FILM RADIOGRAPHY I 1 33
laboratories as well as radiographic departments within amount of radiation reaching the specimen, the proportion
manufacturing plants. The radiographic inspection per- of this radiation that passes through the specimen; and the
formed by industry is frequently monitored for quality by its intensifying action of screens, if used.
customers - other manufacturers or governmental agen-
cies - who use specifications or codes agreed to by contract Emission from X-Ray Source
and provided by several technical societies or other regula-
tory groups. The total amount of radiation emitted by an X-ray tube
depends on tube current (milliamperage), kilovoltage, and
To meet the growing and changing demands of industry, the time the tube is energized.
research and development in the field of radiography are
continually producing new sources of radiation such as When the other operating conditions are held constant, a
neutron generators and radioactive isotopes; lighter, more change in milliamperage causes a change in the intensity
powerful, more portable X-ray equipment as well as multi- (quantity of radiation leaving the X-ray generator per unit
megavolt X-ray machines designed to produce highly pene- time) of the radiation emitted, the intensity being approxi-
trating radiation; new and improved X-ray films and mately proportional to the milliamperage. The high voltage
automatic film processors; and improved or specialized transformer saturation and voltage waveform can change
radiographic techniques. These factors, plus the activities with tube current but a compensation factor is usually
of many dedicated people, broadly expand radiography's applied to minimize the effects of these changes. In normal
usefulness to industry. industrial radiographic practice, the variation from exact
proportionality is not serious and may usually be ignored.
Factors Governing Exposure
Figure 2 shows spectral emission curves for an X-ray
Generally speaking, the density of any radiographic tube operated at two different currents, the higher being
image depends on the amount of radiation absorbed by the twice the milliamperage of the lower. Therefore, each wave
sensitive emulsion of the film. This amount of radiation in length is twice as intense in one beam as in the other. Note
tum depends on several factors: the total amount of radia- that no wavelengths present in one beam are absent in the
tion emitted by the X-ray tube or gamma ray source; the other. Hence, there is no change in X-ray quality or pene-
trating power.
FIGURE2. Curvesillustratingeffect of change
in milliamperageon intensityof X-ray beam As would be expected, the total amount of radiation
emitted by an X-ray tube operating at a certain kilovoltage
HIGH and milliamperage is directly proportional to the time the
MILLIAMPERAGE tube is energized.
~ Because the X-ray output is directly proportional to both
zvi milliamperage and time, it is directly proportional to their
product. (This product is often referred to as the exposure.)
UJ Algebraically, this may be stated E = MT, where E is the
exposure, M the tube current and T the exposure time. The
~1-- amount of radiation will remain constant if the exposure
remains constant, no matter how the individual factors of
LOW tube current and exposure time are varied. This permits
MILLIAMPERAGE specifying X-ray exposures in terms of milliampere-minutes
or milliampere-seconds, without stating the specific individ-
WAVELENGTH ual values of tube current and time.
The kilovoltage applied to the X-ray tube affects not only
the quality but also the intensity of the beam. As the kilo-
voltage is raised, X-rays of shorter wavelength, and hence of
more penetrating power, are produced. Shown in Fig. 3 are
spectral emission curves for an X-ray tube operated at two
different kilovoltages but at the same milliamperage. Note
that, in the higher kilovoltage beam, some shorter wave-
lengths are absent from the lower kilovoltage beam. Fur-
ther, all wavelengths present in the lower kilovoltage beam
are present in the more penetrating beam and in greater
amount. Thus, raising the kilovoltage increases both the
penetration and the intensity of the radiation emitted from
the tube.
1 34 I NONDESTRUCTIVETESTING OVERVIEW
Emission from Gamma Ray Source Geometric Principles
The total amount of radiation emitted from a gamma ray
Because X-rays and gamma rays obey the common laws
source during a radiographic exposure depends on the activ- of light, their shadow formation may be simply explained i~1
ity of the source (in becquerels or curies) and the time of terms of light. It should be borne in mind that the analogy IS
exposure. For a particular radioactive is~tope, the inte~s~ty not perfect because all objects are, to a greater or lesser
of the radiation is approximately proportional to the activity degree, transparent to X-rays and gamma r~ys and becau~e
(in becquerels or curies) of the source. If it were not for scattering presents greater problems in radiography than m
absorption of gamma rays within the radioactive mater!al optics. However, the same geometri~ laws ~f s~iadowforma-
itself, this proportionality would be exact. In normal radio- tion hold for both light and penetrating radiation.
graphic practice, the range of source size~ u~ed in a particu-
lar location is small enough so that vanations from exact Suppose that, as in Fig. 4, there is hght from ~ point ~
proportionality are not serious and may usually be ignored. falling on a white card C and that an opaque object O is
Thus, the gamma ray output is directly proportional to interposed between the light source and the card. A shadow
both activity of the source and time and hence is directly of the object will be formed' on the surface of the card.
proportional to their product. Analogously to the X-ray
exposure, the gamma ray exposure E may be sta~edE = MT, This shadow cast by the object will naturally show some
where M is the source activity in curies and T IS the expo- enlargement because the object is not in contact with the
sure time; the amount of gamma radiation remains constant card; the degree of enlargement will vary according to the
so long as the product of source activity and time remai~s relative distances of the object from the card and from the
constant. This permits specifying gamma ray exposures in light source. The law governing the size of the shadow may
becquerel-hours or curie-hours without stating specific val- be stated: the diameter of the object is to the diameter of the
ues for source activity or time. shadow as the distance of the light from the object is to the ,
distance of the light from the card.
Because gamma ray energy is fixed by the nature of the
particular radioactive isotope, there is no v~able to ~orre- Mathematically, the degree of enlargement may be cal-
spond to the kilovoltage factor encountered in X-radiogra- culated by use of the following equ_ations:
phy. The only way to change penetra~ng power wh~n using
gamma rays is to change the source, i.e., cobalt-60 in place (Eq. l)
of iridium-192.
which may also be expressed as
FIGURE 3. Curves illustraitng effect of change
in kilovoltage on composition and intensityof s.(~Js = (Eq. 2)
X-ray beam O
I Di
WAVELENGTHS INCREASED
IN INTENSITY SY Where:
INCREASING KILOVOLTAGE
S0 size of object;
~ S1 size of shadow (or radiographic image);
zVl D0 distance from radiation source to object; and
~·fL-U D1 distance from radiation source to recording surface
WAVELENGTH (or radiographic film).
The degree of sharpness of any shadow depends on _the
size of the light source and on the position of the object
between the light and the card - whether nearer to or far-
ther from one or the other. When the source of light is not a
point but a small area, the shadows cast are not perfe.ctly
sharp (Fig. 4b-d) because each point in the source of light
casts its own shadow of the object and each of these over-
lapping shadows is slightly displaced from the others, pro-
ducing an ill defined image.
The farm of the shadow may also differ according to the
angle that the object makes with the incident light rays.
FILM RADIOGRAPHY I 135
Deviations from the true shape of the object as exhibited in Radiographic Shadows
its shadow image are referred to as distortion.
The basic principles of shadow formation must be given
Figure 4a to 4f shows the effect of changing the size of primary consideration to ensure satisfactory sharpness and
the source and of changing the relative positions of source, freedom from distortion in the radiographic image. A certain
object and card. From an examination of these drawings, it degree of distortion will exist in every radiograph because
will be seen that the following conditions must be fulfilled some parts will always be farther from the film than others,
to produce the sharpest, truest shadow of the object. the greatest magnification being evident in images of those
parts at the greatest distance from the recording surface.
1. The source of light should be small, that is, as nearly
a point as can be obtained (compare Fig. 4a and 4c). Note, also, that there is no distortion of shape in Fig. 4e
- a circular object having been rendered as a circular
2. The source of light should be as far from the object shadow. However, under circumstances similar to those
as practical (compare Fig. 4b and 4c). shown in Fig. 4e, it is possible that spatial relations can be
distorted. In Fig. 5 the two circular objects can be rendered
3. The recording surface should be as close to the either as two circles (Fig. Sa) or as a figure-eight shaped
object as possible (compare Fig. 4b and 4d). shadow (Fig. Sb). It should be observed that both lobes of
the fi.gure eight have circular outlines.
4. The light rays should be directed perpendicularly to
the recording surface (see Fig. 4a and 4e). Distortion cannot be eliminated entirely but, with an
appropriate source-to-film distance, can be lessened to a
5. The plane of the object and the plane of the
recording surface should be parallel ( compare
Fig. 4a and 4f).
FIGURE 4. Geometric principles of shadow formation: (a) planes of object and film perpendicular to
X-ray direction from point source; f b) perpendicular, near non point source; f c) perpendicular, distant
nonpoint source; {d) perpendicular, midrange nonpoint source; (e) oblique, parallel object and film
planes, point source; (f) oblique, object and film planes not parallel, point source.
faJ fbJ -(cJ
~A
IAI L \ II /I
II I I /I
II \fl I \/ I ____.\
II \\ IVI
II \\ I'{ I I II I
I\ I I\ I I II I
Ii II \I lI II
II II II
I\ o~ II II
of~\ II I I
If \\ II I\
I\ II 11
Ici /1~... ·\·, o,~1
c ~_/ _/ .___,)\ /J ,1
c ~/-·'.__'.' _\'_·1
fdJ (eJ \II\I\\ ffJ L A
1i\n/ I, I \\ III\
\ II
II
I/\ I I\ '\
'1I I\ I I\
o \I; ,1,1 \\ \
J I\
ti \\ \ I\
1\ I\ I\
o~I \ \
\ II \ .>I \
/~.\ I \ I\
\ o~\
c~
136 I NONDESTRUCTIVETESTING OVERVIEW
point where it will not be objectionable in the radiographic 2. The distance between the source and the material
image. examined should always be as great as practical.
Comparatively long source-to-film distances should
Application to Radiography be used in the radiography of thick materials to
minimize the fact that structures farthest from the
The application of the geometric principles of shadow film are less sharply recorded than those nearer to it.
formation to radiography leads to five general rules. At long distances, radiographic definition is
Although these rules are stated in terms of radiography with improved and the image is more nearly the actual
X-rays, they also apply to gamma radiography. size of the object. Figure 6 shows the effects of
source-to-film distance on image quality. As the
1. The focal spot should be as small as other source-to-film distance is decreased from 1, 730 mm
considerations will allow, for there is a definite (68 in.) to 305 mm (12 in.) the image becomes more
relation between size of the focal spot of the X-ray distorted until at 305 mm (12 in.) it is no longer a
tube and definition in the radiograph. A large focus true representation of the casting. This is particularly
tube, although capable of withstanding large loads, evident at the edges of the casting where the
does not permit the delineation of as much detail as a distortion is greatest.
small focus tube. Long source-to-film distances will
aid in showing detail when a large focus tube is 3. The film should be as close as possible to the object
employed but it is advantageous to use the smallest being radiographed. In practice, the film (in its
focal spot permissible for the exposures required.
cassette or expos,ure holder) is placed in contact ~th
FIGURE 5. Dependingon directionof radiation, the object. In Fig. 6b, the effects of object-to-film
two circular objects can be rendered: fa) as two distance are evident. As the object-to-film distance is .
separate circles; (bJ as two overlapping circles increased from zero to 102 mm (4 in-), the image. '
becomes larger and the definition begins to degrade.
(aJ 1, Again, this is especially evident at chamber edges that
/11\ are no longer sharp.
II ,!\\ \ \ 4. The central ray should be as nearly perpendicular to
the film as possible to preserve spatial relations.
5. As far as the shape of the specimen will allow the
plane of maximum interest should be parallel to the
plane of the film.
I II \
I I\ \
I 11 \ Calculation of Geometric Unsharpness
I I11\ \\
I The width of the fuzzy boundary of the shadows in
I II \ Fig. 4c and 4d is known as the geometric unsharpness U{!,.
Because the geometric unsharpness can strongly affect the
o, ;~\ \ appearance of the radiographic image, it is frequently nec-
essary to determine its magnitude. From the laws of similar
1--1·\I1I~t.-i. \ 02 triangles (see Fig. 7), it can be shown that:
I
(bJ Ug d
F Do
(Eq. 3)
or Ug F !!:_ (Eq. 4)
Where:
Do
U, = geometric unsharpness,
F = size of radiation source;
D,, = source-to-object distance; and
d = is the object-to-film distance.
FILM RADIOGRAPHY I 1_,37
Because the maximum unsharpness involved in any usually stated in millimeters and Ug will also be in millime-
radiographic procedure is usually the significant quantity, ters. If the source size is stated in inches, Ug will be inches.
the object-to-film distanced is usually taken as the distance
from the source side of the specimen to the film. For rapid reference, graphs of the type shown in Fig. 8 can
be prepared with these equations. The graphs relate source-
D0 and d must be measured in the same units - say, mil- to-film distance, object-to-film distance and geometric
limeters or inches. So long as D0 and dare in the same units, unsharpness. Note that the lines of Fig. 8 are all straight.
Eq. 3 or 4 will always give the geometric unsharpness Ug in Therefore, for each source-to-object distance, it is only nec-
whatever units were used to measure the dimensions of the
source. Projected sizes of the focal spots of X-ray tubes are essary to\ calculate the value of U, for a single specimen
thickness and then draw a straight line through the point so
FIGURE 6. E.ffects on image quality when geometric exposure factorsare changed:(aJ 1.75 m
(68 in.] source-to-film distance, 0 object-to-filmdistance; fb) 1.5 mm (0.06 in.] focal spot,
O object-to-flim distance; [c] 100 mm {4 in.) object-to-film distance; (d) perpendicularfilm-to-source
angJe and 45 degree object-to-flim angle
(a) fc)
fbJ fdJ
138 I NONDESTRUCTIVETESTING OVERVIEW
FIGURE 7. Geometric constructionfor determined and the origin. It should be emphasized, how-
determininggeometricunsharpness U9 where ever, that a separate graph of the type shown in Fig. 8 must
source is smaller than object be prepared for each size of source.
Geometric Enlargement
F I
Inmost radiography, it is desirable to have the specimen
SOURCE IV\.----------------- and the film as close together as possible to minimize geo-
metric unsharpness. An exception to this rule occurs when
I I / I x\ \\ \ the source of radiation is extremely minute, that is, a small
fraction of a millimeter, as in a betatron. In such a case, the
I \ film may be placed at a distance from the specimen rather
than in contact with it (see Fig. 9). Such an arrangement
// \\ Do results in an enlarged radiograph without introducing objec-
tionable geometric unsharpness. Enlargemeiits of as much
/;II I I\ \\~ as three diameters by this technique are useful in the detec-
\\;----T tion of structures otherwise invisible radiographically.
OBJECT/
FIGURE 9. With ve~y small focal spot, enlarged
// \\ d image can be o.btained:degree of enlargement
depends uponratio of source-to-filmand
FILM PLANE I I \\ I source-to-specimendistances
t ug FILM AND
._ .... lliiiialllllliiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiriiiCiiiAiiSiiSiiEiiIiiTE
FIGURE 8. Graph relating geometric
unsharpness U9 to specimen thickness and
source-to-object distance, for 5 mm (0.2 in.J
source size
l.O
0.9
0.8 I~
lti
Vl 0.7
Vl 06 a!Q
0.5
zI.JJ 0.4 &
0.3
:E~~Q.. @
2
tu-u~zV1 QQJI
ii,
E::::>
o2 E ~
~
0 0
oI.JJ VJ'
0.2
0.1
25 50 75 JOO 125 150
(I) (2) (3) (4) (5) (6)
SPECIMEN THICKNESS
millimeters
(inches)
FILM RADIOGRAPHY I 1 39
Inverse Square Law directly but must be obtained from the exposure chart of
the equipment or the operator's log book.
When the X-ray tube output is held constant or when a Milliamperage-Distance Relation
particular radioactive source is used, the radiation intensity
reaching the specimen is governed by the distance between The milliamperage employed in any exposure technique
the tube (or source) and the specimen, varying inversely with should conform with the manufacturer's rating of the X-ray
the square of this distance. The explanation that follows is in tube. In most laboratories, however, a constant value of mil-
terms of X-rays and light but applies to gamma rays as well. liamperage is usually adopted for convenience.
Because X-rays conform to the laws of light they diverge Rule: the milliamperage (M) required for a given expo
when they are emitted from the anode and cover an increas- sure is directly proportional to the square of the focusto
ingly larger area with lessened intensity as they travel from film distance (D). The equation is expressed as follows:
their source. This principle is illustrated in Fig. 10. In this
example, it is assumed that the intensity of the X-rays emit- (Eq. 6)
ted at the anode A remains constant and that the X-rays or
passing through the aperture B cover an area of 25.8 cm2
(4 in.2) on reaching the recording surface C1, which is 30.5 For example, suppose that with a given exposure time
cm (12 in.) from the anode (distance D). and kilovoltage, a properly exposed radiograph is obtained
When the recording surface is moved 305 mm ( 12 in.) FIGURE 10. Schematic diagramillustrating
farther from the anode, to C2, so that the distance (2D) from inverse square law
the anode is 610 mm (24 in.) or twice its earlier value, the
X-rays will cover 103.4 cm2 (16 in.2) - an area four times as A~._ _ _,
great as that at C1. It follows, therefore, that the radiation per
square centimeter on the surface at C2 is only one-quarter of 1
that at the level C1. The exposure that would be adequate at
C1 must be increased four times in order to produce at C2 a lD
radiograph of equal density. In practice, this can be done by
increasing the time or by increasing the milliamperage. 20
The inverse square law can be expressed algebraically as
follows:
D2 (Eq. 5)
D212
where I 1 and 12 are the intensities at the distances D1 and
D2, respectively.
Relations of Milliamperage(Source
Strength), Distance and Time
With a given kilovoltage of X-radiation or with the
gamma radiation from a. particular isotope, the three factors
governing the exposm;ti' are the milliamperage (for X-rays)
or source strength (for gamma rays), time, and source-to-
film distance. The numerical relations among these three
quantities are demonstrated below, using X-rays as an exam-
ple. The same relations apply for gamma rays, provided the
number of becquerels (curies) in the source is substituted
wherever milliamperage appears in an equation.
The. necessary calculations for any changes in focus-to-
film distance D, milliamperage M or time Tare matters of
simple arithmetic and are illustrated in the following exam-
ple. As noted earlier, kilovoltagechanges cannot be calculated
140 I NONDESTRUCTIVETESTING OVERVIEW
with 5 mA (M1) at a distance of D1 of 305 mm (12 in.) and milliamperage and time is constant for the same radio-
that it is desired to increase the sharpness of detail in the graphic effect. Thus:
image by increasing the focus-to-film distance D2 to
610 mm (24 in.). The correct milliamperage M2 to obtain (Eq. 10)
the desired radiographic density at the increased distance
D2 may be computed from the proportion: C (a constant)
5:lv12 = 502 :1002 (Eq. 7) This is commonly referred to as the reciprocity law. (Impor-
tant exceptions are discussed below.)
or
Tabular Solution of Milliamperage-Time and
502 Distance Problems
1002
Problems of the types discussed above may · also be
5 x 1002 solved by the use of a table similar to Table 1. The factor
~ between the new and the old exposure time, mtlliamperage,
or milliampere-minute (mA·min) value appears in the box at
When very low kilovoltages, say 20 kV or less, are used, the intersection of the column for the new source-to-film
the X-ray intensity decreases with distance more rapidly than distance and the row for the old source-to-film distance.
calculations based on inverse square law would indicate
because of absorption of the X-rays by air. Most industrial Suppose, for example, a properly exposed ratliograph is
radiography, however, is done with radiation so penetrating produced with an exposure of 20 mA-min with a source-to-
that air absorption need not be considered. These comments film distance of 762 mm (30 in.). The goal is to increase the
also apply to the time-distance relations discussed below. source-to-film distance to 1.14 m (45 in.) in order. 'to
decrease the geometric unsharpness in the radiograph, The
Time-Distance Relation factor appearing in the box at theintersection of the column
for 1.14 m (45 in.) (new source-to-film distance) and the
Rule: The exposure time T required fora given exposure is row for 0.76 m (30 in.) (old source-to-film distance) is 2.3.
directly proportional to the square of the focustofilm dis Therefore, the old milliampere-minute value (20) should be
tance D: multiplied by 2.3 to give the new value: 46 m.A-min.
T1 :T2 D; :D~ (Eq. 8) Note that some approximation is involved in the use of
such a table because the values in the boxes are rounded off
or to two significant digits. However, the errors involved are
always less than 5 percent and, in general, are insignificant
i: _021_ in actual practice. Also, a table of this type cannot include all
source-to-film distances. However, in any one radiographic
Tz D22 department, only a few source-to-film distances are used in
the great bulk of the work, and a table of reasonable size can
Milliamperage-Time Relation be made using only these few distances.
Rule: The milliamperage M required for a given expo The Reciprocity Law
sure is inversely proportional to the time T:
In the preceding text, it has been assumed that exact
lv11 ; lv12 r, :T1 (Eq. 9) compensation for a decrease in the time of exposure can be
made by increasing the milliamperage according to the rela-
or
tion M1T1 = M2T2. This may be written MT= Candis an
lv11 IL
example of a general photochemical law: the same effect is
is, T1 produced for IT= constant, where I is intensity of the radi-
ation and T is the time of exposure. This is called the
Another way of expressing this is to say that for a given reciprocity law and is true for direct X-ray and lead screen
set of conditions (voltage, distance etc.), the product of exposures. For exposure to light, it is not quite accurate and,
FILM RADIOGRAPHY I 141
TABLE 1. Milliamperage-time and distance relationships
New Distance
(meters or inches)
Old Distance 25 30 35 40 45 50 55 60 65 70 75 80
(meters or inches)
1.s1.0 5.6 6.8 7.8 9.0 10.0
25 1.4 2.0 2.6 3.2 4.0
30
35 0.70 1.0 1.4 1.8 2.3 2.8 3.4 4.0 4.8 5.4 6.3 7. l
40
45 0.51 0.74 1.0 1.3 1.6 2.0 2.5 3.0 3.4 4.0 4.6 5.2
50
55 0.39 0.56 0.77 1.0 1.3 l .6 1.9 2.2 2.6 3.1 3.5 4.0
60
65 0.31 0.45 0.60 0.79 i.O 1.2 1.5 1.8 2.1 2.4 2.8 3.2
70
75 0.25 0.36 0.49 0.64 0.81 1.0. 1.2 l.4 1. 7 2.0 2.2 2.6
80
0.21 0.30 0.40 0.53 0.67 0.83 1.0 1.2 1.4 1.6 1.9 2. l
0.17 0.25 0.34 0.44 0.56 0.69 0.84 1.0 1.2 1.4 J .6 1.8
0.15 0.21 0.29 0.38 0.48 0.59 0.72 0.85 l.O 1.2 1.3 1.5
0.13 0.18 0.25 0.33 0.41 0.51 0.62 0.74 0.86 1.0 I.I l.3
0. JI 0.16 0.22 0.28 0.36 0.45 0.54 0.64 0.75 0.87 l.O 1. l
0. JO 0. 14 0. 19 0.25 0.32 0.39 0.47 0.56 0.66 0.77 0.88 1.0
because some radiographic exposures are made with the and gamma ray (Eq. 12)
light from fluorescent intensifying screens, the law cannot exposure factor
be strictly applied. curies x time
distance"
Formally defined, the Bunsen-Roscoe reciprocity law
states that the result of a photochemical reaction is depen- Radiographic techniques are sometimes given in terms
dent only on the product of radiation intensity (I) and the ofkilovoltage and exposure factor, or radioactive isotope and
duration of the exposure ( T) and is independent of absolute exposure factor. In such a case, it is necessary merely to
multiply the exposure factor by the square of the distance in
values of either quantity. order to find, for example, the milliampere-minutes or the
Errors that result from assuming the validity of the curie hours required.
reciprocity law are usually so small that they are not notice- Determinationof Exposure Factors
able in examples of the types given here. Departures may be
apparent, however, if the intensity is changed by a factor of 4 X-Rays
or more. Because intensity may be changed by changing the
source-to-film distance, failure of the reciprocity law may The focus-to-film distance is easy to establish by actual
appear to be a violation of the inverse square law. Applica- measurement, the milliamperage can conveniently be deter-
tions of the reciprocity law over a wide intensity range some- mined by the milliameter supplied with the X-ray machine
times arise and the relation between results and calculations and the exposure time can be accurately controlled by a good
may be misleading unless the possibility of reciprocity law time switch. The tube voltage, however, is difficult and
failure is kept in mind. Failure of the reciprocity law means inconvenient to measure accurately. Furthermore, designs of
that the efficiency of a light sensitive emulsion (in using the individual machines differ widely and may give X-ray outputs
light energy) depends on tl)e light intensity. of a different quality and intensity even when operated at the
nominal values of peak kilovoltage and milliamperage.
.§
Consequently, although specified exposure techniques
Exposure Factor can be duplicated satisfactorily in the factors of focus-to-
film distance, milliamperage and exposure time, one appa-
The exposure factor is a quantity that combines mil- ratus may differ materially from another in the kilovoltage
liamperage (X-rays) or source strength (gamma rays), time setting necessary to produce the same radiographic density.
and distance. Numerically the exposure factor equals
milliamperes x time X-ray
distance" exposure factor (Eq. ll)
142 I NONDESTRUCTIVETESTING OVERVIEW
Because of this, the kilovoltage setting for a given technique Contrast
should be determined by trial on each X-ray generator. In
the preliminary tests, published exposure charts may be fol- In a radiograph, the various intensities transmitted by
lowed as an approximate guide. It is customary for equip- the specimen are rendered as different densities in the
ment manufacturers to calibrate X-ray machines at the image. The density differences from one area to another
factory and to furnish suitable exposure charts. For the constitute radiographic contrast. Any shadow or detail
unusual problems that arise, it is desirable to record in a log- within the image is visible by reason of the contrast between
book all the data on exposure and techniques. In this way, it and its background of surrounding structures. Within
operators will soon build up a source of information that will appropriate limits, the greater the contrast or density differ-
make them more competent to deal with difficult situations. ences in the radiograph, the more definitely various details
will stand out. However, if overall contrast is increased too
For developing trial exposures, a standardized technique much, there is an actual loss in detail visibility in both the
should always be used. If this is done, any variation in the thick and the thin regions of the specimen.
quality of the trial radiographs may then be attributed to the
exposure alone. This method obviates many of the variable Radiographic contrast is the result of both subject con-
factors common to radiographic work. trast and film contrast. Subject contrast is governed by the
range of radiation intensities transmitted by the specimen
Because an increase of kilovoltage produces a marked (see Fig. 11). A flat sheet of homogeneous material of nearly
increase in X-ray output and penetration (see Fig. 3), it is uniform thickness would have very low subject contrast.
necessary to maintain a close control of this factor in order to Conversely, a specimen with large variations in thickness,
~ecure .radio&raphs of uniform density. In many types of whi~h transmits a wide range of intensities, would have high
mdustnal radiography where it is desirable to maintain con- subject contrast. . Overall subject contrast could be defined
stant exposure conditions for focus-to-film distance, mil- as the ratio of the highest to the lowest radiatio~'intensities ,
ham~erage and exposure time, it is common practice to vary falling on the film. Contrast is also affected by scattered
the kilovoltage in accordance with thickness of the material radiation, removal of which increases subject contrast.
to be examined in order to secure proper density in the
radiographic image. Suppose, for example, it is desired to Choice of Film
change from radiographing 38 mm (1.5 in.) steel to radio-
graphing 50 mm (2 in.) thick steel. The 50 mm (2 in.) steel Different films have different contrast characteristics.
will require more than 10 times the exposure in milliampere- Thus, a film of high contrast may give a radiograph of rela-
minutes at 170 kV However, increasing the kilovoltage to a tively low contrast if subject contrast is very low; conversely,
little more than 200 will yield a comparable radiograph with a film of low contrast may give a radiograph of relatively
the same milliampere-minutes. Thus, kilovoltage is an high contrast if subject contrast is very high. With any given
important variable because economic considerations often
require that exposure times be kept within fairlynarrow lim- FIGURE 11 . As kilovoltage increases, subject
its. It is desirable, as a rule, to use as low a kilovoltage as contrast decreases: more wavelengths
otherfactors will permit. In the case of certain high voltage penetrate subject in both thick and thin
X-ray machines, the technique of choosing exposure condi- sections, thus reducing overall difference in
tions may be somewhat modified. For instance, the kilovolt- exposure between the two
age may be fixed rather than adjustable at the will of the
operator, leaving only milliamperage, exposure time, ftlm
type, screen type and focus-to-film distance as variables.
Gamma Rays
With radioactive materials, the variable factors are more
limited than with X-rays. Not only is the quality of the radia-
tion fixed by the nature of the radiation emitter, but also the
intensity is fixed by the amount of radioactive material in the
particular source. The only variables under the control of
operators, and the only quantities they need to determine,
are the source-to-film distance, film type, screen type and
the exposure time. As in the case of X-radiography, it is
desirable to develop trial exposures using the gamma ray
sources under standardized conditions and to record all data
on exposures and techniques.
FILM RADIOGRAPHY I 143,
specimen, the contrast of the radiograph will depend on image quality indicators (IQis), are used as a check on
kilovoltage of the X-rays or quality of the gamma rays, con- radiographic sensitivity.
trast characteristics. of the film, type of screen, density to
which the radiograph is exposed and processing. In radiography of materials of approximately uniform
thickness, where the range of transmitted X-ray intensities is
Radiographic Sensitivity small, a technique producing high contrast will satisfactorily
render all portions of the area of interest, and the radio-
Radiographic sensitivity refers to the size of the smallest graphic sensitivity will be greater than with a technique pro-
detail that can be seen in a radiograph. Penetrameters, or ducing low\ contrast. If, however, the part radiographed
transmits a Wide range of X-ray intensities, then a technique
producing lower contrast may be necessary to record detail
(achieve radiographic sensitivity) in all regions of the part.
144 I NONDESTRUCTIVE TESTING OVERVIEW
PART 2
ABSORPTION AND SCATTERING
Radiation Absorption in the on a compilation of data from many sources. In a particular
Specimen instance, the exact value of the radiographic equivalence
factor will depend on the quality of the X-radiation and the
When X-rays or gamma rays strike an absorber (Fig. 12), thickness of the specimen. It will be noted from this table
some radiation is absorbed and another portion passes that the relative absorptions of the different materials are
through undeviated. It is the intensity variation of the unde- not constant but change with kilovoltage and as the kilovolt-
viated radiation from area to area in the specimen that age increases the differences between all materials tend to
forms the useful image in a radiograph. However, not all the become less. In other words, as kilovoltage is increased, the
radiation is either completely removed from the beam or radiographic absorption of a material is less and less depen-
transmitted. Some is deviated within the specimen from its dent on the atomic numbers of its constituents.
original direction - that is, it is scattered and is nonimage-
forming. The nonimage-forming, scattered radiation, if not FIGURE 12. Schematic diagram of some ways
carefully controlled, will expose the film and thus tend to X-ray or gamma ray energyis dissipated on
obscure the useful radiographic image. (Scattered radiation passing through matter: electrons from
and the means for reducing its effects are discussed in detail specimens are usually unimportant
later in this section.) Another portion of the original beam's radiographically; those from lead foil screens
energy is spent in liberating electrons from the absorber. are very important
The electrons from the specimen are unimportant radio-
graphically; those from lead screens are very important.
X-Ray Equivalency !PRIMARY RADIATION
If industrial radiography were done with an X-ray beam SECONDARY I
containing a single wavelength and if there were no scatter- X-RAYS OR GAMMA RAYS
ing, the laws of absorption of X-rays by matter could be i ~~
stated mathematically with great exactness. However,
because a broad band of wavelengths is used and because ELECTRONS
considerable scattered radiation reaches the film, the laws
must be briven in a general way. UNABSORBED PRIMARY
RADIATION
The X-ray absorption of a specimen depends on its thick-
ness, on its density and, most important of all, on the atomic (IMAGE FORMING)
nature of the material. Comparing two specimens of similar
composition, the thicker or the more dense will absorb
more radiation and so require more kilovoltage or exposure,
or both, to produce the same photographic result. However,
the atomic elements in a specimen usually exert a far greater
effect upon X-ray absorption than either the thickness or
the density. For example, lead is about 1.5 times as dense as
ordinary steel, but at 220 kV, 2.5 mm (0.1 in.) oflead absorbs
as much as 30.5 mm (1.2 in.) of steel. Brass is only about 1.1
times as dense as steel, yet, at 150 kV, the same exposure is
required for 6.4 mm (0.25 in.) of brass as for 8.9 mm (0.35in.)
of steel. Table 2 gives approximate radiographic equivalence
. factors. It should be emphasized that this table is approxi-
mate and is intended merely as a guide because it is based
FILM RADIOGRAPHY I 145
TABLE2. Approximate radiographic equivalence factors"
Material X-Rays Gamma Rays
(kilovolts) lridium-192 Cesium-137 Cobalt-60 Radium
50 100 150 220 400 1,000 2,000 4 to 25
Magnesium 0.6 0.6 0.5 0.08 0.35 0.35 0.35 0.40
Aluminum 1.0 i.O 0.12 0.18 0.35 0.35 0.35
2024 (aluminum) alloy 2.2 1.6 0.16 0.22
Titanium 1.0 1.0 l,000 1.0 1.0 1.0 1.0
Steel 0.45 0.35 1.0 1.0 l,000 l .0 1.0 1.0 1.0
l 8-8 (steel) alloy 12.0 1.0 1.0 l.O l.l 1.1 1. l 1.1
Copper 12.0 1.0 1.0 1.0 J .2 1,300 l.l 1.0 1.0 1.0
Zinc 18.0 1.6 1.4 1.4 1.3 1,200 1.1 1. 1 1. 1 1. 1
Brass" 1.0 1.2 1,200 1.3 1.3 1.3 1.3
lnconel X alloy-coated 1.4 1.3 L3 5.0 1.3 1,300
Zirconium 1.4 1.3 1.3 4.0 3.2 2.3 2.0
Lead 16.0 1.4 1.3 1.3 2.5 3,000
Uranium 2.3 2.0 3,900 12.6 5.6 3.4
14.0 12.0
25.0
a. ALUMINUM IS THE STANDARD METALAT 50 kV AND 100 kV AND STEEL AT THE HIGHER VOLTAGES AND GAMMA RAYS. THE THICKNESS OF ANOTHER
METAL IS MULTIPLIED BY THE CORRESPONDING FACTOR TO OBTAIN THE APPROXIMATE EQUIVALENT THICKNESS OF THE STANDARD METAJ... THE
EXPOSURE APPLYING TO THIS THICKNESS OF THE STANDARD METAL IS USED. EXAMPLE: TO RADIOGRAPH 12.7 mm 10.s in.] OF COPPER AT 220 kV.
MULTIPLY 12.7 mm 10.5 in.) BY THE FACTOR 1.4, OBTAINING AN EQUIVALENT THICKNESS OF 17.8 mm (0.7 in.) OF STEEL.
b. TIN OR LEAD ALLOYED IN BRASS WILL INCREASE THESE FACTORS.
For X-rays generated at voltages more than 1 MeV and for Radiography of 50 mm (2 in.) aluminum can also be
materials not differing too greatly in atomic number (steel accomplished at these two kilovoltages. Equal densities will
and copper, for example), the radiographic absorption for a result with the following exposures:
given thickness of material is roughly proportional to density
of the material. However, even at high voltages or with pene- 1. 80 keV, 17 ma-min,
trating gamma rays, the effect of composition on absorption 2. 120 keV, 2.4 rnA-min.
cannot be ignored when dealing with materials that differ
widely in atomic number. For instance, absorption of lead for In this case, the same increase in ki.lovoltage results in an
1 MeV X-rays is about five times that of an equal thickness of increase in photographically effective X-ray intensity of only
steel, although its density is only 1.5 times as great. seven times. Many other examples can be found to illustrate
the extreme variability of the effect of ki.lovoltageon X-ray
The ki.lovoltage governs the penetrating power of an intensity.
X-ray beam and hence governs the intensity of the radiation
passing through the specimen. It is not possible, however, to Gamma Ray Equivalency
specify a simple relation between kilovoltage and X-ray
intensity. Such factors as the thickness and the kind of mate- Essentially the same considerations apply to gamma ray
rial radiographed, the characteristics of the X-ray generating absorption because the radiations are of similar nature. It is
apparatus and whether or not the film is used alone or with true that some radioactive materials used in industrial radio-
intensifying screens all exert considerable influence on this graphy emit radiation that is monoenergetic, or almost so
relation. The following,example illustrates this point. (for example, cobalt-60 and cesium-137). However, even
with these sources, scattering is dependent on size, shape
Data from a given exposure chart indicate that radio- and composition of the specimen, which prevents the laws
graphs of equal density can be made of 12.7 mm (0.5 in.) of absorption from being stated exactly. For those gamma
steel with either of the following sets of exposure conditions: ray emitters (for example, iridium-192) that give off a num-
ber of discrete gamma ray wavelengths extending over a
1. 80 keV, 35 mA-min; wide energy range, the resemblance to the absorption of
2. 120 keV, 1.5 ma-min. X-rays is even greater.
In this case, a 50 percent increase in ki.lovoltage results Gamma ray absorption of a specimen depends on its thick-
in a 23-fold increase in photographically effective X-ray ness, density and composition, as does its X-ray absorption.
intensity.
146 I NONDESTRUCTIVE TESTING OVERVIEW
However, the most commonly used gamma ray sources emit tube, this too will scatter X-rays. However, because of the dis-
fairly penetrating radiation corresponding in their proper- tance from the film, scattering from this source is negligible.
ties to high voltage X-radiation. The radiographic equiva- Any other material, such as wall or floor, on the film side of
lence factors in Table 2 show that the absorptions of the the specimen may also scatter an appreciable quantity of
various materials for penetrating gamma rays are similar to X-rays back to the film, especially if the material receives the
their absorptions for high voltage X-rays - that is, the direct radiation from the X-ray tube or gamma ray source
absorptions of materials fairly close together in atomic num- (Fig. 14). This is referred to as backscattered radiation.
ber are roughly proportional to their densities. As with high
voltage X-rays this is not true of materials, such as steel and Reduction of Scatter
lead, that differ widely in atomic number.
Although scattered radiation can never be completely
Scattered Radiation eliminated, a number of means are available to reduce its
effect. The various methods are discussed in terms of
When a beam of X-rays or gamma rays strikes any object,
some of the radiation is absorbed, some is scattered, and FIGURE13. Sourcesof scatteredradiation
some passes straight through. The electrons of the atoms
constituting the object scatter radiation in all directions, FILM AND CASSITTE
much as light is dispersed by fog. The wavelengths of much
of the radiation are increased by the scattering process and LEGEND
hence the scatter has lower kilovoltage and is always some-
what softer, or less penetrating, than the unscattered pri- A = TRANSMIITED SCAITER
mary radiation. Any material -whether specimen, cassette,
tabletop, walls or floor - that receives the direct radiation B = SCAITER FROM CASSITTE
is a source of scattered radiation. Unless suitable measures
are taken to reduce the effects of scatter, it will reduce the C = REFLECTION SCAITER
contrast over the whole image or parts of it.
Scattering of radiation occurs, and is a problem, in radi-
ography with both X-rays and gamma rays. In the text which
follows, the discussion is in terms of X-rays, but the same
general principles apply to gamma radiography.
In the radiography of thick materials, scattered radiation
forms most of the total radiation. For example, in the radio-
graphy of a 19 mm (0.75 in.) thickness of steel, the scattered
radiation from the specimen is almost twice as intense as the
primary radiation; in the radiography of a 50 mm (2 in.)
thickness of aluminum, the scattered radiation is two and a-
half times as great as the primary radiation. Preventing scat-
ter from reaching the film markedly improves the quality of
the radiographic image.
As a mle, the greater portion of the scattered radiation
affecting the film is from the specimen under examination (A
in Fig. 13). However, any portion of the film holder or cas-
sette that extends beyond the boundaries of the specimen and
thereby receives direct radiation from the X-ray tube also
becomes a source of scattered radiation that can affect the
film. Influence of this scatter is most noticeable just inside the
borders of the image (Bin Fig. 13). In a similar manner, pri-
mary radiation striking the film holder or cassette through a
thin portion of the specimen will cause scattering into the
shadows of adjacent thicker portions. Such scatter is called
undercut. Another source of scatter that may undercut a
specimen is shown as C in Fig. 13. If a filter is used near the
FILM RADIOGRAPHY I 147
X-rays. Although most of the same principles apply to Lead Foil Screens
gamma radiography, differences in application arise because
of the highly penetrating radiation emitted by most com- Lead screens, mounted in contact with the film, dimin-
mon industrial gamma ray sources. For example, a mask
(see Fig. 15) for use with 200 kV X-rays could easily be light ish the effect on the film of scattered radiation from all
enough for convenient handling. A mask for use with
cobalt-60 radiation, on the other hand, would be thick, sources. They are beyond doubt the least expensive, most
heavy and probably cumbersome. In any event, with either convenient and most universally applicable means of com-
X-rays or gamma rays, the means for reducing the effects of
scattered radiation must be chosen on the basis of cost, con- bating the effects of scattered radiation. Lead screens lessen
venience and effectiveness.
tshcereesncsattpeer. rmreitacahdinegcrethaese films rezardless of whether the
Some standards recommend checking for excessive or necesOsitate an increase in the
backscatter by looking for the image of a lead letter B placed
on the back of the film cassette. radiographic exposure. The nature of the action of lead
screens is discussed more fully in Part 3 of this Section.
FIGURE 14. Intense backscattered radiation
may originate in the floor or wall; coning, Many X-ray exposure holders incorporate a sheet of lead
masking or diaphragming should be used;
backing the cassette with lead may give foil in the back for the specific purpose of protecting the
adequate protection
FIGURE 1 5. Combined use of metallicshot and
lead mask for lessening scattered radiation is
conduciveto good radiographic quality; if
several round bars are to be radiographed,
they may be separated along their lengthswith
lead strips held on edge by wooden frame and
voids filled with fine shot
FLOOR OR WALL FILM AND CASSETTE
148 I NONDESTRUCTIVETESTING OVERVIEW
film from backscatter. This lead will not serve as an intensi- with copper or steel shot having a diameter of about
fying screen - first because it usually has a paper facing 0.25 mm (0.01 in.) or less (Fig. 15). This material "flows"
and second because it often is not lead of radiographicqual and is very effective for filling cavities in irregular objects,
ity. If intensifying screens are used with such holders, defi- such as castings, where a normal exposure for thick parts
nite means must be provided to ensure good contact. would result in aI1 overexposure for thinner parts. Of course,
it is preferable to make separate exposures for thick and thin
When such a cassette or film holder is used with gamma parts, but this is not always practical.
rays or with megavolt X-rays, the film should always be
enclosed between double lead screens; otherwise, secondary In some cases, a lead diaphragm or lead cone on the tube
radiation from the lead backing is sufficient to penetrate the head may be a convenient way to limit the area covered by
intervening felt or paper and cast a shadow of this material on the X-ray beam. Such lead diaphragms are particularly use-
the film giving a granular or mottled appearance. This effect ful where the desired cross section of the beam is a simple
can also occur at voltages as low as 200 kV unless the film is geometric figure, such as a circle, square or rectangle.
enclosed between lead foil or fluorescent intensifying screens.
Fiiters
Masks and Diaphragms
In general, the use of filters is limited to radiography
Scattered radiation originating in matter outside the with X-rays. A simple metallic filter mounted in the X-ray
specimen is most serious for specimens that have high beam near the X-ray tube (Fig. 16) may adequately serve
absorption for X-rays because the scattering from external the purpose of eliminating overexposure in the thin regions
sources may be large compared to the primary image form- of the specimen a11~tn the area surrounding the part. Such
. ing radiation that reaches the film through the specimen. a filter is particular1y useful for reducing scatter undercut in
Often, the most satisfactory method of lessening this scatter cases where a mask around the specimen is impractical or ,
is by the use of cutout diaphragms or some other form of
mask mounted over or around the object radiographed. If bewhere the specimen would be injured by chemicals or shot
many specimens of the same article are to be radiographed,
it may be worthwhile to cut an opening of the same shape, Of course, an increase in exposure of kilovoltage will
but slightly smaller, in a sheet of lead and place this on the required to compensate for the additional absorption but; in
object. The lead serves to reduce the exposure in surround- cases where the filter method is applicable, this is not seri-
ing areas to a negligible value and to eliminate scattered ous unless the limit of the X-ray machine has been reached.
radiation from this source. Because scatter also arises from
the specimen itself, it is good practice, wherever possible, to The underlying principle of the method is that the addi-
limit the cross section of an X-ray beam to cover only the tion of the filter material-causes a much greater change in
area of the specimen that is of interest in the examination. the amount of radiation passing through the thin parts than
through the thicker parts. Suppose the shape of a certain
For situations where a cutout diaphragm would not be steel specimen is as shown in Fig. 16 and that the thick-
economical, barium clay packed around the specimen will nesses are 6.4 mm (0.25 in.), 12.7 mm (0.5 in.) and 25.4 mm
serve the same purpose. The clay should be thick enough so (1 in.). This specimen is radiographedfirst with no filter and
that the film density under the clay is somewhat less than then with a filter near the tube.
that under the specimen. Otherwise, the clay itself con-
tributes appreciable scattered radiation. Column 3 of Table 3 shows the percentage of the original
X-ray intensity remaining after the addition of the filter,
It may be found advantageous to place the object in alu- assuming both exposures were made at 180 kV. (These val-
minum or thin iron pans and to use a liquid absorber, pro- ues where derived from actual exposure chart data.)
vided the liquid chosen will not damage the specimen. A
combined saturated solution of lead acetate and lead nitrate Note that the greatest percentage change in X-ray inten-
is satisfactory. This is prepared by dissolving about 1.6 kg sity is under the thinner parts of the specimen and in the
(3.5 lb) of lead acetate in one gallon of hot water. When the film area immediately surrounding it. The filter reduces by a
lead acetate is in solution, about 1.4 kg (3 lb) of lead nitrate large ratio the X-ray intensity passing through the thin sec-
may be added. (Warning! The lead solution is harmful if tions or striking the cassette around the specimen and
swallowed or inhaled. Wash thoroughly after handling. Use hence reduces the undercut of scatter from these sources.
only with adequate ventilation.) Because of its high lead Thus, in regions of strong undercut, the contrast is
content this solution is a strong absorber of X-rays. Inmask- increased by the use of a filter because the only effect of the
ing with liquids, care must be used to eliminate bubbles that undercutting radiation is to obscure the desired image. In
may cling to the surface of the specimen. regions where the undercut is negligible, a filter has the
effect of decreasing the contrast in the finished radiograph.
One of the most satisfactory arrangements, combining
effectiveness and convenience, is to surround the object Although the highest possible contrast is often desired,
there are certain instances in which too much contrast is a
disadvantage. For example, it may be desired to render
detail visible in all parts of a specimen having wide variations
FILM RADIOGRAPHY I 149
TABLE 3. Effect of metallic filter on X-ray intensity of thickness. If the exposure is made to give a usable density
under the thin part, the thick region may be underexposed.
Original If the exposure is adjusted to give a suitable density under
the thick parts, the image of the thin sections may be grossly
X-ray Intensity overexposed.
Specimen Remaining after A filter reduces excessive subject contrast (and hence
radiographic contrast) by hardening the radiation. The
Thickness Additionof Filter longer wavelengths do not penetrate the filter to as great an
extent as do the shorter wavelengths. Therefore, the beam
Region millimeters(inches} (percent) emerging from the filter contains a higher proportion of the
more penetrating wavelengths (see Fig. 17). In the sense
Outside specimen 0 (OJ <5 that a more penetrating beam is produced, filtering is analo-
- 30 gous to increasing the voltage. However, it requires a com-
Thin section 6.4 (0.25) - 40 paratively large change in ki.lovoltage to change the
- 55 hardness of an X-ray beam to the extent that will result from
Medium section 12.7 (0.50) adding a small amount of filtration.
Thick section 25.4 (1.0) Although filtering reduces the total quantity of radiation,
most of the wavelengths removed are those that would not
FIGURE 16. Filterplaced near X-ray tube penetrate the thicker portions of the specimen in any case.
reduces subjectcontrastand eliminatesmuch The radiation removed would only result in a high intensity
of secondary radiation,which tendsto obscure in the regions around the specimen and under its thinner
detail in peripheryof specimen sections, with the attendant scattering, undercut and over-
exposure. The harder radiation obtained by filtering the
X-ray beam produces a radiograph of lower contrast, per-
mitting a wider range of specimen thicknesses to be
recorded on a single film than would be possible otherwise.
A filter can act either to increase or to decrease the net
contrast. The contrast and penetrameter , visibility are
increased by the removal of the scatter that undercuts the
specimen and decreased by the hardening or the original
beam. The nature of the individual specimen will determine
which of these effects will predominate or whether both will
occur in different parts of the same specimen.
FIGURE 1 7. Curves illustrating effect of filter
on compositionand intensity of X-ray beam
~
vzi
LU
~I-
SPECIMEN
FILM AND CASSETTE
WAVELENGTH
1 50 I NONDESTRUCTIVETESTING OVERVIEW
The choice of a filter material should be made on the effective filter for the scatter from the bulk of the specimen.
basis of availability and ease of handling. For the same fil- Additional filtration between specimen and film only tends
tering effect, the thickness of filter required is less for those to contribute additional scatter from the filter itself. The
materials having higher absorption. In many cases, copper scatter undercut can be decreased by adding an appropriate
or brass is the most useful, because filters of these materials filter at the tube as mentioned before. Although the filter
will be thin enough to handle easily yet not so thin as to be near the tube gives rise to scattered radiation, the scatter is
delicate (see Fig. 18). emitted in all directions; and because the film is far from the
filter, scatter reaching the film has ve:ry low intensity.
Rules for filter thicknesses are difficult to formulate
exactly because the amount of filtration required depends Further advantages of placing the filter near the X-ray
not only on the material and thickness range of the speci- tube are that specimen-to-film distance is kept to a mini-
men but also on the distribution of material in the specimen mum and that scratches and dents in the filter are so blurred
and on the amount of scatter undercut to be eliminated. In that their images are not apparent on the radiograph.
the radiography of aluminum, a filter of copper about 4 per-
cent of the greatest thickness of the specimen should prove Grid Diaphragms
the thickest necessary. With steel, a copper filter should
ordinarily be about 20 percent, or a lead filter about 3 per- One of the most effective ways to reduce scattered radi-
cent, of the greatest specimen thickness for the greatest use- ation from an object being radiographed is through the use
ful filtration. The foregoing values are maximum values; of a Potter-Bucky diaphragm. This apparatus (Fig. 19) con-
depending on circumstances, useful radiographs can often sists of a moving grid, composed of lead strips held in posi-
be made with far less filtration. tion by intervening strips of a material transparent to X-rays.
The lead strips are tilted, so that the plane of each is in line
In radiography with X-rays up to at least 250 kV, the with the focal spot of the tube. The slots between the lead
0.125 mm (0.005 in.) front lead screen customarily used is an strips are several times as deep as they are wide. The paral-
lel lead strips have the function of absorbing the ve:ry diver-
FIGURE 18. Maximumfilter thicknessfor gent scattered rays from the object being radiographed, so
aluminum and steel that most of the exposure is made by the primary rays ema-
nating from the focal spot of the tube and passing between
2.5 the lead strips. During the course of the exposure, the grid
is moved, or oscillated, in a plane parallel to the film as
h shown by the black arrows in Fig. 19. Thus, the shadows of
the lead strips are blurred to the point that they do not
I~ appear in the final radiograph.
l2.25 "- I I I I I I
I STEEL (LEAD FILTER) Use of the Potter-Bucky diaphragm in industrial radiog-
2.0 I I raphy complicates the technique to some extent and neces-
.! '!.75 I sarily limits the flexibility of the arrangement of the X-ray
tube, the specimen and the film. Grids can, however, be of
j great value in the radiography of beryllium more than
75 mm (3 in.) thick and in the examination of other low
I .' i's. absorption materials of moderate and great thicknesses. For
I -, ALUMINUM (COPPER FILTER) - - these materials, kilovoltages in the medical radiographic
! II' ,I range (approximately 50 to 150 kV) are used, and the medi-
,:I cal forms of Potter-Bucky diaphragms are appropriate. Grid
'I// ,. , I, ratios (the ratio of height to width of the openings between
the lead strips) of 12 or more are desirable.
,I ,,
,'/, The Potter-Bucky diaphragm is seldom used elsewhere
.,,0.75 ' in the industrial field, although special forms have been
,, . I"''l , ,0.5 .' designed for the radiography of steel with voltages as high as
',J 200 to 400 kV These diaphragms are not used at higher volt-
,II STEEL (COPPER FILTER) ages or with gamma rays because relatively thick lead strips
would be needed to absorb the radiation scattered at these
,,l,: ,.,0.25. I energies. This in tum would require a Potter-Bucky
diaphragm, with the associated mechanism, of an uneco-
:. nomical size and complexity.
0 i.o 2.0 3.0
FILTER THICKNESS
(relative units)
FILM RADIOGRAPHY I 1 51
Mottling Caused by X-Ray successive radiographs, with the specimen rotated slightly (1
Diffraction to 5 degrees) between exposures, about an axis perpendicu-
lar to the central beam. A pattern caused by porosity or seg-
A special form of scattering caused by X-ray diffraction is regation will change only slightly; however, one caused by
encountered occasionally. It is most often observed in the diffraction will show a marked change. The radiographs of
radiography of fairly thin metallic specimens whose grain some specimens will show a mottling from both effects, and
careful observation is needed to differentiate between
size is large enough to be an appreciable fraction of the part them.
thickness. The radiographic appearance of this type of scat- Relatively large crystal or grain in a relatively thin speci-
tering is mottled and may be confused with the mottled men may in' some cases diffract an appreciable portion of
appearance sometimes produced by porosity or segregation. the X-ray energy falling on the specimen, much as if it were
It can be distinguished from these conditions by making two a small mirror. This will result in a light spot on the devel-
oped radiograph corresponding to the position of the partic-
FIGURE 19. Schematic diagramshowinghow ular crystal and may also produce a dark spot in another
primaryX-rays pass between lead strips of location if the diffracted, or reflected, beam strikes the film.
Potter-Bucky diaphragm: most of scattered Should this beam strike the film beneath a thick part of the
X-rays are absorbed because they strikesides of specimen, the dark spot may be mistaken for a void in the
strips thick section. This effect is not observed in most industrial
radiography, for most specimens are composed of a multi-
tude of very minute crystals or grains variously oriented;
hence scatter by diffraction is essentially uniform over the
film area. In addition, the directly transmitted beam usually
reduces the contrast in the diffraction pattern to a point
where it is no longer visible on the radiograph.
The mottling caused by diffraction can be reduced, and
in some cases eliminated, by raising the kilovoltage and by
using lead foil screens. The former is often of positive value
even though the radiographic contrast is reduced. Because
definite rules are difficult to formulate, both approaches
should be tried in a new situation, and perhaps both used
together.
It should be noted, however, that in some instances, the
presence or absence of mottling caused by diffraction has
been used as a rough indication of grain size and thus as a
basis for the acceptance or the rejection of parts.
FILM AND CASSETTE Scattering in High Voltage
Megavolt Radiography
Lead screens should always be used in the 1 or 2 MeV
range. The common thicknesses, 0.125 mm (0.005 in.) front
and 0.25 mm (0.010 in.) back, are both satisfactory and con-
venient. Some users, however, find a 0.25 mm (0.010 in.)
front screen of value because of its greater selective absorp-
tion of the scattered radiation from the specimen.
A very important point is to block off all radiation except
the useful beam with heavy (12.7 to 25.4 mm [0.5 in. to
1 in.]) lead at the tubehead. Unless this is done, radiation
striking the walls of the X-ray room will scatter back in such
quantity as to seriously affect the quality of the radiograph.
This will be especially noticeable if the specimen is thick or
has parts projecting relatively far from the ftlm.
1 52 I NONDESTRUCTIVETESTING OVERVIEW
PART 3
RADIOGRAPHIC SCREENS
Functions of Screens the secondary radiation generated in the lead; (2) it absorbs
Radiographic screens are employed to use more fully the the longer wavelength scattered radiation more than the pri-
X-ray or gamma ray energy reaching the film. The physical
principles underlying the action of both lead foil and fluo- mary; and (3) it intensifies the primary radiation more than
rescent screens are discussed elsewhere and only the practi- the scattered radiation. The differential absorption of the
cal applications are discussed here.
secondary radiation and the differential intensification of the
When an X-ray or gamma ray beam strikes a film, usually
less than one percent of the energy is absorbed. Because the primary radiation result in diminishing the effect of scattered
formation of the radiographic image is primarily governed
by the absorbed radiation, more than 99 percent of the radiation, producing greater contrast and clarity in the radio-
available energy in the beam performs no useful photo- graphic image. This reduction in the effect of the scattered
graphic work. Obviously, any means of more fully using this
wasted energy, without complicating the technical proce- radiation decreases the total intensity of the radiation reach-
dure, is highly desirable. Two types of radiographic screens
are used to achieve this end - lead and fluorescent. Lead ing the film and lessens the net intensification factor of the
screens, in turn, are of two different forms. One form is screens. The absorption of primary radiation by the front
sheets of lead foil, usually mounted on cardboard or plastic,
which are used in pairs in a conventional cassette or expo- lead screen also diminishes the net intensifying effect, and, if
sure holder. The other consists of a compound (usually an
oxide), evenly coated on a thin support. The film is placed the incident radiation does not have sufficient penetrating
between the leaves of a folded sheet of this oxide coated
material with the oxide in contact with the film. The combi- power, the actua,.e,cposure required may be even gr~ater
nation is supplied ina sealed, lightproof envelope. than without screens. At best, the exposure time is one half
to one third of that without screens but the.gdvantage of i
Lead Foil Screens
screens in reducing scattered radiation still holds. '
For radiography in the range 150 to 400 kV, lead foil in
direct contact with both sides of the film has a desirable The quality of the radiation necessary to obtain an appre-
effect on the quality of the radiograph. In radiography with
gamma rays and with X-rays below 1 MeV, the front lead foil ciable intensification from lead foil screens depends mi the
need be only 0.1 to 0.15 mm (0.004 to 0.006 in.) thick; con-
sequently, its absorption of the primary beam is not serious. type of film, the kilovoltage, and the thickness of the material
The back screen should be thicker to reduce backscattered
radiation. Such screens are available commercially. The FIGURE 20. Effects of kilovoltageon
choice of lead screen thiclmesses for multimegavolt radiog- intensificationpropertiesof lead screens
raphy is much more complicated, and the manufacturers of
the equipment should be consulted for their recommenda- lINTENSIFICATIONw0::: 2
tions. 1.0 3
Effects of Lead Screens zuw 0.8
Lead foil in direct contact with the film has three princi- w 0.6
pal effects: (1) it increases the photographic action on the 0LL.
film, largely by reason of the electrons emitted and partly by 0.4
LL.
zV)j~ 0.2
w 0
0
-0.2
-0.4 100 125 150 175 200 225
ABSORPTION so 75 KILOVOLTAGE
LEGEND
I. 0.05 mm LEADOXIDE(0.01 mm LEADEOUIVAI..ENT)
2. 0.12mm
3. 0.25 mm LEAD
FILM RADIOGRAPHY I 1 53
through which the rays must pass (Fig. 20). In the radiogra- usefulness of the screen, but large blisters or cavities should
phy of aluminum, for example, using a 0.125 mm (0.005 in.) be avoided.
front screen and a 0.25 mm (0.010 in.) back screen, the
thickness of aluminum must be about 150 mm (6 in.) and Most of the intensifying action of a lead foil screen is
the kilovoltage as high as 160 kV to secure any advantage in caused by the electrons emitted under X-ray or gamma ray
exposure time with lead screens. Inthe radiography of steel, excitation. Because electrons are readily absorbed even in
lead screens begin to give appreciable intensification with thin or light materials, the surface must be kept free of
thicknesses in the neighborhood of 6.3 mm (0.25 in.), at grease and lint, which will produce light marks on the radio-
voltages of 130 to 150 kV In the radiography of 32 mm graph. Small flakes of foreign material - for example, dan-
(1.25 in.) thick steel at about 200 kV, lead screens permit an druff or tobacco - will likewise produce light spots on the
exposure of about one-third of that without screens (intensi- completedradiograph (Fig. 23). To minimize the absorption
fication factor of 3). With cobalt-60 gamma rays, the intensi- of electrons and keep the intensification factor as high as
fication factor of Lead screens is about 2. Lead foil screens, possible, protective coatings should not be used.
however, do not detrimentally affect the definition or grain-
iness of the radiographic image to any material degree so Deep scratches on lead foil screens, on the other hand,
long as the lead and the film are in intimate contact. will produce dark lines on the radiograph (Fig. 24).
Lead foil screens diminish the effect of scattered radia- Grease and lint may be removed from the surface of lead
tion, particularly that which undercuts the object when the foil screens with a mild household detergent or cleanser and
primary rays strike the portions of the film holder or cassette a soft, lint-free cloth. If the cleanser is one that dries to a
outside the area covered by the object. powder, care must be taken to remove all the powder and to
prevent its being introduced into the cassette or exposure
Scattered radiation from the specimen itself is cut almost holder. The screens must be completely dry before use; oth-
in half by lead screens, contributing to maximum clarity of erwise, the film will stick to them. If more thorough clean-
detail in the radiograph; this advantage is obtained even ing is necessary, screens may be very gently rubbed with the
under conditions where the lead screen makes an increase finest grade of steel wool. If this is done carefully, the shal-
in exposure necessary. low scratches left by the steel wool will not produce dark
lines in the radiograph.
In radiography with gamma rays or high voltage X-rays,
films loaded in metal cassettes without screens are likely to Films could be fogged if left between lead foil screens
record the effect of secondary electrons generated in the longer than is reasonably necessary, particularly under con-
lead covered back of the cassette. These electrons, passing ditions of high temperature and humidity. When screens
through the felt pad on the cassette cover, produce a mot- have been freshly cleaned with an abrasive, this effect will
tled appearance because of the structure of the felt. Films be increased; prolonged contact between film and screens
loaded in the customary lead backed cardboard exposure should be delayed for 24 hours after cleaning.
holder may also show the structure of the paper that lies
between the lead and the film (Fig. 21). To avoid these FIGURE 21 . Upper area shows decreased
effects, film should be enclosed between double lead densitycaused by paper between lead screen
screens, care being taken to ensure good contact between '. and film: electronshadowpictureof paper
film and screens. Thus, lead foil screens are essential in structurehas also been introduced
practically all radiography with gamma rays or megavolt
X-rays. If, for any reason, screens cannot be used with these
radiations, a lightproof paper or cardboard holder with no
metal backing should be used.
Contact between the film and the lead foil screens is
essential to good radiographic quality. Areas lacking contact
produce fuzzy images, as.shown in Fig. 22.
)'"
Selection and Care of Lead Screens
Lead foil for screens must be selected with extreme care.
Commercially pure lead is satisfactory. An alloy of 6 percent
antimony and 94 percent lead, being harder and stiffer, has
better resistance to wear and abrasion. Tin-coated lead foil
should be avoided, because irregularities in the tin cause a
variation in the intensifying factor of the screens, resulting
in mottled radiographs. Minor blemishes do not affect the
1 54 I NONDESTRUCTIVETESTING OVERVIEW
FIGURE 22. Between film and lead foil screens: (a) good contactgives sharp image; fbJ poor contact
gives fuzzy image
faJ fbJ
FIGURE 23. Static marks resultingfrom poor Fluorescent Screens
film handling;static marks may also be treelike
or branching Certain chemicals fluoresce; that is, they have the ability
to absorb X-rays and gamma rays and immediately emit
light; the intensity of the emitted light depends on the
intensity of the incident radiation. These phosphorescent
materials can be used in radiography by first being finely
powdered, mixed with a suitable binder, then coated in a
thin, smooth layer on a special cardboard or plastic support.
For the exposure, film is clamped firmly between a pair of
these fluorescent screens. The photographic effect on the
film, then, is the sum of the effects of the X-rays and of the
light emitted by the screens. A few examples will serve to
illustrate the importance of intensifying screens in reducing
exposure time. In medical radiography, the exposure is from
1/10 to 1/60 as much with fluorescent intensifying screens as
without them. In other words, the intensificationfactorvaries
from 10 to 60, depending on the kilovoltage and type of
screen used. In the radiography of 12.7 mm (0.5 in.) thick
steel at 150 kV, a factor as high as 125 has been observed, and
in radiography of 19 .1 mm (0. 75 in.) thick steel at 180 kV, fac-
tors of several hundred have been obtained experimentally.
Under these latter conditions, the intensification factor
has about reached its maximum and diminishes both for
lower voltage and thinner steel and for higher voltage and
thicker steel. Using cobalt-60 gamma rays for very thick
steel, the factor may be 10 or less.
FILM RADIOGRAPHY I 1 55
FIGURE 24. Numberof electrons emitted {per Despite their great effect in reducing exposure time, flu-
surface unit of l~ad) is essentially uniform:more orescent screens are not widely used in industrial radiogra-
electrons can reach film in vicinityof scratch, phy. This is in part because they may give poor definition,
compared to a radiograph made directly or with lead
resultingin dark line on radiograph{for screens. The poorer definition can result from the spreading
illustrativeclarity, electron paths have been of the light emitted from the screens, as shown in Fig. 25.
shownas straightand parallel; actually, The light from any particular portion of the screen spreads
electrons are emitted diffusely) out beyond the confines of the X-ray beam that excited the
fluorescence. This spreading of light from the screens may
X-RAYS FILM account fdr the blurring of outlines in the radiograph.
,I. ... A .t. ,I. .l.t.l.A.1.,+..t. ,I. ,I. ,I. ,I. ,!. ,I. The other reason fluorescent screens are seldom used in
II · industrial radiography is because they may produce screen
II I I l I I I I II I I I I I I rrwttle on the finished radiograph. This mottle is character-
II istic in appearance, very much larger in scale and much
II I ELECTRONS FROM LEAD FOIL 1 1 1 softer in outline than the graininess associated with the film
itself. It is not associated with the actual structure of the
I II III I II screen; that is, it is not a result of the size of the fluorescent
crystals themselves or any unevenness in their dispersion in
III III I II the binder. Rather, screen mottle is associated with purely
statistical variations in the numbers of absorbed X-ray pho-
tons, from one tiny area of the screen to the next. The fewer
the number of X-ray photons involved, the stronger the
appearance of the screen mottle. This explains, for example,
why the screen mottle produced by a particular type of
screen tends to become greater as the kilovoltage of the
radiation increases. The higher the voltage, the more ener-
getic, on the average, are the X-ray photons. Therefore, on
absorption in the screen, a larger burst of light is produced.
The larger the bursts, the fewer that are needed to produce
FIGURE 25. light and ultravioletradiationfrom typicalfluorescentscreen spreads beyondX-ray beam
that excites fluorescence
1 56 I NONDESTRUCTIVETESTING OVERVIEW
a given density and the greater is the purely statistical varia- At kilovoltages higher than those necessary to radiograph
tion in the number of photons from one small area to the about 12.7 mm (0.5 in.) of steel, the fastest available screens
next. are usually used, because the major function of fluorescent
intensifying screens is to minimize the exposure time.
Intensifying screens may be needed in the radiography
of steel thicknesses greater than 50 mm (2 in.) at 250 kV, There are a few radiographic situations that demand a
75 mm (3 in.) at 400 kV and 125 mm (5 in.) at 1 MeV speed higher than the fastest film designed for direct expo-
sure (or exposure with lead screens) yet do not require the
Fluorescent screens are not used with gamma rays speed of film designed for use with fluorescent intensifying
because, apart from the screen mottle, failure of the screens. In such cases a high speed, direct exposure film
reciprocity law may result in relatively low intensification may be used with fluorescent screens. The speed of this
factors with the longer exposure times usually necessary in combination will be intermediate between those of the two
gamma radiography. In the radiography of light metals, flu- combinations mentioned above. However, the contrast and
orescent screens are normally not used; but, should they be the maximum density will be higher than that obtained with
required, the best choice would be fluorescent screens of a film designed for fluorescent screen exposure., and the
the slowest type compatible with an economical exposure screen mottle will be less because of the lower speed of the
time - if possible, those designed specifically for sharpness screen-film combination.
of definition in medical radiography.
FILM RADIOGRAPHY I 1 57
PART4
INDUSTRIAL RADIOGRAPHIC FILMS
Modern radiographic films for general radiography consist Although an image may be formed by light and other
of an emulsion (gelatin containing a radiation sensitive silver forms of radiation, as well as by gamma rays or X-rays, the
compound) and a flexible, transparent base that sometimes properties of the latter two are of distinct character, and, for
contains a tint. Usually,the emulsion is coated on both sides this reason, the sensitive emulsion must be different from
of the base in layers about 0.0125 mm (5 x 10-4 in.) thick (see those used in other types of photography.
Figs. 26 and 27). Putting emulsion on both sides of the base
doubles the amount of radiation sensitive silver compound FIGURE 27. Crosssection of unprocessed
and thus increases the speed. At the same time, emulsion lay- emulsionon one side of radiographic film (note
ers are thin enough so that developing, fixingand drying can large numberof grainsas compared to
take place in a reasonable time. However, some radiographic developedgrainsof Fig. 28)
films in which the highest detail visibility is required have
emulsion on only one side of the base.
When X-rays, gamma rays or light strike the grains of the
sensitive silver compound in the emulsion, a change takes
place in the physical structure of the grains. This change
cannot be detected by ordinary physical methods. However,
when the exposed film is treated with a chemical solution
(called a developer), a reaction takes place, causing the for-
mation of black. metallic silver. It is this silver, suspended in
the gelatin on both sides of the base, that constitutes the
image (see Fig. 28).
FIGURE 26. Silver bromidegrains of
radiographicfilm emulsion (2,500 diameters):
grains have been dispersed to show shape and
relativesizes more clearly; in actual coating,
crystals are much more closely packed
FIGURE 28. Crosssection showingdistribuiton
of developed grainsin radiographicfilm
emulsion exposed to give moderatedensity
1 58 I NONDESTRUCTIVE TESTING OVERVIEW
Selection of Films for Industrial film particularly sensitive to blue light, rather than a direct
Radiography exposure film with lead screens.
As pointed out above, industrial radiography now has Figure 29 indicates the direction that these substitutions
many widely diverse applications. There are many consider- take. The direct-exposure films may be used with or without
ations to be made in obtaining the best radiographic results, lead screens, depending on the kilovoltage and the thickness
for example: (1) the composition, shape and size of the part and shape of the specimen.
being examined - and, in some cases, its weight and loca-
tion as well; (2) the type of radiation used-whether X-rays Fluorescent intensifying screens must be used in radiog-
from an X-ray machine or gamma rays from a radioactive
material; (3) the kilovoltages available with the X-ray equip- raphy requiring the highest possible photographic speed.
ment; (4) the intensity of the gamma radiation; (5) the kind The light emitted by the screens has a much greater photo-
of information sought - whether it is simply an overall graphic action than the X-rays either alone or combined
inspection or the critical examination of some especially with the emission from lead screens. To secure adequate
important portion, characteristic or feature; and (6) the exposure within a reasonable time, screen type X-ray films
resulting relative emphasis on definition, contrast, density sandwiched between fluorescent intensifying screens are
and time required for proper exposure. All of these factors often used in radiography of steel in thicknesses greater
are important in the determination of the most effective than about 50 mm (2 in.) at 250 kV and greater than 75 mm
combination of radiographic technique and film. (3 in.) at 400 kV.
Selection of a film for the radiography of any particular PhotographicDensity
part depends on the thickness and material of the specimen
and on the voltage range of the available X-ray machine. In Photographic density refers to the quantitative measure
addition, the choice is affected by the relative importance of of film blackening. When no danger of confusion exists,
high radiographic quality or short exposure time. Thus, an photographic density is usually spoken of merely as density.
attempt must be made to balance these two opposing fac- Density is defined by the equation
tors. As a consequence, it is not possible to present definite
rules on the selection of a film. If high quality is the deciding D logI -0 (Eq.13)
factor, a slower and finer grained film should be substituted It
for a faster one - for instance, for the radiography of steel
up to 6.3 mm (0.25 in.) thick at 120 to 150 kV, Film Y Where:
(Fig. 33) might be substituted for Film X. If short exposure
times are essential, a faster film (or film-screen combina- D = density;
tion) can be used. For example, 38 mm (1.5 in.) steel might I,, = light intensity incident on film; and
be radiographed at 200 kV using fluorescent screens with a I1 = light intensity transmitted.
FIGURE 29. Change in choice of film, Table 4 illustrates some relations between transmittance,
depending on relative emphasis on high speed percent transmittance, opacity and density. It showsthat an
or high radiographic quality increase in density of 0.3 reduces the light transmitted to
one-half its former value. In general, because density is a
logarithm, a certain increase in density always corresponds
to the same percentage decrease in transmittance.
IMPROVING QUALITY ----....ii• Densitometers
SCREEN TYPE FILM FAST SLOW A densitometer is an instrument for measuring photo-
WITH FLUORESCENT graphic densities. A number of different types, both visual
DIRECT EXPOSURE DIRECT EXPOSURE and photoelectric, are available commercially. For purposes
SCREENS TYPE FILM TYPE FILM of practical industrial radiography there is no great pre-
mium on high accuracy in a densitometer. A much more
INCREASING SPEED important property is reliability - the densitometer should
reproduce readings from day to day.
FILM RADIOGRAPHY I 1 59
TABLE 4. Transmittance, percent transmittance, Another method, requiring fewer stepped wedge expo-
opacity and density relationships sures but more arithmetical manipulation, is to make one
step tablet exposure at each kilovoltage and to measure the
Percent densities in the processed stepped wedge radiographs. The
exposure that would have given the chosen density (in this
Transmittance Transmittance Opacity Density case 1.5) under any particular thickness of the stepped
wedge can then be determined from the characteristic
It I 10 It I 10 x 100 10 I It Log 10 I It curve of the film used. The values for thickness, kilovoltage
and exposure are then plotted.
1.00 100 1 0
0.50 50 2 0.3 Note that thickness is on a linear scale and that mil-
0.25 25 4 0.6 liampere-minutes are on a linear scale. The logarithmic
0.10 10 10 1.0 scale is not necessary but is very convenient because it com-
0.01 100 2.0 presses an otherwise long scale. A further advantage of the
0.001 1 1,000 3.0 logarithmic exposure scale is that it usually allows the loca-
0.0001 0.1 10,000 4.0 tion of the points for any one kilovoltage to be well approxi-
0.01 mated by a straight line.
X-Ray Exposure Charts FIGURE 30. Typical X-ray exposure chart for
steel may be appliedto film X (see Fig. 33),
An exposure chart is a graph showing the relation with lead foil screens, at 1 . 5 film densityand
between material thickness, kilovoltage and exposure. In its 1.02 m (40 in.) source-to-film distance
most common form, an exposure chart resembles Fig. 30.
These graphs are adequate for determining exposures in the .---~-.--r--~--.----,-~....---r-~.........-.....-~-,--r-~100
radiography of uniform plates but they serve only as rough t--~--+-f-~-+---f-~H-~--f-1'~~-+---,j~~ 80
guides for objects, such as complicated castings, having
wide variations of thickness. 1.5
Exposure charts are usually available from manufactur-
ers of X-ray equipment. Because, in general, such charts
cannot be used for different X-ray machines unless suitable
correction factors are applied, individual laboratories some-
times prepare their own.
Preparing an Exposure Chart Ll.J-
A simple method for preparing an exposure chart is to §0,:: .Q
make a series of radiographs of a stack of metal plates con-
sisting of a number of steps. This step tablet, or stepped 10 ~
wedge, is radiographed at several different exposure times
at each of a number of kilovoltages. The exposed films are 8 0LX0..l..uJ:-05
all processed under conditions identical to those that will
later be used for routine work. Each radiograph consists of a 6
series of photographic densities corresponding to the X-ray
intensities transmitted by the different thicknesses of metal. 3
A certain density, for example 1.5, is selected as the basis for
the preparation of the chart. Wherever this density occurs 0'---~----~~_.__~~'---~-'-~~-'-~--'
on the stepped wedge radiographs, there are corresponding
values of thickness, milliampere-minutes and kilovoltage. It 0 6.4 12.7 19 25.4 31.8 38.1
is unlikely that many of the radiographs will contain a value (150)
of exactly 1.5 in density but the correct thickness for this (0) (0.25) (0.50) (0.75) (100) (l.25)
density can be found by interpolation between steps. Thick-
ness and milliampere-minute values are plotted for the dif- EQUIVALENT THICKNESS
ferent kilovoltages in the manner shown in Fig. 30. millimeters of steel
(inches of steel)
160 I NONDESTRUCTIVETESTINGOVERVIEW
An exposure chart usually applies only to a single set of 5. The chart gives exposures to produce a certain den-
conditions, determined by: sity. If a different density is required, the correction
factor may be calculated from the film's characteristic
1. the X-ray machine used; curve.
2. a certain source-to-film distance;
3. a particular film type; 6. If the type of screen is changed - for example, from
4. processing conditions used; lead foil to fluorescent - it is easier and more accu-
5. the film density on which the chart is based; and rate to make a new exposure chart than to determine
6. the type of screens (if any) that are used. correction factors.
Only if the conditions used in making the radiograph In some radiographic operations, the exposure time and
agree in all particulars with those used in preparation of the the source-to-film distance are set by economic considera-
exposure chart can values of exposure be read directly from tions or on the basis of previous experience and test radio-
the chart. Any change requires the application of a correc- graphs. The tube current is, of course, limited by the design
tion factor. The correction factor applying to each of the of the tube. The specimen and the kilovoltage are variables.
above conditions is discussed separately. When these conditions exist, the exposure chart may take a
· simplified form as shown in Fig. 31, which allows the kilo-
voltage for any particular specimen thickness to be chosen.
1. It is sometimes difficult to find a correction factor to FIGURE 31 . Typical X-ray exposure chart for
use when exposure and distance are held
make an exposure chart prepared for one X-ray constantand kilovoltageis varied to conform
machine applicable to another. Different X-ray to specimen thickness: film X (see Fig. 33),
machines operating at the same nominal kilovoltage exposed with lead foil screensto densityof
and milliamperage settings may give not only differ- 1.5, source-to-film distanceis 1.02 m (40 in.)
ent intensities but also different qualities of radiation. and exposure is 50 mAmin
2. A change in source-to-film distance may be compen-
sated for by use of the inverse square law. Some 220
exposure charts give exposures in terms of exposure
factor rather than in terms of milliampere-minutes 200 /
or milliampere-seconds. Charts of this type are read-
ily applied to any value of source-to-film distance. 180 v/ /
3. The use of a different type of film can be corrected by
comparing the difference in the amount of exposure 160 / ,/
necessary to give the same density on both films (from
relative exposure charts such as those described tV:1J Iv
below). For example, to obtain a density of 1.5 using 00> 140
Film Y, 0.6 more exposure is required than for Film X.
:-:I;;z
This log exposure difference corresponds to an 120
exposure factor of 3.99. To obtain the same density on
Film Y as on Film X, multiply the original exposure 100
by 3.99 to get the new exposure. Conversely, if going
from Film Y to Film X, divide the original exposure 80
by 3.99 to obtain the new exposure.
60
These procedures can be used to change densities 7 I 3 I 9 25 3 I 38
on a single film as well. Simply find the log E differ-
ence needed to obtain the new density on the film roJ ro.25J ro.5oJ ro. 75J r i.oo, r 1.25J r 1 .so,
curve; read the corresponding exposure factor from
the chart; then multiply to increase density or divide STEEL THICKNESS
to decrease density. millimeters
4. A change in processing conditions causes a change in (inches)
effective film speed. If the processing of the radio-
graphs differs from that used for the exposures from
which the chart was made, the correction factor must
be found by experiment.
FILM RADIOGRAPHY I 1 61
Such a chart will be particularly useful when uniform sec- the various factors of specimen thickness, source strength,
tions must be radiographed in large numbers by relatively and source-to-film distance can be set and from which expo-
untrained persons. This type of exposure chart may be sure time can be read directly.
derived from a chart similar to Fig. 30 by following the hor-
izontal line corresponding to the chosen milliampere- The Characteristic Curve
minute value and noting the thickness corresponding to this
exposure for each kilovoltage. These thicknesses are then The characteristic curve, sometimes referred to as the
plotted against kilovoltage. sensitometric curve or the Hand D curve (after Hurter and
Driffield, who first used it in 1890), expresses the relation
Gamma Ray Exposure Charts between the exposure applied to a photographic material
and the resulting photographic density. The characteristic
A typical gamma ray exposure chart is shown in Fig. 32. curves of three typical films, exposed between lead foil
It is somewhat similar to Fig. 30; however, with gamma rays, screens to X-rays, are given in Fig. 33. Such curves are
there is no variable factor corresponding to the kilovoltage. obtained by giving a film a series of known exposures, deter-
Therefore, a gamma ray exposure chart contains one line, or mining the densities produced by these exposures, and then
several parallel lines, each of which corresponds to a partic- plotting density against the logarithm of relative exposure.
ular film type, film density or source-to-film distance.
Gamma ray exposure guides are also available in the form of Relative exposure is used because there are no conve-
linear or circular slide rules. These contain scales on which nient units, suitable to all kilovoltages and scattering condi-
tions, in which to express radiographic exposures. The
FIGURE 32. Typical gamma-ray exposure chart FIGURE 33. Characteristic curves of three
for iridium-192,based upon the use of film X typicalX-ray films, exposed between lead foil
(see Fig. 33) screens
,------.----,------,------,10.0
501-,-----i------t-----t-----1 8.0
1-----+-----+----+-----1 6.0
.__ __ ___._ __,__ .....____ ___0.,1
fO)
25 50 75 100
fl) f2) (3) f4)
0 0.5 1.0 1.5 2.0 2.5 3.0
STEEL THICKNESS LOG RELATIVE EXPOSURE
millimeters
(inches)
162 I NONDESTRUCTIVE TESTING OVERVIEW
exposures given a film are expressed in terms of some par- TABLE 5. Equivalent exposure ratios
ticular exposure, giving a relative scale. In practical radiog-
raphy, this lack of units for X-ray intensity or quantity is no Relative Log Relative Interval in Log
hindrance, as will be seen below. Use of the logarithm of the Exposure Exposure Relative Exposure
relative exposure, rather than the relative exposure itself,
has a number of advantages. It compresses an otherwise }1 0.0 0.70
long scale. Furthermore, in radiography, ratios of exposures
or intensities are usually more significant than the exposures 5 0.70
or the intensities themselves. Pairs of exposures having the
same ratio will be separated by the same interval on the log }2 0.30 0.70
relative exposure scale, no matter what their absolute value
may be. Consider the pairs of exposures in Table 5. 10 l .00
As can be seen in Fig. 33, the slope (or steepness) of }30 1.48 0.70
the characteristic curves is continuously changing through-
out the length of the curves. For example, two slightly dif- 150 2. l 8
ferent thicknesses in the object radiographed transmit
slightly different exposures to the film. These two expo- where on the characteristic curve they fall; the steeper the
sures have a certain small log E interval between them; slope of the curve, the greater is this density difference.
that is, they have a certain ratio. The difference in the den- For example, the curve of Film Z (Fig. 33) is steepest in its
sities corresponding to the two exposures depends on just middle portion. This means that a certain log E interval in
the middle of the curve corresponds to a greater density
FIGURE 34. Characteristic curve of film Z (see FIGURE 35. Characteristic curves of two X-ray
Fig. 33) films exposed with lead foil screens
4.0 ----------------~ 4.0
3.5 3.5
3.0 3.0
2.5 2.5
~ ~
zvi zvi
w 2.0 w 2.0
0 0
1.5 1.5
1.0 1.0
0.5 0.5
0 0.5 1.0 1.5 2.0 2.5 3.0 0.5 1.0 1.5 2.0 2.5 3.0
LOG RELATIVE EXPOSURE
LOG RELATIVE EXPOSURE
LEGEND LEGEND
I. LOGEATD=2.0ISJ.62 1 LOGEATD=2.0FORFILMXIS 1.91
2 LOG EAT D = 0.8 IS 1.00
3. DIFFERENCE IN LOG E IS 0.62 2. LOG EAT D = 2.0 FOR FILM Z IS 1.62
3. DIFFERENCE IN LOGE IS 0.29
FILM RADIOGRAPHY I 163
difference than the same log E interval at either end of the Quantitative use of characteristic curves are worked out
curve. In other words, the film contrast is greatest where in Figs. 34 and 35. Note that Dis used for density and log E
the slope of the characteristic curve is greatest. For Film for logarithm of relative exposure.
Z, as has been pointed out, the region of greatest slope is
in the central part of the curve. For Films X and Y, how- In the first example (see Fig. 34), suppose a radiograph
ever, the slope - and hence the film contrast - continu- made of Film Z with an exposure of 12 m.A-min has a density
ously increases throughout the useful density range. The of 0.8 in the region of maximum interest. It is desired to
curves of most industrial X-ray films are similar to those of increase the density to 2.0 for the sake of the increased con-
Films X and Y. trast there available.
Use of the Characteristic Curve 1. Log E at D = 2.0 is 1.62.
2. Log E at D = 0.8 is 1.00.
The characteristic curve can be used in solving quantita- 3. Difference in log E is 0.62. Antilogarithm of this
tive problems arising in radiography, in the preparation of
technique charts and in radiographic research. Characteris- difference is 4.2.
tic curves made under actual radiographic conditions
should be used in solving practical problems. However, it is Therefore, the original exposure is multiplied by 4.2 giving
not always possible to produce characteristic curves in a 50 mA-min to produce a density of 2.0.
radiography department and curves prepared elsewhere
must be used. Such curves prove adequate for many pur- In the second example (see Fig. 35), film X has higher
poses although it must be remembered that the shape of contrast than Film Z at D = 2.0 and also a finer grain. Sup-
the characteristic curve and the speed of a film relative to pose that, for these reasons, it is desired to make the radio-
that of another depend strongly on developing conditions. graph on Film X with a density of 2.0 in the same region of
The accuracy attained when using readymade characteris- maximum interest.
tic curves is governed largely by the similarity between
developing conditions used in producing the characteristic 1. Log Eat D = 2.0 for Film Xis 1.91.
curves and those for the films whose densities are to be 2. Log E at D = 2.0 for Film Z is 1.62.
evaluated. 3. Difference in log E is 0.29. Antilogarithm of this
difference is 1.95.
Therefore, the exposure for D = 2.0 on Film Z is multiplied
by 1.95, giving 97.5 mA-min for a density of 2.0 on Film X.
164 I NONDESTRUCTIVETESTING OVERVIEW
PART 5
RADIOGRAPHIC IMAGE QUALITY AND
DETAIL VISIBILITY
Controlling Factors and definition (the abruptness and the smoothness of the
density transition). See Fig. 36.
The relationships of the various factors influencing
radiographic sensitivity are shown in Table 6. Because the Radiographic contrast is the difference in density
purpose of most radiographic testing is to examine speci- between two areas of a radiograph. It depends on both sub-
mens for inhomogeneity, a knowledge of the factors affect- ject contrast and film contrast.
ing the visibility of detail in the finished radiograph is
essential. Table 6 shows the relationships of the various fac- Subject contrast is the ratio of X-ray or gamma ray inten-
tors influencing image quality and radiographic sensitivity; sities transmitted by two selected portions of a specimen.
following are a. few important definitions. Subject contrast depends on the nature of the specimen, the
energy (spectral composition, hardness or wavelengths) of
Radiographic sensitivity is a general or qualitative term the radiation used, and the intensity and distribution of the
referring to the size of the smallest detail that can be seen in scattered radiation but.is independent of time, milliamper-
a radiograph. Phrased differently, it is a reference to the age or source strengtl1, distance and the characteristics or
amount of information in the radiograph. Note that radio- treatment of the tilm.
graphic sensitivity depends on the combined effects of two
independent sets of factors: radiographic contrast (the den- Film contrast refers to the slope (steepness)of the char-,
sity difference between a small detail and its surroundings) acteristic curve of the film. It depends on the type of film,
on the processing it receives and density. It also depends on
whether the film's exposure is direct, with lead screens or
with fluorescent screens. Film contrast is independent, for
FIGURE 36. Radiographic definition: (aJ advantageof higher contrastis offset by poor definition;
(bJ despite lower contrast better renditionof detail is obtained by improveddefinition
(aJ (bJ
FILM RADIOGRAPHY I 165
TABLE 6. Factors controlling radiographic sensitivity
RadiographicContrast RadiographicDefinition
Subject Contrast Film Contrast Geometrical Factors Graininess Factors
Affectedby Affectedby Affectedby Affectedby
Thickness differences Type of film Focal-spot size Type of film
in specimen Development time, temperature Focus-film distance Type of screen
Specimen-to-film distance Radiation quality
Radiation quality and agitation Abrupt thickness Development
Scattered radiation Density
Activity of the developer changes in specimen
Screen-film contact
most practical purposes, of the wavelengths and distribution TABLE 7. Exposure of steel part containing two
of the radiation reaching the film and hence is independent thicknesses
of subject contrast.
Thickness Exposure to
Definition refers to the sharpness of outline in the image.
It depends on the types of screens and film used, the radia- millimeters Give O = 1.5 Relative Ratio of
tion energy (wavelengths etc.) and the geometry of the
radiographic setup. kilovolts (inches) mA-min Intensity Intensities
Subject Contrast 160 20 (0.75) 18.5 3.8} 3.8
70.0 1.0
25 i I.OJ
200 20 (0.75) 4.9 14.3} 2.5
25 (I .OJ 1 l.O 5.8
Subject contrast decreases as the kilovoltage is Film Contrast
increased. The decreasing slope (steepness) of the lines of
the exposure chart (Fig. 30) as kilovoltage increases illus- The dependence of film contrast on density must be kept
trates the reduction of subject contrast as the radiation in mind when considering problems of radiographic sensi-
becomes more penetrating. For example, consider a steel tivity. In general, the contrast of radiographic films, except
part containing two thicknesses, 19 and 25 mm (0.75 and those designed for use with fluorescent screens, increases
continuously with density in the usable density range.
1 in.), which is radiographed first at 160 kV and then at Therefore, for films that exhibit this continuous increase in
contrast, the best density to use (or the upper limit of the
200 kV. density range) is the highest that can be conveniently
In Table 7, column 3 shows the exposure in milliampere- viewed with the illuminators available. Adjustable high
intensity illuminators are commercially available and greatly
minutes required to reach a density of 1.5 through each increase the maximum density that can be viewed.
thickness at each kilovoltage. These data are from the expo-
sure chart in Fig. 30. It is apparent that the milliampere- The use of high densities has the further advantage of
minutes required to produce a given density at any increasing the range of radiation intensities that can be use-
kilovoltage are inversely proportional to the corresponding fully recorded on a single film. InX-radiography, this in tum
X-ray intensities passingiithrough the different sections of permits the use of lower kilovoltage, resulting in increased
the specimen. Column 4 gives these relative intensities for subject contrast and radiographic sensitivity.
each kilovoltage. Column 5 gives the ratio of these intensi-
ties for each kilovoltage. The maximum contrast of fluorescent screen type films
is at a density of about 2.0. Therefore, other things being
Column 5 shows that, at 160 kV, the intensity of the equal, the greatest radiographic sensitivity will be obtained
X-rays passing through the 19 mm (0.75 in.) section is 3.8 when the exposure is adjusted to give this density.
times greater than that passing through the 25 mm (1 in.)
section. At 200 kV, the radiation through the thinner portion
is only 2.5 times that through the thicker. Thus, as the kilo-
voltage increases, the ratio of X-ray transmission of the two
thicknesses decreases, indicating a lower subject contrast.
166 I NONDESTRUCTIVETESTING OVERVIEW
Film Graininess and Screen Mottle relation to the thickness of the part being tested. The image
of the penetrameter on the radiograph is permanent evi-
The image on an X-ray film is formed by countless dence that the radiographic examination was conducted
minute silver grains, the individual particles being so small under proper conditions.
that they are visible only under a microscope. However,
these small particles are grouped together in relatively large Codes or agreements between customer and vendor may
masses visible to the naked eye or with a magnification of specify the type of penetrameter, its dimensions and h?~ it
only a few diameters. These masses result in a visual impres- is to be employed. Even if penetrameters are not specified,
sion called graininess. their use is advisable because they provide an effective
check on the quality of the radiographic inspection and evi-
All films exhibit graininess to a greater or lesser degree. dence that radiographic sensitivity is achieved.
In general, the slower films have lower graininess. Thus,
Film Y (Fig. 33) would have a lower graininess than Film X. Hole and Wire Type Penetrameters
The graininess of all films increases as the penetration of The most common penetrameter consists of a plaque, a
the radiation increases, although the rate of increase may be small rectangular piece of metal containing several (usually
different for different films. The graininess of the images three) holes, the diameter of which are related to the thick-
produced at high kilovoltages makes the slow, inherently ness of the penetrameter (Fig. 37).
fine grain films especially useful in the megavolt and multi-
megavolt range. When sufficient exposure can be given, fine The ASTM (American Society for Testing and Materials)
grain films are also useful with gamma rays. plaque type penetrameter3 contains three holes of diameters
T, 2T, and 4T, where Tis the thickness of the penetrameter.
Lead screens have no significant effect on film graininess. Because of the practical difficulties in drilling minute holes
However, graininess is affected by processing conditions, in thin materials, the minimum diameters of. these three
being directly related to the degree of development. For holes are 0.25, 0.50 and 1.00 mm (0.01, 0.02, and 0.04 in.),
instance, if development time is increased for the purpose of respectively. Thick penetrameters of the hole type would be
increasing film speed, graininess of the resulting image is like- very large because of the diameter of the 4T hole. Therefore,
wise increased. Conversely, a developer or developing tech- penetrameters more than 0.46 mm (0.180 in.) thick are in
nique that results in an appreciable decrease in graininess_will the form of disks, the diameters of which are four times the
also cause an appreciable loss in film speed. However, adjust- thickness (4T) and which contain only two holes, of diame-
ments in development technique made to compensate for ters T and 2T. Each penetrameter is identified by a lead
changes in temperature or activityof a developer will have lit- number showing the thickness in thousandths of an inch.
tle effect on graininess. Such adjustments are made to
achieve the same degree of development as would be The ASTM plaque penetrameter permits the specifica-
obtained in fresh developer at a standard processing temper- tion of a number of levels of radiographic sensitivity,
ature, so graininess of the film will be essentially unaffected. depending on the requirements of the job. For example, the
specifications may call for a radiographic sensitivity level of
Another source of irregular density in uniformly exposed 22T. The first symbol (2) indicates that the penetrameter
areas is the screen mottle encountered in radiography with shall be 2 percent of the thickness of the specimen; the sec-
fluorescent screens. Screen mottle increases markedly as ond symbol (2T) indicates that the hole having a diameter
hardness of the radiation increases. This mottle limits use of twice the penetrameter thickness shall be visible on the fin-
fluorescent screens at high voltage and with gamma rays. ished radiograph. The quality level 2-2T is the one most
commonly specified for routine radiography. However, crit-
Penetrameters ical components may require more rigid standards and
require a level of l-2T or 1-1T. On the other hand, the
A standard test piece is usually included in every radio- radiography of less critical specimens may be satisfactory if a
graph as a check on the adequacy of the radiographic tech- quality level of 2-4T or 4-4T is achieved. The more critical
nique and to show that required radiographic sensitivity has the radiographic examination - that is, the higher the level
been achieved. The test piece is commonly referred to as a of radiographic sensitivity required - the lower the numer-
penetrameter or an image quality indicator (IQI). The pen- ical designation for the quality level.
etrameter is a simple geometric form made of the same
material as, or a material similar to, the specimen being Another ASTM penetrameter design used by ASME is
radiographed. It contains some small structures (holes, the wire type that consists of sets of wires arranged in ord~r
wires etc.), the dimensions of which bear some numerical of increasing diameter. The penetrameter shall be fabn-
cated in accordance with the ASTM design (Fig. 38).
All sections of the ASME Boiler and Pressure Vessel
Code require a penetrameter identical to the ASTM plaque
or wire type penetrameter.
FILM RADIOGRAPHY I 167
FIGURE37. American Societyfor Testingand Materials (ASTMJpenetrameter according to ASTM
StandardE 1025: (aJ design for penetrameter type numbers 5 to 20, with tolerances of z0.0005;
(bJ design for penetrameter type numbers 21 to 59 with tolerances of z0.0025 in. and for
penetrameter type numbers 60 to 179, with tolerance of z0.005 in.; (cJ design for penetrameter type
=numbers over 180, with tolerances of zO.O10 in. (exceptfor relative thickness T, all measurementsare
in inches; 1.00 in. 25.4 mm)
(a) 4 T DIAMETER
T DIAMETER
2 T DIAMETER
(bJ IDENTIFICATION NUMBER 4 T DIAMETER
T DIAMETER
T2 T DIAMETER
1 in.
--+---+-----+---~1--l 1-3/8 in.
3/8 in. T
3/4 in.
~----- I 3/8 in. ---~
Ii-4---------2 1/4 in. -------~
t(cJ
1.33 T
0.83 T
T I-
168 I NONDESTRUCTIVE TESTING OVERVIEW
Equivalent Penetrameter Sensitivity grouping materials with similar radiographic absorptions.3•4
In addition, a penetrameter made of a particular material
Ideally, the penetrameter should be made of the same may be used in the radiography of materials having greater
material as the specimen. However, this is sometimes radiographic absorption. In such a case, there is a certain
impossible because of practical or economic difficulties. In penalty on the radiographic testers because they are setting
such cases, the penetrameter may be made of a radiograph- more rigid radiographic quality standards for themselves
ically similar material - that is, material having the same than those which are actually required. This penalty is often
radiographic absorption as the specimen but which is better outweighed by avoiding the problems of obtaining pene-
suited to the making of penetrameters. Tables of radio- trameters for an unusual material.
graphically equivalent materials have been published,
In some cases the materials involved do not appear in
FIGURE38. Examplesof wire type published tabulations. Under these circumstances the com-
penetrameters:(a) ASTMStandardE 747 (set B, parative radiographic absorption of two materials may be
Alternate 2); (b) DeutschelndustrieNorm determined experimentally. A block of the material under
541 09, German standardpenetrameter test and a block of the material proposed for penetrameters,
equal in thickness to the part being examined, can be radio-
(a) 6.35 mm (0.25 in.] MINIMUM graphed side by side on the same film with the technique to
LEAD LETTERSAND NUMBERS be used in practice. If the film density under the proposed
penetrameter material is equal to or greater than the film
LARGEST density under the specimen material, that proposed mate-
rial is suitable for fabrication of penetrameters.
WIRE NUMBER
In practically all cases, the penetrameter is placed on the
TM source side of the specimen, in the least advantageous geo-
03 metric position. In some instances, however, this location
MATERIAL ENCAPSULATED for the penetrameter is not feasible and a film side pene-
GRADE BETWEENCLEAR VINYL trameter must be used. Some codes. such as ASME, specify
PLASTIC OF 1.52 mm the film side penetrameter that is equivalent to the source
NUMBER (0.06 in.] MAXIMUM side penetrameter normally required and require that it be
identified as such. When such a specification is not made,
SET THICKNESS the required film side penetrameter may be found experi-
IDENTIFICATION mentally. In the example above, a short section of tube of
-1 the same dimensions and materials as the item under test
NUMBER would be used in the experiment. The required penetrame-
ter would be placed on the source side and a range of pene-
LENGTH MINIMUM 25.4 mm trameters on the film side. If the source side penetrameter
indicated that the required radiographic sensitivity was
(1.0 in.] FOR SETS AAND B being achieved, the image of the smallest visible hole or
__ _l wire size in the film side penetrameters would be used to
determine the penetrameter and the hole or wire size to be
6 WIRES 5.08 mm (0.200 in.] used on the production radiographs.
EQUALLY (MINIMUM DISTANCE BETWEENAXIS OF
WIRES IS NOT LESS THAN 3 TIMES WIRE Sometimes the shape of the part being examined pre-
SPACED DIAMETERAND NOT MORE THAN cludes placing the penetrameter on the part. When this
5.08 mm [0.200 in.]) occurs, the penetrameter may be placed on a block of radio-
(bJ graphically similar material of the same thickness as the
D•HED specimen. The block and the penetrameter should be
placed as close as possible to the specimen.
53
Other Wire Penetrameters
A number of wire penetrameter designs are also in use.
ASTM E 747 penetrameter4 and the German DIN (Deutsche
Industrie-Norm) penetrameter5 (Fig. 38) are widely used.
It consists of a number of wires of various diameters sealed
in a plastic envelope that carries the necessary identifica-
tion symbols. The image quality is indicated by the thinnest
FILM RADIOGRAPHY I 169
wire visible on the radiograph. The system is such that only film across the thickness of the crack, its image on the film
may not be visible because of the very gradual transition in
three penetrameters, each containing seven wires, can cover photographic density. Thus, a penetrameter is used to indi-
a very wide range of specimen thicknesses. Sets of DIN cate the quality of the radiographic technique and sensitivity
penetrameters are available in aluminum, copper and steel. and not to measure the size of cavitythat can be shown.
Thus a total of nine penetrameters is sufficient for the radi-
ography of a wide range of materials and thicknesses. In the case of a wire image quality indicator, the visibility
of a wire of a certain diameter does not ensure that a dis-
Comparison of Penetrameter Design continuity of the same cross section will be visible. The
human eye perceives much more readily a long boundary
The hole type penetrameter is, in a sense, a go/nogo gage; than it does a short one, even if the density difference and
that is, it indicates whether or not a specified quality level has the sharpness of the image are the same. However, the
been attained but, in most cases, does not indicate whether equivalency between the hole and wire ASTM penetrame-
ters was developed on the basis of empirical data as well as
requirements have been exceeded or by how much. The wire theoretical numbers.
penetrameter on the other hand is a series of penetrameters Viewing and Interpreting
in a single unit. Similarly,the ASTM wire type is a series of six Radiographs
penetrameters in a single unit. As such, they have the advan- The examination of the finished radiograph should be
made under conditions that favor the best visibility of detail
tage that the radiographic quality level achieved can often be combined with a maximum of comfort and a minimum of
read directly from the processed radiograph. · eye fatigue for the observer. To be satisfactory for use in
viewing radiographs, an illuminator must fulfill two basic
The hole penetrameter can be made of any desired requirements. First, it must provide light having an intensity
that will illuminate the areas of interest in the radiograph to
material but the wire penetrameter is made from only a few their best advantage, free from glare. Second, it must dif-
fuse the light evenly over the entire viewing area. The color
materials. A quality level of 2-2T may be specified for the of the light is of no optical consequence but most observers
prefer bluish white. An illuminator incorporating several
radiography of, for example, commercially pure aluminum fluorescent tubes meets this requirement and is often used
and 2024 aluminum alloy, even though these have apprecia- for viewing industrial radiographs of moderate density.
bly different compositions and radiation absorptions. The For routine viewing of high densities, one of the com-
hole penetrameter would, in each case, be made of the mercial high intensity illuminators should be used. These
provide an adjustable light source, with a maximum inten-
appropriate material. To achieve the same quality of radio- sity which allows viewing of densities of 4.0 or even higher.
graphic inspection for equal thicknesses of these two mate-
Such a high intensity illuminator is especially useful for
rials, it would be necessary to specify different wire examination of radiographs having a wide range of densities
diameters - that for 2024 alloy would probably have to be corresponding to a wide range of thicknesses in the object. If
the exposure was adequate for the greatest thickness in the
determined by experiment. specimen, the detail reproduced in other thicknesses can be
visualizedwith illumination of sufficient intensity.
Special Penetrameters
Contrast sensitivity of the human eye (the ability to dis-
Special penetrameters have been designed for certain tinguish small brightness differences) is greatest when the
classes of radiographic inspection. An example is the radiog- surroundings have about the same brightness as the area of
raphy of small electronic components in which some of the interest. Thus, to see the finest detail in a radiograph, the
significant factors are the continuity of fine wires or the illuminator must be masked to avoid glare from bright light
presence of tiny balls of solder. Special image quality indica- at the edges of the radiograph or transmitted by areas of low
tors have been designed consisting of fine wires and small density and the inspector's eyes must have adapted to the
metallic spheres within a plastic block. The block is covered dark. Subdued lighting, rather than total darkness, is prefer-
on top and bottom with steel approximately as thick as the able in the viewing room. Room illumination must be such
case of the electronic component. that there are no troublesome reflections from the surface
of the film under examination.
Penetrameters and \(isibility of Discontinuities
It should be remembered that even if a certain hole or
wire in a penetrameter is visible on the radiograph, an equiv-
alent cavity may not be visible. Penetrameter holes or wires,
having sharp boundaries, result in abrupt, though small,
changes in metal thickness whereas a natural cavity having
more or less rounded sides causes a gradual change. There-
fore, the image of the penetrameter hole or wire is sharper
and more easily seen in the radiograph than is the image of
the cavity. Similarly, a fine crack may be of considerable
extent, but if the X-rays or gamma rays pass from source to
1 70 I NONDESTRUCTIVETESTING OVERVIEW
PART 6
FILM HANDLING AND STORAGE
Radiographic film should always be handled carefully to means of identifying radiographs. They may also be used as
avoid physical strains, such as pressure, creasing, buckling, reference marks to determine the location of discontinuities
friction, etc. The normal pressure applied in a cassette to within the specimen. Such markers can be conveniently fas-
provide good contacts is not enough to damage the film. tened to the film holder or object with adhesive tape. A code
However, when films are loaded in semiflexible holders and can be devised to minimize the amount of lettering needed.
external clamping devices are used, care should be taken to Lead letters are commercially available in a variety of sizes
ensure that this pressure is uniform. If a film holder bears and styles. The thickness of the chosen letters should be
against a few high spots, such as those that occur on an great enough so that their image is clearly visible on expo-
unground weld, the pressure may be great enough to pro- sures with the most penetrating radiation routinely used.
duce desensitized areas in the radiograph. Precaution is par- Under some circumstances it may be necessary to put the
ticularly important when using envelope packed films. lead letters on a radiation absorbing block so that their
image will not be burned out. The block should be consider-
Crimp marks or marks resulting from contact with fin- ably larger than the legend itself.
gers that are moist or contaminated with processing chemi-
cals can be avoided if large films are grasped by the edges Shipping of Unprocessed Films
and allowed to hang free. A convenient supply of clean tow-
els is an incentive to dry the hands often and well. Use of If unprocessed film is to be shipped, the package should
envelope packed films avoids these problems until the enve- be carefully and conspicuously labeled, indicating the con-
lope is opened for processing. Thereafter, of course, the tents, so that the package may be segregated from any
usual care must be taken. radioactive materials. It should further be noted that cus-
toms inspection of shipments crossing international bound-
Another important precaution is to avoid drawing film aries sometimes includes fluoroscopic inspection. To avoid
rapidly from cartons, exposure holders or cassettes. Such damage from this cause, packages, personnel baggage, and
care will materially help to eliminate objectionable circular the like containing unprocessed film should be plainly
or treelike black markings in the radiograph, the results of marked and the attention of inspectors drawn to their sensi-
static electric discharges. tive contents.
The interleaving paper should be removed before the film Storage of Unprocessed Film
is loaded between either lead or fluorescent screens. When
using exposure holders in direct exposure techniques, how- X-Ray Film Storage
ever, the paper should be left on the film for the added pro-
tection that it provides. At high voltage, direct exposure With X-rays generated to 200 kV, it is feasible to use film
techniques are subject to the problems mentioned earlier: storage compartments lined with a sufficient thickness of
electrons emitted by the lead backing of the cassette or expo- lead to protect the film. At higher kilovoltages, protection
sure holder may reach the film through the intervening paper becomes increasingly difficult; film should be protected not
or felt and record an image of this material on the film. This only by the radiation barrier for protection of personnel but
effect is avoided by the use of lead or fluorescent screens. In also by increased distance from the source.
the radiography of light metals, direct exposure techniques
are the rule and the paper folder should be left on the inter- At 100 kV, a 3 mm (0.125 in.) thickness of lead should
leaved film when loading it in the exposure holder. normally be adequate to protect film stored in a room adja-
cent to the radiation room if the film is not in the line of the
Ends of a length of roll film factory packed in a paper direct beam. At 200 kV, the lead thickness should be
sleeve should be sealed in the darkroom with black pressure increased to 6.4 mm (0.25 in.).
sensitive tape. Tape should extend beyond edges of the strip 6
to 13 mm (0.25 to 0.5 in.) to provide a positive light tight seal. With megavolt X-rays, films should be stored beyond the
concrete or other protective wall at a distance at least five
IdentifyingRadiographs
Because of their high absorption, lead numbers or letters
affixed to the film holder or test object furnish a simple
FILM RADIOGRAPHY I 1 71
times farther from the X-ray tube than the area occupied by 2 mg-cm-2 on each side of coarse grain radiographs. For
personnel. The storage period should not exceed the times short term storage requirements, the residual thiosulfate
recommended by the manufacturer. content can be at a higher level, but this level is not speci-
fied by ANSI.
Medical X-ray films should be stored at about 12 times
the distance of the personnel from the megavolt X-ray tube, Washing of the film after development and fixing, there-
for a total storage period not exceeding two weeks. fore, is most important. The methylene blue test and silver
densitometric test are laboratory procedures performed on
Storage near Gamma Rays clear areas of the processed film.
When radioactive material is not in use, the lead container Storage Suggestions
in which it is stored helps provide protection for film. In many
cases, however, the container for a gamma ray source will not Regardless of the length of time a radiograph is to be
provide satisfactoryprotection to stored radiographic film. In stored, these suggestions should be followed to provide for
such cases, the emitter and stored film should be separated maximum stability of the radiographic image.
by a sufficient distance to prevent fogging.
1. Avoid storage in the presence of chemical fumes.
Storage of Exposed and Processed 2. Avoidshort term cyclingof temperature and humidity.
Film 3. Place each radiograph in its own folder to prevent
Archival Keeping Quality possible chemical contamination by the glue used in
making the storage envelope (negative preserver).
Archival storage is a term commonly used to describe the Several radiographs may be stored in a single storage
keeping quality of radiographs. It is defined by the American envelope if each is in its own interleaving folder.
National Standards Institute (ANSI) as "those storage condi- 4. Never store unprotected radiographs in bright light
tions suitable for the preservation of photographic film hav- or sunlight.
ing permanent value." ANSI does not define archival storage 5. Avoid pressure damage caused by stacking a large
in years but in terms of the thiosulfate content (residual number of radiographs in a single pile or by forcing
fixer) permissible for storage of radiographs. more radiographs than can comfortably fit into a sin-
gle file drawer or shelf.
Although many factors affect the storage life of radio-
graphs, one of the most important is the residual thiosulfate Radiographic film offers a means of precise discontinuity
left in the radiograph after processing and drying. Deter- detection and documentation. Despite the introduction of
mined by the methylene blue test, the maximum level is digital means of image capture, display and storage, film
radiography will continue to be an important part of nonde-
structive testing well into the twenty-first century.
17 2 I NONDESTRUCTIVETESTING OVERVIEW
REFERENCES
1. Quinn, R.A. and C.C. Sigl, eds. Radiography in Philadephia, PA: American Society for Testing and
Modem Industry, fourth edition. Rochester, NY: Materials.
Eastman Kodak Company (1980).
4. E 747, Standard Practice for Design, Manufacture
2. Nondestructive Testing Handbook, second edition: and Material Grouping Classification of Wire
Vol. 3, Radiography and Radiation Testing. Colum- Image Quality Indicators (IQI) Usedfor Radiology.
bus, OH: American Society for Nondestructive Test- Philadephia, PA: American Society for Testing and
ing (1985). Materials.
3. E 1025, Standard Practicefor Design, Manufacture, 5. DIN 54109. Berlin, Germany: German Institute for
and Material Grouping Classification of HoleType Standardization.
Image Quality Indicators (IQI) Usedfor Radiology.
6SECTION
RADIOSCOPY AND TOMOGRAPHY
Richard H. Bossi, Boeing Defense and Space Group, Seattle, Washington
174 I NONDESTRUCTIVETESTING OVERVIEW
PART 1
FUNDAMENTALS OF RADIOSCOPY
Principles golf balls, cables, candy bars, shoes, light alloy castings and
packages.1·2
Radioscopy, or real time radiography, is a nondestructive
testing method that uses penetrating radiation to produce Modem radioscopy systems operate on the same princi-
images viewed concurrent with the irradiation. In the case ples as earlier systems but with considerable improvement
of dynamic systems, radioscopy allows radiographic inter- in the radiation sources, object handling, image detection
pretation to be performed simultaneously with the progress performance and personnel radiation protection.
of the event. Arrangement of the radiation source, object
and image plane is similar to conventional radiography. Advances in the electronics industry have been primarily
responsible for the improvements in radioscopy. Image
The most important process in radioscopy is the conver- amplifiers and television systems were first introduced
sion of radiation to light by means of a fluorescent screen. around 1950. Visual acuity improvements, no dark adapta-
The light signal may then be observed directly, amplified tion time and image contrast improvements are all impor-
and/or converted to a video signal for presentation on a tele- tant technical advances that image amplifiers have provided.
vision monitor and subsequent recording. Television systems have made radioscopy safer by allowing
the inspector to be completely removed from the object and
Fluoroscopy is the original term used to describe the the radiation field. The digital electronic advances over the
direct viewing of fluorescent screens. Real time radiography last few decades have increased the possibilities of image
and radioscopy are the terms now used to describe indus- enhancement and information storage. Remote operations
trial imaging systems. with sophisticated image enhancement capability are typical
of modem systems.
Radioscopy is often applied to objects on assembly lines
for rapid inspection. Remote adjustment of the object posi- FIGURE 1. European customs fluoroscope
tion allows inspectors the freedom to review details of inter- inspection (1897)
est or to move on to other locations. Accept-or-reject
decisions may be made immediately without the delay or
expense of film development.
For dynamic events, radioscopy is typically used in the
range of several seconds to several minutes, and longer
events, on the order of hours, also can be monitored. Events
may be imaged in real time and replayed at slower rates;
television frame rates of 30 frames per second can cover
events in the fraction of a second range. Even flash radio-
graphic events can be imaged using frame grabbing tech-
niques.
Background
Radioscopy has its roots in the discovery of X-rays;
Roentgen, for example, used phosphor screens for X-ray
detection. Figure 1 shows inspection of baggage at the Brus-
sels railroad station in 1897.
Barium platinocyanide, willemite, calcium tungstate,
cadmium zinc sulfide, and cesium iodide are a few of the
phosphors developed for fluoroscopy systems used over the
years in medicine and industry. Typical industrial applica-
tions in the first half of this century included examination of
RADIOSCOPYAND TOMOGRAPHY I 175
Basic Technique by the inherent resolution of the detector. Resolutions from
4 to 10 line pairs per millimeter are possible, depending on
The basic technique of radioscopic systems is shown in system components. Scatter on the detector often limits
Fig. 2. The radiation source, object and detector are ori- detail sensitivity and is controlled by collimators and shut-
ented in the same fashion as in conventional film radiogra- ters. Figure 3 shows the geometry of such a configuration.
phy. The X-ray geometry of source-to-object distance
(SOD) and source-to-image distance (SID) are important The second method for radioscopy is to use a very small
variables in radioscopy. The object is placed on a manipula- spot, or microfocus, X-ray source and very high magnifica-
tor, allowing up to five axes of motion in order to view the tion (2x to 20x) to achieve detail sensitivity. Figure 2 shows
object from any number of orientations. Positioning of this configuration. The optimum magnification is given by:
object relative to the source and detector can optimize mag-
nification for greatest detail sensitivity. A cabinet arrange- ( SID)Mopt (Eq. l)
ment containing the entire system is typical of most designs. SOD opt
The advantages of radioscopic systems are low operating 1 + 1u2.1
cost, simplicity and speed of inspection. Compared to film
radiography systems, radioscopic units have a higher capital d
investment but much greater throughput. Radioscopy also
provides the answer to the inspection question immediately. Where:
The ability to manipulate an object remotely provides a
comprehensive inspection and an optimization of X-ray inherent unsharpness of detector's fluorescent
viewing orientation. screen layer; and
focal spot size.
Recommended Practice
The observable discontinuity size s is given by:3
Good technique in radioscopy may proceed along two
lines. One method uses conventional focal spot tubes, high =s uf (Eq. 2)
current and high resolution imaging. The other method uses
a small focal spot and large magnification. The geometry of --2-
both these methods can be optimized for the detail required
in the object of interest. Af3opt
In the first method the detail sensitivity is achieved by Large magnification can provide sensitivity to very small
high image contrast using the lower kilovoltage but high mil- details. With high magnification, the object is located far
liamperage output of a conventional X-ray source. Magnifi- from the detector screen so scattered radiation is signifi-
cation is usually less than 1.5 x and image resolution is driven cantly reduced at the detector.
FIGURE 2. Radioscopicsystem The microfocus source operates at low current, so high
kilovoltage and high gain intensification are used with this
configuration. At high magnification the inspected field size
can be very small, requiring careful manipulation and obser-
vation to inspect an entire object. However, sensitivities of
better than 1 percent contrast with resolution better than 20
line pairs per millimeter are possible using microfocus
radioscopy.
CHARGE SOURCE-TO-l/'v1AGE Image Intensifiers
COUPLED· DISTANCE
DEVICE X-Ray Image Intensifiers
CAMERA ------1
The image intensifier tube converts photons to electrons,
SOURCE-TO-OBJECT accelerates the electrons and then reconverts them to light.
DISTANCE Figure 4 shows a generalized diagram of an intensifier tube.
Intensifiers typically operate in the range of 30 to 10,000
CABINET light amplification factors. The intensification is not neces-
SHIELD sarily solely electronic but may also include a reduction in
image area (electrons from a large area input screen are
COMPUTER/l/'v1AGE focused on a small area output screen).
PROCESSOR
STAGE
176 I NONDESTRUCTIVETESTING OVERVIEW
FIGURE3. Remote radioscopicviewing system FLUORESCENTSCREEN
X-RAY SOURCE
~
~ -, SHIELDING
COLLIMATION
/ X-RAY CELL WALL
X-RAY D
CONTROL
MONITOR
VIDEOTAPE
FIGURE4. X-ray image intensifiertube design The earliest type of image intensifiers for X-ray applica-
tions used a zinc-cadmium sulfide (ZnCdS) layer inside the
X-RAY FOCUSING glass envelope to convert the X rays to light. The photocath-
WINDOW ELECTRODES ode adjacent to the fluorescent layer converted the light to
electrons. The original X-ray tube operated at 25 kV poten-
. OUTPUT tial between the photocathode and output phosphor. Even
PHOSPHOR though only 10 percent of the light photons from the fluo-
rescent screen would generate electrons at the photocath-
INPUT INTENSIFIED ode and only 10 percent of the accelerated electrons would
X-RAYS LIGHT produce light at the output phosphor, a 10 to 15 times
OUTPUT increase in luminous fluxwas generated by the acceleration.
The tube had a curved input screen with a 130 mm (5.0 in.)
PHOTOCATHODE diameter and an output screen with a 15 mm (0.6 in.) diam-
LAYER eter. The nine-times reduction in diameter from the fluores-
cent screen to the viewing screen provided an additional
factor of 80 in brightness gain. The total gain was between
800 and 1,200.4
Technical improvements in electronic gain, fluorescent
and photocathode layer efficiencies and electron optics have
made X-ray image intensifiers very useful in both medical
and industrial applications.v" Csl(Na) is now commonly
used as the fluorescent layer because it has twice the X-ray
RADIOSCOPY AND TOMOGRAPHY I 177
absorption of ZnCdS and the crystalline structure mnu- the channel. The gain of the channel multiplier depends on
the applied voltage and the ratio of length to diameter.
mizes lateral light diffusion. Rare earth phosphors such as
Gd202S are also found to be superior to ZnCdS. It appears With 1,000 Vanda length to diameter ratio of 50, a typi-
that, at X-ray energies below 100 kV, Csl(Na) is very good; cal gain is about 104• The resolution of the device is limited
at higher kilovoltages, the rare earths may be useful. 9 by the size of the channel spacing.11·12
Tubes are available with 100 to 400 mm (4 to 16 in.) The microchannel plate is often used in conjunction with
input diameters, multiple modes (which electronically the electrostatic image intensifier tubes to form what is
select variable field size of the input), and fiber optic output called a second generation image converter. The microchan-
for direct camera coupling. A typical 230 mm (9 in.) tube nel array follows a first stage electrostatic lens.
performs with resolution better than 4 line pairs per mil-
limeter and gains on the order of 10,000. Resolution is at a For radiography, the light from a fluorescent screen is
maximum at the center of these intensifiers and decreases input to the photocathode of the intensifier using a lens sys-
somewhat at the edges. tem. In some cases direct contact is made between the pho-
tocathode and fluorescent screen. With a proper lens
The advantages of these tubes are the relatively low cost, system, the image converter can be used with any size fluo-
generally compact size and high resolution/contrast. A disad- rescent screen. For a large input screen, the loss of intensity
vantage is that minification will increase image unsharpness. experienced in focusing to the smaller diameter intensifier
Also, because a length to diameter ratio of 1.0 to 1.5 is may be limiting, depending on the radiographic parameters.
required for the electron optics, large diameter inputs The term image converter is used because the light input
require large tubes. This not only increases bulk but creates and light output may not necessarily be at the same wave-
a potential implosion hazard. The curved input screens in length. Gain characteristics are approximate values in this
these tubes cause distortion. The tubes are sensitive to volt- case because the input and output spectra are different.
age drifts, stray mar.:etic fields, and space charge defocusing
at high dose levels. Electron scattering, thermonic emission Spectral Matching
and light reflection on interior surfaces are causes for loss of
contrast from intensifiers. Fabrication techniques in the lat- Image intensifiers rely on a photocathode to convert
est generation tubes minimize these problems. input light radiation to electrons. The X-ray image intensi-
fiers have a fluorescent screen ahead of the photocathode to
Tubes with 360 mm (14 in.) input have been manufac- convert X-rays to light. The spectral response and sensitivity
tured.l? The advanced vacuum tube technology requires a vary among photocathode materials (see Fig. 5 for the spec-
metal tube body. A titanium membrane is used for the tral response of several typical photocathodes). Desirable
entrance window to withstand atmospheric pressure and characteristics include high efficiency at the wavelength of
maintain transparency to X-rays. The titanium produces less light being observed, and a low dark current (the signal level
scatter than a glass window which improves contrast. An accel- when no light is falling on the photocathode). The light emit-
eration voltage of 35 kV is used. limiting resolution in the ted from the intensifier is generated by the action of elec-
large format is specified at 3.6 line pairs per millimeter. Tubes trons on a phosphor. The spectral emission characteristics of
as large as 400 mm (16 in.) diameter have also been marketed. some common phosphors are shown in Fig. 6.
Image Converters and Light Amplifiers The spectral performance of phosphors and photocath-
odes is important for radioscopic systems. The photocath-
Another type of image intensifier, often called the image ode of the intensifier must be capable of functioning over
converter, uses a light input and light output system. The the range of light emitted by the fluorescent screen. Good
electrons from the photocathode are accelerated by high volt- matching of these spectra can make a significant improve-
age before striking a phosphor screen. Luminous gains of 50 ment in system performance. Selection of the X-ray fluo-
to 100 times are typical. Coupling of the tubes in stages can rescent screen must be based on knowledge of the
result in gains of 1()5 to 106. Typicalinput diameters for com- photocathode response of the light pickup system. Alterna-
mercial tubes are 18, 25 and 40 mm (0.7, 1.0, and 1.6 in.), tively, the input phosphor of the light pickup should
respond well to the light produced by the selected fluores-
The channel electron multiplier or microchannel array is cent screen. A further consideration may be made to sup-
an assembly of small tubes for amplifying an electron signal press certain wavelengths, because some phosphors emit
using secondary emission. The channels are glass coated or promptly at one wavelength and decay with time at
ceramic coated with a high resistance material on the inside. another. An advantage might be gained by matching well to
A potential difference of 500 to 1,000 Vis applied across the the prompt light and poorly to the decaying signal. Gener-
channel. An electron entering the channel will strike a wall ally, good spectral matching exists between commercial
causing one or more secondary electrons to be released. intensifiers and television systems.
These will continue to strike the channel walls yielding more
electrons as they are accelerated by the electric field along
178 I NONDESTRUCTIVE TESTING OVERVIEW
FIGURE 5. Photocathoderesponse spectrum Statistics
~s The image intensifier system can improve imaging by
boosting the light output so that the statistical limitation in
j:: the image process is not at the eye, as in direct viewing fluo-
roscopy, but at the input fluorescent screen. The intensifier
vzi itself operates on a statistical process for the generation of
electrons and the regeneration of light. The sources of fluc-
UJ tuation are essentially independent.
Vl
Amplification may be used for improving detail sensitiv-
ity in an intensifier system. This is done by choosing that
amplification which makes the number of light quanta used
by the observer's eye equal to the number of radiation
quanta used by the input fluorescent screen.
0 200 400 600 800 1.00.0 1.200 Television Cameras, Image Tubes
z ~wcw,:: and Peripherals
~ c:w3o 0>:w:::-l 0wc,::
0> \'.) A wide variety of television cameras and image tubes are
available for use on radioscopic imaging systems. Many dif-
WAVELENGTH ferent camera configurations can be used to accommodate
inspection requirements. On one extreme is the small com-
(nanometers) pact camera with no user adjustments. At the other extreme
LEGEND is the larger, two-piece camera, with many controls for opti-
I. CS3Sb, DARK CURRENT I 0-4 TO I 0-13 A-cm-2 mizing image quality. Solid state cameras are widely used. A
variety of image tubes are also available. The most common
o-2. Na2KSb+Cs, DARK CURRENT Io- I 5 A-cm-2 types for radiographic applications are charge coupled
device (CCD) array, vidicon, silicon-intensifier target (SIT),
3. NaKSb+Cs3Sb. DARK CURRENT I 13 A-cm-2 image isocon and X-ray sensitive tubes.
4. Ag+csp, DARK CURRENT 10-10 to 10-13 Acrrr?
Solid State Cameras
FIGURE 6. Spectra of commonphosphors
Solid state cameras use an array of photodiodes or
ZnS:Ag charged coupled devices as the sensitive layer. These arrays
may be linear or area arrays of individually addressable ele-
Czi ments. The solid state cameras are much smaller and have
wider spectral response (see Fig. 7), reduced lag, higher
UJ quantum yields (50 percent), and (depending on the appli-
cation) may have equivalent resolution capabilities when
0u:: compared to vidicon cameras. Solid state cameras are
rugged and are not damaged by intense light images.
LL
UJ The photodiode arrays in solid state cameras are simple
__J photon detectors, typically reverse biased silicon photodi-
odes, which absorb incident photons and liberate current
b~ carriers. This gives rise to a current referred to as the pho
tocurrent signal, which is proportional to the arrival rate of
UJ the incident photon. The efficiency of the photodiode
CL strongly depends on its material and construction as well as
Vl the wavelength of the incident photons. The diodes consist
of p-type islands in an n-type substrate. Standard arrays are
~300400 :wc3o 50zwwco0,:: 600 700 available with 128 x 1,024 diodes with a center spacing as
0> 8
c,::
WAVELENGTH
(nanometers)
RADIOSCOPYAND TOMOGRAPHY I 1 79
small as 25 urn (0.001 in.). A dynamic range of 100:1 has 9 µm (3.5 x 10-4 in.). Dynamic range is typically 1,500:1.
Larger arrays of 1,024 x 1,024 and greater are available but
been reported. are significantly more expensive.
Charge coupled devices (CCDs) work in a manner ve:ry
The image on the solid state array is coupled to video cir-
similar to photodiodes.lv'" A photon, incident on the deple- cuitry by the horizontal and vertical scan generators, which
tion region of a charge coupled device, will create an elec- read the charge level at the detector elements. Figure 8
tron-hole pair if absorbed. This creates a current flow shows the schematic of a charge coupled device array. The
which, in a charge coupled device, is stored in the potential output of the video can be specified to fit a particular video
well of the device. The amount of charge collected at the format. The clock sequence, which sequentially reads the
potential well is in direct proportion to the amount of local charge level on each device, is started after a suitable image
light intensity. The charge coupled device is fabricated with integration time. This integration _time can be adjusted,
a combination of thin film technology and silicon technol- making solid state cameras useful for low light level applica-
ogy. Arrays are available at ve:ry reasonable cost with 760 tions, provided the detection element can retain the charge
rows of 520 points with a photoelement center spacing of over the integration period.
FIGURE7. Sensitivityof a photodiodearray The interesting feature of solid state cameras is that indi-
camera versusvidiconcamera vidual pixel elements may be addressed and the signals pro-
cessed digitally.With individually addressable elements, the
10.0 integration time and the video output format may be simply
specified. The cost of solid state cameras increases signifi-
SILICON PHOTODIODE ARRAY cantly with increasing array size and the electronic circuit
complexity required to scan large arrays. Each element
must be individually calibrated for uniform response
throughout the field. This feature can be used to correct
nonuniform fields in radiography. Because each element is
independent, blooming can be controlled; a bright element
does not spill over into a neighboring dark element.
Imaging in radioscopic applications may be accomplished
with optical focusing of the light from a scintillation screen
onto the solid state detector. In this case, the image format
FIGURE8. Schematicof chargedcoupled
device array
!SENSORARRAY
0.01 .___ ......_ __. _
I .0 I.I
0.4 0.5 0.6 0.7 0.8 0.9 VIDEO COUPLING CIRCUITS
HORIZONTAL SCAN GENERATOR
OPTICAL WAVELENGTH
(micrometer)
180 I NONDESTRUCTIVE TESTING OVERVIEW
and resolution are similar to conventional television cameras. the target. The SIT tubes and intensified SIT tubes (ISIT)
Also, scintillation materials may be deposited directly on the are used extensively for low light level application.
array. Each element becomes an independent radiation
detector and the resolution depends on the element spacing. The image isocon tube (Fig. 10) is widely used in radio-
graphic applications without intensifiers because of its high
Image Tubes sensitivity. The image on its photocathode forms a photo-
electron pattern that is focused by an axial magnetic field
The vidicon is a small, rugged and simple tube. An elec- onto a thin, moderately insulating target. The photoelec-
tron beam scans a light sensitive photoconductive target. A trons striking the target cause secondary emission electrons,
signal electrode of transparent material is coated onto the which are collected in a nearby mesh, leaving a net positive
front of the photoconductor. The scanning electron beam charge on the target. The beam from an electron gun scans
charges the target to the cathode potential. When light is the target, depositing electrons on the positively charged
focused on the photoconductor, the target conductivity areas. The scattered and reflected components in the return
increases, changing the charge to more positive values. The beam are separated. Only the scattered component enters
signal is read by the electron beam, which deposits electrons the electron multiplier surrounding the electron gun. This
on the positively charged areas, causing a capacitively cou- signal is amplified to become the video output.
pled signal at the signal electrode (see Fig. 9).
Camera System Characteristics
The vidicon has a number of variations, depending on
the selection of the photoconductive material. The standard Performance criteria for camera tubes are based on sen-
vidicon uses an antimony trisulfide layer. The plumbicon sitivity, dynamic 'range, resolution, dark current and lag. A
uses a lead oxide junction layer, the newvicon uses cadmium plot of signal output versus face plate illuminance for some
and zinc telluride and the silicon diode tube uses a silicon typical camera tubes is shown in Fig. 11 and the slope of
diode array target structure. these curves is called the tube gamma. The light source. for
illuminance is important in the response characteristics of
Another type of tube called the silicon intensifier target the tubes.
(SIT) uses a photocathode as an image sensor and focuses
the photoelectrons onto a silicon mosaic diode target. Table l · lists characteristics for television tubes used in
Readout is similar to the vidicon. The design allows for very radioscopic applications. Lag is given as the percentage of
high light gains in the pickup by accelerating the photoelec- the original signal present after 50 ms.
trons to high energies (perhaps 10 keV) before they strike
FIGURE 9. Vidicon television camera
SEMITRANSPARENT PHOTOCONDUCTOR
CONDUCTING O V IN THE DARK
COATING ON GlASS
(+20 V DC)
FINE MESH SCREEN
ELECTRON BEAM
CONNECTED TO SEMITRANSPARENT + ov +
CONDUCTIVE COATING ON GlASS 20 V 300 V
RADIOSCOPY AND TOMOGRAPHY I 181
TABLE 1. Typical characteristicsof televisioncamera tubes
Tube Type Dynamic Typical Dark Lag Gamma
Range Resolution Current (percentage)
Television Lines (nanoamperes) 1.0
7 0.65
Image lsocon 2,000 1,000 0 20 1.0
Sb2S3 vidicon 300 900 20 20 0.95
Newvicon 100 800 1.0
PbO vidicon 300 700 8 4
SIT 100 700 3 12
8
FIGURE 1 0. Image isocon televisioncamera
FIELD MESH
FOCUSING
COIL
PHOTOCATHODE SCANNING ELECTRON
PHOTOELECTRONS BEAM MULTIPLIER
In X-ray imaging applications, image isocon television FIGURE 11. Television camera output versus
tubes are commonly used because of their low light level light input
sensitivity and high dynamic range; unfortunately, isocons
are expensive. Charge coupled device (CCD) cameras and 104
vidicons often are used in combination with X-ray sensitive
image intensifier tubes; they both are simple as well as inex- ~f::-:, Vl 103
pensive. The newvicon is more sensitive than the plumbicon 102
or the Sb2S3 vidicon and can be used with image intensifiers. CL 101
SIT tubes are used with low light level systems when high f- QJ
dynamic range is not required. :::, Q_
Camera Matching OE
Standard scanning rate for camera tubes is 30 frames per z~<_J l\i
second. Each frame image is created in a 33 ms exposure 0
time. The frame is composed of two fields in which the elec-
tronically scanned 525 vertical lines are interlaced. The first Cv'Ji- c
field in l/60th of a second contains the odd numbered lines
and the second field contains the even numbered lines. oI 0 ....._........._......__._..........__._......_....._ .................._. ._ ............._.......,.....,_._ ...........
102
In some applications, where very low radiation intensi- I 0-5 10-4 I 0-3 I 0-2 IQ-I 101
ties are experienced, it is necessary to use a slower scanning
FACEPLATE ILLUMINANCE
(2,856 K tungsten source)
182 I NONDESTRUCTIVE TESTING OVERVIEW
rate. The target of the television camera can be made to Optical lenses provide good coupling, depending on thef
integrate the incoming signal for several minutes and then
scan it to provide one frame of information. Charge coupled number and transmission characteristics. The illuminance E
device cameras can be used in this way. on a pickup surface, coupled by a lens in a simple optical
system, is given by:
To avoid noise problems when slow scanning is needed,
CCD cameras are cooled to reduce inherent dark current E nBT (Eq. 3)
noise. Faster scanning rates on cameras may be used to
image rapid dynamic system. There must be sufficient light 4f2 (l+M)2
intensity present and the lag features of the cameras must
be acceptable. Where:
X-Ray Sensitive Vidicon B luminance of output phosphor;
T lens transmission;
Although the usual input to a television camera is a light M magnification; and
signal, for radiographic purposes it is possible to make the
camera sensitive directly to X-radiation. The vidicon camera f = relative aperture oflens (or f number).
in particular may be modified for X-ray sensitivity and has
been found useful for obtaining direct radioscopic images. The lower the f number, the more light collected for
Two alterations of the vidicon are needed for good results:
an X-ray window and an efficient target. Thin glass or beryl- imaging. Simple lenses are not often used in coupling
limn X-ray windows located close to the target replace the because of the low optical efficiency. For example, if a reduc-
heavy optical glass windows in conventional tubes. Although tion in image size by a factor of two is required from the cou-
the normal vidicon photoconductive targets will respond to pling, the phosphor lo lens distance must be twice the lens to
X-ray, they are so thin that absorption of radiation is mini- pickup surface distance. From the lens formula, the phos-
mal. Suitably thick targets must be used. Selenium has been phor to lens distance will be three times the focal length of
found very effective, having adequate response and low the lens, giving poor light collection efficiency. With a one-
lag.15 Lead oxide targets are common, having a high density to-one coupling, this distance is still twice the focal length.
for good X-ray absorption and resulting sensitivity.16_
Collimated optics are superior because the objective lens,
The X-ray sensitive vidicon is an imaging system for small focused at infinity, is located at its focal length from the phos-
objects and low kilovoltages, 150 kV or less. X-ray intensity phor. Collection efficiency is nine and four times greater,
must be high, in the range of 0.01 to 0.1 Oy-s' (50 to respectively, for the two magnifications discussed above
500 rad-min:"). The vidicon tube typically has a sensing area (two-to-one, one-to-one). The second lens will determine
of only 9.5 x 12.5 mm (0.37 x 0.5 in.). Presentation of the the image size at the pickup surface by ratio of its focal
image on a 480 mm (19 in.) television screen results in better length to focal length of the objective lens. Vignetting, which
than 30 times magnification. With a 525 line scan rate, reso- is a reduction in light intensity at the edges, does occur in
lution in the object is better than 0.02 mm (8 x 10-4 in.). The collimated optics. This is minimized when the lenses are
X-ray sensitive vidicon camera has a gamma on the order of close to each other. Some radioscopic imaging systems use
0. 7 to 1.0. Penetrameter sensitivities of 2 percent have been specialized optics such as Bouwers/Schmidt lenses, where
obtained. The cameras have experienced problems with
deterioration, possibly due to local overheating in the target concentric mirrors me employed to provide a low (0.65) f
layer, poor bonding to the heat sink layer, substrate irregu-
larities or incompatibility between beryllium and target nurnber.17
materials. Fiber optics may also be used for coupling. Intensifier
Optical Coupling tubes often have fiber optic input and output surfaces. These
may be coupled directly to other components with similar
Radioscopic imaging systems using fluorescent screens surfaces by using an optical gel. Fiber-optic light-guides can
for X-ray to light conversion, or systems with intensifier to be considered for m Jving light from one location to another
television camera chains, require coupling of the optical sig- (i.e., from the fluorescent or phosphor screen to the camera).
nals between components. The most common technique The potential advani:ages of this are greater retention of the
uses mirrors or lenses. light, one-to-one size: transfer, improved contrast by suppres-
sion of undesirable reflections and shortening of the system
Front faced silver mirrors must be used to avoid ghost dimensions. The use of fiber optics over considerable dis-
images caused by multiple reflections in back faced mirrors. tance or around unusual obstructions is also a possibility.
Fibers for fiber optic systems are glass or plastic with
diameters of 1 to 50 urn (4 x 10-·5 to 2 x 10-3 in.). The fiber
optic array operates on total internal reflection. To reduce
leakage each fiber is coated with a material having a lower
index of refraction. Losses in the fiber optic system are due
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ASNT NDT Handbook Volume 10 OVERVIEW
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ASNT NDT Handbook Volume 10 OVERVIEW
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