image surface is part of the measurement
picture with right eye, the object appears calculation. Unless the reticle is flush with
again in three dimensions. the test surface, magnification is also a
factor. This monocular photogrammetry
For various points on an object, resembles viewing with a loupe such as
relative differences in distance from the the measuring magnifier described in the
camera can be found by determining their chapter on direct visual testing.
parallax differences. With the aid of a
parallax bar or stereo plotter, parallaxes The visual inspector using monocular
can be measured with a precision of photogrammetry needs to account for
10 µm (0.0004 in.). If the distance several factors.
between camera stations (base), camera
focal distance and object-to-camera 1. When held touching the test surface, a
distance are known, absolute contact reticle can measure surface
measurements on the object can be made. features directly. If the reticle is
positioned at a distance, the
In petrochemical furnaces, close range differences in focal distance (distance
photogrammetry can be applied to to image surface versus distance to
noncontact, on-stream monitoring of the reticle) must be known if features of
condition of furnace tubes through the test surface are to be measured.
measurement of bulging, bowing and
creep as well as deterioration of 2. If the surface measured is a
components such as cracks in hangers and photograph, then its scale relative to
tubesheets or spalling of refractory and the depicted surface must also be
brickwork. Photogrammetry could prove known before features on the test
useful as a quantitative tool applied surface can be measured.
periodically for charting the condition of
furnace internals. 3. A further consideration for precise
measurement is viewing angle. The
There are two kinds of viewing angle from feature to feature
photogrammetry: monocular and stereo. varies from one spot on the image to
another. The visual inspector,
FIGURE 12. Stereo effect: (a) schematic; (b) view from left however, needs to know if the camera
eye; (c) view for right eye. has introduced distortion by using a
fish eye lens, for instance, or by
(a) R (b) F panning the scene.

L L 4. The photograph is flat, but is the test
surface flat or curved? If curved, like
(c) the inside of a pipe, did the camera
rotate to pan the scene?
F
FR These factors that affect measurements
must be weighed in relation to the
Legend demands of rejection criteria. Visual tests
typically call for verification by other
F = object in foreground methods before rejection. In many cases,
L = left object these factors will be trivial and can be
R = right object ignored.

Stereo Photogrammetry

Stereo viewing is related to the optical
phenomenon called parallax, in which the
position of an observed object appears to
vary with the viewer’s line of sight
(Fig. 12). In binocular vision, two
overlapping fields of views enable depth
perception and distance estimates. For
photogrammetry, the purpose of
measuring parallax difference between
two object points is to develop the
contour or shape of a surface so that
information on deformation, relief
(contour) or movement of the component
can be determined.

Visual Test Imaging 93

Parallax difference between incandescent carbon particles tend to hide
corresponding points on stereo details of the subject if it is located behind
photographs can be measured by means the flame. Uneven heat distribution also
of a calibrated gage called a parallax bar, creates problems resulting from local
essentially an optical micrometer with variations in flue gas density causing heat
one fixed and one movable measuring waves that distort images of objects
mark. pictured in the firebox.

After appropriate positioning under a Because furnace firing conditions can
stereoscopic viewer, the parallax bar is be varied, some control can be exerted on
placed on a stereo pair of photographs, the visual environment. More favorable
aligning the two reference plates on the periods, such as when gas rather than oil
parallax bar to exactly coincide with one is being burned, offer lighting conditions
point on the two photographs. When this more suitable for taking photographs. The
condition is achieved on the stereoscope, stereo photographs shown in Fig. 13 were
both marks on the parallax bar appear to taken in a refinery furnace. Even though
fuse together into one point apparently this furnace was oil fired at the time, good
floating into space above the model of the definition and image quality are apparent.
object. If two points of known elevation
can be identified in a stereo model, the Still Photography and
distance of other points on the model can Videography
be calculated. This technique is also used
in radiography. Calculations and Movies, including digital videos, consist
examples are illustrated elsewhere.1 of successive frames of still photographic
images. Still photography and moving
Greater measuring accuracies are videography share much of the same
probably required for determining creep digital technology: hardware, processes
growth of high alloy castings where the and terminology. Digital cameras designed
total creep growth to failure is on the for still photography can also make
order of 5 percent. For evaluating movies, and vice versa. The charge
photogrammetry as a means of coupled device, a sensor common in
determining the condition of furnaces, digital cameras, is discussed in the section
tube sheets, hangers or refractories, an on video, below.
assessment should be made of the
minimum distortion, deformation, FIGURE 13. Refinery pipestill furnace:
cracking or other significant precursor to (a) stereo photography setup; (b) stereo
failure. This size estimate can in turn be views.
compared to the calculated accuracy
limits of a particular photogrammetric (a)
setup used to decide whether results are
acceptable. 1.38 m 5.41 m
(54.5 in.) (213 in.)
High Temperature
Limitations A B

Cameras for documenting conditions in (b)
high temperatures are subject to several
constraints and limitations. The most
serious problem in furnaces, for example,
is the protection of the camera from high
intensity radiant energy emitted through
the viewing ports. This high temperature
environment usually limits the placement
period and exposure times to a few
minutes. The relatively small size of the
viewing ports cuts down the angle of view
and limits the area that can be pictured
within the furnace. Lighting conditions
significantly affect the production of
shadows and modeling of the surface of
subjects. In this respect, a single lighting
source is desirable. However, in a furnace
the many burners, as lighting sources,
tend to produce diffuse lighting effects
that reduce overall shadow contrast and
surface modeling. Flame luminosity,
particularly from oil fired burners, is
undesirable because the unburned

94 Visual Testing

PART 2. Digital Processing and Archiving for
Visual Testing5,6

Pixels are not considered here. Raster images, or
bit mapped images, assign hue and
Digital photographs are designed for a intensity values to each pixel and are used
computer display. A display consists of a for photography. Table 2 lists common file
grid of squares called picture elements, or formats for digital images. Some cameras
pixels (Fig. 14).10 Displays range from 320 use proprietary formats similar to these
× 240 pixels to over 3840 × 2400. A common file formats.
standard display of 800 × 600 pixels is
called a super video graphics array. It is A bit map defines an image and color
superseded in most computers but for all pixels in the image. TIF and JPG
remains popular in mobile devices. files are bit mapped. A bit map does not
need to contain color coded information
Each pixel is part of a larger image, and for each pixel in every row. It needs to
more pixels provide a more accurate contain information indicating a new
representation of the original. Pixels are color as the display scans along the row.
rectangular and on computer monitors are
usually square. The square shape is an TABLE 2. Digital file formats of still images.
artifact of the display; the original
electronic datum is dimensionless. Pixels Designation Format
may be translated into dots for printing.
Dot and pixel are interchangeable terms: Lossless Raster Formats
for example, dots per inch (DPI) and
pixels per inch (PPI) are equivalent BMP bit mapped picture
measures of resolution, discussed below.
DIB device independent bitmap
There are two classes of graphic file
formats. Line drawings use mathematical PIC three-letter file extension for PICT
calculations called vectors to plot lines and
PCT three-letter file extension for PICT
FIGURE 14. Array of pixels in digital display.10
PICT picture metafile
Pixel
RAW extension for undeveloped image from digital camera
(virtually lossless)

TIF three-letter file extension for TIFF

TIFF tagged image file format

Lossy Raster Formats

GIF graphics interchange format, native to CompuServe®
internet service provider

IFF Interchange file format

JFF JPEG file format

JFIF JPEG file interchange format

JIF JPEG image format

JPEG Joint Photographic Experts Group, a standards
organization

JPG three-letter file extension for JPEG

PBM portable bit map

PGM portable gray map

PNG portable network graphics

RAW extension for undeveloped image from digital camera
(virtually lossless)

XBM X bit map

Vector Formats (for line drawings)

CGM computer graphics metafile

EMF enhanced metafile, version of WMF

EPS encapsulated postscript, native to Adobe® software

PDF portable document format, native to Adobe®
software

SVG scalable vector graphics

SWF Shockwave Flash®

WMF Windows® metafile, native to Microsoft® software

Visual Test Imaging 95

For this reason, an image with much solid inherent in JPG files. RAW files have
color requires a small bit map. many advantages over JPG files, including
the following:
The term pixel can also mean an
element in a sensor array. An array is a 1. Image quality is higher.
group of adjacent receptors each of which 2. RAW conversion software gives the
records a single point of radiation and
which together provide a composite user fine control of more parameters
picture. Such arrays are in cameras and such as lightness, color balance, hue
other radiation detectors (X-ray and saturation.
fluoroscopes and infrared cameras) at 3. Edits are performed nondestructively,
various wavelengths. without degrading image quality.
4. If the file is not compressed, the
File Formats maximum amount of image detail is
always kept within it.
File formatting has been evolving rapidly
with advances in digital technology. Camera manufacturers use different
Several standards may be consulted or versions of RAW format. As of 2009, there
referenced for digital image archives.8-11 is no widely accepted standard for RAW
format, so more specialized software may
The file formats for digital photographs be required to open RAW files than to
fall into two classes: lossy and lossless open standardized formats like JPG or TIF.
(Table 2). Lossy formats compress files to Cameras that support RAW files typically
save hard disk space. A format is said to come with proprietary software for
be lossy when digital information and conversion of their RAW format to TIF or
hence image detail are lost. JPG files are JPG. To retrieve an image from a RAW file,
lossy and are used in many digital the data must first be converted into a
cameras and web pages because their red, green and blue (RGB) image.
compression makes small files and
because they can be viewed on personal Because of proprietary formatting,
computers. Lossless TIF and PICT and some formats listed in Table 2 are in fact
nearly lossless RAW formats provide families of related formats. There are
similar files but use more disk memory. several forms of TIF and of PNG, for
example, and a given version may not
Inspectors who archive digital work with a given program. Before using
photographs need to be aware that new equipment, the inspector will want
converting files from a lossless to a lossy to check that test images can be
format (for example, from a TIF to a JPG) transferred to and viewed with hardware
can degrade images so that some details, at the archiving location.
such as crack tips, are harder to see. There
are several occasions when this sort of Image Characteristics
image degradation can take place:
(1) when a file is changed from a lossless Many computer programs are widely
to a lossy format, such as TIF to JPG; available for processing digital images.
(2) when a lossy image is converted to a They let the user adjust settings that, in
file with lower size or resolution; (3) when the twentieth century, used to rely on the
an image is imported into a document skill of a photographer at the moment of
created by a word processing or layout image capture or film development.
program. This reduction may be
inadvertent, caused by program settings. Color
Of course, an image conversion may have
no effect on the usefulness of the Hue. A color is described by its hue,
resulting image: though degraded, it may saturation and value (HSV). Hue is what is
still show all features of interest. normally thought of as color. A digital
color in a computer display is a blend of
Procedures need to be clear so that three primary hues: red, green and blue.
they can be followed. The procedures Saturation is the intensity or dominance
describing how photographs are archived of a hue: a hue can be increased or
should include definitions of lossy and diminished in an image just as black
lossless and of file formats so that the versus white can be adjusted in an image
results can be interpreted later. through all shades of gray. Value is the
lightness or darkness of a hue and is
The purpose of the camera RAW format usually called brightness when the three
is to record all of the exact data from the hues are combined in white light. Image
camera’s sensor and any other metadata, processing software permits the intensity
such as time and camera settings. Camera and value of these hues to be adjusted
RAW files are much larger than JPG files. independently within each image.
Some RAW formats do not use
compression; others implement lossy data Conversion to Gray. If the image is
compression to reduce file size without converted to gray scale, as in a report
affecting image quality. Camera RAW printout, care must be taken that features
formats avoid the compression artifacts

96 Visual Testing

of interest remain visible: a fluorescent manipulated independently but to
nondestructive test indication in brilliant prevent distortion usually undergo the
green may disappear entirely if it changes same processes as the horizontal.
to a shade of gray like that in an adjacent
area. The number of pixels in an image can
be decreased or increased in a step called
Color Balance. The camera setting of resampling or conversion. Resampling
white balance, discussed above, can cannot add more data to a photograph
improve the realism of color photographs once it has been shot.
by preventing unnatural tints. Image
processing programs also enable Resolution
improvement of color balance. Program
menus may refer to it by other terms, The resolution of an image is its ability to
such as color intensity or hue saturation. distinguish small adjacent objects. Two
thin adjacent lines appear as two thin
If a photograph is taken in a digital lines, for instance, and not as one thicker
camera’s RAW file format, image colors line. Resolution depends on the number
can be fixed without information loss. of pixels in the image and hence on its
Most cameras take RAW photos in twelve- file size. In Fig. 16, two versions of the
bit color (4096 shades per color) instead same image show the difference between
of eight-bit color (256 shades per color), high and low resolution. Details in an
enabling powerful balance adjustments image cannot be restored simply by
without visible loss in quality. converting it to a higher resolution or
from a lossy to a lossless format. Once an
Size image’s resolution is reduced, details are
lost. If a display’s resolution exceeds an
The more pixels in an image, the more image’s resolution, the image will look
information it carries. An image’s quantity blurry.
of pixels is its true size in the
dimensionless electronic space of the Resolution is measured in dots per inch
computer. (DPI) for printers or square pixels per inch
(PPI) for computer displays, measured
To be viewed, the image must be along the horizontal axis. Alternatives
translated to a medium such as a using SI metric units are to specify pixels
computer display or printed page, where per centimeter (pixels·cm–1) or the pixel
details become visible to human eyes in width in micrometers (µm).
order to be interpreted. In a process
known as scaling, the pixels can be Because the number of pixels in a
compressed or spread out to fit the desired scalable image is fixed, however, an
viewing surface (Fig. 15). Scaling affects expression of resolution is meaningless
resolution, discussed below.
FIGURE 16. Visible light photograph showing
Image size and resolution are usually magnetic particle indication under
expressed in terms of image width, in the ultraviolet lamp: (a) high resolution, at
horizontal dimension. The size and 40 pixels·cm–1 (100 PPI); (b) low resolution,
resolution of the vertical scale can be at 10 pixels·cm–1 (25 PPI). Lower resolution
blurs details. Lowering resolution sometimes
FIGURE 15. Example of scaling of digital enhances contrast or increases hue intensity
images. — side effects better achieved by image
processes that do not sacrifice detail.

(a)

228 pixels gives (b)
resolution of

58 pixels·cm–1 (144 DPI)

50 mm (2 in.)

Scaling

228 pixels gives resolution of
29 pixels·cm–1 (72 DPI)

100 mm (4 in.)

Visual Test Imaging 97

unless the viewed image size is also have zero saturation, the image is
specified. If a given digital image is colorless, black.
reduced to half its width, for instance, the
number of pixels per unit of width and Contrast. The setting of contrast controls
the resolution are doubled (Fig. 15). The the degree of difference between light
data in the image file may remain the intensities at different points in the
same, but smaller size makes details image. Like brightness, contrast is
harder to see. Someone processing digital adjusted to affect all pixels in an image at
images must make decisions balancing once. As contrast is increased, for
image size versus resolution. example, a dark indication becomes easier
to see on a bright background and a
Brightness bright indication becomes more visible on
a dark background.
Brightness is the common term for the
luminance of light emitted by each pixel. Contrast and brightness interact
Usually, one control setting adjusts closely, and the inspector must sometimes
brightness to affect all pixels in an image adjust them to find the correct settings for
simultaneously. This parameter can be each photograph. Figure 17 shows the
adjusted to compensate for poor visibility effect of brightness and contrast on an
on overcast days. Too little brightness can image.
make a picture dark as night, and too
much brightness can make it washed out Image Integrity
so no features stand out.
Inspectors may be tempted to enhance
Saturation. In a color image, saturation is images to justify accept/reject decisions.
the intensity of a hue. A hue of zero This temptation should be treated with
saturation is black or gray. If all three hues caution. The inspector may consider
several rules of thumb.

FIGURE 17. Photograph of cracked rod: (a) high contrast, high brightness; (b) high contrast, low brightness; (c) low contrast,
high brightness; (d) low contrast, low brightness.

(a) (b) (c) (d)

98 Visual Testing

1. Local features including indications 1. The vision acuity of the viewing
must not be exaggerated or obscured human can be impaired by medical
using image processing tools having conditions such as myopia and color
descriptors such as pencil, eraser, flood, blindness.
spray or smudge. These alterations of
an image could be considered 2. The visibility of a shot is affected by
falsification of test results. things such as light and camera
position.
2. Image settings such as zoom,
brightness and contrast may be freely 3. The useful detail in an image is
adjusted, as photographers have done affected by its size and by its quality in
for generations. terms of features such as contrast and
resolution.
3. A photograph of the reference
standard should be included in the Technology has given the inspector
archive and used to evaluate options beyond the scope of the present
modifications made to test images. discussion: (1) light measurement and
How do the same modifications affect illumination, (2) optical gaging and range
discontinuity visibility in the reference finding, (3) photography sensitive to
standard? other wavelengths, including ultraviolet
and infrared, and (4) video
For some applications, a written documentation of inspection, more for
statement on image processing may be a procedures than for indications. These
desirable part of an inspection procedure. and other options depend on hardware.
Accept/reject decisions are usually made The present discussion also does not
at the moment of inspection and in the consider marine environments or
presence of the test object. If some extremes of altitude or temperature.
decisions are made later, that protocol can
be documented in the procedure. Most inspectors already use a computer
for communication, for research and for
In some archives, a file’s metadata, writing procedures. With the investment
including its creation date, are part of its of a few hundred dollars for a camera and
documentation. These metadata can be software, an inspector can add digital
lost if an image is saved in another photography to the inspection options.
format. With practice and planning, digital
photography can be a valuable tool for
Closing inspection and quality control.

In practice, the viewing quality of a
digital photograph depends on several
factors.

Visual Test Imaging 99

PART 3. Video10,11

The considerations discussed above for fluctuations in the signal are meaningful.
still images pertain also to motion The primary disadvantage of analog
pictures, or movies. The term video applies signaling is that, if any signal acquires
specifically to movie signals transmitted random noise as it is copied or
electronically. There have been three transmitted, these random variations
different platforms for moving pictures. become dominant. Electrically these losses
are lessened by shielding, good
1. Film projection was used for connections including coaxial and twisted
commercial cinema in the twentieth pair cables.
century. Each frame was in effect an
individual color slide, and they were A digital system uses a series of discrete
sequenced on spools, or reels. A long values rather than a continuum of values.
movie consisted of thousands of Waves are encoded as digital signals in a
frames and required several reels. Film sequence of ones and zeros in the binary
of smaller width, and hence less language of computing. Each selected
resolution, was widely used at home individual point on the line is an integral
and by industry. Some film was in single value. When reproduced on a video
black and white. screen, a large number of single values
appear as a stream of values representing
2. Analog video was used for television. the original wave.
The moving images were converted
electronically into a series of It should be pointed out that videotape
horizontal lines that scanned across presents its analog signal as individual
the screen successively in a raster frames that appear as a succession of
pattern. Analog television can be in discrete image signals. These frames,
color or in black and white. however, are an artifact of the display
medium. What appears to be a single
3. In digital displays, a movie consists of analog frame is built out of adjacent lines
a series of still images, or frames, in a raster arrangement. The signal input
viewed in succession to recreate the to the video recorder is continuous, the
appearance of motion (Fig. 18). Digital raster lines succeed each other more
video can be viewed on a computer quickly than the human eye can track
display or on a digital television and the whole screen display is a
screen. composite of these line signals.

Analog versus Digital10 A great variety of analog devices persist
in the twenty-first century: film cameras,
An analog signal is any continuous microphones, speakers, luminaires
variable signal and consists of a (lamps), switches, machinery controls (for
continuum of values (Table 3). It differs sorting in assembly lines, for example),
from a digital signal in that small cathode ray tubes and closed circuit
monitors. In electronics, a
FIGURE 18. Falling object in video: (a) 30 frames per second; digital-to-analog converter is needed to
(b) 25 frames per second. convert a digital, usually binary, code to
an analog signal, usually an electric
(a) current or voltage. Signals are converted
with switches or a network of resistors or
current (electricity) sources. An
analog-to-digital converter performs the
reverse operation. Many digital devices,

(b) TABLE 3. Analog versus digital formatting.
0.20 s
Format Sampling Interval

Analog continuous stream of data

Digital with low sampling values for discrete moments

Digital with higher sampling discrete values in briefer intervals

100 Visual Testing

including household appliances, (2) a two-dimensional imagers of visual
incorporate circuitry to convert signals data over an area.
from a digital interface to analog
performance, or vice versa, automatically. A major category of photoelectric
sensors, solid state image amplifiers, is
Discontinuity Size Visibility11 discussed below.

In the twentieth century, the resolution of Photoemissive devices include
an analog television system was expressed photoemissive cells and photomultipliers,
as the number of lines in the picture. The photoconductive cells, photovoltaic cells
electron beam produces a picture by and various devices that measure radiant
drawing repeated lines of varying energy directly, such as bolometers
brightness across the tube. In analog (thermal radiation detectors) and
video broadcasting, a signal with 525 lines thermocouples. In the second category are
has been used. About 480 lines actually various image converter tubes, image
form the picture and the balance are used amplifier screens and television pickup
to return the beam from the bottom to tubes. Photoemissive devices feature
the top of the screen. There is also a kind materials that emit electrons under the
of resolution in the horizontal direction, influence of electromagnetic radiation,
because television monitors are designed such as light. These electrons are then
to have equivalent horizontal and vertical drawn away from the emitting surface by
resolution. Closed circuit television an electric field and used as the signal
systems used for visual nondestructive current, which may actuate relays to be
tests sometimes had resolution of 500 amplified electronically. The emission of
lines, higher than consumer broadcast electrons in response to light is explained
systems of about 200 lines. by the quantum theory of radiation. A
potential barrier at the surface is the total
Usually, a video system cannot resolve energy an electron must have to escape.
narrower than a pixel. The smallest detail When its energy exceeds this threshold,
that can be resolved is predicted without an electron is emitted.
magnification. If a lens with 2:1
magnification is used, the requirements The photoelectric cell or multiplier
on the electronic system are not so high phototube is used in many ways in
and the size of the detail that can be industrial nondestructive testing. One of
resolved is smaller. In practice, it is best to the most common uses is the
magnify by moving the camera closer to measurement of radiant flux. In many
the test object than merely to plug the respects, the phototube exceeds the
video output into a larger screen. Also, capabilities of the human eye. It can
because of considerations such as detect not only radiation invisible to the
orientation and lighting, indications may eye but can also accurately measure
be detected that are narrower than a quantities of light without reference to a
resolution calculation would predict. standard, as required in a visual
Moreover, resolution is a characteristic of photometer. The familiar photoelectric
an entire system — the recording exposure meter is no more than a barrier
medium, the video camera and the layer photovoltaic cell and a sensitive
monitor. The best way to quantify the meter.
resolution of a system is to use vision
acuity charts and test objects with known In addition to measuring light flux,
discontinuities. photoelectric devices permit
measurements of reflectance and
Photoelectric Sensors11 transmission of materials, comparisons of
two or more sources of light and (with the
The photoelectric effect is the emission of aid of filters or some type of spectrometer)
electrons from some materials in the colorimetric measurements. Phototubes
presence of electromagnetic radiation. find a natural application in
Photoelectric sensors tuned to radiation spectrophotometry. Next to the
were used for television cameras in the measurement of radiant flux, perhaps the
twentieth century. They converted most widespread use of photoelectric
information in the form of light into devices is in monitoring and control
electrical signals that could then be applications: street lamps, smoke
amplified or processed to let the observer detectors, door openers and safety
gather and interpret the test data. The interlocks on punch presses. They inspect
information gathered may be from the bottles after washing and detect foreign
visible region or in a form invisible to the matter in filled soft drink bottles. They
eye, such as X-ray, ultraviolet or infrared sort products such as beans, peas and
radiation. There are two broad coffee and actuate mechanisms for
classifications of photoelectric devices: rejecting discolored products.
(1) detectors or measuring devices and
Many materials (primarily metals)
permit photoemission. By far the most
important such materials are compounds
and alloys of the alkali metals, principally
cesium. Of these, the most widely used

Visual Test Imaging 101

are cesium antimony alloys and cesium coupled devices use a special
oxygen silver compounds. The cesium manufacturing process to create the
antimony emitters are true alloys, while ability to transport to the chip without
the cesium oxygen silver emitters are distortion. This process leads to sensors of
complex surfaces made from layers of high quality in terms of fidelity and light
cesium, cesium oxide and silver oxide in sensitivity.
varying proportions on a silver base.
Cesium antimony surfaces may have a The charge coupled device imager is a
quantum efficiency of 10 to 30 percent. solid state silicon chip similar in structure
Photoemitters of this type have responses to a photovoltaic cell but more complex.
that are selective to wavelength of The charge coupled device is widely used
illumination, so some respond to infrared for digital memory and analog signal
and some to visible radiation or some processing, as well as optical imaging. A
other waveband. This selectivity varies charge coupled device camera system
greatly with processing. consists of a lens system, a charge coupled
device imager and processing electronics.
Solid State Image The device includes a monolithic array of
Amplifier11 many loosely spaced, light sensitive,
metal oxide semiconductor capacitors that
The solid state image amplifier is a transfer an analog signal charge from one
sandwich of a photoconductive layer and capacitor to the next. Each capacitor
an electroluminescent layer. The corresponds to a picture element, or pixel,
electroluminescent layer is composed of a in a display.
material that emits light in response to an
applied voltage. The photoconductor and The capacitors that collect the electrical
the electroluminescent material are charge are colorblind: they track only the
essentially in series across a suitable total intensity of the light that hits the
alternating current voltage supply. In surface. To get a full color image, filters
darkness, the photoconductor is highly are needed. These filters allow the light to
resistive and passes no current. When be seen in the three primary colors, which
light falls on it, it becomes conductive are then added together to attain a full
and current flows through the sandwich, color image in the full spectrum of colors.
causing the luminescent material to emit Each photosite collects varying amounts
light in the illuminated regions. Light of electrical charge and therefore
amplifications up to 1000× have been represent brighter and darker sections of
obtained. the image. The electrical charge in each
photosite is analog information, measured
A modification of this light amplifier is and digitized into binary form.
the amplifying fluoroscope. In this device,
the photoconductive material responds Figure 19 shows in detail how each
directly to X-ray radiation and, by the pixel works. The lens focuses the image of
same principle as the light amplifier, an object on the surface of the charge
converts it to visible light. An coupled device, where the light is
amplification of 100× over the output of a converted to electrons. This effect is
conventional fluoroscope has been photoelectric, the basis of common
obtained. devices such as automatic night lights and
certain types of paper copiers. The pixels
Charge Coupled Device10 are separate from each other, and the
number of electrons produced in each one
Digital cameras are used to capture still is proportional to the intensity of light
images. The image can then be saved to focused on that particular pixel. This
memory and transferred to a computer. differential charging contains information
From the computer, the image can be needed to form an image.
manipulated, shared, stored, printed and
much more. FIGURE 19. Activation of one pixel from charge coupled
device.
Most digital cameras use a charge
coupled device (CCD) as an image sensor. Electron collection Silicon
The charge coupled device is a collection region substrate
of photosites, light sensitive diodes that
convert light into electrical charge Color filter Photons from Insulator
(photons into electrons). The brighter the image scene
light that hits a single photosite, the
greater the electrical charge that will be Doped electrode
accumulated. This electrical charge is then
transported across the chip and read in
one corner of the array. Each pixel’s value
is then turned into a digital value by an
analog-to-digital converter. The charge

102 Visual Testing

The amount of detail that can be resolution image than do conventional
captured is called the resolution and is fiberscopes. Because of the charge coupled
measured in pixels. The more pixels there device’s diminutive size, the silicon chip
are, the more detail that can be captured. can be placed within the tip of a small
The more detail there is, the more a diameter probe capable of penetrating the
picture can be enlarged without becoming smallest apertures. Its advanced
grainy and looking out of focus. microelectronic technology enables the
charge coupled device to function as a
Some typical resolutions include the miniature television camera able to record
following: 256 × 256 pixels, resolution is and display images with great clarity on a
so low that the picture quality is almost video monitor. The device’s ability to
always unacceptable, 65 000 total pixels; generate, detect, and shift distinct packets
640 × 480 pixels, 307 000 total pixels; of electrons makes charge coupling
1216 × 912 pixels, 1 109 000 total pixels; suitable for many different information
1600 × 1200 pixels, almost 2 million total processing applications, including image
pixels; 1920 × 1080 pixels, 2 073 600 sensing in video camera technology.
pixels is the pixel count for high
definition television displays. Digital File Formats

Unlike common photovoltaic cells, Codecs
charge coupled device pixels feature a
storage cell in each element that collects A codec is software that encodes or
and stores the electrons as they are decodes a data stream for digital
produced. The image of the object is then processing. Video codecs compress or
stored in the form of electrons on the decompress video files for transmission,
charge coupled device, but is of no use storage or viewing. Most codecs are lossy,
unless the location and number of sacrificing data for the sake of efficiency.
electrons in each pixel can be identified. Many codecs are proprietary, designed to
work with particular commercial software
At this point, the charge coupling or hardware products; compatability
effect of the charge coupled device comes problems can arise among versions,
into play. Finite amounts of electrical platforms and competing software.
charge (that is, packets of electrons) are
transferred from one pixel site to the A codec determines the format of the
next, just as water is moved from one video file created with it, so the visual
bucket to the next in a bucket brigade inspector selecting a codec for visual
(Fig. 20). The force required to move these testing purposes must consider what
electrons is produced in the pixels by format is desired for short term evaluation
means of a voltage that alternately turns of files and for their long term archiving.
on and off. As this clock voltage The selection of codec also affects the
alternates, the electrons move from one selection of hardware and vice versa.
site to the other, and after the last pixel
the electrons are converted to a voltage Codecs define the video settings such
proportional to their number, this voltage as the frame rate and size. There are many
is amplified, filtered, and then applied to video codecs and formats. Some are listed
the input of a video monitor. in Table 4.

Charge coupled device image sensor Moving Pictures Experts Group
systems produce a brighter, higher (MPEG)

FIGURE 20. Electron transfer in charge coupled devices — The Moving Pictures Experts Group is a
analogy of bucket brigade. body in the International Organization
for Standardization that develops
consensus standards for the encoding of
audiovisual information in digital
formats. Codecs conforming to their
standards use high levels of compression
to achieve small file sizes; they format and
read digital video files with extensions
such as MPG, MOV and AVI.

MPEG-1. This codec is available in all
personal computers in the twenty-first
century.17

MPEG-2. A refined version of MPEG-1,
MPEG-2 allows networking and
multiple channels and is used on
digital video disks and the internet.18

Visual Test Imaging 103

MPEG-4. This codec offers more efficient Ethernet

compression and is used in digital Ethernet cables are made of copper or
fiber optic filaments and widely used for
television and interactive media. land based transmission of digital
signals.23 Ethernet was introduced as an
MPEG-4 facilitates protection of upgrade to coaxial cable.
intellectual property.19
Thumb Flash Drives10
MPEG-7. This codec specifies standardized
Thumb flash drives are convenient for
metadata to enable search, tagging, plugging into your laptop for transfer of
cataloging and indexing.20 data or image files. In 2009, they use a
universal serial bus connector and can be
MPEG-21. This codec standardizes hung around the inspector’s neck on a
delivery across multiple platforms.21 lanyard, attached to a key ring or kept in
a pocket of the carrying case for a laptop
MPEG-A. This codec integrates the computer. Inexpensive models in 2010
features of its predecessors.22 hold several gigabytes of data.

Connectors Universal Serial Bus (USB)

Table 5 lists several kinds of media for The universal serial bus is widely used for
data transfer and storage. connected computer components. There
are several generations and configurations
Coaxial Cables10 of the bus. Four configurations developed
by year 2010 are discussed below.
Coaxial cable is used for transmission of
analog or digital signals. In coaxial cables, 1. The USB 1.0 has a slow transfer speed
a center conductor is surrounded by of 1.5 megabytes per second and does
heavy dielectric insulation that is braided not allow for streaming video. The
or foil shielded. The center-to-shield USB 1.0 has been superseded by later
distance is controlled so that the designs.
impedance of the coaxial line matches the
design load of the video circuitry feeding 2. Called full speed, the USB 1.1 has a
the signal. This matching minimizes transfer speed of 12 megabytes per
losses and enables maximum power second and allows for video transfer
transfer. Most video coaxial cable has an with an audio track. It has been widely
impedance of 75 Ω. used for most accessory devices in the
first decade of the twenty-first century.
TABLE 4. Digital formats and codecs of video files.
3. Called high speed, the USB 2.0 has a
Designation Format transfer speed of 480 megabytes per
second and requires a correspondingly
Formats Audio Video Interleave fast processor. It is backwardly
AVI Shockwave Flash® file extension compatible with USB 1.1 and uses the
FLV Moving Picture Experts Group, a standards body same connection.
MPEG video format specification for compressing NTSC
MPEG 1 input 4. Called super speed, the USB 3.0 has a
video specification, compatible with analog transfer speed of 4.8 gigabytes per
MPEG 2 interlacing second and requires a correspondingly
video format specification fast processor. It is backwardly
MPEG 3 video format specification compatible with most earlier USB
MPEG 4 video format from MPEG 3 devices and uses the same connection.
MP3 video format from MPEG 4
MP4 QuickTime® file extension TABLE 5. Media for data storage.
MOV Windows® Media Encoder
WME Windows® Media Video Medium Portability Capacity
WMV Shockwave Flash®
SWF video object, container format in DVD-Video media Floppy disk poor poor
VOB Windows® metafile, native to Microsoft® software Compact disk (CD) good good for images;
WMF
Digital video disk (DVD) good poor for video
Disk formats Blu-RayTM disk (BD) good good
FlashTM drive excellent excellent
BD Blu-Ray disk good for images;
External drive, cabled awkward
CD compact disk External drive, wireless connectivity limits poor for video
good
DVD digital video disk connectivity limits

ISO extension for a file container that boots as virtual
video disk

104 Visual Testing

IEEE 1394 The images are produced on flat
surfaces by shining light through liquid
The standard IEEE 139424 defines a cable crystals and colored filters. The liquid
and connector similar to the universal crystal display allows a flat display device
serial bus. Its popular trade names include made up of pixels in rows and columns to
Firewire® and Lynx®. The IEEE 1394 display an image. The individual pixels
connector preceded the universal serial may emit colors or be monochromatic.
bus and was intended to have similar The low levels of electric power produces
goals. Using twisted pair wiring to move a lit pixel by backlighting the screen,
the data, IEEE 1394 has a serial bus that passing light through a layer of liquid
can handle over 400 megabits per second crystal between two polarizing filters and
and over 60 devices per bus. IEEE 1394 the transparent electrodes between them.
supports devices that stream data in real When the electrodes relax, the liquid
time, including video. IEEE 1394 crystal is without electrical charge and
connections are inherently faster than dims. Because the axes of polarity are
USB 1.0 and so have been popular for fast perpendicular to each other, the light will
transfer of large data streams, as in video not pass through. When the electrical
recording. charge is applied to the pixel, the
molecules align themselves in helixes so
Analog Video Connectors that the light may pass from one side
with vertical filters to the other side with
Analog video connectors are used for horizontal filters. Hence the polarized
transmitting signals to or from an analog filters oriented perpendicular to one
device such as a telephone cable, another no longer block passage of the
television screen or video cassette player. light once the liquid crystals have gained
a helical twist going from horizontal to
1. Connections referred to as composite vertical. Uncharged crystals will be
video have one connector. completely untwisted and the polarized
Supplementary connectors may carry perpendicular filters will block all light
audio or control signals. transmission. The more the crystal is
charged, the more the helical twist will
2. Connections referred to as component occur and allow more light to pass
video carry analog signals and through the transparent crystal and more
terminate in two or more plugs or intensely light the pixel.
sockets (sometimes colored red, green,
blue and black). Plasma Display

3. Connections referred to as separate Plasma displays operate similarly to liquid
video (or S video) split the color signal crystal displays. Electric charges stimulate
among three or more wires in one minuscule packets of gas on the viewing
cable and transmit images inferior to surface so that they glow with desired
those provided by component video. color and intensity and together form a
two-dimensional image.
Visual inspectors may have to use
analog connectors when working with Light Emitting Diode
video players. Analog videotapes may
have been archived to document the A light emitting diode (LED) is a kind of
service history of pressure vessels in a transistor. Two-dimensional arrays of
power plant, for example. The analog these transistors are actuated by the video
signals may be converted from analog to signal and emit light of desired
digital files through a modem. frequencies and intensities.

Coaxial cables are used for video
signals and can serve as control cables,
actuating remote equipment.

Video Displays TABLE 6. Dimensions (in pixels) of some fixed grid
displays. These specifications are sometimes called
Digital screen displays are available in a resolution, but true resolution depends also on physical
wide variety of sizes. Some popular sizes, dimensions of display.
measured in vertical pixels by horizontal
pixels, are listed in Table 6. Arrays Total Aspect Application
(pixels) Pixels Ratio
Liquid Crystal Display10
—— —— 3 to 2 analog television
Liquid crystal displays (LCDs) are used for 64 000 8 to 5 thumbnails and previews
color displays in many television screens, 320 × 200 307 000 4 to 3 portable devices
some computer monitors and many 640 × 480 786 432 4 to 3 extended graphics resolution
portable devices. Liquid crystal allows the 1024 × 768 1 920 000 4 to 3 television
display to be much thinner than cathode 1600 × 1200 2 073 600 16 to 9 high definition television
ray tube technology and consumes much 1920 × 1080
less power.

Visual Test Imaging 105

Frame Rate A video camera converts the
two-dimensional optical image into a
In digital video, moving pictures are series of electrical impulses, the video
produced by successive display of what signal. The video signal is converted by
are essentially still images, each image the monitor back into a two-dimensional
corresponding to a frame. Figure 18 uses picture that reproduces the dynamic scene
the example of a falling object to show of the original image in real time.
the difference between two frame rates.
Capture rates of 60 frames per second are To dissect the optical image so that it
common in digital video cameras. Human can be represented as a series of electrical
perception performs well in viewing signals, narrow strips of the image are
frames rates over 20 frames per second. In scanned. In principle, the image can be
the second half of the twentieth century, scanned by any electrooptical transducer
most movies from digital files are each that rapidly converts light intensities into
encoded to play at a particular rate, in electrical voltages or currents.
synchrony with the computer’s integral
clock. Personal computers can display Line Scanning
video recordings at any frame rate the
visual inspector is likely to encounter. In displaying a video image, analog video
uses the raster scanning technique
Analog Image whereby electrons are beamed onto the
Transmission and phosphor coating on the screen a line at a
Recording10,11 time from left to right starting at the top
left corner. At the end of the line, the
The following brief discussion of analog beam is turned off and moved back to the
technology is included for visual left and down one line (Fig. 21), When
inspectors who must work with quality the bottom right corner is reached, the
assurance archives of service history. gun is returned to the top left corner.
Fuller discussion is available elsewhere.11
Interlacing illuminates a screen by
Cathode Ray Tubes displaying all odd lines in the frame first
and then all even lines (262.5 scan lines
Cathode ray tubes (CRTs) were found in each) which are combined and interlaced
most computer monitors and television to produce a complete frame of 525 lines.
sets in the second half of the twentieth The 2:1 standard interlace ratio means
century. The cathode is a heated filament that even fields of horizontal scan fit
inside a glass tube. The ray is a stream of neatly into odd fields of horizontal scan.
electrons that pour off the heated cathode
in a vacuum. Because the anode is FIGURE 21. Electron beam scan path on analog video screen.
positive, it attracts the electrons pouring
off of the cathode. In cathode ray tubes, Return trace Path of scanning
the radiation is a beam of electrons. In a electron beam
black and white screen, there is only one
phosphor that glows white when struck 1
by the beam of electrons. In a color
screen, there are three types of phosphors 2 A
— red, green and blue — and three B
different electron beams to illuminate the 3 C
colors together. D
4 E
Video Tape Technology F
5 G
The main task of video technology is to H
convert an optical image of a dynamic 6 I
scene into an electrical signal that can be J
transmitted in real time to another 7 K
location, and converted back without L
delay into an optical image that faithfully 8
reproduces the original scene. Sound,
synchronized to the scene, commonly 9
accompanies the video image.
10

11

12

13

106 Visual Testing

Analog Video Systems 3. The SECAM system antedates the
other two; it has been used in France
Before the move to digital video in 2009, and parts of Africa, Asia and Eastern
three systems were used worldwide for Europe.
analog video: the National Television
System Committee (NTSC), the phase Figure 18 shows the difference between
alternating line (PAL) system and the NTSC and PAL formats in terms of frame
SECAM (séquentiel couleur à mémoire, rate, NTSC being 25 and PAL being 30
“sequential color with memory”) system. frames per second. The two formats were
incompatible in the years of analog video
1. NTSC broadcasts are fully compatible tape. Personal computers can display
with the video home system (VHS) digitized video recordings at either
standard for video tape recording and frame rate.
playback. In the United States, electric
power is provided at 120 V at 60 Hz. The analog video is not a compilation
The National Television System of successive still image as in digital video
Committee developed the television but rather is a series of cycles in a raster
standards in the United States — signal. What appears to be a single image
broadcasts at 60 half frames per when a video tape is paused is in fact an
second of 262.5 lines interlaced twice image that the video player has
(odd and even) to make 30 frames per constructed from successive lines of the
second of image, or 525 horizontal continuous raster signal. Discussions of
lines of resolution with 484 visible video electronics often refer to frame rate
lines of image at 30 frames and 60 in hertz (Hz) — that is, cycles per second.
fields per second interlaced, the rest The two measures are equivalent; 1 frame
being used for other information such per second equals 1 Hz.
as synchronizing, data and captioning.
The NTSC system has been used in Closing
North America and parts of Asia.
Digital technology has revolutionized the
2. PAL broadcast signals are fully practice of visual testing. Personal
compatible with video tape standards computers are used for processing,
of the same name. PAL is the standard archiving and transmitting images.
for color television that was developed Remote and portable devices integrate
in Germany; it has been used in many microprocessors for recording and
nations of Europe, Africa and Asia. transmission of test images on site. And
Electricity in Europe typically operates advances in battery life, hard disk
at 220 V and 50 Hz. German memory and processor speed have made
broadcasts have been at 25 half frames possible the acquisition of test
per second of 312.5 interlaced lines documentation, including video records,
twice (odd and even) to make impractical to obtain in the twentieth
50 frames per second image, or century.
625 horizontal lines of resolution with
576 visible lines of image at
25 frames/50 fields per second
interlaced, the rest being used for
other information such as
synchronizing data and captioning.
Two frames equal one field of image.

Visual Test Imaging 107

References

1. Pasquazzi, D.[A.] and W.R. Tweddell. 10. Nondestructive Testing: Remote Visual
Chapter 11, “Other Applications of Inspection (training modules).
Visual and Optical Tests”: Part 2, Skaneateles, NY: General Electric
“Applications of Photography in Corporation, General Electric
Visual Testing.” Nondestructive Testing Inspection Technologies [2005].
Handbook, second edition: Vol. 8,
Visual and Optical Testing. Columbus, 11. Generazio, E.R., D.J. Roth and
OH: American Society for V. Torrey. Chapter 5, “Other
Nondestructive Testing (1993): Instrumentation and Electronic Aids
p 313-319, 336. for Visual Testing”: Part 2, “Video
Technology.” Nondestructive Testing
2. Schmidt, J.T. Section 11, “Recording of Handbook, second edition: Vol. 8,
Magnetic Particle Test Indications.” Visual and Optical Testing. Columbus,
Nondestructive Testing Handbook, OH: American Society for
second edition: Vol. 6, Magnetic Particle Nondestructive Testing (1993):
Testing. Columbus, OH: American p 131-147, 154.
Society for Nondestructive Testing
(1989): p 271-290. 12. Sayler, G.[C.] ASNT Level III Study
Guide: Visual and Optical Testing
3. Schmidt, J.T. “Color Photography of Method. Columbus, OH: American
Magnetic Particle and Penetrant Society for Nondestructive Testing
Indications.” Materials Evaluation. (1998, revised 2006).
Vol. 31, No. 3. Columbus, OH:
American Society for Nondestructive 13. Mikhail, E.M., J.S. Bethel and
Testing (March 1973): p 39-42. J.C. McGlone. Introduction to Modern
Photogrammetry. New York, NY: Wiley
4. Mosher, T.A. “Ultraviolet Photography: (2001).
A System for the Nonprofessional
Photographer.” Materials Evaluation. 14. Manual of Photogrammetry, fifth
Vol. 45, No. 7. Columbus, OH: edition. Bethesda, MD: American
American Society for Nondestructive Society for Photogrammetry and
Testing (July 1987): p 778-779, 781. Remote Sensing (2004).

5. Chapter 9, “Recording of Magnetic 15. Luhmann, T., S. Robson, S. Kyle and
Particle Indications”: Part 5, “Digital I. Harley. Close Range Photogrammetry:
Photography and Image Archiving.” Principles, Methods and Applications.
Nondestructive Testing Handbook, third Dunbeath, Caithness, United
edition: Vol. 8, Magnetic Testing. Kingdom: Whittles (2006).
Columbus, OH: American Society for
Nondestructive Testing (2008): 16. Linder, D. Digital Photogrammetry:
p 243-250, 257. A Practical Course. New York, NY:
Springer (2009).
6. Moore, P.O. “Digital Photography and
Image Archiving.” Materials Evaluation. 17. ISO/IEC 11172, Information Technology
Vol. 66, No. 7. Columbus, OH: — Coding of Moving Pictures and
American Society for Nondestructive Associated Audio for Digital Storage
Testing (July 2008): p 717-722. Media at up to about 1.5 Mbit/s
[MPEG-1]. Geneva, Switzerland:
7. Stephens, P. “Liquid Penetrant Panel International Organization for
Calibration.” Materials Evaluation. Standardization (1993, 2008).
Vol. 65, No. 10. Columbus, OH:
American Society for Nondestructive 18. ISO/IEC 13818, Information Technology
Testing (October 2007): p 1016-1018. — Generic Coding of Moving Pictures and
Associated Audio Information [MPEG-2].
8. Ang, T. Advanced Digital Photography: Geneva, Switzerland: International
Techniques and Tips for Creating Organization for Standardization
Professional-Quality Images, revised (1996, 2009).
edition. Garden City, NY: Amphoto
Books (2007). 19. ISO/IEC 14496, Information Technology
— Coding of Audio-Visual Objects
9. Kelby, S. The Digital Photography Book. [MPEG-4]. Geneva, Switzerland:
Berkeley, CA: Peachpit (2006). International Organization for
Standardization (2004, 2009).

108 Visual Testing

20. ISO/IEC TR 15938, Information ISO DIS 12232, Photography — Digital Still
Technology — Multimedia Content Cameras — Determination of Exposure
Description Interface [MPEG-7]. Geneva, Index, ISO Speed Ratings, Standard
Switzerland: International Output Sensitivity, and Recommended
Organization for Standardization Exposure Index. Geneva, Switzerland:
(2004, 2009). International Organization for
Standardization (2004).
21. ISO/IEC 21000, Information Technology
— Multimedia Framework [MPEG-21]. ISO/IEC 10918, Information Technology —
Geneva, Switzerland: International Digital Compression and Coding of
Organization for Standardization Continuous-Tone Still Images. Geneva,
(1996, 2009). Switzerland: International
Organization for Standardization
22. ISO/IEC 23000, Information Technology (2005).
— Multimedia Application Format
[MPEG-A]. Geneva, Switzerland: ISO TS 22028, Photography and Graphic
International Organization for Technology — Extended Colour Encodings
Standardization (2007, 2009). for Digital Image Storage, Manipulation
and Interchange. Geneva, Switzerland:
23. IEEE 802, Information Technology, International Organization for
Telecommunications and Information Standardization (2006).
Exchange between Systems — Local and
Metropolitan Area Networks — Specific Moving Pictures
Requirements. New York, NY: Institute
of Electrical and Electronic Engineers Kingslake, R. SPIE Monograph PM06,
(2009). Optics in Photography. Bellingham, WA:
International Society for Optical
24. IEEE 1394, A High-Performance Serial Engineering (Society of Photo-Optical
Bus. New York, NY: Institute of Instrumentation Engineers) (1992).
Electrical and Electronic Engineers
(2008). Ray, S.E. Applied Photographic Optics, third
edition. Burlington, MA: Elsevier,
Bibliography Focal Press (2002).

Still Photography Robin, M. Little Digital Video Book, second
edition. Berkeley, CA: Peachpit (2008).
ANSI/I3A IT10, Photography — Digital Still
Cameras. Washington, DC: American Underdahl, K. Digital Video for Dummies,
National Standards Institute (2004). fourth edition. Indianapolis, IN: Wiley,
For Dummies (2006)

SMPTE 2019, Mapping VC-3 Coding Units
into the MXF Generic Container. White
Plains, NY: Society of Motion Picture
and Television Engineers (2009).

Visual Test Imaging 109

5

CHAPTER

Direct Visual Testing

Gregory C. Sayler, MD Helicopters, Mesa, Arizona
Douglas G. Krauss, Huddleston Technical Services,
Redstone Arsenal, Huntsville, Alabama (Part 5)

PART 1. Circumstances of Viewing

Direct visual testing is a technique of the in a scene far exceeds the typical
visual testing method of nondestructive description of the scene. And a great deal
testing. The American Society for of information is potentially available to
Nondestructive Testing divides the observer immediately after viewing. If
nondestructive testing into methods and an observer has the intention of looking
subdivides methods into techniques. for certain aspects of a scene, only certain
Direct viewing is a technique of visual visual information enters the awareness,
testing in the same way that bubble yet the total picture is certainly imaged
testing is a technique of leak testing and on the retina. If a scene or an object is
gamma radiography is a technique of the viewed a second time, many new
radiographic test method. characteristics can be discerned. This new
information directly influences perception
Direct visual testing differs from of the object, yet such information might
indirect techniques, sometimes called not be available to the viewer without a
remote visual testing, because the direct second viewing.
inspector is in the presence of the test
object and has an unmediated view of the Vision is selective in many common
test surface, even if he looks through a situations. An individual can walk into a
device such as a magnifier or camera. In room full of people and notice only the
remote, or indirect, techniques, the face of an expected friend. The same
inspector uses a borescope or propelled individual can walk right by another
camera to view surfaces otherwise friend without recognition if the
inaccessible. The distinction is fine, for encounter is unexpected. Vision is
the inspector may use a “remote” strongly selective and guided almost
apparatus on a test object held in his entirely by what the observer wants and
hand, and an inspection may be viewed does not want to see. Any additional
“directly” through a system of lenses and details beyond the very broadest have
mirrors. been built up by successive viewing. Both
the details and the broad image are
Observer’s Attitude1 retained as long as they are needed, and
then they are quickly erased.
A complete representation of the visual
field probably is not present in the brain The optical image on the retina is
at any one time. The brain must contain constantly changing and moving as the
electrochemical activity representing some eye moves rapidly from one point to
major aspects of a scene, but such a another — the sensing rods and cones are
picture typically does not correspond to stimulated in ways that vary widely from
how the observer describes the scene. This one moment to the next. The mental
discrepancy occurs because the observer image is stationary for stationary objects
adds experience and prejudices that are regardless of the motion of the optical
not themselves part of the visual field. image or, for that matter, the motion of
Such sensory experience may reflect the observer’s head. It is very difficult to
physical reality or may not. determine how a unique configuration of
brain activity can result from a particular
Sensory data entering through the eye set of sensory experiences. A unique visual
are irretrievably transformed by their configuration must be a many-to-one
contexts — an image on the retina is relationship requiring complex
perceived differently if its background or interpretation. If an observer does not
context changes. Perceptually, the image apply experience and the intellect, it is
might be a dark patch in a bright likely that a nondestructive visual test will
background that can, in turn, appear to be inadequate.
be a white patch if displayed against a
dark background. No single sensation Viewing Angle1-3
corresponds uniquely to the original
retinal area of excitation. Eye muscles may manipulate the eye to
align the image on the lens axis. Different
The context of a viewed object can sensors in the retina receive images of
affect perception and, in addition, the different objects in the field of view
intention of the viewer may also affect (Fig. 1), and different banks of sensors
perception. The number of visible objects

112 Visual Testing

basically require different stimuli to best mirrors or borescopes. The field of view
perform their functions. Also, light rays should be maintained much in the same
entering the lens at angles not parallel to way that it is when viewed directly. If the
the lens axis are refracted to a greater examination surface is immovable and
degree. The angle changes the quality and situated so that the eye cannot be placed
quantity of the light energy reaching the within this region, suitable visual aids,
retina. Even the color and contrast ratios such as mirrors, must be used.
vary and affect depth perception.
FIGURE 2. Eye position affects apparent size and location of
The included angle of five minutes of object.
arc is commonly cited as optimum; it is
the average in which an individual sees a Eye positions
sharp image. There are other angles to be
considered when discussing visual testing. 1 23
The angle of peripheral vision is not a
primary consideration when performing Table level Object
detailed visual tests. It is of value under
certain inspection conditions: (1) when Projected Relative image size and location 1
surveying large areas for a discontinuity images Image 2
indication that (2) has a high contrast Image 3
ratio with the background and (3) is Ratio 1:1
observed to one side of the normal lens Ratio 1:1.40
axis. The inspector’s attention is drawn to Ratio 1:1.65
this area and it can then be scrutinized by
focusing the eyes on the normal plane of
the lens axis.

The angle of view is very important
during visual testing. The viewer should
in all cases attempt to observe the target
on the center axis of the eye. Figure 2
shows how the eye perceives an object
from several angles and how the object
appears to change or move with a change
in viewing angle.

The angle of view should vary ideally
not more than 45 degrees from normal,
and a recommended viewing distance and
angle for visual testing is to have the eye
within 600 mm (24 in.) of the object and
positioned at an angle not less than
30 degrees to the inspection surface as
shown in Fig. 3.

The same principle applies to objects
being viewed through accessories such as

FIGURE 1. Peripheral vision.

Peripheral angle FIGURE 3. Range of viewing angle.
View aligned with lens
Eye

Retina with image Peripheral angle 305 mm (12 in.) No closer than
locations at different 150 mm (6 in.)
angles

30 degrees 30 degrees
Test surface
Test site

Direct Visual Testing 113

Pitfalls Related to Viewing Angle Some specifications (in nuclear and
aerospace industries) forbid the use of
Posture affects the way an object is mirrors.
observed. Appropriate posture and
viewing angle minimize fatigue, eyestrain Light Manipulation by Mirrors
and distraction. The viewer’s posture
should make it easy to maintain the Mirrors change the direction of light by
optimum view on the axis of the lens. reflection and can be flat, convex or
concave.
On reflective backgrounds, the viewing
angle should be off normal but not 1. Flat or plane mirrors are arranged
beyond 45 degrees. This angle is alone or in series to transmit light or
maintained so that the light reflected off an image.
the surface is not directed toward the
eyes, reducing the contrast image of the 2. Convex mirrors provide an enlarged
surface itself. It also allows the evaluation field of view of the reflected image.
of discontinuities without distorting their
size, color or location. The angle is 3. Concave mirrors each have a reflecting
important when using optical devices to focal point. If a light is projected onto
view surfaces not available to direct line a spherical mirror normal to the curve
of sight. of the surface, the light will be focused
slightly in front of the mirror. If a
Elevated objects are hard to test point light source is placed at its focal
visually. Without adequate access, a point, the light will be reflected from
surface beyond the reach of the inspector the mirror so that it is parallel to the
cannot be thoroughly inspected except in normal of the curve. A concave mirror
cases such as object verification or can also be used for image
approximate location — to answer a enlargement. An image that is small
question such as “Have the girders been compared to the width of the mirror
painted yet?” will be reflected back in a diverging
and optically reversed image.

Mirrors2 FIGURE 4. Examples of inspection mirrors:
(a) 32 mm (1.25 in.) diameter mirror,
Mirrors are common inspection aids. Easy 375 mm (17 in.) long, with pocket clip and
to use, mirrors make inspection possible swiveling neck; (b) 60 mm (2.25 in.)
inside pipes and apertures and inside or diameter mirror with swiveling neck and
behind objects obstructing the inspector’s length telescoping from 250 to 350 mm
view. (10 to 14 in.); (c) 54 × 89 mm
(2.13 × 3.5 in.) mirror with swiveling neck
Inspection mirrors come in a variety of and length telescoping from 286 to
shapes (Fig. 4). The head, holding the 387 mm (11.25 to 15.25 in.).
reflective surface, can be round or (a)
rectangular and range in size from 20 to
100 mm (0.8 to 4.0 in.). The handle, or (b)
stem, includes a scored surface or rubber
sheath for easy handling. Smaller mirrors (c)
resemble those used by dentists and
usually have a 150 mm (6 in.) long
handle with a round head. The diameter
of the handle can range from about 3 to
25 mm (0.1 to 1.0 in.).

Inspection mirrors frequently have
telescoping handles that can almost
double the handle length. Most have a
double ball joint between the mirror and
handle that lets the mirror swivel to any
convenient angle.

The illumination of the area being
inspected should be the same as that
specified for the rest of the inspection.

Flashlights or other small portable light
sources can provide adequate
illumination, but strong direct lighting
can cause contrastive shadows and
reflected glare.

Many industrial environments are hard
on the mirror’s glass surface. A scratched
mirror is a hindrance during the
inspection. The mirror or its head should
be replaced when marred.

114 Visual Testing

Interpretation of Mirror Images angle is something other than normal
to the surface being inspected. These
Several precautions must be remembered factors can cause indications to be
in interpreting mirror images. interpreted as smaller than they are.
Measuring the size of the indication at
1. Curved mirrors can distort the the reflection in the mirror is only
apparent shape and size of an object. appropriate when pinpoint accuracy is
Concave mirrors make objects look unimportant or when the mirror is
smaller or farther away; convex close to the surface being inspected. A
mirrors (sometimes called fish eye set of pliable wires of known
mirrors) make objects look larger or diameters is valuable when measuring
closer to the viewer. Glass and plastic indications in difficult-to-reach areas.
may be polarized to achieve these 3. A mirror image is reversed, so an
effects with a flat surface. object on the right appears on the left
and one on the left appears right. This
2. The inspection distance is equal to the reversal may affect documentation —
distance from the area being inspected photographs or descriptions in
to the mirror plus the distance from inspection reports.
the mirror to the inspector’s eye.
When using a mirror, the inspection

Direct Visual Testing 115

PART 2. Illumination2

Visual Inspection LightingAccuracy (percentage) Quality of lighting or illumination in
the inspection area refers to the
The purpose of lighting in a visual distribution of the light sources in the
inspection area is to provide adequate area and implies that these devices aid
contrast so that relevant objects or visual performance and comfort. Quality
discontinuities are detected. Contrast illumination is composed of area lighting
detection is the most basic of visual tasks. and specific test lighting.
It is a property of the difference between
an object and its background of either To avoid inspector eye fatigue and to
luminance or color. enhance the probability of detection due
to size, the luminance ratios of the
Luminance contrast is the difference in inspection area should be controlled.
intensity of reflected light between the Table 1 lists the maximum recommended
discontinuity and its background. luminance ratio between the test object
Luminance contrast as related to and the environment and for different
optometry is discussed in this volume’s areas of the environment. This table
chapter on vision acuity. recommends maximum luminance ratios
for areas where reflectances in the work
The contrast value is constant for any area can be controlled when control of
value of luminance but, like reflection, the surrounding area is limited.
varies with the position of the subject
(observer) and object. The probability of Disability glare reduces visibility and
detection increases as the relative contrast visual performance. Glare is caused by
value increases. Figure 5 illustrates this as light sources or reflections from light
a percentage of accuracy and relates it to sources in the field of view. Discomfort
the probability of detection. glare produces visual discomfort. To
reduce glare, it may be necessary to take
Chromatic contrast is the difference in one of more of the following steps:
hue and saturation between an object and (1) decrease the intensity of the light
its background. Chromatic contrast source, (2) reduce the area of the light
produces visibilities less than 20 percent source, (3) increase the angle between the
of the detectability of luminance contrast. light source and the field of view or
Chromatic contrast may augment or (4) decrease reflections by using a light
detract from the achromatic luminance source with a larger area and lower
contrast, depending on the perceptive luminance.
ability of the eye to detect the colors
involved. Illuminance is very important in visual
tasks. The Illuminating Engineering
FIGURE 5. Probability of detection versus target contrast Society recommends the illuminance
ratio. levels listed in Table 2 for the
performance of visual tasks.
100
The spectral distribution of lighting
90 can enhance or subdue a color. To
enhance a color, the light source should
80 be rich in the color. In most visual
inspection situations, the color quality
70 and spectral distribution of the light
sources have no effect on vision acuity.
60 When color discrimination or color

50 TABLE 1. Maximum recommended luminance ratios.

40 Ratio Environment

30 3 to 1 between test surface and adjacent, darker surfaces
1 to 3 between test surface and adjacent, lighter surfaces
20 20 to 1 between test surface and distant, darker surfaces
1 to 20 between test surface and distant, lighter surfaces
10

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Relative target contrast (ratio)

116 Visual Testing

matching is part of the inspection process, Localized general lighting provides
the color of light should be chosen uniform general lighting and is oriented
carefully. Generally, the light color and to provide optimum illumination in the
spectral distribution should approximate inspection area. Commonly, ceiling lights
sunlight. are positioned close to the inspection
area.
When performing visual inspections
under field conditions, it can be difficult Local lighting includes desk lamps and
to get enough light. Shadows in the other portable lighting equipment. Local
inspection area can be controlled by a lighting produces very high illuminance
horizontal light source with a large and is useful when shadow formation
surface area or by reflection from produces enhanced contrast. Local
luminant walls. Harsh shadows generally lighting can cause direct and reflected
cause eye strain, but some shadow effect glare. To prevent eye fatigue due to
can accentuate contrast and aid in the constant eye adaptation, the general room
detection of depth and form. It is lighting should provide at least 20 to
important to weigh the effects of 30 percent of the total illumination.
illuminance on the probability of
detecting a significant indication. Flashlights, called torches in the United
Kingdom, are common, inexpensive and
Area Lighting useful (Fig. 6). They let the inspector
direct light onto shaded and interior
Area or room lighting can be direct, surfaces.
semidirect or diffuse. In room lighting,
ceiling lamps direct 90 to 100 percent of FIGURE 6. Flash lights: (a) household;
the output downward. This type of (b) high power; (c) integrated in helmet.
lighting produces high illuminance in the
vertical direction but also can produce (a)
shadows, glare and veiling reflections.
Highly reflective walls are generally (b)
recommended to control these effects.
(c)
Semidirect lighting directs 60 to
90 percent of the total light output
downward with the balance of the light
directed upward. This type of lighting
eliminates ceiling shadows, but bright
ceilings should be avoided.

Ambient lighting is diffuse, directing
light upward, downward and sideways.
This type of lighting generally provides
good brightness relationships throughout
the room and produces horizontal
illuminance to soften shadows. Glare is
minimal with this type of lighting.

Task Lighting

Task lighting may be classified based on
its layout or orientation to the task to be
performed. General task lighting provides
approximately uniform illuminance over
a broad area. It is generally produced by
the area or room lighting.

TABLE 2. Iluminance levels for visual tasks.2

Activity Illuminance (lx)

General Lighting 100 to 200
Occasional visual tasks 200 to 500
Tasks with high contrast or large size
500 to 1000
Task Lighting 1000 to 2000
Tasks with medium contrast or small size
Tasks with low contrast or very small size

Direct Visual Testing 117

Lighting for Automated 2. Absorptive characteristics can be
Inspection selective or nonselective. Nonselective
materials appear black or gray.
When choosing lighting for automated Selective materials absorb some
inspection and machine vision wavelengths more than others and so
applications, the goal is to optimize the have a distinct color.
contrast between a potential discontinuity
and its surroundings by providing stable 3. Transparent and translucent materials
illumination that reduces signal have transmissive characteristics.
processing complexity. Lighting that takes Transparent materials transmit the
advantage of the spectral sensitivity of the light spectrum with no apparent
light sensor and the optical characteristics scatter. Translucent materials transmit
of the test material are recommended. a large percentage of the light, but
some light scatters by diffusion in the
The spectral sensitivity of sensors is material. Opaque materials transmit
different from that of the human eye. no light — all of the spectrum is
This difference allows for light sources absorbed or reflected.
ineffective during direct visual testing or
light harmful to the eye: polarized, The placement of the light source or
monochromatic and coherent (laser) light. sources and the amount of direct or
Sensors can be optimized for self-radiating diffuse light produced by each light
materials such as high temperature metals source are critical for accurate inspections.
or glass. Most light sensors cannot adapt The type of illuminator controls the
to differing levels of illumination as easily amount of direct and diffuse light
as the human eye, which makes produced. Types of illuminators include
consistent illumination critical in condenser reflectors, spot projectors,
automated inspection. diffuse light sources and collimators
(Fig. 7).
When determining the lighting
geometry and the type of light source for 1. A condenser projector has a lens that
an inspection, the reflective, absorptive focuses a cone of light on a target
and transmissive characteristics of the test surface.
material are the primary factors.
2. A spot projector has a reflective
1. Reflective characteristics are attributes surface designed to produce a
of a diffuse or specular surface and of gradually expanding cone of light.
materials that reflect either selectively
or nonselectively. Spectrally selective 3. Collimators use a small hole to
reflectors reflect uneven amounts of produce a direct cone of light that
the incident light’s initial wavelength expands very little.
distribution and absorb or transmit
the balance. The nonuniform 4. Diffuse surface light sources use a
reflective characteristics of a spectrally translucent material to scatter the
selective reflector may be illuminated incident light.
by a monochromatic source.
Various illumination geometries are
used with microscopes and machine
vision. Translucent test objects may be

FIGURE 7. Types of light sources: (a) condenser reflector; (b) spot projector; (c) diffuse light
source; (d) collimated light source.

(a) (b) (c) (d)

Opaque
material

Reflector Translucent Hole
lens material

118 Visual Testing

illuminated from the side opposite the up to a factor of 100 000 to 1. This
detector, for instance. Diffuse front arrangement is used in some older
illumination (Fig. 8) floods the area of photometers. Polarizing filters are also
interest with as much light as possible used in many glare reducing products
and minimizes shadow formation. It such as sunglasses.
typically is used on opaque materials
where there are prismatic reflections or Some specialized optical techniques use
where the contrast values are very high. A polarized light because of its ability to
directional light source makes differences produce uniform patterns of constructive
in the surface texture more apparent. and destructive interference of the light
waves. This allows the characteristics of
Light Sources many products to be measured by
assessing the interference patterns when
Light sources for visual and optical polarized light is transmitted through or
inspection may be divided into five reflected from a test object. Techniques
categories: solar, incandescent, using polarized light include moiré fringe
luminescent, polarized and coherent light. and birefringence techniques.

Sunlight Coherent Light

The brightness of sunlight varies with Coherent light, such as light from a laser,
weather and time of day. is visible radiant energy or light with a
high degree of phase coherence. Phase
Direct sunlight can be a health hazard, coherence requires both frequency
burning skin and eyes. Glare from bright coherence and the spatial coherence of
sunlight can interfere with nondestructive polarized light. Phase coherence requires
testing by obscuring indications and by the individual waves of light to be of the
fatiguing the inspector. same frequency and the sinusoidal curve
of the waves to be aligned.
Incandescent Light
The light produced by most light
Incandescence is the emission of light due sources has a broad spectrum and
to the thermal excitation of atoms or produces a diverging luminous area, but
molecules. Sources of incandescent light the rays of a laser or phased light are
include filament lamps, pyroluminescence aligned in parallel. Phased light is
(such as glow in furnaces or foundries), produced by a ruby laser in a manner
gas mantles and carbon arc lamps. similar to the production of fluorescent
light. The difference is that in the
Luminescent Light coherent light producing material, the
excited electrons pause momentarily in an
Luminescence results from the excitation orbit with more energy than the ground
of a single valence electron. Luminescent state but less energy than the most
light is more monochromatic in nature excited state. This intermediate stage is
than incandescent light sources. Sources known as the metastable state. The
of luminescent light include gaseous photons of light energy are released from
discharge lamps, lasers, light emitting the metastable state simultaneously
diodes and fluorescent lamps. producing spatial or time phased
coherence.
Polarized Light
FIGURE 8. Diffuse front illumination.
The vibrations of polarized light have
been oriented to show preference. This Camera
means that the vector describing the
direction of the lightwave form is Light source Light source
constant in time. Linear polarization
means that either the vector or the
waveforms have been aligned so that both
are in the same plane. Although polarized
light can be produced directly, it is most
commonly produced using a conventional
light source and a polarizing light filter.

Polarizing filters are used to control the
intensity, color and glare of light. The
intensity of light is controlled by using a
pair of linear polarizing filters that can be
rotated. This two-filter arrangement is
capable of a smooth range of attenuation

Direct Visual Testing 119

Photometry

The light used for visual testing is a
nondestructive testing parameter that
may be specified in a contract or written
procedure. Hand held photometers
provide digital measurements of ambient
or direct light in lux. Photographers call
them light meters. Photometry is discussed
in this volume’s chapter on light.

120 Visual Testing

PART 3. Magnification2,4,5

Magnification is commonly achieved by 6. Distracting rainbows and colored
the refracting of light through curved bands are seen when different
lenses. Refraction and basic lens types are wavelengths of light refract in
discussed in this volume’s chapter on the different directions. Chromatic
physics of light. correction mitigates this spectral
divergence, often by using two lenses
Devices for magnification used in sandwiched together (a doublet), two
visual and optical inspections range in layers with different indices of
magnifying power from 1.5× to over refraction.
2000×. Magnifiers are described by
interrelated factors: (1) magnification 7. Binoculars are used for stereoscopic
power, (2) field of view, (3) working viewing from a distance. Stereoscopic
distance, (4) eye relief, (5) depth of field, microscopes are used to inspect
(6) chromatic correction and (7) binocular miniature assemblies such as printed
or monocular vision. circuit boards. Nearly all other
magnifiers are monocular.
1. The magnifying ability of a lens
depends on the amount of curvature These magnifier attributes are
in the converging lens. Greater interrelated. A high power magnifier, for
curvature produces a greater angle of example, has a short working distance, a
refraction, reducing the focal length small field of view and cannot be used for
and so increasing magnification. The binocular observation; a low power
power of a magnifier describes the magnifier, such as a rectangular reader
amount of enlargement compared to lens, has a long working distance, a large
viewing the object 250 mm (10 in.) field of view and can looked through by
from the eye. An object 25 mm (1 in.) both eyes. To attain chromatic correction
from the eye would appear to be ten (to eliminate color fringing), the high
times as large as the same object power lens typically contains a cemented
viewed at 250 mm (10 in.). The doublet or triplet of different optical
magnifier that produces enough glasses whereas the low power reader lens
refraction to allow the eye to focus at is sufficiently achromatic as a simple lens.
this distance has 10× magnification
(10 power) and a focal length of There are many variations of these
25 mm (1 in.). characteristics. Commercial magnifiers
can be 30× or higher in power, and there
2. Field coverage of conventional are many special mountings for particular
magnifiers ranges from 90 mm applications.
(3.5 in.) down to 0.15 mm (0.006 in.)
wide. The ideal magnifier has a large field of
view, a large depth of field and an image
3. The working distance of the magnifier without refractive or color aberrations,
is the same as the focal length. The but in practice all magnifiers present a
distance from the magnifier to the compromise. Generally, using as little
object being inspected is adjusted to magnification as possible will cause the
produce good focus. Close working fewest technical problems and provide the
distances accompany higher simplest inspection.
magnification. When choosing the
magnification and working distance, Magnifiers
the amount of room needed to
perform the inspection and the tools Resolving powers range from 0.05 mm
to be used should be considered. (0.002 in.) to 0.2 µm (8 × 10–6 in.). Powers
of magnification refer to enlargement in
4. The eye relief is the distance from the one dimension only. A two-dimensional
inspector’s eye to the device’s lens. image magnified × 2, for example, doubles
in width and in height while its area
5. The depth of field is the maximum quadruples.
range of distances in focus at the same
time without a change in the viewing Magnifying Lenses
position. The field of view decreases as
the magnification increases. A large A test surface is viewed through a lens to
depth of field eliminates the need for obtain a magnification as great as desired.
constant adjustments to the viewing
position to maintain focus.

Direct Visual Testing 121

The distance from lens to object is with the power of the lens and is
adjusted until the object is in the lens’s comparatively greater in lower power
depth of field and is in focus. The magnifiers, decreasing as the power of the
simplest form of a microscope is a single lens increases.
converging lens, often referred to as a
simple magnifier. Magnification M of a Conventional Magnifiers and
single lens is determined by Eq. 1: Readers

(1) M = 250 Table 3 shows the characteristics of a few
fmm typical magnifiers. These values are
approximations because eye
= 10 accommodation can cause each of the
fin values to vary. Except for the reader lens,
all magnifiers are used with the eye fairly
In this equation, fmm is the focal length of close to the magnifier, giving the largest
the lens in millimeters, fin is the focal field of view. The reader lens is used
length of the lens in inches and 250 (or binocularly and is normally held some
10) is a constant that represents the distance away from the eyes.
average minimum distance at which
objects can be distinctly seen by the Single lens magnifiers provide between
unaided eye. 1.5× and 10× magnification. They may be
handheld, mounted on flexible or rigid
Using the equation, a lens with a focal stands or mounted with a headband to
length of 125 mm (5 in.) has a allow free hands during viewing. In low
magnification of two widths or is said to cost magnifiers, the distortion caused by
be a two-power (2×) lens. For a simple highly refracting lenses is controlled by
magnifier, the focal length and working decreasing the diameter of the lens. This
distance are about the same. For example, decrease reduces the field of view.
suppose that a component must be
inspected without moving it and that a For magnification above 7× to 10×, it is
magnifier cannot be placed nearer than common for magnifiers to use more than
75 mm (3 in.). A lens with a working one lens to provide for the increased
distance (focal length) of at least 75 mm magnification while limiting the spherical
(3 in.) is required. From Eq. 1, that is and chromatic aberrations produced by a
shown to be a 3× or a three power lens. highly refractive lens. Magnifiers with two
or three lenses are referred to as doublets
The field of view is the area seen or triplets; these lens types are illustrated
through the magnifier. With a simple in this volume’s chapter on light. Another
magnifier, the diameter of the field of type of lens, the coddington magnifier,
view is less than its focal length. Selection has a thick groove cut into the outer
of a magnifier with the proper field of diameter of a thick lens to eliminate
view is important. For example, if the test spherical aberration (Fig. 9).
object is large, it takes too much time to
use a 20× magnifier, with a field of view A loupe (or lupe) is a single or double
slightly greater than 10 mm (0.37 in.). lens magnifier where the lenses are held
The proper procedure is to use a low at the recommended working distance
power magnifier first, marking with a transparent cylinder. The clear
questionable areas and then to inspect spacing cylinder allows the use of ambient
questioned areas with a higher powered lighting to illuminate the test piece.
magnifier. Illuminating loupes have a battery
powered light source for illumination.
Depth of field is the distance a Loupes frequently include a contact
magnifier can be moved toward or away reticle. Jewelers’ loupes generally use the
from a subject with the subject remaining cylinder for purposes of eye relief rather
in good focus (sharply defined). At other than working distance. They are inserted
distances, the subject is out of focus and into the eye socket and held in place with
not sharply defined. Depth of field varies the facial muscles around the eye or
attached to the frame of an eyeglass.

TABLE 3. Characteristics of typical magnifiers.

Magnifier _W__o_r_k_i_n_g__F_ie_l_d__o_f_V__ie_w__ _R_e_s_o_l_v_in_g__D__is_t_a_n_c_e_ _____P_o_w__e_r____
mm (in.) Power mm (in.) µm (in.)

Reader lens 90 × 40 (3.5 × 1.5) 1.5× 100 (4) 50 (0.002)
Eyeglass loupe 60 (2.375) 2× 90 (3.5) 40 (0.0015)
Doublet magnifier 60 (2.375) 3.5× 75 (3) 25 (0.001)
Coddington magnifier 19 (0.75) 7× 25 (1) 10 (0.0004)
Triplet magnifier 22 (0.875) 10× 20 (0.75) (0.0003)
7.5

122 Visual Testing

Because of its large diameter, the 3.5× (0.0003 in.). The field of view is about
doublet magnifier has a field as large as 1 mm (0.4 in.) diameter.
that of the 2× loupe. The double convex
lens of the doublet magnifier with its Measuring Magnifier
central iris has a comparatively small
field. The triplet is a three-element design A measuring magnifier, or contact reticle
having excellent optical correction for (Fig. 11), has a graduated measuring scale
field coverage and reduction of color in a transparent layer held against the test
fringing. Its resolving power is the limit of object to measure tiny details on its flat
detection for fine structures. In surfaces. Measuring magnifiers have one-,
comparison, the doublet magnifier can two- or three-lens magnifiers and a
barely differentiate two points 25 µm transparent housing that lets light fall on
(0.001 in.) apart. the measured surface. Contact reticles are
available in a variety of scales and
Surface Comparators measuring systems including linear
measurement, angular measurement, grid
The surface comparator is a magnifier that patterns and thread gages. Magnification
provides a means for comparing a test is often between 6× and 20×; the diameter
surface against a standard surface finish. of the field of view is often about 25 mm
The observer views the two surfaces side
by side, as shown in Fig. 10. The surface FIGURE 10. Surface comparator: (a) schematic; (b) two
comparator uses a small battery powered surfaces magnified for comparison.
light source, a semitransparent beam
divider and a 10× triplet. (a) Comparison
standard
The light is divided between the Test surface
reference surface and the standard surface. Eye
Flat and shiny surfaces reflect the filament
image directly into the pupil of the eye so 10× magnifier
that these parts look bright. Sloping or Beam divider
rough surfaces reflect the light away from
the pupil and such areas appear dark. This
form of illumination sharply delineates
surface pattern characteristics. The
resolving power is about 7.5 µm

FIGURE 9. Cross sections: (a) triplet
magnifier; (b) coddington magnifier.

(a)

Light source

(b)
(b)

Direct Visual Testing 123

(1 in.); the resolving power is often about printed circuits, or can be used to
1 mm (4 × 10–5 in.). examine an area of a test object without
sectioning or scraping it.
Illuminated Magnifiers
Low Power Microscopes
Illuminated magnifiers range from large
circular reader lenses, equipped with When magnifications above 10× are
fluorescent lighting and an adjustable required, the short working distance of
stand, to a small battery powered 10× the magnifier becomes a problem and a
magnifier shaped like a pencil. Some low power compound microscope is
illuminated magnifiers can be obtained in preferred. Typical resolving power is about
either a battery powered model or 7.5 µm (3 × 10–4 in.). At magnifications
equipped for 115 V line operation. Such above 10×, low power microscopes are
triplet magnifiers give about a 50 mm more convenient than simple magnifiers.
(2 in.) field of view. Resolving power is Low power microscopes for shop or other
about 1.5 µm (6 × 10–5 in.). stationary use include a large variety of
devices ranging in complexity from
Microscopes2,4,5 simple to wide field stereoscopic
microscopes. They use lensing systems
A magnifying glass magnifies what is that increase in complexity with the
visible to the unaided eye but is too small amount of magnification. At low and
to evaluate; a microscope makes details medium levels of magnification, a single
visible that otherwise would remain lensing system can be used. At higher
invisible to the human eye. levels, compound lenses with two sets of
lenses are common.
In its simplest form, a microscope is a
single biconvex lens in a housing Field, pocket, pen or measuring
adjustable for focus. Many forms of microscopes are all terms used to describe
illumination are available, including small handheld magnifiers that have
bright field, dark field, oblique, polarized, magnifying powers between 10× and 50×
phase contrast and interference. and more complex lens arrangements
than simple magnifiers. These are
Microscopy is usually destructive: a powerful for field inspections but are
sample scraping removed from the surface difficult to use because of limited stability,
of an object is discarded after analysis. small depths of field, rudimentary
There are many varieties of microscopes, focusing systems, narrow fields of view
however, and some can be used to and the need for intense illumination.
examine very small products, such as
Stereo Microscopes (Medium
FIGURE 11. Typical measuring scales and reticles of Power)
measuring magnifier.
The wide field stereoscopic microscope is
90 75 very complex. It is basically two erect
60 image microscopes, one for each eye,
comprising two objectives, two erecting
15 prisms, two inclination prisms and two
30 eyepieces. The useful power range of the
stereomicroscope is limited to 100×. The
45 resolving power is about 5 µm
(2 × 10–4 in.). Field coverage is inverse to
0 its power: at 10× field coverage, the field
is about 25 mm (1 in.). The instrument
—38 13—12 1—56 3—92 —14 3—72 1—36 3—52 —18 3—32 1—16 provides binocular vision, which makes
possible its prolonged use for visual
1 DIV = 0.005” testing. As with low power magnifiers,
manual manipulation during observation
0” .1 .2 .3 .4 .5 is practical. The stereoscopic microscope
provides a true view of depth, so that test
1 DIV = 01 MM objects may be inspected in three
dimensions.
0 1 2 3 4 5 6 7 8 9 10
Shop Microscope (Medium
MM Power)

.001 .002 The shop microscope is similar to a wide
field tube. It has a power of 40× and
.003 contains a built-in light source that may
be operated from a battery or 115 V line
current. The shop microscope contains a

124 Visual Testing

scale permitting direct measurement on lens, is a semireflecting, thin, transparent
the object plane to 0.025 mm (0.001 in.) plate. It directs light down through the
or estimates to 6 µm (2.5 × 10–4 in.), over objective lens onto the test object. The
a scale length of 4 mm (0.15 in.). The microscope is normally equipped with a
field of view is 5 mm (0.22 in.) and the built-in light source and has field and
resolving power is about 3.3 µm aperture iris controls in the illuminating
(1.3 × 10–4 in.). The instrument weighs arm. Because thick preparations are
little, only 500 g (18 oz) including common in opaque test objects, the stage
batteries. may be focused. Focusing permits an
intense external light source without
Applications of the shop microscope upsetting the illumination centering.
include (1) on-site tests of plated, painted
or polished surfaces, (2) detection of Although this microscope finds its
cracks, blowholes and other principal applications in metallurgy, it can
discontinuities and (3) measurement be used on almost any opaque material
checking wear in mechanical having a reasonably high reflectivity.
components. Welding of machine tool When test objects are dark by nature (dark
frames, piping, structural members, plastics, paints, minerals) or have
pressure vessels, jigs and fixtures, can be excessive light scattering (fabrics, paper,
quickly inspected. wood, or biological specimens), a form of
incident dark field illumination is superior
In finishing and electroplating to regular vertical illumination.
operations, surface tests with the shop
microscope can detect cracks, blister, Metallographic Microscope
irregular deposits, pitting and poor quality
buffing or polishing. It can reveal slag Microscopes designed to provide images
inclusions and poor surfacing of base of opaque objects have provisions for
metals before plating. On painted surfaces vertical illumination — that is, by
it permits quick and accurate evaluation reflected light rather than light
of quality, uniformity and pigment transmitted through the test object from
distribution. In the graphic arts, it is used the other side. A metallographic
to check halftones for size, shape and microscope is a common example of this
distribution of dots. type of microscope. A metallographic
microscope is a metallurgical microscope
Textile mills use shop microscopes for designed to integrate a camera for
identification of fiber textures, photographic documentation. The
distribution of coloring matter and test of metallographic microscope’s light source
weave, twist and other general may have filters to alter the spectrum or
characteristics. Fabric finishes, markings, polarize light to reduce glare.
lusters and dye transfers can be inspected
for penetration and quality. In the paper Common metallographic techniques
industry, the shop microscope is used to include dark field illumination, polarized
check fiber uniformity, evenness of light, phase contrast and differential
coating and wear of Fourdrinier wires. interference. These techniques are
increasingly being applied to the analysis
Laboratory Microscope of the microstructures of many
nonmetallic and composite materials.
Laboratory microscopes normally range in
power from 100× to 2000× and over. They Polarizing Microscope
commonly use compound lensing systems
with two or more separate sets of lenses. It The addition of two polarizing elements
is becoming more common for these and a circular stage converts a laboratory
microscopes to include provisions for a microscope into an elementary polarizing
still or video camera. A video camera microscope. A polarizing element is a
allows the image to be viewed on a device that restricts light vibration to a
monitor and provides an excellent means single plane. This form of light is useful
of documentation. Adapters are available for studying most materials with
to adapt conventional microscopes for use directional optical properties, including
with a video or still camera. fibers, crystals, sheet plastic and materials
under strain. As such materials are rotated
Most laboratory microscopes were between crossed polarizers on the
developed for medical use and rely on the microscope stage, they change color and
transmission of light through the object, intensity in a way that is related to their
so they are limited to translucent objects. directional properties.

Metallurgical Microscope Interference Microscope

The metallurgical microscope is similar to The interference microscope is a tool
a laboratory microscope with the addition using the wavelength of light as a unit of
of top or vertical illumination to permit measure for surface contour and other
viewing of opaque materials. The vertical characteristics. In one form of interference
illuminator, directly above the objective

Direct Visual Testing 125

microscope, the stage is inverted and the including highly polished or glossy
test object is placed face downward. The finished surfaces, where the degree of
image appears as a contour map, with a surface roughness is within a few
separation of one half-wave or about wavelengths of light. With coarser
0.25 µm (1 × 10–5 in.) between contour surfaces, the contour lines are close
lines. Extremely precise measurements can together and interpretation is difficult. An
be made with such equipment. advantage of the interference microscope
is that the test object is not moved
Applications of the interference manually during inspection.
microscope include the measurement,
testing and control of very fine finishes,

126 Visual Testing

PART 4. Surface Characteristics2

Surface characteristics that can be visually 3. When the object of controlling surface
tested include surface texture (roughness roughness is to control waviness and
or waviness), color (including gloss) and roughness, the profile variation may
cleanliness. Cleanliness is a prerequisite be removed by using a line that
for certain processes such as painting and describes the average surface. This line
liquid penetrant testing. Surfaces may also can then be applied as a profile filter.
be visually tested closely to identify Surface roughness may be measured
discontinuities and damage mechanisms,
such as porosity, blistering, corrosion, with photogrammetry or profilometry. In
flaking and fatigue. The visual inspector photogrammetry, surface details are
must flag apparent cracks for evaluation measured by comparing two photographic
by other methods. images offset from each other. In
profilometry, points on the surface are
The variation from a specified nominal successively measured and a series of
surface condition is controlled by optical or mechanical measurements are
dimensional tolerances and surface compiled.
roughness specifications. Surface texture is
usually measured in accordance with Average roughness Ra is the average
published standards.6-11 distance of the profile to the mean line.
When controlling the height of individual
Surface Texture peaks and valleys is important,
measurements of maximum peak-to-valley
The surface features of any object, height Rmax and mean peak-to-valley
regardless of scale, share three
independent characteristics: form, FIGURE 12. Comparison of profiles for average roughness Ra:
roughness and waviness. (a) Ra = 2.4 µm; (b) Ra = 2.5 µm; (c) Ra = 2.4 µm. Surfaces of
Figs. 12a and 12c have same average but different profiles.
1. Variations in form or profile are
typically controlled by the (a)
dimensional or geometric tolerance
specifications. Surface profiles for (b)
larger tolerances, ±0.03 mm
(±0.001 in.) and greater, are typically (c)
measured using standard dimensional
equipment or optical comparators and
should be measured with respect to a
datum surface.

2. Surface roughness is critical wherever
friction must be reduced, as in
propellers, ball bearings or prosthetic
joints. When measuring and reporting
surface roughness, it is essential to
choose a measurement parameter that
assigns a numerical value to the
characteristics that must be controlled.
If only one parameter is chosen, there
is a risk that some unimportant aspect
of the surface may be overcontrolled,
adding unnecessary cost to assure
adequate control of the key
characteristic. For example, the
surfaces in Fig. 12 have the same
roughness average Ra but different
roughness peak Rp and roughness
depth RY.

Direct Visual Testing 127

height Rz can control the total variation 3. Saturation (chroma) measures the
without adding the cost of an overly tight distance from the corresponding
neutral color. It is often referred to as
Ra requirement. the color intensity.
Typically, Rz is determined by dividing
Human perception of color varies with
the measured length into five equal the illumination and with the perceptive
abilities of the observer. The
lengths and using the formula: communication of color requirements and
the assessment of color against
∑(2) Rz = 1 5 requirements can be imprecise. Color
5 requirements can be effectively
Zi communicated by visual comparison to
reference standards, a color order system
i =1 or a color collection. Color order systems
place colors into an orderly
where Zi is the waviness variation. three-dimensional arrangement with a
The total waviness parameter is used to standard nomenclature to describe each
color in the system.
control the larger horizontal scale
variation, waviness. Surface waviness and 1. Based on the three sets of opposing
surface roughness measurements are colors, the Natural Color System® is a
similar and differ primarily in scale. In proprietary system available from the
addition, the direction of the surface Scandinavian Color Institute AB,
roughness should be considered. Isotropic Stockholm, Sweden. The chromatic
surfaces are random. They are typically colors are arranged in a circle with
produced by processes such as casting or yellow, red, blue and green spaced at
any kind of abrasive blasting. Anisotropic 90 degree intervals. The opposing
surfaces have a periodic irregularity, monochromatic colors of black and
usually in one direction. They are white are represented by the central
produced by most machining operations. axis. The hue is described by its
The measured value of anisotropic resemblance to the primary colors on
surfaces will change greatly with the a scale of 0 to 100 where the sum of
direction of the measurement. the resemblances adds up to 100.
Because red opposes green and blue
Table 4 lists the surface roughness opposes yellow, only two chromatic
produced by some manufacturing colors may be used at one time. The
operations. value of the other two chromatic
colors is always zero.
Color and Gloss
TABLE 4. Surface roughness produced by some
Color manufacturing operations.2

The color of many materials is a Process _A__p_p_r_o_x_i_m__a_te__R_a_n__g_e__fr_o_m___S_u_r_f_a_c_e_
significant part of commercial value. In µm (10–6 in.)
cold rolled and surface treated steels, too,
the surface colors are an important Casting and Forging 30 to 10 (1100 to 400)
consideration in quality control.12 Sand casting 30 to 10 (1100 to 400)
Hot rolling 15 to 2.5
Colored light is reflected from a test Forging (600 to 100)
object and perceived by an instrument or Permanent mold casting 4 to 1.5 (150 to 50)
human observer. Color is a characteristic Investment casting 4 to 1.5 (150 to 50)
of light, and each hue corresponds to a Extruding 4 to 0.5 (150 to 25)
waveband of electromagnetic radiation. Cold rolling, drawing 4 to 0.5 (150 to 25)
The study of color is a branch of optics, Die casting 2 to 0.5
which is a branch of physics and is (75 to 25)
discussed in this volume’s chapter on Finishing 30 to 10
light. Flame cutting 30 to 1.5 (1100 to 400)
Sawing 15 to 1.5 (1100 to 50)
Color order systems describe color in Planing, shaping
terms of hue, value and saturation. Drilling 7.5 to 0.5 (600 to 50)
Chemical milling 7.5 to 0.5 (300 to 50
1. Hue, or chromaticity, indicates the Milling 7.5 to 0.5 (300 to 50)
value that separates the color in terms Broaching, reaming 4 to 0.8 (300 to 25)
of its primary color constituents or its Grinding 2 to 0.2 (150 to 25)
mix of primary constituents. The hue Polishing 0.5 to 0.2
describes the color’s redness, blueness Superfinishing 0.1 (75 to 5)
and so forth. (35 to 5)

2. Value describes the lightness or (4)
darkness of the color. Colors with a
large portion of white or lightness
have a high value. Dark colors have a
low value.

128 Visual Testing

2. Introduced by Albert H. Munsell These various color systems may be used
around 1910, the munsell color system to specify a desired color in a contract.
is similar in concept to the Natural
Color System but uses ten hues Color may also be measured with a
equally spaced around the outside of a colorimeter. A colorimeter is a photometer
circle. The munsell color system is a that attempts to duplicate the perception
common system used in the United of color by a human observer: three
States and Japan. Color trees are colored filters simulate the effects of a
available in vertical slices that show light source and observer. They measure
the variation of chroma and value for the tristimulus values or the color
a specified hue. Horizontal slices are coordinate value of the reflected light.
also available that show the variation
in hue and chroma from a given point Color measuring devices must be
on the neutral axis. properly calibrated and maintained
because they are affected by variations in
3 International Commission on temperature and humidity. These devices
Illumination (CIE) has promulgated a are simple and inexpensive but are limited
color system as an international to single combinations of light source and
standard.13 When measured observer variables. Many digital cameras
quantitatively, the reflected light of an integrate colorimetric functions.
object is measured in terms of its
ability to create a response in three Gloss
ideal color sensors that represent an
ideal color observer. This measurement Gloss, or reflectance, is a measure of the
may be expressed in terms of the specular or mirror reflectance of a surface.
tristimulus value of the color or the Reflectance is measured by comparing the
chromaticity coordinates. The reflected light of the test piece to the
tristimulus value is calculated by reflection produced by a perfectly
multiplying the spectral power diffusing, perfectly reflective plane surface
distribution by a reflectance factor. that is illuminated and viewed at the
The tristimulus primaries specified by same angle as the test piece. The
the International Commission on reflectance standard is usually a metal
Illumination (CIE) are imaginary plate coated with magnesium oxide with
values of light and cannot be a reflectance of about 98 percent.
represented by an actual color
stimulus. The illumination angle and the angle
of the light receiver must be equal when
4. Color collections are produced by measured from the surface normal. The
many government and private sources angle chosen for the measurement is
to demonstrate fabric colors, paint based on the relative glossiness to best
colors or ceramic colors. The color match the visual perception. Most
collection is an excellent way to measurements are made at 60 degrees
specify the exact color requirements from the surface normal. Very high gloss
for a specific product where the surfaces, such as automotive paints, are
properties of gloss and texture are measured at 20 degrees; measurements of
similar. They can be very difficult to flat finishes are made at up to 85 degrees.
correlate to other product forms or to
other color collections or color scales.

Direct Visual Testing 129

PART 5. Dimensional Measurement

Inspection methods that measure mass or and depth, and they can be either direct
length are often excluded from reading or indirect reading.
nondestructive testing because, although
mass and length are material properties, 1. Indirect reading, or transfer type,
the methods do not seek discontinuities. calipers (Fig. 13) are used to transfer
Nevertheless, the visual inspector is often the dimension of an item from the
given the task of measuring test objects item to a steel rule. For example, the
for various purposes. measurement of an outside diameter is
made by adjusting the caliper so that
1. In receiving inspection, the visual both legs lightly touch the widest
inspector may confirm that portion of the item. This distance is
components received were the then transferred to a steel rule to
components ordered. For instance, obtain the measurement. If performed
does the tubing have the needed properly, this type of measurement is
diameter? accurate to 0.04 mm (0.016 in.).

2. In assembly, the visual inspector may FIGURE 13: Indirect calipers: (a) for outside
confirm that the correct parts are measurement; (b) for inside measurement.
being used. For instance, are bolts of (a)
the correct size being installed?
(b)
3. In machining, the visual inspector
may confirm that the operation is
performed to specification. For
instance, are rivets placed far enough
from the edge of a plate?

4. In finishing, the visual inspector may
confirm that the surface is treated to
specification. For instance, has sheet
metal been burnished or coated?

5. In nondestructive testing, the visual
inspector needs to describe a visual
indication. How long is the crack?
How extensive is the blistering on the
boiler’s surface?

In many industries, the direct visual
testing of bolts requires steel rulers,
micrometers, vernier calipers, depth
micrometers, thread gages and
magnifying glasses.

Rulers and Tape Measures

Rulers and tape measures are familiar and
easy to use. In the United States, these
tools almost always show units of the
English system (inches) on one side and
of the International System (centimeters,
millimeters) on the other side.
Increasingly, the International System is
specified for goods and services.

Calipers3

Calipers are used to obtain accurate linear
measurements. Calipers come in a wide
variety of sizes and configurations for
measuring length, width, height, diameter

130 Visual Testing

2. Direct reading calipers vary. A direct To better understand the operation of
reading caliper can be simple, a rule the mechanical gage, it is useful to know
with jaws for coarse measurements; or the components of the instrument. This
it could be of the vernier, dial, or includes the frame, anvil, spindle, barrel,
electronic digital type, which are used thimble, screw, ratchet and clamp ring
for very accurate measurements (Fig. 15). Measurement with the
(Fig. 14). All types of direct reading mechanical gage occurs between the anvil
calipers consist of a fixed jaw on a and spindle. As the thimble screw is
beam along which a moveable jaw rotated counterclockwise, the spindle and
slides. The measurements are taken anvil separate and the barrel graduations
with the item between the jaws of the are revealed in succession.
instrument.
To make a measurement, rotate the
Dial and electronic calipers are simple thimble counterclockwise until the
to use and read. Electronic calipers are the spindle is far enough away from the anvil
easiest to read because the actual to allow the test object to fit between
measurement is displayed on a digital them. With the part between the anvil
readout. The dial caliper may require and spindle, slowly turn the thimble
some interpretation if the beam scale and clockwise to obtain contact between the
the rotating indicator on the dial are anvil, part and spindle. A gentle pressure
graduated in different increments. is all that is necessary to make an accurate
measurement. Pressure can be applied in
Vernier calipers are more difficult to fine increments with a ratchet. Too much
use because the scale requires care during pressure could distort the frame and
interpretation. The vernier consists of the reduce accuracy; too little pressure could
fixed main scale, etched into the beam result in improper contact with the part,
and the sliding vernier scale attached to producing an inaccurate measurement.
the moveable jaw. To take a measurement The part should be able to be rotated
with the vernier caliper, open the jaws about the spindle axis with the feeling of
larger than the maximum dimension of a slight drag. Barrel graduations are
the item to be measured and slowly close revealed by the outward travel of the
the jaws around the item until light thimble. To read the dimension, note the
contact is made. For the greatest accuracy, largest major division uncovered, the
the item must make even contact all graduation closest to the thimble and the
along the thickness of the jaw faces. thimble division aligned with the barrel
reference line.
Gages3
Weld Gages3
Some gages commonly used during visual
testing include mechanical gages, weld A common tool used in visual
fillet gages and cambridge gages. examination of weldments is the weld
fillet gage. This simple, easy-to-use device
Mechanical Gages (Micrometers) measures leg lengths and determines if
there is sufficient throat in weld fillets.
Mechanical gages perform extremely This gage is basically a comparator — the
precise measurements of linear acceptable size is etched into the gage and
dimensions. Mechanical gages are arcs are cut into the gage to allow space
available in a wide variety of for the weld bead. The gage is placed
configurations for inside and outside square against the welded components
measurements of flat, curved, threaded and the actual weld is compared to the
and cylindrical dimensions. The
mechanical gage is a caliper that operates FIGURE 15. Mechanical gage, or micrometer.
by determining how far the end of a
screw travels in one complete revolution. Clamp ring Screw Ratchet
Spindle
FIGURE 14: Direct reading caliper. Anvil

0

Barrel Thimble

Frame

Direct Visual Testing 131

standards of the gage (Fig. 16). This type attributes of each characteristic including
of gage offers a quick and precise means tolerances. Requirements are specified and
of measuring concave and convex fillet variations are assessed by using geometric
welds from 3 mm (0.13 in.) to 25 mm dimensions and tolerances, uniformly
(1.0 in.). communicated in a consensus standard.
Dimensional tolerances are related to
Weld gages (Fig. 17) designed to issues of surface roughness and waviness,
measure offset displacement can be used discussed above, and are governed by
to measure the size of fillet welds, the some of the same standards.6,7,11 Working
actual throat size of convex and concave to a consensus standard realizes the
fillet welds, reinforcement of butt welds following benefits.
and root openings of 8 mm (0.3 in.) and
3 mm (0.13 in.) FIGURE 17. Displacement weld gaging:
(a) leg length of fillet weld; (b) convexity of
Another more versatile device used for fillet weld; (c) concavity of fillet weld;
weld inspection is the welding gage, (d) bead height of butt weld.
commonly referred to as the cambridge
gage. The Welding Institute, Cambridge, (a)
United Kingdom, developed this versatile
tool — hence the name. With this device,
joint preparation angles, joint
misalignment, weld fillet size and depth
measurements can be easily obtained.
Figure 18 shows some typical applications.

This volume’s chapter on electric
power applications includes some
discussion of weld gaging.

Tolerance Standards2 (b)

The physical features of an object (form,
profile, orientation, location and size)
must be controlled. The blueprint or
drawing for the object must specify the

FIGURE 16. Fillet weld gage: (a) concave
weld; (b) convex weld.

(a)

(c)

(b) (d)

Leg length

132 Visual Testing

1. Computerized design tools, such as 50 2. The part may be allowed to have the
computer aided design (CAD) systems, 60 maximum possible tolerances while
are more feasible. The designer must MM IN ensuring interchangeability and fit to
have a complete understanding ofIN other pieces. This will enhance the
inspection principles to properly apply MM IN producibility and lower costs.
geometric tolerancing principles. For MM IN
designers to use points in space as 3. The specification of a consensus
construction points in their drawing MM tolerance standard in the working
can be a liability during procedures helps to answer concerns
manufacturing and inspection. about quality and, in the event of
material failure, about liability.
FIGURE 18. Cambridge gage: (a) zero to
60 degree angle of preparation; (b) excess 4. Controversy during manufacturing
weld metal in reinforcement; (c) pitting or and inspection is reduced when the
depth of undercut; (d) fillet weld throat size; application of the design requirements
(e) high/low misalignment. is done consistently. Part tolerances
can be established with regard to the
(a) part’s functional surfaces. This
consistency allows functional
(b) inspection gaging and manufacturing
fixturing.
(c)
Consistency is generally accomplished
(d) MM IN in relation to datums and datum surfaces
(e) that serve as orientation or zero points.
The datum plane or surface is the actual
feature of the part used to establish the
datum. A datum is a theoretically perfect
point, axis, or plane derived from actual
features. Any feature has variation.
Datums are specified on the drawing and
are controlled by establishing three
separate datums to create an X,Y,Z
coordinate system. Geometrically, three
points establish a plane, two points
establish a line and one point establishes
a point in space. This arrangement is also
called a 3,2,1 coordinate system.

Tolerances of orientation include
requirements for perpendicularity,
angularity or parallelism.

Tolerances of form include
requirements for flatness, straightness,
circularity and cylindricity. A feature has
form only in terms of its relationship to
itself. For example, the requirement for a
flat surface means that each point on the
surface must be within the specified band
in terms of every other point of that
surface. Any point on the surface is
independent of any other surface.

Profile tolerances include the
requirements for the profile of a line and
the profile of a surface.

Basic dimensions are used to describe
the theoretically exact size, profile,
orientation, or location of a feature. They
prevent ambiguity when describing the
variation allowed in a feature. Size can be
specified as a linear dimension, a
diameter, a radius, an angle or any other
quantity of size.

Direct Visual Testing 133

References

1. Bailey, W.[H.] Part 3, “Vision and 12. Aizawa, H. and K. Miyagawa.
Light.” Section 1, “Fundamentals of Section 8, “Applications of Visual and
Visual and Optical Testing.” Optical Tests in the Metals Industries”:
Nondestructive Testing Handbook, Part 2, “Visual and Optical Testing in
second edition: Vol. 8, Visual and the Steel Industry.” Nondestructive
Optical Testing. Columbus, OH: Testing Handbook, second edition:
American Society for Nondestructive Vol. 8, Visual and Optical Testing.
Testing (1993): p 9-21. Columbus, OH: American Society for
Nondestructive Testing (1993):
2. Sayler, G.[C.] ASNT Level III Study p 228-243.
Guide: Visual and Optical Testing
Method. Columbus, OH: American 13. CIE S 014-2; ISO 1164-2, CIE Standard
Society for Nondestructive Testing llluminants for Colorimetry. Wien,
(1998, revised 2006). Österreich [Vienna, Austria]:
Commission Internationale de
3. Krauss, D.G. ASNT Level II Study Guide: l’Éclairage [International Commission
Visual and Optical Testing Method. on Illumination (CIE)] (2006).
Columbus, OH: American Society for
Nondestructive Testing (1998). Bibliography

4. Chapter 4, “Basic Visual Aids and Bureau of Naval Personnel. Basic Optics
Accessories for Visual Testing”: Part 3, and Optical Instruments. New York, NY:
“Magnifiers.” Nondestructive Testing Dover (1969).
Handbook, second edition: Vol. 8,
Visual and Optical Testing. Columbus, Dunn, D. Chapter 3, “The Visual and
OH: American Society for Optical Testing Environment”: Part 2,
Nondestructive Testing (1993): “Environmental Factors.”
p 76-81. Nondestructive Testing Handbook,
second edition: Vol. 8, Visual and
5. Benford, J.R., L.E. Florry and Optical Testing. Columbus, OH:
A.A. Levin. Section 11, “Visual American Society for Nondestructive
Inspection Equipment.” Nondestructive Testing (1993): p 54-56.
Testing Handbook, first edition.
Columbus, OH: American Society for Hecht, E. Schaum’s Outline of Theory and
Nondestructive Testing (1959). Problems of Optics. New York, NY:
McGraw-Hill (2007).
6. ISO 2768, General Tolerances. Geneva,
Switzerland: International IESNA Lighting Handbook: Reference and
Organization for Standardization Application, ninth edition. New York,
(1989). NY: Illuminating Engineering Society
of North America (2000).
7. BS EN 22768-1, General Tolerances.
Brussels, Belgium: European ISO 3058, Non-Destructive Testing — Aids to
Committee for Standardization (1993). Visual Inspection — Selection of
Low-Power Magnifiers. Geneva,
8. ANSI B46.1, Surface Texture (Surface Switzerland: International
Roughness, Waviness, and Lay). New Organization for
York, NY: American National Standardization (1998).
Standards Institute (2000).
Johnson, B.K. Optics and Optical
9. ANSI Y14.5M, Dimensions and Instruments: An Introduction, third
Tolerancing. New York, NY: American edition. New York, NY: Dover (1960).
National Standards Institute (2000).
Superseded by ASME Y14.5M. Ness, S. Chapter 3, “The Visual and
Optical Testing Environment”: Part 3,
10. ISO 1302, Geometrical Product “Physiological Factors.” Nondestructive
Specifications (GPS) — Indication of Testing Handbook, second edition:
Surface Texture in Technical Product Vol. 8, Visual and Optical Testing.
Documentation. Geneva, Switzerland: Columbus, OH: American Society for
International Organization for Nondestructive Testing (1993):
Standardization (2002). p 57-62.

11. ASME Y14.5M, Dimensions and
Tolerancing. New York, NY: American
Society of Mechanical Engineers
(2009). Supersedes ANSI Y14.5M.

134 Visual Testing

6

CHAPTER

Indirect Visual Testing

Michael W. Allgaier, Mistras Group, Princeton Junction,
New Jersey
Thomas D. Britton, General Electric Sensing and
Inspection Technologies, Skaneateles, New York
(Part 3)
Trevor Liddell, General Electric Sensing and Inspection
Technologies, Lewistown, Pennsylvania (Part 3)

Portions of Parts 1 and 2 are reprinted with permission from Visual Examination Technologies (EPRI learning modules), © [ca. 1982 and] 1996,
the Electric Power Research Institute (EPRI), Charlotte, NC. ASNT has revised the text in 1993 and 2010, and deficiencies are not the
responsibility of the Electric Power Research Institute.

Portions of Part 3 are reprinted with permission from Nondestructive Testing: Remote Visual Inspection, training modules © [2005] General
Electric Corporation, Lewistown, PA. ASNT has revised the text in 2010, and deficiencies are not the responsibility of General Electric
Corporation.

PART 1. Introduction to Indirect Visual Testing

Visual tests comprise five basic elements: Effects of Test Object1,2 Test object
the inspector, the test object,
illumination, a recording method and The test surface determines the
usually an optical instrument. Each of specifications for (1) the instrument used
these elements interacts with the others during the visual test and (2) the required
and affects the test results. illumination. Objective distance, object
size, discontinuity size, reflectivity, entry
The human eye is an important port size, object depth and direction of
component for performing visual view are all critical aspects of the test
nondestructive tests. However, there are object that affect the visual test.
situations where the eye does not have 1. Objective distance (Fig. 1) is important
access to the test surface. In these cases
mechanical and optical instruments can in determining the illumination
supplement the eye in a family of source, as well as the required
techniques called indirect visual testing. objective focal distance for the
Indirect visual testing is sometimes called maximum power and magnification.
remote visual testing and is distinct from 2. Object size, combined with distance,
direct visual testing, where the inspector determines what lens angle or field of
views the test surface with the naked eye view is required to observe an entire
and simple magnifiers and gages. Direct test surface (Fig. 2).
visual testing is discussed in a separate 3. Discontinuity size determines the
chapter. magnification and resolution required
for visual testing. For example, greater
Adjustable Focus1 resolution is required to detect hairline
cracks than to detect undercut (Fig. 3).
Vision acuity affects the visual test, so it is
important for borescopes to allow the FIGURE 2. Arrows indicate portion of test
eyepiece to be focused when used, just as object falling within field of view for forward
binoculars are. Frequently, eyeglasses are viewing borescope.
inconvenient when the inspector is using
a borescope: it is difficult to place the eye Forward view borescope
at the ideal distance from the eyepiece,
and the view may be obscured by external Entry port
glare and reflections. Rubber eyeshields
on borescopes are designed to shut out
external light but are not as effective
when glasses are worn. For these reasons,
it is critical that the inspector be able to
adjust the instrument without wearing
glasses to compensate for variations in
vision acuity.

FIGURE 1. Objective distance (arrows) for FIGURE 3. Discontinuity size affects
forward and side viewing borescopes. resolution limits and magnification
requirements.
Side view borescope
Test object Discontinuity

Forward view borescope Test object
Entry port

Forward view borescope

136 Visual Testing

4. Reflectivity is another factor affecting Some of the factors affecting visual
illumination. Dark surfaces such as tests with borescopes are in conflict and
those coated with carbon deposits compromise is often needed. For example,
require higher levels of illumination a wide field of view reduces magnification
than light surfaces do (Fig. 4). but has greater depth of field (Fig. 7). A
narrow field of view produces higher
5. Entry port size determines the magnification but results in shallow depth
maximum diameter of the instrument of field. Interaction of these effects must
that can be used for the visual test be considered in determining the
(Fig. 5). optimum setup for detection and
evaluation of discontinuities in the test
6. Object depth affects focusing. If object.
portions of the object are in different
planes, then the borescope must have Parts of Indirect Visual Test
sufficient focus adjustment or depth of Instruments
field to visualize these different planes
sharply (Fig. 6). Instruments for indirect visual testing
typically have components for three
7. Direction of view determines functions.
positioning of the borescope,
especially with rigid borescopes. 1. A means of illumination is needed for
Viewing direction also affects the most applications.
required length of the borescope.
2. A viewing medium conveys or
FIGURE 4. Reflectivity helps determine levels presents the image to the inspector’s
of illumination. eye.

Dark surface 3. One or more control circuits let the
inspector control a camera’s or
Forward view borescope vehicle’s motion.

Entry port An instrument widely used for indirect
inspections is the borescope, a long
Light surface instrument that may be rigid or flexible.
The distal end has the objective lens
FIGURE 5. Entry port size (arrows) limits size directed at the test surface; the eyepiece,
of borescope. or ocular end, is close to the inspector
and may include output to a video screen
for viewing. Many borescopes also
incorporate clamps or tweezers for object
manipulation or retrieval.

Forward view borescope Test object FIGURE 7. Effects of viewing angle on other
Entry port test parameters: (a) narrow angle with high
magnification and shorter depth of field;
(b) wide angle with low magnification and
greater depth of field.

(a)

FIGURE 6. Object depth (arrows) is critical (b)
factor affecting focus.

Forward view borescope Test object

Indirect Visual Testing 137

Illumination probe may be positioned in real time with
a moving image; then a still image can be
In indirect visual tests, the inspector may recorded to document the inspection.
work in a well lighted area. The test
surface, however, is inside an object or on Telemetric Instrument. In another sort of
the far side of a barrier. Indirect visual video probe, the video signal may be
tests can be classified according to the transmitted as radio signals to a receiver.
source of light that illuminates the test Wireless transmission, however, opens up
surface. a range of possibilities. Such systems have
been used for telemetry and robotics —
1. Light may be ambient in the chamber the remote operation of vehicles such as
— if it is another room, for instance, submarines, aviation drones and outer
or inside an operating furnace. Such space probes. Radio control can permit
applications might include the nondestructive inspector to direct
hydroelectric penstock inspection, for robotic inspections from a distance of
example, but are unusual for most many kilometers.
visual inspectors.
Visual test instruments in which a
2. Light may be introduced by a lamp or camera is mounted on a robot or crawler
flash unit at the distal (objective) end are not borescopes, and many of their
of the probing instrument. A lamp can applications, such as marine exploration,3
be powered through a cable integral to are not nondestructive testing.
a borecope. This design is problematic
where there are hazardous fumes. Applications of Indirect
Techniques4
3. Light may be generated at the ocular
(eyepiece) end of the instrument and Machine Shops
conveyed to the test surface through
fiber optic cable. This means of Borescopes find applications in
illumination is integral to the design production machine shops, tool and die
of virtually all borescopes. departments and in ferrous, nonferrous
and alloy foundries. In production
4. A camera based system may operate in machine operations, borescopes of various
darkness if the camera is sensitive to sizes and angles of view are used to
infrared or ultraviolet radiation at the examine internal holes, cross bored holes,
wavelengths of interest. In condition threads, internal surface finishes and
monitoring, for example, an infrared various inaccessible areas encountered in
camera may look for hot spots in an machine and mechanical assembly
electric motor. Such applications are operations. Specific examples are visual
rare for most visual inspectors. tests of machine gun barrels, rifle bores,
cannon bores, machine equipment and
Image Transmission hydraulic cylinders.

There are various designs for indirect In tool and die shops, borescopes are
visual testing, and they can be classified used to examine internal finishes, threads,
according to the medium used to transmit shoulders, recesses, dies, jigs, fixtures,
the image from the objective lens to the fittings and the internal mating of
viewing eyepiece or display. mechanical parts. In foundries, borescopes
are widely used for internal inspections to
Rigid Borescope. The light image may be locate discontinuities, cracks, porosity and
transmitted through air inside the tube of blowholes. Borescopes are also used for
a rigid borescope. This borescope is rigid tests of many types of defense materials,
so that the instrument’s stiff body keeps including the internal surface finish of
the optical elements (lenses and possibly rocket heads, rocket head seats and guided
mirrors) aligned. missile components.

Fiber Optic Borescope. Light may be Electric Power Industry
transmitted through a fiber optic
borescope, having transparent (glass or In steam power plants, borescopes are
plastic) fibers in a cable. A jargon term for used for visual tests of boiler tubes for
such an instrument is fiber scope. The pitting, corrosion, scaling or other
cable is thin, and the cable’s flexibility lets discontinuities. Borescopes used for this
it be threaded around interior corners to type of work are usually made in 2 or 3 m
view surfaces difficult to access. At the (6 or 9 ft) sections. Each section is
objective end, the image may be captured designed so that it can be attached to the
by a camera and viewed on a video screen preceding section, providing an
such as a computer display. instrument of any required length.

Camera Borescope. Light may be sensed Other borescopes are used to examine
by a small camera, converted to an turbine blades, generators, motors,
electronic signal and sent through a cable
to a video receiver. A single system called
a video probe can offer a choice of still
photography, motion video or both. The

138 Visual Testing

pumps, condensers, control panels and blowholes in castings and forgings.
other electrical or mechanical Machined components such as cross
components without dismantling. In bored holes can be examined for internal
nuclear plants, borescopes offer the discontinuities. Borescopes are used to
advantage that the inspector can be in a inspect cylinders for internal surface
low radiation field while the distal, or finish after honing. Tapped holes,
sensor, end is in a high radiation field. shoulders or recesses also can be observed.
Inaccessible areas of hydraulic systems,
Photography of the interiors of large small pumps, motors and mechanical or
power plant furnaces during operation has electrical assemblies can be visually tested
been done since the 1940s using a without dismantling the engine.
periscope and camera. The periscope
extends through the furnace wall and Aviation Industry
relays the optical image to the camera. A
water cooled jacket protects the optical The use of borescopes for tests of airplane
system and the camera from the furnace’s engines and other components without
high temperatures. With this equipment, disassembly has resulted in substantial
still and motion picture studies have been savings in costs and time. A borescope of
made of the movement of the fuel bed 11 mm (0.44 in.) diameter by 380 mm
and the action of the powdered fuel (15 in.) working length can be used by
burner in furnaces operating at full load. maintenance and service departments for
visual testing of engines through spark
Petroleum and Chemical Industry plug openings, without dismantling the
engines. An excellent view of the cylinder
Borescopes are used for visual tests of high wall, piston head, valves and valve seats is
pressure catalytic cracking units, possible and several hundred hours of
distillation equipment, fractionation labor are saved for each engine test. Spare
units, hydrogenation equipment, pressure engines in storage can also be inspected
vessels, retorts, pumps and similar process for corrosion of cylinder wall surfaces.
equipment. Use of the borescope in the
examination of such structures is doubly Aircraft propeller blades are visually
significant. Not only does it allow the tested during manufacture. The entire
examination of inaccessible areas without welded seam of a blade can be inspected
the lost time and expense incurred in internally for cracks and other
dismantling, it avoids breakdown and the discontinuities. Propeller hubs, reverse
ensuing costly repair. pitch gearing mechanisms, hydraulic
cylinders, landing gear mechanisms and
Visual tests of high pressure distillation electrical components also can be
units are used to determine the internal inspected with borescopes. Aircraft wing
condition of tubes or headers. spars and struts are inspected for evidence
Evaporation tubes, fractionation units, of fatigue cracks and rivets and wing
reaction chambers, cylinders, retorts, sections can be tested visually for
furnaces, combustion chambers, heat corrosion. Borescopes used for tests of
exchangers, pressure vessels and many internal wing tank surfaces and wing
other types of chemical process corrugations subject to corrosion have
equipment are inspected with borescopes saved airlines expense by reducing the
or extension borescopes time aircraft are out of service.

Tank cars are inspected for internal Infrastructure
rust, corrosion and the condition of outlet
valves. Cylinders and drums can be Indirect visual testing is suited for the
examined for internal conditions such as interrogation of channels, cavities,
corrosion, rust or other discontinuities. pockets, crevices and interstitial areas in
all sorts of civil engineering structures. In
Automotive Industry interior areas, it can look for dampness
and signs of corrosion, grout in masonry
Borescopes are widely used in the and places where brackets and braces have
manufacturing and maintenance divisions become detached from structures they are
of the automotive industry. Engine intended to support. Indirect techniques
cylinders can be examined through spark can explore air ducts, drains, cisterns,
plug holes without removing the cylinder water lines, sewers, tunnels, crawl spaces
head. The cylinder wall, valves and piston and wall interiors.
head can be visually tested for excess
wear, carbon deposits and surface Fiber optic borescopes have been used
discontinuities. Crankcases and in the conservation of public statues
crankshafts are examined through wall (Fig. 8)5,6 and to look inside a crypt
plug openings without removing the (Fig. 9).7
crankcase. Transmissions and differentials
are similarly inspected.

Borescopes are also useful for locating
discontinuities such as cracks or

Indirect Visual Testing 139

FIGURE 8. Indirect visual testing of statues: (a) corrosion FIGURE 9. Indirect visual testing of crypt: (a) introduction of
detection5; (b) checking for separation of weld joints in borescope from above; (b) view in borescope eyepiece.7
Statue of Liberty.6
(a) Fiber optic borescope
(a)
Hidden Stone floor
entrance Light source Drilled hole of church

(b)
(b)

140 Visual Testing

PART 2. Borescopy4,8

Rigid Borescopes Focusing of Rigid Borescope

The rigid borescope (Fig. 10) was invented The focus control in a rigid borescope
to inspect the bores of rifles and cannons. greatly expands the depth of field over
It was a thin telescope with a small lamp nonfocusing or fixed focus designs. At the
at the objective end for illumination. same time, focusing can help compensate
Most rigid borescopes now use a fiber for wide variations in eyesight among
optic light guide for illumination. inspectors.

The image is brought to the eyepiece Figures 11 and 12 emphasize the
by an optical train consisting of an importance of focus adjustment for
objective lens, sometimes a prism, relay expanding the depth of field. Figure 11
lenses and an eyepiece lens. The image is was taken at a variety of distances with
not a real image but an aerial image: it is fixed focus. Figure 12 was taken at the
formed in the air between the lenses. This same distances as in Fig. 11 but with a
means that it is possible both to provide variable focus, producing sharper images.
diopter correction for the observer and to
control the objective focus with a single Need for Specifications
adjustment to the focusing ring at the
eyepiece. Because rigid borescopes lack flexibility
and the ability to scan areas,
FIGURE 10. Lens system in representative rigid borescope. specifications regarding length, direction
of view and field of view become more
critical for achieving a valid visual test.
For example, the direction of view should
always be specified in degrees rather than
in letters or words such as north, up,
forward or left. Tolerances should also be
specified.

FIGURE 11. Borescope images with fixed FIGURE 12. Borescope images with variable
focus (compare Fig. 12): (a) at 75 mm focus (compare Fig. 11): (a) 75 mm (3 in.);
(3 in.); (b) at 200 mm (8 in.); (c) at (b) 200 mm (8 in.); (c) 300 mm (12 in.).
300 mm (12 in.).
(a)
(a)

(b) (b)

(c) (c)

Indirect Visual Testing 141

Some borescope manufacturers have Miniature Borescope
considered the eyepiece to be zero degrees
and therefore a direct view rigid borescope One variation of the rigid borescope is
is 180 degrees. Other manufacturers start called the miniature borescope. In this
with the borescope tip as 0 degrees and design, the relay lens train is replaced
then count back toward the eyepiece, with a single, solid fiber. The fiber diffuses
making a direct view 0 degrees (Fig. 13). ions in a parabola from the center to the
periphery of the housing, giving a graded
Setup of Rigid Borescope index of refraction. Light passes through
the fiber and forms an image at specific
To find the direction and field of view intervals. The aperture is so small that the
during visual testing with a rigid lens has an infinite depth of field (like a
borescope, place a protractor scale on a pinhole camera) and no focusing
board or worktable. Position the mechanism is needed.
borescope parallel to the zero line, with
the lens directly over the center mark on Accessories
the protractor. Remember that the optical
center of a borescope is usually 25 to Many accessories are available for rigid
50 mm (1 to 2 in.) behind the lens borescopes. Still and video cameras can be
window. added to provide a permanent record of a
visual test. Closed circuit television
By sighting through the borescope, displays are common as well. Also
stick pins into the board at the edge of available are attachments at the eyepiece
the protractor to mark the center and permitting dual viewing or right angle
both the left and right edges of the view viewing for increased accessibility.
field. This simple procedure gives both the
direction of view and the field of view Fiber Optic Borescopes
(Figs. 14 and 15).
The industrial fiber optic borescope is a
FIGURE 13. Borescope direction of view: flexible, layered sheath protecting two
(a) forward; (b) side; (c) forward oblique; fiber optic bundles, each comprising
(d) retrospective. thousands of glass fibers. One bundle
serves as the image guide, and the other
(a) bundle illuminates the test object.

View Light travels in straight lines but
optical glass fibers bend light by internal
(b) View reflection to carry light around corners
(Fig. 16). Such fibers are 9 to 30 µm
Light (0.0004 to 0.0013 in.) in diameter,
roughly one tenth of the thickness of a
human hair.

A single fiber transmits very little light,
but thousands of fibers may be bundled
for transmission of light and images. To

(c) View FIGURE 14. Field of view for rigid borescope.

Light 60
60 70 80 90 80 70 60
(d) View 50 50
40 40
Light 30 30
20 20
10 10

0

142 Visual Testing

FIGURE 15. Distance to test surface versus field of view.

Standard fields of view (degrees)

80 70 60 5550 45 40 30 20 20 30 40 455055 60 70 80

Distance to test surface, m (ft)

225 200 175 150 125 100 75 50 25 50 mm (2 in.)
(9) (8) (7) (6) (5) (4) (3) (2) (1)
25 50 75 100 125 150 175 200 225
(1) (2) (3) (4) (5) (6) (7) (8) (9)

Field of view width, mm (in.)

FIGURE 16. Internal reflection of light in FIGURE 17. Light paths in fiber bundles:
optic fiber can be used to move light path (a) uncoated fibers allow light to travel
in curve. laterally through bundle and (b) coated
fibers restrict light’s path to original fiber.

(a)

Cladding

prevent the light from diffusing, each (b) Cladding Core
fiber consists of a central core of high
quality optical glass coated with a thin Fiber Image Guides
layer of another glass with a different
refractive index (Fig. 17). This cladding The fiber bundle used as an image guide
acts as a mirror — all light entering the (Fig. 18) carries the image formed by the
end of the fiber is reflected internally as it objective lens at the distal end, or tip, of
travels (Fig. 16) and cannot escape by the borescope back to the ocular end, pr
passing through the sides to an adjacent eyepiece. The image guide must be a
fiber in the bundle. coherent bundle: the individual fibers
must be precisely aligned so that they are
Although the light is effectively in identical relative positions at their
trapped within each fiber, not all of it terminations.
emerges from the opposite end. Some of
the light is absorbed by the fiber itself, Image guide fibers range from 9 to
and the amount of absorption depends on 17 µm (0.0004 to 0.0007 in.) in diameter.
the length of the fiber and its optical Their size affects resolution, although the
quality. For example, plastic fiber can preciseness of alignment is far more
transmit light and is less expensive to important.
produce than optical glass, but plastic is
less efficient in its transmission and is
unsuitable for fiber optic borescopes.

Indirect Visual Testing 143