rt07

Timothy J. Kinsella, Carpenter Technology Corporation,Reading, Pennsylvania (Part 7)

Principles ofFilm Radiography1-3

Parts 1 to 6 from Radiography in Modern Industry. © 1980, Eastman Kodak Company.Reprinted with permission by the American Society for Nondestructive Testing.

7C H A P T E R

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140 Radio

PART 1. Film Exposure

{ {

FIGURE 1. Diagram of setup for making industrial radiographwith X-rays.

Anode

Focal spot

Diaphragm

Specimen

Front screen

Film

Low densityin radiograph

Back screen

High densityin radiograph

Sheet of lead

MOVIE.Conventionalradiographygives shadowimage.

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Making RadiographsRadiography is one of the oldest and mostwidely used of nondestructive testingtechniques. Despite its establishedposition, new developments areconstantly modifying the radiographictechniques applied by industrial andscientific users, thereby producingtechnical or economic advantages, orboth, over previous techniques. Thisprogressive trend continues with suchspecial equipment and techniques asmicrofocus X-ray generators, portablelinear accelerators, radioscopy, neutronradiography, imaging on paper, digitalimage analysis and image enhancement.

A radiograph is a photographic recordproduced by the passage of penetratingradiation through an object onto a film(Fig. 1). When film is exposed to X-rays,gamma rays or light, an invisible changecalled a latent image is produced in thefilm emulsion. The areas so exposedbecome dark when the film is immersedin a developing solution, the degree ofdarkening depending on the amount ofexposure. After development, the film isrinsed, preferably in a special bath, to stopdevelopment. The film is next put into afixing bath, which dissolves theunexposed parts of the emulsion’ssensitive salt. The film is washed toremove the fixer and dried so that it maybe handled, interpreted and filed. Thedeveloping, fixing and washing of theexposed film may be done manually or inautomated processing equipment.

The diagram in Fig. 1 shows theessential features in the exposure of aradiograph. The focal spot is a small areain the X-ray tube from which theradiation emanates. In gammaradiography, it is the capsule containingthe radioactive material that is the sourceof radiation (for example, cobalt-60). Ineither case the radiation proceeds instraight lines to the object; some of therays pass through and others are absorbed— the amount transmitted depending onthe nature of the material and itsthickness. For example, if the object is asteel casting having a void formed by agas bubble, the void produces a reductionof the total thickness of steel to bepenetrated. Hence, more radiation willpass through the section containing thevoid than through the surrounding metal.

graphic Testing

A dark spot, corresponding to theprojected position and shape of the void,will appear on the film when it isdeveloped. Thus, a radiograph is a kind ofshadow picture — the darker regions onthe film representing the more penetrableparts of the object and the lighter regionsrepresenting those more opaque togamma radiation or X-radiation.

Industrial radiography is tremendouslyversatile. Radiographed objects range, insize, from microscopic electronic parts tomammoth missile components, inproduct composition through virtuallyevery known material and inmanufactured form over an enormouslywide variety of castings, weldments andassemblies. Radiographic examination hasbeen applied to organic and inorganicmaterials, to solids, liquids and even to

FIGURE 2. Curves illustrating effect of changein milliamperage on intensity of X-ray beam.

Inte

nsity

(re

lativ

e un

it)

Lowmilliamperage

Wavelength (µm)

Highmilliamperage

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gases. An industry’s production ofradiographs may vary from the occasionalexamination of one or several pieces tothe examination of hundreds ofspecimens per hour. This wide range ofapplications has resulted in theestablishment of independent,professional X-ray laboratories as well asradiographic departments withinmanufacturing plants. Radiographictesting performed by industry usescustomer specifications or industrystandards provided by technical societiesand regulatory bodies.

To meet the growing and changingdemands of industry, research anddevelopment in the field of radiographyare continually producing new sources ofradiation such as neutron generators andradioactive isotopes; lighter, morepowerful, more portable X-ray equipmentas well as multimegavolt X-ray machinesdesigned to produce highly penetratingradiation; new and improved radiographicfilms and automatic film processors; andimproved or specialized radiographictechniques. These factors, plus theactivities of many dedicated people,broadly expand radiography’s usefulnessto industry.

Factors GoverningExposureGenerally speaking, the optical density(called photographic density or simplydensity) of any radiographic imagedepends on the amount of radiationabsorbed by the sensitive emulsion of thefilm. This amount of radiation in turndepends on several factors: the totalamount and type of radiation emitted bythe X-ray tube or gamma ray source; theamount of radiation reaching thespecimen; the amount of radiationspecifically absorbed that is characteristicof the test material; secondary andscattered radiation; filtration; and theintensifying action of screens, if used.Photographic density is discussedelsewhere in this chapter.

Emission from X-Ray SourceThe total amount of radiation emitted byan X-ray tube depends on tube current(milliamperage), voltage, target (source)material and the time the tube isenergized.

When the other operating conditionsare held constant, a change inmilliamperage causes a change in theintensity (quantity of radiation leaving theX-ray generator per unit time) of theradiation emitted, the intensity beingapproximately proportional to themilliamperage. The high voltage

transformer losses and voltage waveformcan change with tube current but acompensation factor is usually applied tominimize the effects of these changes. Innormal industrial radiographic practice,the variation from exact proportionality isnot serious and may usually be ignored.

Figure 2 shows spectral emission curvesfor an X-ray tube operated at twodifferent currents, the higher being twicethe milliamperage of the lower. Therefore,each wavelength is twice as intense in onebeam as in the other. Note that nowavelengths present in one beam areabsent in the other. Hence, there is nochange in X-ray quality or penetratingpower.

As would be expected, the totalamount of radiation emitted by an X-raytube operating at a certain kilovoltage andmilliamperage is directly proportional tothe time the tube is energized.

Because the X-ray output is directlyproportional to both milliamperage andtime, it is directly proportional to theirproduct. (This product is often referred toas the exposure in units such asmilliampere minutes.) Algebraically, thismay be stated E = MT, where E is theexposure, M the tube current and T theexposure time. The amount of radiationwill remain constant if the exposureremains constant, no matter how theindividual factors of tube current andexposure time are varied. This permitsspecifying X-ray exposures in terms suchas milliampere minutes (mA·min) ormilliampere seconds (mA·s), withoutstating the specific individual values oftube current and time.

The kilovoltage applied to the X-raytube affects not only the quality but alsothe intensity of the beam. As thekilovoltage is raised, X-rays of shorterwavelength and hence of morepenetrating power, are produced as well asmore X-rays of the same wavelength as at

141Principles of Film Radiography

142 Radio

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lower voltages. Shown in Fig. 3 arespectral emission curves for an X-ray tubeoperated at two different kilovoltages butat the same milliamperage. Notice thatsome shorter wavelengths present in thehigher kilovoltage beam are absent fromthe lower kilovoltage beam. Further, allwavelengths present in the lowerkilovoltage beam are present in the morepenetrating beam and in greater amount.Thus, raising the kilovoltage increasesboth the penetration and the intensity ofthe radiation emitted from the tube.

Emission from Gamma Ray SourceThe total amount of radiation emittedfrom a gamma ray source during aradiographic exposure depends on theactivity of the source (in becquerels orcuries) and the time of exposure. For aparticular radioactive isotope, theintensity of the radiation is approximatelyproportional to the activity (in becquerelsor curies) of the source. If it were not forabsorption of gamma rays within theradioactive material itself, thisproportionality would be exact. In normalradiographic practice, the range of sourcesizes used in a particular location is smallenough so that variations from exactproportionality are not serious and mayusually be ignored.

Thus, the gamma ray output is directlyproportional to both activity of the sourceand time and hence is directlyproportional to their product.Analogously to the X-ray exposure, thegamma ray exposure E may be statedE = MT, where M is the source activity inbecquerels or curies and T is the exposuretime; the amount of gamma radiationremains constant so long as the productof source activity and time remains

graphic Testing

FIGURE 3. Curves illustrating effect ofchange in kilovoltage on composition andintensity of X-ray beam.

Inte

nsity

(re

lativ

e un

it)

Wavelength (µm)

Wavelengthsadded byincreasing

kilovoltage

Wavelengthsincreased inintensity byincreasingkilovoltage

Lowkilovoltage

Highkilovoltage

constant. This permits specifying gammaray exposures in becquerel hours or curiehours without stating specific values forsource activity or time.

Because the gamma ray energy dependson the isotope, there is no variable tocorrespond to the kilovoltage factorencountered in X-radiography. The onlyway to change the radiation penetratingpower when using gamma rays is tochange the source, for example, higherpenetration cobalt-60 in place of lowerpenetration iridium-192.

Geometric PrinciplesBecause X-rays and gamma rays obey thecommon laws of light, their shadowformation may be simply explained interms of light. It should be borne in mindthat the analogy is not perfect because allobjects are, to a greater or lesser degree,transparent to X-rays and gamma rays andbecause scattering presents greaterproblems in radiography than in optics.However, the same geometric laws ofshadow formation hold for both light andpenetrating radiation.

Suppose that, as in Fig. 4a, there islight from a point L falling on a whitecard C and that an opaque object O isinterposed between the light source andthe card. A shadow of the object will beformed on the surface of the card.

This shadow cast by the object willnaturally show some enlargement becausethe source is smaller than the object andthe object is not in contact with the card;the degree of enlargement will varyaccording to the relative distances of theobject from the card and from the lightsource. For a point source, or one muchsmaller than the object, the law governingthe size of the shadow may be stated: thediameter of the object is to the diameter ofthe shadow as the distance of the light fromthe object is to the distance of the light fromthe card.

Mathematically, the degree ofenlargement may be calculated with thefollowing equations:

(1)

which may also be expressed as Eq. 2:

(2)

where Do = distance from radiation sourceto object; Di = distance from radiationsource to image recording surface (orradiographic film); So = size of object; andSI = size of shadow (or radiographicimage).

S SDDo i

o

i==

SS

DD

o

i

o

i==

FIGURE 4. Ge(a) planes offrom point s(c) perpendimidrange nofilm planes, not parallel,

(a)L

LegendC = film planL = radiationO = test obje

O

C

L

O

C

(b)

L

O

C

(c)

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The degree of sharpness of any shadowdepends on the size of the light sourceand on the position of the object betweenthe light and the card — whether nearerto or farther from one or the other. When

ometric principles of shadow formation: object and film perpendicular to X-ray directionource; (b) perpendicular, near nonpoint source;cular, distant nonpoint source; (d) perpendicular,npoint source; (e) oblique, parallel object and

point source; (f) oblique, object and film planespoint source.

e sourcect

(d)

L

O

C

(e)

L

O

C

(f)

L

O

C

the source of light is not a point but asmall area, the shadows cast are notperfectly sharp (Figs. 4b to 4d) becauseeach point in the source of light casts itsown shadow of the object and each ofthese overlapping shadows is slightlydisplaced from the others, producing anill defined image.

When the source is larger than theobject, as when imaging a crack, theshadow will be smaller than the object.Depending on the distance from object tofilm the image may be undetectablebecause only the black shadow is usuallydetectable.

The form of the shadow may also differaccording to the angle that the objectmakes with the incident light rays.Deviations from the true shape of theobject as exhibited in its shadow imageare referred to as distortion ormisalignment.

Figure 4a to 4f shows the effect ofchanging the size of the source and ofchanging the relative positions of source,object and card. From an examination ofthese drawings, it will be seen that thefollowing conditions must be fulfilled toproduce the sharpest, truest shadow of theobject.

1. The source of light should be small,that is, as nearly a point as can beobtained (compare Fig. 4a and 4c).

2. The source of light should be as farfrom the object as practical (compareFig. 4b and 4c).

3. The recording surface should be asclose to the object as possible(compare Fig. 4b and 4d).

4. The light rays should be directedperpendicularly to the recordingsurface (see Fig. 4a and 4e).

5. The plane of the object and the planeof the recording surface should beparallel (compare Fig. 4a and 4f).

Radiographic ShadowsThe basic principles of shadow formationmust be given primary consideration toensure satisfactory sharpness and freedomfrom distortion in the radiographic image.A certain degree of distortion will exist inevery radiograph because some parts willalways be farther from the film thanothers, the greatest magnification orimage shrinkage being evident in theimages of those parts at the greatestdistance from the recording surface.

Note, also, that there is no distortion ofshape in Fig. 4e — a circular object havingbeen rendered as a circular shadow.However, under circumstances similar tothose shown in Fig. 4e, it is possible thatspatial relations can be distorted. In Fig. 5the two circular objects can be renderedeither as two circles (Fig. 5a) or as a figure


144 Radi

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eight shaped shadow (Fig. 5b). It shouldbe observed that both lobes of the figureeight have circular outlines.

Distortion cannot be eliminatedentirely but, with an appropriatesource-to-film distance, can be lessened toa point where it will not be objectionablein the radiographic image.

Application to RadiographyThe application to the geometricprinciples of shadow formation toradiography leads to five general rules.Although these rules are stated in terms ofradiography with X-rays, they also applyto gamma radiography.

ographic Testing

FIGURE 5. Depending on direction ofradiation, two circular objects can berendered: (a) as two separate circles; (b) astwo overlapping circles.

LegendC = film planeO1 = first test objectO2 = second test object

(a)

O1

C

O2

(b)

O1

C

O2

1. The focal spot should be as small asother considerations will allow, forthere is a definite relation between thesize of the focal spot of the X-ray tubeand the definition in the radiograph. Alarge focus tube, although capable ofwithstanding large loads, does notpermit the delineation of as muchdetail as a small focus tube. Longsource-to-film distances will aid inshowing detail when a large focus tubeis used but it is advantageous to usethe smallest focal spot permissible forthe exposures required. Figures 6b and6h show the effect of focal spot sizeon image quality. As the focal spot sizeincreases from 1.5 mm (0.06 in.) inFig. 6b to 4.0 mm (0.16 in.) in Fig. 6h,the definition of the radiograph startsto degrade. This change is especiallyevident at the chamber edges that areno longer sharp.

2. The distance between the anode andthe material examined should alwaysbe as great as practical. Comparativelylong source-to-film distances shouldbe used in the radiography of thickmaterials to minimize the fact thatstructures farthest from the film areless sharply recorded than those nearerto it. At long distances, radiographicdefinition is improved and the imageis more nearly the actual size of theobject. Figures 6a to 6d show theeffects of source-to-film distance onimage quality. As the source-to-filmdistance is decreased from 1730 mm(68 in.) to 305 mm (12 in.) the imagebecomes more distorted until at305 mm (12 in.) it is no longer a truerepresentation of the casting. This isparticularly evident at the edges of thecasting where the distortion isgreatest.

3. The film should be as close as possibleto the object being radiographed. Inpractice, the film (in its cassette orexposure holder) is placed in contactwith the object. In Fig. 6b and 6e, theeffects of object-to-film distance areevident. As the object-to-film distanceis increased from zero to 102 mm(4 in.), the image becomes larger andthe definition begins to degrade.Again, this is especially evident atchamber edges that are no longersharp.

4. The central ray should be as nearlyperpendicular to the film as possibleto preserve spatial relations.

5. As far as the shape of the specimenwill allow the plane of maximuminterest should be parallel to the planeof the film.

In Fig. 6f and 6g, the effects ofobject-film-source orientation are shown.When compared to Fig. 6b, the image inFig. 6f is extremely distorted; although the

FIGURE 6. Effects on image quality when geometric exposure factors are changed: (a) 1.75 m (68 in.) source-to-film distance,0 mm (0 in.) object-to-film distance; (b) 1.5 mm (0.06 in.) focal spot, 0 mm (0 in.) object-to-film distance; (c) intermediatefocal spot size, intermediate source-to-film distance; (d) 0.30 m (12 in.) source-to-film distance; (e) 100 mm (4 in.)object-to-film distance; (f) perpendicular film-to-source angle and 45 degree object-to-film angle; (g) perpendicularfilm-to-source angle, parallel object-to-film angle; (h) 4.0 mm (0.10 in.) focal spot.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

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film is perpendicular to the central ray,the casting is at a 45 degree angle to thefilm and spatial relationships are lost. Asthe film is rotated to be parallel with thecasting (see Fig. 6g), the spatialrelationships are maintained and thedistortion is lessened.

Calculation of GeometricUnsharpnessThe width of the fuzzy boundary of theshadows in Fig. 4c and 4d is known as thegeometric unsharpness Ug. Because thegeometric unsharpness is a calculablemeasure of the sharpness of the imageand can strongly affect the appearance ofthe radiographic image, it is frequentlynecessary to determine its magnitude.From the laws of similar triangles (seeFig. 7), it can be shown that:

(3)

or

(4)

where Do = source-to-object distance;F = size of radiation source; d = theobject-to-film distance; andUg = geometric unsharpness.

Because the maximum unsharpnessinvolved in any radiographic procedure isusually the significant quantity, theobject-to-film distance d is usually takenas the distance from the source side of thespecimen to the film.

Do and d must be measured in the sameunits — say, millimeters or inches. So longas Do and d are in the same units, Eq. 3 or

U Fd

Dgo

=

U

Fd

Dg

o==


146 Rad

LegendDo = sourcd = objecF = radiatUg = geom

FIGURE 7. GunsharpnessEq. 4.

Film plane

O

FIGURE 8. Graph relating geometric unsharpness Ug tospecimen thickness and source-to-object distance, for 5 mm(0.2 in.) source size.

1.0 (40)

(10

in.)

(20

in.)

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4 will always give the geometricunsharpness Ug in whatever units wereused to measure the dimensions of thesource. The projected sizes of the focalspots of X-ray tubes are usually stated inmillimeters and Ug will also be inmillimeters. If the source size is stated ininches, Ug will be inches.

For rapid reference, graphs of the typeshown in Fig. 8 can be prepared withthese equations. The graphs relatesource-to-film distance, object-to-filmdistance and geometric unsharpness. Notethat the lines of Fig. 8 are all straight.Therefore, for each source-to-objectdistance, it is only necessary to calculatethe value of Ug for a single specimenthickness and then draw a straight linethrough the point so determined and theorigin. It should be emphasized, however,that a separate graph of the type shown inFig. 8 must be prepared for each size ofsource.

Geometric EnlargementIn most radiography, it is desirable tohave the specimen and the film as closetogether as possible to minimizegeometric unsharpness. An exception tothis rule occurs when the source ofradiation is extremely minute, that is, afraction of a millimeter, as in a microfocussource or betatron. In such a case, the

iographic Testing

e-to-object distancet-to-film distanceion sourceetric unsharpness

eometric construction for determining geometric Ug where source is smaller than object. See

F

Do

d

Ug

bject

Source

film may be placed at a distance from thespecimen rather than in contact with it(Fig. 9). Such an arrangement results in anenlarged radiograph without introducingobjectionable geometric unsharpness.Enlargements of three to ten diameters bythis technique are useful in the detectionof fine structures otherwise invisibleradiographically. As the enlargementincreases, the effective field of view(inspection area) decreases. This can resultin the requirement of multiple exposuresto cover an entire part. A benefit ofgeometric enlargement is a decrease in theamount of object scattered radiationreaching the image plane. This effect canimprove contrast sensitivity.

Inverse Square LawWhen the X-ray tube output is heldconstant or when a particular radioactivesource is used, the radiation intensityreaching the specimen (object) is

Geo

met

ric u

nsha

rpne

ss,

mm

(10

–3in

.)

0.9 (36)

0.8 (32)

0.7 (28)

0.6 (24)

0.5 (20)

0.4 (16)

0.3 (12)

0.2 (8)

0.1 (4)

0

Sour

ce-t

o-ob

ject

dis

tanc

e 25

0 m

m

500

mm

750

mm

(30

in.)

1.00

m (4

0 in

.)

1.25

m (5

0 in.

)1.50 m

(60 in

.)

1.75 m (7

0 in.)

2.00 m (8

0 in.)

0 25 50 75 100 125 150(1) (2) (3) (4) (5) (6)

Specimen thickness, mm (in.)

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governed by the distance between thetube (source) and the specimen, varyinginversely with the square of this distance.The explanation that follows is in termsof X-rays and light but applies to gammarays as well.

Because X-rays conform to the laws oflight they diverge when they are emittedfrom the anode and cover an increasinglylarger area with lessened intensity as theytravel from their source. This principle isillustrated in Fig. 10. In this example, it isassumed that the intensity of the X-raysemitted at the anode A remains constantand that the X-rays passing through theaperture B cover an area of 25.8 cm2

(4 in.2) on reaching the recording surfaceC1, which is 305 mm (12 in.) from theanode (distance D).

When the recording surface is moved305 mm (12 in.) farther from the anode,to C2, so that the distance (2D) from theanode is 610 mm (24 in.) or twice itsearlier value, the X-rays will cover103.4 cm2 (16 in.2) — an area four timesas great as that at C1. It follows, therefore,that the radiation per square centimeteron the surface at C2 is only one fourth of

FIGURE 9. With very small focal spot,enlarged image can be obtained. Degree ofenlargement depends upon ratio ofsource-to-film and source-to-specimendistances.

Anode

Focal spot

Diaphragm

Void

Film andcassette

Specimen

that at the level C1. The exposure thatwould be adequate at C1 must beincreased four times to produce at C2 aradiograph of equal density. In practice,this can be done by increasing the time orby increasing the milliamperage.

The inverse square law can beexpressed algebraically as follows:

(5)

where I1 and I2 are the intensities atdistances D1 and D2, respectively.

Relations of SourceStrength (Milliamperage),Distance and TimeWith a given kilovoltage of X-radiation orwith the gamma radiation from aparticular isotope, the three factorsgoverning the exposure are themilliamperage (for X-rays) or sourcestrength (for gamma rays), time and

II

D

D1

2

22

12

==


FIGURE 10. Schematic diagram illustratinginverse square law.

A

B

C1

C2

D

2D

LegendA = radiation sourceB = focal pointC1 = first film planeC2 = second film planeD = source-to-film distance

148 Rad

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source-to-film distance. The numericalrelations among these three quantities aredemonstrated below, using X-rays as anexample. The same relations apply forgamma rays, provided the number ofbecquerels (curies) in the source issubstituted wherever milliamperageappears in an equation.

The necessary calculations for anychanges in focus-to-film distance D,milliamperage M or time T are matters ofsimple arithmetic and are illustrated inthe following example. As noted earlier,kilovoltage changes cannot be calculateddirectly but must be obtained from theexposure chart of the equipment or theoperator’s log book.

Relationship of Source Strengthand DistanceRule: If exposure time is held constant, themilliamperage (M) required for a givenexposure is directly proportional to the squareof the source-to-film distance (D). Theequation is expressed as follows:

(6)

or

For example, suppose that with a givenexposure time and kilovoltage, a properlyexposed radiograph is obtained with 5 mA(M1) at a distance of D1 of 120 mm(30 in.) and that it is desired to increasethe sharpness of detail in the image byincreasing the focus-to-film distance D2 to240 mm (60 in.). The correctmilliamperage M2 to obtain the desiredradiographic density at the increaseddistance D2 may be computed from theproportion:

(7)

or

or

When very low kilovoltages, say 20 kVor less, are used, the X-ray intensitydecreases with distance more rapidly than

clorract

RRteo

(

o

RReMp

(

o

tdmt

(

rd

TSPPmTtmatds

i

M2

2

25

60

305

3600900

5 4 20

= × = ×

= × = mA

5 30

602

2

2M=

5 30 6022 2: :M =

MM

D

D1

2

12

22

==

M M D D1 2 12

22: :==

iographic Testing

alculations based on the inverse squareaw would indicate because of absorptionf the X-rays by the air. Most industrialadiography, however, is done withadiation so penetrating that the airbsorption need not be considered. Theseomments also apply to theime-to-distance relations discussed below.

elationship of Time and Distanceule: If tube current (mA) is held constant,

he exposure time T required for a givenxposure is directly proportional to the squaref the focus-to-film distance D:

8)

r

elation of Milliamperage to Timeule: If distance is held constant butxposure must be changed, the milliamperage

required for a given exposure is inverselyroportional to the time T:

9)

r

Another way of expressing this is to sayhat for a given set of conditions (voltage,istance and others), the product ofilliamperage and time is constant for

he same photographic effect. Thus:

10)

This commonly referred to as theeciprocity law. (Important exceptions areiscussed below.)

abular Solution of Sourcetrength, Time and Distanceroblemsroblems of the types discussed aboveay also be with a table similar to

able 1. The factor between the new andhe old exposure time, milliamperage, or

illiampere minute (mA·min) valueppears in the box at the intersection ofhe column for the new source-to-filmistance and the row for the oldource-to-film distance.

Note that some approximation isnvolved in such a table because the

M T M T M T

C1 1 2 2 3 3== ==

== (a constant)

MM

TT

1

2

2

1==

M M T T1 2 2 1: :==

TT

D

D1

2

12

22

==

T T D D1 2 12

22: :==

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values in the boxes are rounded off to twosignificant digits. However, the errorsinvolved are always less than 5 percentand, in general, are insignificant in actualpractice. Also, a table of this type cannotinclude all source-to-film distances.However, in any one radiographicdepartment, only a few source-to-filmdistances are used in the great bulk of thework and a table of reasonable size can bemade using only these few distances.

Reciprocity LawIn the preceding text, it has been assumedthat exact compensation for a decrease inthe time of exposure can be made byincreasing the milliamperage according tothe relation M1T1 = M2T2. This may bewritten MT = C and is an example of ageneral photochemical law: the sameeffect is produced for IT = constant, whereI is intensity of the radiation and T is thetime of exposure. This is called thereciprocity law and is true for direct X-rayand lead screen exposures. For exposure tolight, it is not quite accurate and, becausesome radiographic exposures are madewith the light from fluorescentintensifying screens, the law cannot bestrictly applied.

Formally defined, the Bunsen-Roscoereciprocity law states that the result of aphotochemical reaction is dependent onlyon the product of radiation intensity I andthe duration of the exposure T and isindependent of absolute values of eitherquantity.

Errors that result for assuming thevalidity of the reciprocity law are usuallyso small that they are not noticeable inexamples of the types given here.

TABLE 1. Value of source strength–time (mA·msource-to-film distance is changed. (The samexample, multiply by same factor if both oldmillimeters.)

OldSource-to-Film ___________________________Distance (mm) 250 300 350 400

250 1.0 1.4 2.0 2.6300 0.70 1.0 1.4 1.8350 0.51 0.74 1.0 1.3400 0.39 0.56 0.77 1.0450 0.31 0.45 0.60 0.7500 0.25 0.36 0.49 0.6550 0.21 0.30 0.40 0.5600 0.17 0.25 0.34 0.4650 0.15 0.21 0.29 0.3700 0.13 0.18 0.25 0.3750 0.11 0.16 0.22 0.2800 0.10 0.14 0.19 0.2

Departures may be apparent, however, ifthe intensity is changed by a factor of 4 ormore. Because intensity may be changedby changing the source-to-film distance,failure of the reciprocity law may appearto be a violation of the inverse square law.Applications of the reciprocity law over awide intensity range sometimes arise andthe relation between results andcalculations may be misleading unless thepossibility of reciprocity law failure is keptin mind. Failure of the reciprocity lawmeans that the efficiency of a lightsensitive emulsion in responding to thelight energy depends on the lightintensity.

Exposure FactorThe exposure factor is a quantity thatcombines milliamperage (X-rays) or sourcestrength (gamma rays), time and distance.Numerically the exposure factor equals

(11)

and

(12)

Activity is measured in becquerels (Bq) orcuries (Ci), where 3.7 × 1010 Bq = 37 GBq= 1.0 Ci.

Radiographic techniques are sometimesgiven in terms of kilovoltage andexposure factor, or radioactive isotope andexposure factor. In such a case, it isnecessary merely to multiply the exposurefactor by the square of the distance to

Activity Time

Distance

Gamma rayexposure

factor

×× ==2

Milliamperes Time

Distance

X - rayexposure

factor

×× ==2


in) is multiplied by factor shown in this table whene factors apply regardless of unit of distance — for and new distance are measured in inches instead of

New Source-to-Film Distance (mm)_____________________________________________________________450 500 550 600 650 700 750 800

3.2 4.0 4.8 5.6 6.8 7.8 9.0 10.02.3 2.8 3.4 4.0 4.8 5.4 6.3 7.11.6 2.0 2.5 3.0 3.4 4.0 4.6 5.21.3 1.6 1.9 2.2 2.6 3.1 3.5 4.0

9 1.0 1.2 1.5 1.8 2.1 2.4 2.8 3.24 0.81 1.0 1.2 1.4 1.7 2.0 2.2 2.63 0.67 0.83 1.0 1.2 1.4 1.6 1.9 2.14 0.56 0.69 0.84 1.0 1.2 1.4 1.6 1.88 0.48 0.59 0.72 0.85 1.0 1.2 1.3 1.53 0.41 0.51 0.62 0.74 0.86 1.0 1.1 1.38 0.36 0.45 0.54 0.64 0.75 0.87 1.0 1.15 0.32 0.39 0.47 0.56 0.66 0.77 0.88 1.0

150 Rad

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find, for example, the milliampereminutes or the curie hours required.

Determination of ExposureFactors

X-RaysThe focus-to-film distance is easy toestablish by actual measurement, themilliamperage can conveniently bedetermined by the milliammeter suppliedwith the X-ray machine and the exposuretime can be accurately controlled by agood time switch. The tube voltage,however, is difficult and inconvenient tomeasure accurately. Furthermore, designsof individual machines differ widely andmay give X-ray outputs of a differentquality and intensity even when operatedat the nominal values of peak kilovoltageand milliamperage.

Consequently, although specifiedexposure techniques can be duplicatedsatisfactorily in the factors ofsource-to-film distance, milliamperageand exposure time, one apparatus maydiffer materially from another in thekilovoltage setting necessary to producethe same radiographic density. Because ofthis, the kilovoltage setting for a giventechnique should be determined by trialon each X-ray generator. In thepreliminary tests, published exposurecharts may be followed as an approximateguide. It is customary for equipmentmanufacturers to calibrate X-ray machinesat the factory and to furnish suitableexposure charts. For the unusual problemsthat arise, it is desirable to record in alogbook all the data on exposure andtechniques. In this way, operators willsoon build up a source of informationthat will make them more competent todeal with difficult situations.

For developing trial exposures, astandardized technique should always beused so that any variation in the qualityof the trial radiographs may then beattributed to the exposure alone. Thistechnique obviates many of the variablefactors common to radiographic work.

Because an increase of kilovoltageproduces a marked increase in X-rayoutput and penetration (see Fig. 3), it isnecessary to maintain a close control ofthis factor to secure radiographs ofuniform density. In many types ofindustrial radiography where it isdesirable to maintain constant exposureconditions for source-to-film distance,milliamperage and exposure time, it iscommon practice to vary the kilovoltagein accordance with the thickness of thematerial to be examined to secure properdensity in the radiographic image.

iographic Testing

Suppose, for example, it is desired tochange from radiographing 38 mm(1.5 in.) thick steel to radiographing50 mm (2 in.) thick steel. For a givenX-ray machine, the 50 mm (2 in.) thicksteel will require more than 10 times theexposure in milliampere minutes at170 kV than the 38 mm (1.5 in.) thicksteel requires. However, increasing thekilovoltage to a little more than 200 willyield a comparable radiograph with thesame milliampere minutes.

Therefore, kilovoltage is an importantvariable because economic considerationsoften require that exposure times be keptwithin fairly narrow limits. It is desirable,as a rule, to use as low a kilovoltage as otherfactors will permit. In the case of certainhigh voltage X-ray machines, thetechnique of choosing exposureconditions may be somewhat modified.For instance, the kilovoltage may be fixedrather than adjustable at the will of theoperator, leaving only milliamperage,exposure time, film type and focus-to-filmdistance as variables.

Gamma RaysWith radioactive materials, the variablefactors are more limited than with X-rays.Not only is the quality (energy orwavelength) of the radiation fixed by thenature of the radiation emitter, but alsothe intensity is fixed by the amount ofradioactive material in the particularsource. The only variables under thecontrol of operators and the onlyquantities they need to determine are thesource-to-film distance, film type and theexposure time. As in the case ofX-radiography, it is desirable to developtrial exposures using the gamma raysources under standardized conditionsand to record all data on exposures andtechniques.

Radiographic ContrastIn a radiograph, the various intensitiestransmitted by the specimen are renderedas different densities in the image. Thedensity differences from one area toanother constitute radiographic contrast.Details in the image are visible by reasonof the contrast between them and theirbackground. Within appropriate limits,the greater the contrast or densitydifferences in the radiograph, the moredefinitely various details will stand out.However, if overall contrast is increasedtoo much, there may be an actual loss indetail visibility in both the thick and thethin regions of the specimen as the imageis too light or too dark to display usefulcontrast (see discussion of film contrast,below).

L

FIGURE 11. Adecreases. Mand thin secexposure behalf value laget two half

(a)

II= 016

Hal

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Radiographic contrast is the result ofboth subject contrast and film contrast.Subject contrast is governed by the rangeof radiation intensities transmitted by thespecimen. A flat sheet of homogeneousmaterial of nearly uniform thicknesswould have very low subject contrast.Conversely, a specimen with largevariations in thickness, which transmits awide range of intensities, would have highsubject contrast. Overall subject contrastcould be defined as the ratio of thehighest to the lowest radiation intensitiesfalling on the film. The subject contrast isaffected by the X-ray kilovoltage. Asshown in Fig. 11, a lower kilovoltage willincrease subject contrast and so increasesensitivity to small variations in theobject. Contrast is also affected byscattered radiation, removal of whichincreases subject contrast, and by theenergy of the primary radiation.

Choice of FilmDifferent films have different contrastcharacteristics. Thus, a film of highcontrast may give a radiograph ofrelatively low overall contrast if thesubject contrast is very low; conversely, afilm of low contrast may give aradiograph of relatively high overallcontrast if the subject contrast is veryhigh. With any given specimen, thecontrast of the radiograph will depend onthe kilovoltage or quality of the X-rays orgamma rays, the contrast characteristics ofthe film, the type of screen, scatter, the

ow kV

s kilovoltage increases, subject contrastore wavelengths penetrate subject in both thick

tions, thus reducing overall difference intween them: (a) low kilovoltage selected for fouryers in thick section; (b) kilovoltage increased to value layers in thick section.

High kV

(b)

II= 04

II= 04

II= 02

f value layer

Half value layer

density to which the radiograph isexposed and film processing.

The classification of film types andtheir speeds are discussed in the chapteron film processing.

Radiographic SensitivityRadiographic sensitivity refers to the sizeof the smallest detail that can be seen in aradiograph or to the ease with which theimages of small details can be detected.

Sensitivity depends on the sharpnessand the contrast of the radiograph. Thus,the grain size of the film, as well as itscontrast and other factors such as theexposure geometry and radiation energy,affect sensitivity.

In radiography of materials ofapproximately uniform thickness, wherethe range of transmitted X-ray intensitiesis small, a technique producing highcontrast may satisfactorily render allportions of the area of interest and theradiographic sensitivity will usually begreater than with a technique producinglow contrast. If, however, the partradiographed transmits a wide range ofX-ray intensities, then a techniqueproducing lower contrast may benecessary to achieve radiographicsensitivity in all regions of the part.


152 Radi

PART 2. Absorption and Scattering

FIGURE 12. Sgamma ray eElectrons froradiographicimportant.

Se

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Radiation Absorption inSpecimenWhen X-rays or gamma rays strike anabsorber (Fig. 12), some radiation isabsorbed or deflected and some passesthrough undeviated. It is the intensityvariation of the undeviated radiation fromarea to area in the specimen that formsthe useful image in a radiograph. Theradiation that is scattered is not imageforming. Scattered radiation will exposethe film and thus tend to obscure theuseful radiographic image. Therefore,scatter must be carefully controlled.(Scattered radiation and the means forreducing its effects are discussed in detailbelow.) Another portion of the originalbeam’s energy is spent in liberatingelectrons from the absorber. The electrons

ographic Testing

chematic diagram of some ways X-ray ornergy is dissipated on passing through matter.

m specimens are usually unimportantally; those from lead foil screens are very

Primary radiation

Absorber

Electronscondary X–rays or

gamma rays

Unabsorbedprimary radiation(image forming)

from the specimen are usuallyunimportant radiographically; those frommaterials in contact with the film, such asscreens of lead or other materials, are veryimportant.

Radiographic Equivalency ofMaterialsBecause various wavelengths exist inX-rays and gamma rays and becauseconsiderable scattered radiation reachesthe film, the laws of radiation absorptionmust be given in a general way.

The absorption of a specimen dependson its thickness, on its density and on theatomic composition of the material.Comparing two specimens of the samecomposition, the thicker or the moredense will absorb more radiation and sorequire more kilovoltage or exposure, orboth, to produce the same photographicresult.

However, the atomic elements in aspecimen often exert a far greater effectupon X-ray absorption than either thethickness or the density. For example, leadis about 1.5 times as dense as ordinarysteel 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 asdense as steel, yet, at 150 kV, the sameexposure is required for 6.4 mm (0.25 in.)of brass as for 8.9 mm (0.35 in.) of steel.

Table 2 gives approximate radiographicequivalence factors. It should beemphasized that this table is approximateand is intended merely as a guide becauseit is based on a compilation of data frommany sources. In a particular instance, theexact value of the radiographicequivalence factor will depend on thequality of the X-radiation and thethickness of the specimen. It will be notedfrom this table that the relativeabsorptions of the different materials arenot constant but change with kilovoltageand that as the kilovoltage increases thedifferences between all materials tend tobecome less. In other words, askilovoltage is increased, the radiographicabsorption of a material becomes lessdependent on the atomic numbers of itsconstituents.

For X-rays generated at voltages morethan 1 MeV and for materials notdiffering too greatly in atomic number(steel and copper, for example), the

TABLE 2. Approximate radiographic equivalence factors.a

X-Rays (kV) Gamma Rays___________________________________________________ ______________________________________________Material 50 100 150 220 400 1000 2000 4 to 25b Iridium-192 Cesium-137 Cobalt-60 Radium

Magnesium 0.6 0.6 0.5 0.08Aluminum 1.0 1.0 0.12 0.18 0.35 0.35 0.35 0.402024 aluminum alloy 2.2 1.6 0.16 0.22 0.35 0.35 0.35Titanium 0.45 0.35Steel 12.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0Steel alloyc 12.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0Copper 18.0 1.6 1.4 1.4 1.3 1.1 1.1 1.1 1.1Zinc 1.4 1.3 1.3 1.0 1.1 1.0 1.0 1.0Brassd 1.4 1.3 1.3 1.2 1.2 1.0 1.1 1.1 1.1 1.1Nickel alloye 16.0 1.4 1.3 1.3 1.3 1.3 1.0 1.3 1.3 1.3 1.3Zirconium 2.3 2.0 1.0Lead 14.0 12.0 5.0 2.5 3.0 4.0 3.2 2.3 2.0Uranium 25.0 3.9 12.6 5.6 3.4

a. Aluminum is the standard metal at 50 kV and 100 kV; steel is the standard metal with high voltages and gamma rays. The thickness of another metal ismultiplied by the corresponding factor to obtain the approximate equivalent thickness of the standard metal. The exposure applying to this thickness of thestandard metal is used. Example: to radiograph 12.7 mm (0.5 in.) of copper at 220 kV, multiply 12.7 mm (0.5 in.) by the factor 1.4, obtaining an equivalentthickness of 17.8 mm (0.7 in.) of steel.

b. 4 to 25 MeV.c. Alloy consisting of 18 percent chromium, 8 percent nickel.d. Tin or lead alloyed in brass will increase these factors.e. Alloy consisting of 73 percent nickel, 15 percent chromium.

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radiographic absorption for a giventhickness of material is roughlyproportional to the density of thematerial. However, even at high voltagesor with penetrating gamma rays, theeffect of composition on absorptioncannot be ignored when dealing withmaterials that differ widely in atomicnumber. For instance, the absorption oflead for 1 MeV X-rays is about five timesthat of an equal thickness of steel,although its density is only 1.5 times asgreat.

Scattered RadiationWhen a beam of X-rays or gamma raysstrikes any object, some of the radiation isabsorbed, some is scattered and somepasses straight through. The electrons ofthe atoms constituting the object scatterradiation in all directions, much as light isdispersed by fog. The wavelengths ofmuch of the radiation are increased bythe scattering process and hence thescatter is of longer wavelength and issomewhat softer, or less penetrating, thanthe unscattered primary radiation. Anymaterial — whether specimen, cassette,tabletop, walls or floor — that receives thedirect radiation is a source of scatteredradiation. Unless suitable measures aretaken to reduce the effects of scatter, itwill reduce the contrast over the wholeimage or parts of it. Scatter forms fog ofnonuniform density.

Scattering of radiation occurs and is aproblem in radiography with both X-raysand gamma rays. In the text whichfollows, the discussion is in terms ofX-rays but the same general principlesapply to gamma radiography.

In the radiography of materials that arethick relative to the radiation energy,scattered radiation forms most of the totalradiation. For example, in the radiographyof a 19 mm (0.75 in.) thickness of steel,the scattered radiation from the specimenis almost twice as intense as the primaryradiation; in the radiography of a 50 mm(2 in.) thickness of aluminum, thescattered radiation is 2.5 times as great asthe primary radiation. Preventing scatterfrom reaching the film markedly improvesthe quality of the radiographic image.

As a rule, the greater portion of thescattered radiation affecting the film isfrom the specimen under examination (Ain Fig. 13). However, any portion of thefilm holder or cassette that extendsbeyond the boundaries of the specimenand thereby receives direct radiation fromthe X-ray tube also becomes a source ofscattered radiation that can affect thefilm. The influence of this scatter is mostnoticeable just inside the borders of theimage (B in Fig. 13) and is often referredto as undercut. In a similar manner,primary radiation striking the film holderor cassette through a thin portion of thespecimen will cause scattering into theshadows of the adjacent thicker portions.


154 Rad

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Another source of scatter that mayundercut a specimen is shown as C inFig. 13. If a filter is used near the tube,this too will scatter X-rays. However,because of the distance from the film,scattering from this source is negligible.Any other material, such as wall or floor,on the film side of the specimen may alsoscatter an appreciable quantity of X-raysback to the film, especially if the materialreceives the direct radiation from theX-ray tube or gamma ray source (Fig. 14).This is referred to as backscatteredradiation.

Reduction of ScatterAlthough scattered radiation can never becompletely eliminated, a number ofmeans are available to reduce its effect.The various techniques are discussed interms of X-rays. Although most of thesame principles apply to gamma andmegavolt X-ray radiography, differences inapplication arise because of the highlypenetrating radiation emitted by megavolt

iographic Testing

FIGURE 13. Sources of scattered radiation.

Anode

Focal spot

Diaphragm

Filter

Specimen

Film and cassette

LegendA = transmitted scatterB = scatter from cassetteC = diffraction scatter

AA

A AC

B B B

and gamma ray sources. For example, amask (see Fig. 15) for use with 200 kVX-rays could easily be light enough forconvenient handling. A mask for use withcobalt-60 radiation, on the other hand,would be thick, heavy and probablycumbersome. In any event, with eitherX-rays or gamma rays, the means forreducing the effects of scattered radiationmust be chosen on the basis of cost,convenience and effectiveness.

Lead Foil ScreensLead screens, mounted in contact withthe film, diminish the effect on the filmof scattered radiation from all sources.They are beyond doubt the leastexpensive, most convenient and mostuniversally applicable means ofcombating the effects of scatteredradiation. Lead screens lessen the scatterreaching the films regardless of whetherthe screens permit a decrease ornecessitate an increase in the radiographicexposure. The nature of the action of lead

FIGURE 14. Intense backscattered radiationmay originate in the floor or wall.Collimating, masking or diaphragmingshould be used. Backing the cassette withlead may give adequate protection.

Anode

Focal spot

Diaphragm

Floor or wall

Specimen

Film

FIGURE 15. lessening scradiographiradiographewith lead stfilled with fi

Fine metallic

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screens is discussed more below. Definitemeans must be provided to ensure goodcontact with the film to ensure imagesharpness.

Many X-ray holders or cassettesincorporate a sheet of lead foil in the backfor the specific purpose of protecting thefilm from backscatter from the table orother objects. This lead will not serve asan intensifying screen — first because itusually has a paper facing and secondbecause it often is not lead of radiographicquality.

When radiographic film cassettes fittedwith a sheet of lead foil in the back forprotection against backscatter are usedwith gamma rays or with X-rays above200 kV, the film should always beenclosed between double lead screens;otherwise, the secondary radiation fromthe lead backing is sufficient to penetratethe intervening felt or paper and cast ashadow of this material on the film givinga granular or mottled appearance.

Combined use of metallic shot and lead mask forattered radiation is conducive to goodc quality. If several round bars are to bed, they may be separated along their lengthsrips held on edge by wooden frame and voidsne shot.

shot

Anode

Focal spot

Diaphragm

Lead mask

Specimen

Film and cassette

Masks and DiaphragmsScattered radiation originating in matteroutside the specimen is most serious forspecimens that have high absorption forX-rays because the scattering fromexternal sources may be large compared tothe primary image forming radiation thatreaches the film through the specimen. Ifmany specimens of the same article are tobe radiographed, it may be worthwhile tocut an opening of the same shape, butslightly smaller, in a sheet of lead andplace this on the object. The lead serves toreduce the exposure in surrounding areasand thus to reduce scattered radiationfrom this source. Because scatter alsoarises from the specimen itself, it is goodpractice, wherever possible, to limit thecross section of an X-ray beam to coveronly the area of the specimen that is ofinterest in the examination.

For occasional pieces of work with lowenergy radiation, where a cutoutdiaphragm would not be economical,barium clay packed around the specimenmay serve the same purpose. The clayshould be thick enough so that the filmdensity under the clay is somewhat lessthan that under the specimen. Otherwise,the clay itself contributes appreciablescattered radiation.

One of the most satisfactoryarrangements, combining effectivenessand convenience, is to surround theobject with copper or steel shot having adiameter of about 0.25 mm (0.01 in.) orless (Fig. 15). Steel is best for objects oflow atomic number; copper, for steel andobjects of higher atomic number thaniron. The materials flow and are effectivefor filling cavities or irregular edges ofobjects, such as castings, where a normalexposure for thick parts would result in anoverexposure for thinner parts. Of course,it is preferable to make separate exposuresfor thick and thin parts but this is notalways practical.

In some cases, a lead diaphragm or leadcone on the tube head may be aconvenient way to limit the area coveredby the X-ray beam. Such lead diaphragmsare particularly useful where the desiredcross section of the beam is a simplegeometric figure, such as a circle, squareor rectangle.

FiltersIn general, filters are limited toradiography with X-rays below 1 MeV. Asimple metallic filter mounted in theX-ray beam near the X-ray tube (Fig. 16)may adequately serve the purpose ofeliminating overexposure in the thinregions of the specimen and in the areasurrounding the part (Table 3). Such afilter is particularly useful for reducingscatter undercut in cases where a mask


156 Rad

TABLE 3. Ef

Regio

Outside speThin sectionMedium seThick sectio

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around the specimen is impractical. Ofcourse, an increase in exposure ofkilovoltage will be required to compensatefor the additional absorption.

The underlying principle of thetechnique is that the filter absorbs moreof the softer radiation of the primarybeam than it does the harder radiation.This causes a greater change in theamount of radiation passing through thethin parts than through the thicker parts.

iographic Testing

FIGURE 16. Filter placed near X-ray tubereduces subject contrast and eliminatesmuch of secondary radiation, which tends toobscure detail in periphery of specimen.

AnodeFocal spot

Diaphragm

Filter

Specimen

Film and cassette

fect of metallic filter on X-ray intensity.

OriginalX-Ray Intensity

Specimen Remaining afterThickness Addition of Filter_______________ __________________

n mm (in.) (percent)

cimen 0 (0) < 106.4 (0.25) ~ 30

ction 12.7 (0.50) ~ 40n 25.4 (1.0) ~ 55

In regions of strong undercut, thecontrast is increased by a filter because theonly effect of the undercutting radiationis to obscure the desired image. In regionswhere the undercut is negligible, a filterhas the effect of decreasing the contrast inthe finished radiograph.

A filter reduces excessive subjectcontrast (and hence radiographic contrast)by hardening the radiation. The longerwavelengths do not penetrate the filter toas great an extent as do the shorterwavelengths. Therefore, the beamemerging from the filter contains a higherproportion of the more penetratingwavelengths (see Fig. 17).

The choice of a filter material shouldbe made on the basis of availability andease of handling. For the same filteringeffect, the thickness of filter required isless for those materials having higherabsorption. In many cases, copper or brassis the most useful, because filters of thesematerials will be thin enough to handleeasily yet not so thin as to be delicate (seeFig. 18).

Rules for filter thicknesses are difficultto formulate exactly because the amountof filtration required depends not only onthe material and thickness range of thespecimen but also on the distribution ofmaterial in the specimen and on theamount of scatter to be eliminated. In theradiography of aluminum, a filter ofcopper about 4 percent of the greatestthickness of the specimen should providethe thickness necessary. With steel, acopper filter should ordinarily be about20 percent, or a lead filter about 3percent, of the greatest specimenthickness for the greatest useful filtration.The foregoing values are maximumvalues; depending on circumstances,useful radiographs can often be madewith far less filtration.

FIGURE 17. Curves illustrating effect of filteron composition and intensity of X-ray beam.

Inte

nsity

(re

lativ

e un

it)

Wavelengthsreduced inintensity byaddition of filter

Unfilteredbeam

Filteredbeam

Wavelength (relative unit)

FIGURE 18. M

Mat

eria

l thi

ckne

ss (

rela

tive

unit)

2.5

2.25

2.0

1.75

1.5

1.25

1.0

0.75

0.5

0.25

FIGURE 19. Schematic diagram showing how primary X-rayspass between lead strips of potter-bucky diaphragm. Most ofscattered X-rays are absorbed because they strike sides ofstrips.

Anode

Focal spot

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In radiography with X-rays up to atleast 250 kV, a 0.125 mm (0.005 in.) frontlead screen is an effective filter for thescatter from the bulk of the specimen.Additional filtration between specimenand film only tends to contributeadditional scatter from the filter itself andharden the beam unnecessarily.

Grid DiaphragmsOne of the most effective ways to reducescattered radiation from an object beingradiographed at energies up to 400 kV iswith a potter-bucky diaphragm. Thisapparatus (Fig. 19) consists of a movinggrid, composed of lead strips held inposition by intervening strips of amaterial transparent to X-rays. The leadstrips are tilted, so that the plane of eachis in line with the focal spot of the tube.The slots between the lead strips areseveral times as deep as they are wide. Thelead strips have the function of absorbingthe very divergent scattered rays from theobject being radiographed, so that most ofthe exposure is made by the primary raysemanating from the focal spot of the tubeand passing between the lead strips.During the course of the exposure, thegrid is moved, or oscillated (out ofsynchronization with the X-ray pulse) in aplane parallel to the film as shown by the

aximum filter thickness for aluminum and steel.

0 1.0 2.0 3.0

Filter thickness (relative unit)

Steel (lead filter)

Aluminum (copper filter)

Steel (copper filter)

black arrows in Fig. 19. Thus, the shadowsof the lead strips are blurred to the pointthat they do not appear in the finalradiograph.

The potter-bucky diaphragmcomplicates industrial radiographic testingand necessarily limits the flexibility of thearrangement of the X-ray tube, thespecimen and the film. Grids can,however, be of great value in theradiography of beryllium more than75 mm (3 in.) thick and in theexamination of other low absorptionmaterials of moderate and greatthicknesses.

Special forms also have been designedfor the radiography of steel with voltagesas high as 200 to 400 kV. Thesediaphragms are not used at highervoltages or with gamma rays becauserelatively thick lead strips would beneeded to absorb the radiation scatteredat these energies. This in turn wouldrequire a potter-bucky diaphragm, with


Diaphragm

Potter-bucky diaphragm

Specimen

Film and cassette

158 Radio

3RT07 LAYOUT(139_184) 10/14/02 2:58 PM Page 158

the associated mechanism, of anuneconomical size and complexity.

Mottling Caused by X-RayDiffractionA special form of scattering caused byX-ray diffraction is encounteredoccasionally. It is most often observed inthe radiography of fairly thin metallicspecimens whose grain size is largeenough to be an appreciable fraction ofthe part thickness. The radiographicappearance of this type of scattering ismottled and may be confused with themottled appearance sometimes producedby porosity or segregation. It can bedistinguished from these conditions bymaking two successive radiographs, withthe specimen rotated slightly (1 to5 degrees) between exposures, about anaxis perpendicular to the central beam. Apattern caused by porosity or segregationwill change only slightly; however, onecaused by diffraction will show a markedchange. The radiographs of somespecimens will show a mottling from botheffects and careful observation is neededto differentiate between them.

Relatively large crystal or grain in arelatively thin specimen may in somecases diffract an appreciable portion of theX-ray energy falling on the specimen,much as if it were a small mirror. This willresult in a light spot on the developedradiograph corresponding to the positionof the particular crystal and may alsoproduce a dark spot in another location ifthe diffracted, or reflected, beam strikesthe film. Should this beam strike the filmbeneath a thick part of the specimen, thedark spot may be mistaken for a void inthe thick section. This effect is notobserved in most industrial radiography,for most specimens are composed of amultitude of very minute crystals orgrains variously oriented; hence scatter bydiffraction is essentially uniform over thefilm area. In addition, the directlytransmitted beam usually reduces thecontrast in the diffraction pattern to apoint where it is no longer visible on theradiograph.

The mottling caused by diffraction canbe reduced and in some cases eliminatedby raising the kilovoltage and by usinglead foil screens. The former is often ofpositive value even though theradiographic contrast is reduced. Becausedefinite rules are difficult to formulate,both approaches should be tried in a newsituation and perhaps both used together.

It should be noted, however, that insome instances, the presence or absenceof mottling caused by diffraction has beenused as a rough indication of grain size

graphic Testing

and thus as a basis for the acceptance orthe rejection of parts.

Scattering in High VoltageMegavolt RadiographyLead screens should always be used in the1 or 2 MeV range. The commonthicknesses, 0.125 mm (0.005 in.) frontand 0.25 mm (0.010 in.) back, are bothsatisfactory and convenient. Some users,however, find a 0.25 mm (0.010 in.) frontscreen of value because of its greaterselective absorption of the scatteredradiation from the specimen.

At these voltages filtration at the tubeoffers no improvement in radiographicquality. Filters at the film improve theradiograph in the examination of uniformsections but give poor quality at the edgesof an image because of undercut ofscattered radiation from the filter itself.Hence, filtration should not be used inthe radiography of specimens containingnarrow bars, for example, no matter whatthe thickness of the bars in the directionof the primary radiation. Also, filtrationshould be used only where the film canbe adequately protected againstbackscattered radiation.

Lead filters are most convenient forthis voltage range. When used betweenspecimen and film, filters are subject tomechanical damage. Care should be takento reduce this to a minimum, lest filteranomalies be confused with structures inor on the specimen. In radiography withmegavolt X-rays, specimens of uniformsections may be conveniently divided intothree classes. Below 38 mm (1.5 in.) ofsteel, filtration affords little improvementin radiographic quality. Between 38 and100 mm (1.5 and 4.0 in.) of steel, thethickest filter, up to 3 mm (0.125 in.)lead, that allows a reasonable exposuretime, may be used. Above 100 mm(4.0 in.) of steel, filter thicknesses may beincreased to 6.3 mm (0.25 in.) of lead,economic considerations permitting. Itshould be noted that in the radiographyof extremely thick specimens withmegavolt X-rays, fluorescent screens mayincrease the photographic speed to apoint where filters can be used withoutrequiring excessive exposure time.

A very important point is to block offall radiation except the useful beam withheavy (12.7 to 25.4 mm [0.5 in. to 1 in.])lead at the tubehead. This step is calledcollimation. Unless this is done, radiationstriking the walls of the X-ray room willscatter back enough to seriously affect thequality of the radiograph. This will beespecially noticeable if the specimen isthick or has parts projecting relatively farfrom the film.

PART 3. Radiographic Screens

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Functions of ScreensRadiographic screens help radiographersto use more fully the X-ray or gamma rayenergy reaching the film. The physicalprinciples underlying the action of bothlead foil and fluorescent screens arediscussed elsewhere and only the practicalapplications are discussed here.

When an X-ray or gamma ray beamstrikes a film, usually less than onepercent of the energy is absorbed. Becausethe formation of the radiographic image isgoverned by the absorbed radiation, morethan 99 percent of the available energy inthe primary radiation reaching the filmperforms no useful photographic work.Obviously, any means of more fully usingthis wasted energy, without complicatingthe technical procedure, is highlydesirable. Three types of radiographicscreens are commonly used for thispurpose — lead, fluorescent andfluorometallic (metal phosphor). Metalsother than lead are sometimes used inmegavolt radiography. Lead screens maybe in the form of thin foil, usuallymounted on a thin cardboard or plasticsheet, or in the form of a lead compound,usually lead oxide, evenly coated on athin support. The lead compound screensare usually used only for radiographybelow 150 kV.

Lead Foil ScreensFor radiography with X-ray or gamma rayenergies between 150 kV and 2 MeV, leadfoil screens in intimate contact with bothsides of the film, within the film holder,will reduce exposure times and improveradiographic quality by reducing scatter.Foils as thin as 0.10 to 0.15 mm (0.004 to0.006 in.) are commonly used. To reducebackscatter from the table or floor of theroom an additional lead sheet 3 to 6 mm(0.12 to 0.25 in.) thick is usually placedbehind the film holder.

The choice of screens and filters forradiography above 1 to 2 MeV is morecomplicated, as discussed in the sectionon high energy radiography.

Effects of Lead ScreensLead foil in direct contact with the filmhas three principal effects: (1) it increases

the photographic action on the film,largely by reason of the electrons emittedand partly by the secondary X-raysemitted by the lead; (2) it absorbs thelonger wavelength scattered radiationmore than the primary; and (3) itintensifies the primary radiation morethan the scattered radiation. Thedifferential absorption of the secondaryradiation and the differentialintensification of the primary radiationresult in diminishing the effect ofscattered radiation, producing greatercontrast and clarity in the radiographicimage. This reduction in the effect of thescattered radiation decreases the totalintensity of the radiation reaching thefilm and lessens the net intensificationfactor of the screens. The absorption ofprimary radiation by the front lead screenalso diminishes the net intensifying effect;and, if the incident radiation does nothave sufficient penetrating power, theactual exposure required may be evengreater than without screens. At best, theexposure time is one half to one third ofthat without screens but the advantage ofscreens in reducing scattered radiationstill holds.

The quality of the radiation necessaryto obtain an appreciable intensificationfrom lead foil screens depends on the typeof film, the kilovoltage and the thicknessand nature of the material through whichthe rays must pass (Fig. 20). In theradiography of aluminum, for example,using a 0.125 mm (0.005 in.) front screenand a 0.25 mm (0.010 in.) back screen,the thickness of aluminum must be about150 mm (6 in.) and the kilovoltage ashigh as 160 kV to secure any advantage inexposure time with lead screens. In theradiography of steel, lead screens begin togive appreciable intensification withthicknesses in the neighborhood of6.3 mm (0.25 in.), at voltages of 130 to150 kV. In the radiography of 32 mm(1.25 in.) steel at about 200 kV, leadscreens permit an exposure of aboutone third of that without screens(intensification factor of 3). Withcobalt-60 gamma rays, the intensificationfactor of lead screens is about 2. Lead foilscreens, however, do not detrimentallyaffect the definition or graininess of theradiographic image to any material degreeso long as the lead and the film are inintimate contact.


160 Radi

FIGURE 20. Efof lead screen

Den

sity

diff

eren

ce (

rela

tive

unit)

Intensification

Absorption

LegendA. 0.05 mm (B. 0.12 mm (C. 0.25 mm (

FIGURE 21. Upper area shows decreaseddensity caused by paper between leadscreen and film. Electron shadow picture ofpaper structure has also been introduced.

FIGURE 22. Between film and lead foilscreens: (a) good contact gives sharp image;(b) poor contact gives fuzzy image.

(a)

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Lead foil screens also diminish theeffect of scattered radiation. Scatteredradiation from the specimen itself is cutalmost in half by lead screens,contributing to maximum clarity of detailin the radiograph; this advantage isobtained even under conditions where thelead screen makes an increase in exposurenecessary.

In radiography with gamma rays orhigh voltage X-rays, films loaded in metalcassettes without screens are likely torecord the effect of secondary electronsgenerated in the lead covered back of thecassette. These electrons, passing throughthe felt pad on the cassette cover, producea mottled appearance because of thestructure of the felt. Films loaded in thecustomary lead backed cardboardexposure holder may also show thestructure of the paper that lies betweenthe lead and the film (Fig. 21). To avoidthese effects, film should be enclosedbetween double lead screens, care beingtaken to ensure good contact betweenfilm and screens. Thus, lead foil screensare essential in practically all radiographywith gamma rays or megavolt X-rays. If,for any reason, screens cannot be usedwith these radiations, a lightproof plasticholder with no metal backing should beused.

Contact between the film and the leadfoil screens is essential to goodradiographic quality. Areas lacking contactproduce fuzzy images, as shown inFig. 22b.

ographic Testing

fects of kilovoltage on intensification propertiess.

1.0

0.8

0.6

0.4

0.2

0

–0.2

–0.4

A

B

C

50 75 100 125 150 175 200 225

Kilovoltage

0.002 in.) lead oxide, 0.01 mm (0.0004 in.) lead equivalent.0.005 in.).0.01 in.) lead.

(b)

FIGURE 24. Number of electrons emitted (per surface unit oflead) is essentially uniform. More electrons can reach film in

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Selection and Care of LeadScreensLead foil for screens must be selected withextreme care. Commercially pure lead issatisfactory. An alloy of 6 percentantimony and 94 percent lead, beingharder and stiffer, has better resistance towear and abrasion. Tin coated lead foilshould be avoided, because irregularitiesin the tin cause a variation in theintensifying factor of the screens,resulting in mottled radiographs. Minorblemishes do not affect the usefulness ofthe screen but large blisters or cavitiesshould be avoided.

Most of the intensifying action of alead foil screen is caused by the electronsemitted under X-ray or gamma rayexcitation. Because electrons are readilyabsorbed even in thin or light materials,small flakes of foreign material — forexample, dandruff or tobacco — willlikewise produce light spots on thecompleted radiograph. For this samereason, protective coatings on lead foilscreens should be removed before use.The coating should not produce staticelectricity when rubbed against or placedin contact with film (see Fig. 23).

Deep scratches on lead foil screens, onthe other hand, will produce dark lines onthe radiograph (Fig. 24).

Surface contaminants may be removedfrom lead foil screens with a mildhousehold detergent or cleanser and asoft, lint-free cloth. If more thoroughcleaning is necessary, screens may be verygently rubbed with the finest grade of

FIGURE 23. Static marks result from poorfilm handling. Static marks may also betreelike or branching.

steel wool. If this is done carefully, theshallow scratches left by the steel woolwill not produce dark lines in theradiograph.

Films could be fogged if left betweenlead foil screens longer than is reasonablynecessary, particularly under conditions ofhigh temperature and humidity. Whenscreens have been freshly cleaned with anabrasive, this effect will be increased;prolonged contact between film andscreens should be delayed for 24 h aftercleaning.

Fluorescent ScreensCertain chemicals fluoresce; that is, theyhave the ability to absorb X-rays andgamma rays and immediately emit light;the intensity of the emitted light dependson the intensity of the incident radiation.These fluorescent materials can be used inradiography by first being finelypowdered, mixed with a suitable binder,then coated in a thin, smooth layer on aspecial cardboard or plastic support.

For the exposure, film is clampedfirmly between a pair of these fluorescentscreens. The photographic effect on thefilm, then, is the sum of the effects of theX-rays and of the light emitted by thescreens. For example, in the radiographyof 12.7 mm (0.5 in.) steel at 150 kV, afactor as high as 125 has been observed.


vicinity of scratch, resulting in dark line on radiograph. (Forillustrative clarity, electron paths have been shown as straightand parallel; actually, electrons are emitted diffusely.)

X-rays

Film

Electrons from lead foil

ScratchBack lead screen

162 Rad

FIGURE 25. Light and ultraviolet radiationfrom typical fluorescent screen spreadsbeyond X-ray beam that excitesfluorescence.

Fluorescentlayer

X-rays

Visible light

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In radiography of 19.1 mm (0.75 in.) steelat 180 kV, factors of several hundred havebeen obtained experimentally.

Under these latter conditions, theintensification factor has about reachedits maximum and diminishes both forlower voltage and thinner steel and forhigher voltage and thicker steel. Usingcobalt-60 gamma rays for very thick steel,the factor may be 10 or less.

LimitationsDespite their great effect in reducingexposure time, fluorescent screens are notwidely used in industrial radiography.This is mainly because they may give poordefinition, compared to a radiographmade directly or with lead screens. Thepoorer definition results from thespreading of the light emitted from thescreens, as shown in Fig. 25. The lightfrom any particular portion of the screenspreads out beyond the confines of theX-ray beam that excited the fluorescence.

The other reason fluorescent screensare seldom used in industrial radiographyis because they may produce screen mottleon the finished radiograph. This mottle ischaracteristic in appearance, very muchlarger in scale and much softer in outlinethan the graininess associated with thefilm itself. Screen mottle is associated withpurely statistical variations in thenumbers of absorbed X-ray photons, fromone tiny area of the screen to the next.Thus, screen mottle tends to becomegreater as the kilovoltage of the radiationincreases. The higher the kilovoltage, themore energetic, on the average, are theX-ray photons. Therefore, on absorptionin the screen, a larger burst of light isproduced. The larger the bursts, the fewerthat are needed to produce a givendensity and the greater is the purelystatistical variation in the number ofphotons from one small area to the next.

iographic Testing

PART 4. Industrial X-Ray Films

FIGURE 27. Cross section of unprocessedemulsion on one side of radiographic film(2000 diameters). Note greater quantity ofgrains as compared to developed grains ofFig. 28.

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Modern radiographic films for generalradiography consist of an emulsion(gelatin containing a radiation sensitivesilver halide compound) and a flexible,transparent base that sometimes containsa tint. Usually, the emulsion is coated onboth sides of the base in layers about0.0125 mm (5 × 10–4 in.) thick (see Fig. 26and 27). Putting emulsion on both sidesof the base doubles the amount ofradiation sensitive silver compound andthus increases the speed. At the sametime, the emulsion layers are thin enoughso that developing, fixing and drying canbe accomplished in a reasonable time.However, some films for radiography inwhich the highest detail visibility isrequired have emulsion on only one sideof the base.

When X-rays, gamma rays or lightstrike the grains of the sensitive silvercompound in the emulsion, a changetakes place in the physical structure of thegrains. This change cannot be detected byordinary physical techniques. However,when the exposed film is treated with achemical solution (called a developer), areaction takes place, causing theformation of black, metallic silver. It is

FIGURE 26. Silver bromide grains ofradiographic film emulsion(2500 diameters). Grains have beendispersed to show shape and relative sizesmore clearly; in actual coating, crystals aremuch more closely packed.

this silver, suspended in the gelatin onboth sides of the base, that constitutes theimage (see Fig. 28).


FIGURE 28. Cross section showingdistribution of developed grains inradiographic film emulsion exposed to givemoderate density.

164 Rad

FIGURE 29. high speed

Screewith

s

TABLE 4. Transmittance, percent transmittance, opacity

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Although an image may be formed bylight and other forms of radiation, as wellas by gamma rays or X-rays, the propertiesof the latter two are of distinct characterand, for this reason, the sensitiveemulsion must be different from thoseused in other types of photography.

Selection of Films forIndustrial RadiographyAs pointed out above, industrialradiography now has many widely diverseapplications. There are manyconsiderations to be made in obtainingthe best radiographic results, for example:(1) the composition, shape and size of thepart being examined — and, in somecases, its weight and location as well;(2) the type of radiation used — whetherX-rays from an X-ray machine or gammarays from a radioactive material; (3) thekilovoltages available with the X-rayequipment; (4) the intensity of thegamma radiation; (5) the kind ofinformation sought — whether it issimply an overall inspection or the criticalexamination of some especially importantportion, characteristic or feature; and(6) the resulting relative emphasis ondefinition, contrast, density and timerequired for proper exposure. All of thesefactors are important in the determinationof the most effective combination ofradiographic method and radiographicfilm.

The selection of a film for theradiography of any particular partdepends on the thickness and material ofthe specimen and on the voltage range ofthe available X-ray machine. In addition,the choice is affected by the relativeimportance of high radiographic qualityor short exposure time. Thus, an attemptmust be made to balance these twoopposing factors. As a consequence, it isnot possible to present definite rules onthe selection of a film. If high quality isthe deciding factor, a slower (lesssensitive) and finer grained film should besubstituted for a faster (more sensitive)one — for instance, for the radiography of

iographic Testing

Choice of film depends on relative emphasis onor high radiographic quality.

Improving quality

n type filmfluorescentcreens

Fast direct exposure

type film

Slow direct exposure

type film

Increasing speed

steel up to 6.3 mm (0.25 in.) thick at 120to 150 kV, film Y might be substituted forfilm X. If short exposure times areessential, a faster film (or fastercombination of film and screen) can beused. For example, 38 mm (1.5 in.) steelmight be radiographed at 200 kV usingfluorescent screens with a film particularlysensitive to blue light, rather than a directexposure film with lead screens.

Figure 29 indicates the direction thatthese substitutions take. The directexposure films may be used with orwithout lead screens, depending on thekilovoltage and the thickness and shapeof the specimen.

Fluorescent intensifying screens mustbe used in radiography requiring thehighest possible photographic speed. Thelight emitted by the screens has a muchgreater photographic action than theX-rays either alone or combined with theemission from lead screens. To secureadequate exposure within a reasonabletime, screen type radiographic filmssandwiched between fluorescentintensifying screens are often used inradiography of steel in thicknesses greaterthan about 50 mm (2 in.) at 250 kV andgreater than 75 mm (3 in.) at 400 kV.

Photographic DensityPhotographic density refers to thequantitative measure of film blackeningand is also called optical density andsensitometric density. When no danger ofconfusion exists, photographic density isusually spoken of merely as density.Density is defined by the equation:

(13)

where D = density; Io = light intensityincident on film; and It = light intensitytransmitted.

Table 4 illustrates some relationsbetween transmittance, percent

DIIt

= log 0

and density relationships.

PercentTransmittance Transmittance Opacity Density

I t · I o–1 I t · I o

–1 × 100 Io· I t–1 log Io· I t

–1

1.00 100 1 00.50 50 2 0.30.25 25 4 0.60.10 10 10 1.00.01 1 100 2.00.001 0.1 1000 3.00.0001 0.01 10 000 4.0

FIGURE 30. Typical X-ray exposure chart for steel may beapplied to film X (see Fig. 33), with lead foil screens, at1.5 film density and 1.0 m (40 in.) source-to-film distance.

Log

exp

osur

e

1.8

1.5

1.2

0.9

0.6

0.3

0

100

80

60

40

30

20

10

8

6

4

3

2

1

Exp

osur

e (m

A·m

in)

0 6.4 12.7 19 25.4 31.8 38.1(0.25) (0.50) (0.75) (1.00) (1.25) (1.50)

Equivalent thickness, mm (in.) of steel

100

kV

120

kV

140

kV

160

kV18

0 kV

200

kV22

0 kV

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transmittance, opacity and density. Itshows that an increase in density of 0.3reduces the light transmitted to half of itsformer value. In general, because densityis a logarithm, a certain increase in densityalways corresponds to the same percentagedecrease in transmittance.

DensitometersA densitometer is an optical instrumentfor measuring photographic densities.Film density to a required range is usuallyspecified in radiographic procedures. Thedensitometer must be available to see thatspecifications are met. The densitometer isessential for creating characteristic curves,discussed elsewhere.

Different types of densitometers, bothvisual and photoelectric, are availablecommercially. For purposes of practicalindustrial radiography there is no greatpremium on high accuracy in adensitometer. A much more importantproperty is reliability, that thedensitometer should reproduce readingsfrom day to day.

X-Ray Exposure ChartsAn exposure chart is a graph showing therelation between material thickness,kilovoltage and exposure (Figs. 30 to 32).In its most common form, an exposurechart resembles Fig. 30. These graphs areadequate for determining exposures in theradiography of uniform plates but theyserve only as rough guides for objects,such as complicated castings, having widevariations of thickness.

Exposure charts are usually availablefrom manufacturers of X-ray equipment.Because, in general, such charts cannot beused for different X-ray machines unlesssuitable correction factors are applied,individual laboratories sometimes preparetheir own.

Preparing an Exposure ChartA simple technique for preparing anexposure chart is to make a series ofradiographs of a pile of metal plates (ofequal thickness but different lengths)consisting of a number of steps. This steptablet, or stepped wedge, is radiographedat several different exposure times at eachof a number of kilovoltages. The exposedfilms are all processed under conditionsidentical to those that will later be usedfor routine work. Each radiograph consistsof a series of photographic densitiescorresponding to the X-ray intensitiestransmitted by the different thicknesses ofmetal. A certain density, for example 1.5,is selected as the basis for the preparation

of the chart. Wherever this density occurson the stepped wedge radiographs, thereare corresponding values of thickness,milliampere minutes and kilovoltage. It isunlikely that many of the radiographs willcontain a value of exactly 1.5 in densitybut the correct thickness for this densitycan be found by interpolation betweensteps. Thickness and milliampere minutevalues are plotted for the differentkilovoltages in the manner shown inFig. 30.

Another technique, requiring fewerstepped wedge exposures but morearithmetical manipulation, is to make onestep tablet exposure at each kilovoltageand to measure the densities in theprocessed stepped wedge radiographs. Theexposure that would have given thechosen density (in this case 1.5) underany particular thickness of the steppedwedge can then be determined from thecharacteristic curve of the film used. Thevalues for thickness, kilovoltage andexposure are then plotted.

Note that thickness is on a linear scaleand that milliampere minutes are on anonlinear scale. The logarithmic scale isnot necessary but is very convenientbecause it compresses an otherwise longscale. A further advantage of thelogarithmic exposure scale is that itusually allows the location of the points


166 Rad

FIGURE 32. Typical gamma ray exposure chart for

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for any one kilovoltage to be wellapproximated by a straight line.

An exposure chart usually applies onlyto a single set of conditions, determinedby (1) the X-ray machine used; (2) acertain source-to-film distance; (3) aparticular film type; (4) processingconditions used; (5) the film density onwhich the chart is based; (6) the type ofscreens (if any) that are used; and (7) thematerial tested.

Only if the conditions used in makingthe radiograph agree in all particularswith those used in preparation of theexposure chart can values of exposure beread directly from the chart. Any changerequires the application of a correctionfactor. The correction factor applying toeach of the above conditions is discussedseparately.

1. It is sometimes difficult to find acorrection factor to make an exposurechart prepared for one X-ray machineapplicable to another. Different X-raymachines operating at the samenominal kilovoltage andmilliamperage settings may give notonly different intensities but alsodifferent qualities (energies) ofradiation.

iographic Testing

FIGURE 31. Typical X-ray exposure chart foruse when exposure and distance are heldconstant and kilovoltage is varied toconform to specimen thickness. Film X (seeFig. 33), exposed with lead foil screens todensity of 1.5, source-to-film distance is1.0 m (40 in.) and exposure is 50 mA·min.

220

200

180

160

140

120

100

80

60

Kilo

volta

ge (

kV)

0 7 13 19 25 31 38(0.25) (0.50) (0.75) (1.00) (1.25) (1.50)

Steel thickness, mm (in.)

2. A change in source-to-film distancemay be compensated for by theinverse square law. Some exposurecharts give exposures in terms ofexposure factor rather than in terms ofmilliampere minutes or milliampereseconds. Charts of this type are readilyapplied to any value of source-to-filmdistance.

3. A different type of film can becorrected by comparing the differencein the amount of exposure necessaryto give the same density on both films(from relative exposure charts such asthose described below). For example,to obtain a density of 1.5 using film Y,0.6 more log exposure is required thanfor film X (Fig. 33).This log exposure differencecorresponds to an exposure factor of3.99. To obtain the same density onfilm Y as on film X, multiply theoriginal exposure by 3.99 to get thenew exposure. Conversely, if goingfrom film Y to film X, divide theoriginal exposure by 3.99 to obtain thenew exposure.

iridium-192, based upon the use of film X (see Fig. 33).

Exp

osur

e fa

ctor

, G

Bq·m

in·c

m–2

(Ci·m

in·in

.–2 )

0 25 50 75 100(1) (2) (3) (4)

Steel thickness, mm (in.)

500 (10.0)

400 (8.0)

300 (6.0)

200 (4.0)

150 (3.0)

100 (2.0)

50 (1.0)

40 (0.8)

30 (0.6)

20 (0.4)

15 (0.3)

10 (0.2)

5 (0.1)

D = 2.5

D = 2.0

D = 1.5

FIGURE 33. Characteristic curves of three typical X-ray films,exposed between lead foil screens.

Den

sity

D

0 0.5 1.0 1.5 2.0 2.5 3.0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

Log relative exposure

Film Z Film XFilm Y

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These procedures can be used tochange densities on a single film aswell. Simply find the log E differenceneeded to obtain the new density onthe film curve; read the correspondingexposure factor from the chart; thenmultiply to increase density or divideto decrease density.

4. A change in processing conditionscauses a change in effective film speed.If the processing of the radiographsdiffers from that used for theexposures from which the chart wasmade, the correction factor must befound by experiment.

5. The chart gives exposures to produce acertain density. If a different density isrequired, the correction factor may becalculated from the film’scharacteristic curve.

6. If the type of screen is changed — forexample, from lead foil to fluorescent— it is easier and more accurate tomake a new exposure chart than todetermine correction factors

7. Material can be changed by using thematerial equivalence table (Table 2).

In some radiographic operations, theexposure time and the source-to-filmdistance are set by economicconsiderations or on the basis of previousexperience and test radiographs. The tubecurrent is, of course, limited by the designof the tube. The specimen and thekilovoltage are variables. When theseconditions exist, the exposure chart maytake a simplified form as shown in Fig. 31,which allows the kilovoltage for anyparticular specimen thickness to bechosen. Such a chart will be particularlyuseful when uniform sections must beradiographed in large numbers byrelatively untrained persons. This type ofexposure chart may be derived from achart similar to Fig. 30 by following thehorizontal line corresponding to thechosen milliampere minute value andnoting the thickness corresponding to thisexposure for each kilovoltage. Thesethicknesses are then plotted againstkilovoltage.

Gamma Ray ExposureChartsA typical gamma ray exposure chart isshown in Fig. 32. It is somewhat similarto Fig. 30; however, with gamma rays,there is no variable factor correspondingto the kilovoltage. Therefore, a gamma rayexposure chart contains one line, orseveral parallel lines, each of whichcorresponds to a particular film type, filmdensity or source-to-film distance. Gammaray exposure guides are also available inthe form of linear or circular slide rules.

These contain scales on which the variousfactors of specimen thickness sourcestrength and source-to-film distance canbe set and from which exposure time canbe read directly.

Characteristic CurveThe characteristic curve, sometimesreferred to as the sensitometric curve or theH and D curve (after Hurter and Driffield,who first used it in 1890), expresses therelation between the exposure applied toa photographic material and the resultingphotographic density. The characteristiccurves of three typical films, exposedbetween lead foil screens to X-rays, aregiven in Fig. 33. Such curves are obtainedby giving a film a series of knownexposures, determining the densitiesproduced by these exposures and thenplotting density against the logarithm ofrelative exposure.

Relative exposure is used because thereare no convenient units, suitable to allkilovoltages and scattering conditions, inwhich to express radiographic exposures.The exposures given a film are expressedin terms of some particular exposure,giving a relative scale. In practicalradiography, this lack of units for X-rayintensity or quantity is no hindrance, aswill be seen below. The logarithm of the


168 Rad

FIGURE 34. Characteristic curve of film Z (see Fig. 33).

Den

sity

D

4.0

3.5

3.0

2.5

2.0

1.5

1.0

Film z

3

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relative exposure, rather than the relativeexposure itself, has a number ofadvantages. It compresses an otherwiselong scale. Furthermore, in radiography,ratios of exposures or intensities areusually more significant than theexposures of the intensities themselves.Pairs of exposures having the same ratiowill be separated by the same interval onthe log relative exposure scale, no matterwhat their absolute value may be.Consider the pairs of exposures in Table 5.

As can be seen in Fig. 33, the slope (orsteepness) of the characteristic curves iscontinuously changing throughout thelength of the curves. For example, twoslightly different thicknesses in the objectradiographed transmit slightly differentexposures to the film. These twoexposures have a certain small log Einterval between them; that is, they havea certain ratio. The difference in thedensities corresponding to the twoexposures depends on just where on thecharacteristic curve they fall; the steeperthe slope of the curve, the greater is thisdensity difference. For example, the curveof film Z (Fig. 33) is steepest in its middleportion. This means that a certain log Einterval in the middle of the curvecorresponds to a greater density differencethan the same log E interval at either endof the curve. In other words, the filmcontrast is greatest where the slope of thecharacteristic curve is greatest. For film Z,as has been pointed out, the region ofgreatest slope is in the central part of thecurve. For films X and Y, however, theslope — and hence the film contrast —continuously increases throughout theuseful density range. The curves of mostindustrial radiographic films are similar tothose of films X and Y.

Use of Characteristic CurveThe characteristic curve can be used insolving quantitative problems arising inradiography, in the preparation oftechnique charts and in radiographicresearch. Characteristic curves made

iographic Testing

TABLE 5. Equivalent exposure ratios.

Log Interval inRelative Relative Log Relative

Exposure Exposure Exposure

1 0.0 } 0.705 0.70

2 0.30 } 0.7010 1.00

30 1.48 } 0.70150 2.18

under actual radiographic conditionsshould be used in solving practicalproblems. However, it is not alwayspossible to produce characteristic curvesin a radiography department and curvesprepared elsewhere must be used. Suchcurves prove adequate for many purposesalthough it must be remembered that theshape of the characteristic curve and thespeed of a film relative to that of anotherdepend strongly on developingconditions. The accuracy attained whenusing ready made characteristic curves isgoverned largely by the similarity betweenthe developing conditions used inproducing the characteristic curves andthose for the films whose densities are tobe evaluated.

Quantitative use of characteristiccurves are worked out in Figs. 34 and 35.Note that D is used for density and log Efor logarithm of relative exposure.

In the first example (Fig. 34), suppose aradiograph made of film Z with anexposure of 12 mA·min has a density of0.8 in the region of maximum interest. Itis desired to increase the density to 2.0 forthe sake of the increased contrast thereavailable.

0.5

00 0.5 1.0 1.5 2.0 2.5 3.0


Legend1. Log E = 1.62 at D = 2.0.2. Log E = 1.00 at D = 0.8.3. Difference in log E is 0.62.

2

1

FIGURE 35. with lead fo

Den

sity

D

4

3

3

2

2

1

1

0

0

Legend1. Log E = 12. Log E = 13. Difference

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1. Log E at D = 2.0 is 1.62.2. Log E at D = 0.8 is 1.00.3. The difference in log E is 0.62. The

antilogarithm of this difference is 4.2.

Therefore, the original exposure ismultiplied by 4.2 giving 50 mA·min toproduce a density of 2.0.

In the second example (see Fig. 35),film X has higher contrast than film Z atD = 2.0 and also a finer grain. Supposethat, for these reasons, it is desired tomake the radiograph on film X with adensity of 2.0 in the same region ofmaximum interest.

1. Log E at D = 2.0 for film X is 1.91.2. Log E at D = 2.0 for film Z is 1.62.3. The difference in log E is 0.29. The

antilogarithm of this differenceis 1.95.

Therefore, the exposure for D = 2.0 onfilm Z is multiplied by 1.95, giving97.5 mA·min for a density of 2.0 onfilm X.


Characteristic curves of two X-ray films exposedil screens.

.0

.5

.0

.5

.0

.5

.0

.5

0 0.5 1.0 1.5 2.0 2.5 3.0


.91 at D = 2.0.

.62 at D = 2.0. in Log E is 0.29.

Film Z

3

2

1

Film X

170 Radi

PART 5. Radiographic Image Quality and DetailVisibility

TABLE 6. Fac

___________Subject

Affec

Specimen thRadiation quScattered ra

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Controlling FactorsBecause the purpose of most radiographictesting is to examine specimens forheterogeneity, a knowledge of the factorsaffecting the visibility of detail in thefinished radiograph is essential. Table 6shows the relationships of the variousfactors influencing image quality andradiographic sensitivity; following are afew important definitions.

Radiographic sensitivity is a general orqualitative term referring to the size of thesmallest detail that can be seen in aradiograph or to the ease with which theimages of small details can be detected.Phrased differently, it is a reference to theamount of information in the radiograph.Note that radiographic sensitivity dependson the combined effects of twoindependent sets of factors: radiographiccontrast (the density difference between asmall detail and its surroundings) anddefinition (the abruptness and thesmoothness of the density transition). SeeFig. 36.

Radiographic contrast is the difference indensity between two areas of aradiograph. It depends on both subjectcontrast and film contrast.

Subject contrast is the ratio of X-ray orgamma ray intensities transmitted by twoselected portions of a specimen. Subjectcontrast depends on the nature of thespecimen, on the energy (spectralcomposition, hardness or wavelengths) ofthe radiation used and on the intensityand distribution of the scattered radiationbut is independent of time, milliamperageor source strength, distance and thecharacteristics or treatment of the film(Fig. 11).

ographic Testing

tors controlling radiographic sensitivity.

Radiographic Contrast____________________________________________________ Contrast Film Contrastted by Affected by

ickness variations Type of filmality Development time, temperature and

diation DensityActivity of the developer

Film contrast refers to the slope(steepness) of the characteristic curve ofthe film. It depends on the type of film,on the processing it receives and density.It also depends on whether the film’sexposure is direct, with lead screens orwith fluorescent screens. Film contrast isindependent, for most practical purposes,of the wavelengths and distribution of theradiation reaching the film and hence isindependent of subject contrast.

The steepness of the characteristiccurve is sometimes referred to as gamma(Γ). Higher gamma films have morecontrast.

Definition refers to the sharpness ofoutline in the image. It depends on thetypes of screens and film used, theradiation energy (wavelengths) and thegeometry of the radiographic setup.

Subject ContrastSubject contrast decreases as thekilovoltage is increased. The decreasingslope (steepness) of the lines of theexposure chart (Fig. 30) as kilovoltageincreases illustrates the reduction ofsubject contrast as the radiation becomesmore penetrating. For example, consider asteel part containing two thicknesses, 19and 25 mm (0.75 and 1 in.), which isradiographed first at 160 kV and then at200 kV.

In Table 7, column 3 shows theexposure in milliampere minutes requiredto reach a density of 1.5 through eachthickness at each kilovoltage. These dataare from the exposure chart in Fig. 30. Itis apparent that the milliampere minutesrequired to produce a given density at any

Radiographic Definition________ ___________________________________________________Geometrical Factors Graininess Factors

Affected by Affected by

Focal spot size Type of filmagitation Distance from focal point to film Type of screen

Distance from specimen to film Radiation qualityAbrupt specimen thickness variations DevelopmentContact of screen to film

TABLE 7. Exposure of steel part containing twothicknesses.

Exposure toVoltage Thickness Give D = 1.5 Relative Ratio of

(kV) mm (in.) (mA·min) Intensity Intensities

160 20 (0.75) 18.5 3.8 } 3.825 (1.0) 70.0 1.0

200 20 (0.75) 4.9 14.3 } 2.525 (1.0) 11.0 5.8

FIGURE 36. Radiographic definition:(a) advantage of higher contrast is offset bypoor definition; (b) despite lowercontrast better rendition of detail is obtainedby improved definition.

(b)

(a)

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kilovoltage are inversely proportional tothe corresponding X-ray intensitiespassing through the different sections ofthe specimen. Column 4 gives theserelative intensities for each kilovoltage.Column 5 gives the ratio of theseintensities for each kilovoltage.

Column 5 shows that, at 160 kV, theintensity of the X-rays passing throughthe 19 mm (0.75 in.) section is 3.8 timesgreater than that passing through the25 mm (1 in.) section. At 200 kV, theradiation through the thinner portion isonly 2.5 times that through the thicker.Thus, as the kilovoltage increases, theratio of X-ray transmission of the twothicknesses decreases, indicating a lowersubject contrast.

Film ContrastThe dependence of film contrast ondensity must be kept in mind whenconsidering problems of radiographicsensitivity. In general, the contrast ofradiographic films, except those designedfor use with fluorescent screens, increasescontinuously with density in the usabledensity range. Therefore, for films thatexhibit this continuous increase incontrast, the best density to use is thehighest that can be conveniently viewedwith the illuminators available. Adjustablehigh intensity illuminators arecommercially available and greatlyincrease the maximum density that canbe viewed.

High densities have the furtheradvantage of increasing the range ofradiation intensities that can be usefullyrecorded on a single film. InX-radiography, this in turn permits use ofthe lower kilovoltage, resulting inincreased subject contrast andradiographic sensitivity.

The slope of screen film contrastbecomes steep at densities greater than2.0. Therefore, other things being equal,the greatest radiographic sensitivity willbe obtained when the exposure isadjusted to give this density.

Film Graininess and ScreenMottleThe image on an radiographic film isformed by countless minute silver grains,the individual particles being so smallthat they are visible only under amicroscope. However, these small particlesare grouped together in relatively largemasses visible to the naked eye or with amagnification of only a few diameters.These masses result in a visual impressioncalled graininess.


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All films exhibit graininess to a greateror lesser degree. In general, the slowerfilms have lower graininess. Thus, film Y(Fig. 33) would have a lower graininessthan film X.

The graininess of all films increases asthe radiation energy increases, althoughthe rate of increase may be different fordifferent films. The graininess of theimages produced at high kilovoltagesmakes the slow, inherently fine grainfilms especially useful in the megavoltand multimegavolt range. When sufficientexposure can be given, fine grain films arealso useful with gamma rays.

Lead screens have no significant effecton film graininess. However, graininess isaffected by processing conditions, beingdirectly related to the degree ofdevelopment. For instance, ifdevelopment time is increased for thepurpose of increasing film speed, thegraininess of the resulting image islikewise increased and vice versa.However, adjustments in developmenttechnique made to compensate forchanges in temperature or activity of adeveloper will have little effect ongraininess, because they are made toachieve the same degree of developmentas would be obtained in the freshdeveloper at a standard processingtemperature.

Another source of irregular density inuniformly exposed areas is the screenmottle encountered in radiography withfluorescent screens. The screen mottleincreases markedly as hardness of theradiation increases. This mottle limits theuse of fluorescent screens at high voltageand with gamma rays. Yet another sourceof mottle occurs when some films areexposed to megavolt radiation. This ismost noticeable in radiography ofmaterials of uniform thickness.

Image Quality IndicatorsA standard test piece is usually includedin every radiograph as a check on theadequacy of the radiographic method.The test piece is commonly referred to asa penetrameter or an image quality indicator(IQI). The image quality indicator is asimple geometric form made of the samematerial as, or a material similar to, thespecimen being radiographed. It containssome small structures (holes, wires andothers), the dimensions of which bearsome numerical relation to the thicknessof the part being tested. The image of theimage quality indicator on the radiographis permanent evidence that theradiographic examination was conductedunder proper conditions.

Codes or agreements between customerand vendor may specify the type of image

iographic Testing

quality indicator, its dimensions and howit is to be employed. Even if image qualityindicators are not specified, their use isadvisable because they provide aneffective check on the quality of theradiographic inspection and evidence thatradiographic sensitivity is achieved.

Hole Image Quality IndicatorsA common image quality indicator in theUnited States consists of a smallrectangular piece of metal, containingseveral (usually three) holes, the diameterof which are related to the thickness ofthe image quality indicator (Fig. 37).

The ASTM International plaque typeimage quality indicator4 contains threeholes of diameters T, 2T, and 4T, where Tis the thickness of the image qualityindicator. Because of the practicaldifficulties in drilling tiny holes in thinmaterials, the minimum diameters ofthese three holes are 0.25, 0.50 and1.00 mm (0.01, 0.02, and 0.04 in.),respectively. Thick image qualityindicators of the hole type would be verylarge because of the diameter of the 4Thole. Therefore, image quality indicatorsmore than 0.46 mm (0.180 in.) thick arein the form of disks, the diameters ofwhich are four times the thickness (4T)and which contain only two holes, ofdiameters T and 2T. Each image qualityindicator is identified by a lead numbershowing the thickness in thousandths ofan inch.

The ASTM International image qualityindicator permits the specification of anumber of levels of radiographicsensitivity, depending on therequirements of the job. For example, thespecifications may call for a radiographicquality level of 2-2T. The first symbol, 2,indicates that the image quality indicatorshall be 2 percent of the thickness of thespecimen; the second symbol (2T)indicates that the hole having a diametertwice the image quality indicatorthickness shall be visible on the finishedradiograph. The quality level 2-2T isprobably the one most commonlyspecified for routine radiography.However, critical components may requiremore rigid standards and require a level of1-2T or 1-1T. On the other hand, theradiography of less critical specimens maybe satisfactory if a quality level of 2-4T or4-4T is achieved. The more critical theradiographic examination — that is, thehigher the level of radiographic sensitivityrequired — the lower the numericaldesignation for the quality level.

Another ASTM International imagequality indicator design required by somespecifications is the wire type that consistsof sets of wires arranged in order ofincreasing diameter (Fig. 38).


FIGURE 37. Image quality indicator of ASTM International, according to ASTM StandardE 1025: (a) design for image quality indicator type numbers 5 to 20, with tolerances of±0.0005; (b) design for image quality indicator type numbers 21 to 59 with tolerances of±0.0025 in. and for image quality indicator type numbers 60 to 179, with tolerance of±0.005 in.; (c) design for image quality indicator type numbers over 180, with tolerances of±0.010 in. (Except for relative thickness T, all measurements in these diagrams are in inches;1.00 in. = 25.4 mm.)

(a)

Identificationnumber

4 T diameter

0.5 in.

T

0.438 in.

T diameter

2 T diameter

0.25 in.

0.75 in.

0.25 in.

1.5 in.

(b)

(c)

2 T

1.33 T

4 T

0.83 T

T

T

Identificationnumber

4 T diameter

1.0 in.

T

T diameter

2 T diameter

0.375 in.

0.75 in.

2.25 in.

1.375 in.

0.375 in.

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174 Rad

FIGURE 38. Examples of wire type image quality indicators:(a) ASTM Standard E 747 (set B, Alternate 2); (b) DeutscheIndustrie Norm 54109, German standard image qualityindicator.

(a)

(b)

6.35 mm (0.25 in.)minimum lead letters

and numbersLargest wire number

Encapsulatedbetween clear vinylplastic of 1.52 mm(0.06 in.)maximumthickness

Length minimum25.4 mm (1.0 in.)for sets A and B

5.08 mm (0.200 in.) (minimumdistance between axis of wires is

not less than 3 times wirediameter and not more than

5.08 mm [0.200 in.])

6 wiresequallyspaced

Setidentification

number

Materialgrade

number

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All sections of the ASME Boiler andPressure Vessel Code require an imagequality indicator identical to the ASTMplaque or wire type image qualityindicator.5

Equivalent Image QualityIndicator SensitivityIdeally, the image quality indicator shouldbe made of the same material as thespecimen. However, this is sometimesimpossible because of practical oreconomic difficulties. In such cases, theimage quality indicator may be made of aradiographically similar material — thatis, a material having the sameradiographic absorption as the specimenbut which is better suited to the makingof image quality indicators. Tables ofradiographically equivalent materials havebeen published, grouping materials withsimilar radiographic absorptions. Inaddition, an image quality indicator madeof a particular material may be used in theradiography of materials having greaterradiographic absorption. In such a case,there is a certain penalty on radiographictechnicians because they are setting morerigid radiographic quality standards forthemselves than those which are actuallyrequired. This penalty is often outweighedby avoiding the problems of obtainingimage quality indicators for an unusualmaterial.

In some cases the materials involveddo not appear in published tabulations.Under these circumstances thecomparative radiographic absorption oftwo materials may be determinedexperimentally. A block of the materialunder test and a block of the materialproposed for image quality indicators,equal in thickness to the part beingexamined, can be radiographed side byside on the same film with the techniqueto be used in practice. If the film densityunder the proposed image qualityindicator material is equal to or greaterthan the film density under the specimenmaterial, that proposed material issuitable for fabrication of image qualityindicators.

In practically all cases, the imagequality indicator is placed on the sourceside of the specimen, in the leastadvantageous geometric position. In someinstances, however, this location for theimage quality indicator is not feasible. Anexample would be the radiography of acircumferential weld in a long tubularstructure, using a source position withinthe tube and film on the outer surface. Insuch a case film side image qualityindicator must be used. Some codesspecify the film side image qualityindicator that is equivalent to the source

iographic Testing

side image quality indicator normallyrequired.

When such a specification is not made,the required film side image qualityindicator may be found experimentally. Inthe example above, a short section of tubeof the same dimensions and materials asthe item under test would be used in theexperiment. The required image qualityindicator would be placed on the sourceside and a range of image qualityindicators on the film side. If the sourceside image quality indicator indicated thatthe required radiographic sensitivity wasbeing achieved, the image of the smallest

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visible hole or wire size in the film sideimage quality indicators would be used todetermine the image quality indicator andthe hole or wire size to be used on theproduction radiographs.

Although the smallest visible holecriterion is the best one in most cases forplaque image quality indicators, in somecases the best criterion is the smallestvisible hole in the thinnest image qualityindicator. In rare cases neither of theserules of thumb is correct. Therefore, forcritical applications, the best criterion forequivalent sensitivity should bedetermined by calculations based on thevisible features of the film side imagequality indicators and using the equationsgiven in the appendix to ASTM E 1025.4

Sometimes the shape of the part beingexamined precludes placing the imagequality indicator on the part. When thisoccurs, the image quality indicator maybe placed on a block of radiographicallysimilar material of the same thickness asthe specimen. The block and the imagequality indicator should be placed as closeas possible to the specimen.

Wire Image Quality IndicatorsA number of wire image quality indicatordesigns are in use. The ASTM E 747 imagequality indicator6 and the European wireimage quality indicator7,8 (Fig. 38) arewidely used. These consist of a number ofwires of various diameters sealed in aplastic envelope that carries the necessaryidentification symbols. The image qualityis indicated by the thinnest wire visibleon the radiograph. The system is suchthat only three image quality indicators,each containing seven wires, can cover avery wide range of specimen thicknesses.Sets of Deutsche Industrie Norm imagequality indicators are available inaluminum, copper and steel whereasASTM image quality indicators areavailable in three light metal and fiveheavy metal groups.

Comparison of Image QualityIndicator DesignsThe hole type image quality indicator is, ina sense, a go/no-go gage; that is, it indicateswhether or not a specified quality level hasbeen attained but, in most cases, does notindicate whether requirements have beenexceeded or by how much. The wire imagequality indicator on the other hand is aseries of image quality indicators in asingle unit. As such, they have theadvantage that the radiographic qualitylevel achieved can often be read directlyfrom the processed radiograph.

The hole image quality indicator canbe made of any material that can beformed into thin sheets and drilled but

the wire image quality indicator is onlymade from materials that can be formedinto wires. A quality level of 2-2T may bespecified for the radiography of, forexample, commercially pure aluminumand 2024 aluminum alloy, even thoughthese have appreciably differentcompositions and radiation absorptions.The hole image quality indicator would,in each case, be made of the appropriatematerial. To achieve the same quality ofradiographic inspection for equalthicknesses of these two materials, itwould be necessary to specify differentwire diameters — that for 2024 alloywould probably have to be determined byexperiment.

Special Image Quality IndicatorsSpecial image quality indicators have beendesigned for certain classes ofradiographic testing. An example is theradiography of electronic components inwhich some of the significant factors arethe continuity of fine wires or thepresence of tiny balls of solder. Specialimage quality indicators have beendesigned consisting of fine wires andsmall metallic spheres within a plasticblock.9

The block is covered on top andbottom with steel about as thick as thecase of the electronic component.

Image Quality Indicators andVisibility of DiscontinuitiesIt should be remembered that even if acertain hole in an image quality indicatoris visible on the radiograph, a cavity ofthe same diameter and thickness may notbe visible. The image quality indicatorholes, having sharp boundaries, result inabrupt, though small, changes in metalthickness whereas a natural cavity havingmore or less rounded sides causes agradual change. Therefore, the image ofthe image quality indicator hole is sharperand more easily seen in the radiographthan is the image of the cavity.

Similarly, a fine crack may be ofconsiderable extent but if the X-rays orgamma rays pass from source to film in adirection other than parallel to the planeof the crack, its image on the film maynot be visible because of the very gradualor small transition in photographicdensity. Thus, an image quality indicatoris used to indicate the quality of theradiographic method and not to measurethe size of flaw which can be shown.

In the case of a wire image qualityindicator, the visibility of a wire of acertain diameter does not ensure that adiscontinuity of the same cross sectionwill be visible. The human eye perceivesmuch more readily a long boundary than


176 Rad

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it does a short one, even if the densitydifference and the sharpness of the imageare the same. However, the equivalencybetween the hole and wire ASTMInternational image quality indicators wasdeveloped on the basis of empirical dataas well as theoretical numbers.

Viewing and InterpretingRadiographsThe viewing of the finished radiograph isdiscussed elsewhere in this volume.

iographic Testing

PART 6. Film Handling and Storage

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Film HandlingRadiographic film should always behandled carefully to avoid physicalstrains, such as pressure, creasing,buckling, friction and others. The normalpressure applied in a cassette to providegood contacts is not enough to damagethe film. However, when films are loadedin semiflexible holders and externalclamping devices are used, care should betaken to ensure that this pressure isuniform. If a film holder bears against afew high spots, such as those that occuron an unground weld, the pressure maybe great enough to produce desensitizedareas in the radiograph. Precaution isparticularly important when usingenvelope packed films.

Crimp marks or marks resulting fromcontact with fingers that are moist orcontaminated with processing chemicalscan be avoided if large films are graspedby the edges and allowed to hang free. Aconvenient supply of clean towels is anincentive to dry the hands often and well.Envelope packed films avoid theseproblems until the envelope is opened forprocessing. Thereafter, of course, the usualcare must be taken.

Another important precaution is toavoid withdrawing film rapidly fromcartons, exposure holders or cassettes.Such care will materially help to eliminateobjectionable circular or treelike blackmarkings in the radiograph, the results ofstatic electric discharges.

The interleaving paper must beremoved before the film is loadedbetween either lead or fluorescent screens.When using exposure holders withoutscreens, the paper should be left on thefilm for the added protection that itprovides. At high voltage, direct exposuretechniques are subject to the problemsmentioned earlier: electrons emitted bythe lead backing of the cassette orexposure holder may reach the filmthrough the intervening paper or felt andrecord an image of this material on thefilm. This effect is magnified by lead orfluorescent screens. In the radiography oflight metals, direct exposure techniquesare the rule and the paper folder shouldbe left on the interleaved film whenloading it in the exposure holder.

Ends of a length of roll film factorypacked in a paper sleeve should be sealed

in the darkroom with black pressuresensitive tape. The tape should extendbeyond the edges of the strip 7 to 13 mm(0.25 to 0.5 in.) to provide a positive lighttight seal.

Identifying RadiographsBecause of their high absorption, leadnumbers or letters affixed to the filmholder or test object furnish a simplemeans of identifying radiographs. Theymay also be used as reference marks todetermine the location of discontinuitieswithin the specimen. Such markers can beconveniently fastened to the film holderor object with adhesive tape. A code canbe devised to minimize the amount oflettering needed. Lead letters arecommercially available in a variety of sizesand styles. The thickness of the chosenletters should be great enough so thattheir image is clearly visible on exposureswith the most penetrating radiationroutinely used. Under some circumstancesit may be necessary to put the lead letterson a radiation absorbing block so thattheir image will not be burned out. Theblock should be considerably larger thanthe legend itself.

Flash box identification should beincluded where a corner of a radiograph isblocked with lead to minimize exposure.The unexposed corner is flashed withlight transmitted through typed or handwritten information exposed onto thefilm.

Shipping of UnprocessedFilmsIf unprocessed film is to be shipped, thepackage should be carefully andconspicuously labeled, indicating thecontents, so that the package may besegregated from any radioactive materials,high heat or pressure. It should further benoted that customs inspection ofshipments crossing internationalboundaries sometimes includesfluoroscopic inspection. To avoid damagefrom this cause, packages, personnelbaggage and the like containingunprocessed film should be plainlymarked and the attention of inspectorsdrawn to their sensitive contents.


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Storage of UnprocessedFilm

X-Ray Film StorageWith X-rays generated up to 200 kV, it isfeasible to use storage compartments linedwith a sufficient thickness of lead toprotect the film. At higher kilovoltages,protection becomes increasingly difficult;film should be protected not only by theradiation barrier for protection ofpersonnel but also by increased distancefrom the source.

At 100 kV, a 3 mm (0.125 in.) thicknessof lead should normally be adequate toprotect film stored in a room adjacent tothe X-ray room if the film is not in theline of the direct beam. At 200 kV, thelead thickness should be increased to6.4 mm (0.25 in.).

With megavolt X-rays, films should bestored beyond the concrete or otherprotective wall at a distance at least fivetimes farther from the X-ray tube than thearea occupied by personnel. The storageperiod should not exceed the timesrecommended by the manufacturer.

These rules of thumb may be ignored ifsuitable radiation surveys indicateradiation levels low enough to avoidfogging during the maximum time periodthat the film will be stored.

Storage near Gamma RaysWhen radioactive material is not in use,the shielding container in which it isstored helps provide protection for film.In many cases, however, the container fora gamma ray source will not providesatisfactory protection to storedradiographic film. In such cases, theemitter and stored film should beseparated by a sufficient distance toprevent fogging.

Heat, Humidity and FumesThe effects of heat, humidity and fumeson stored film are discussed elsewhere.2

Storage of Exposed andProcessed FilmArchival storage is a term commonly usedto describe the keeping quality ofradiographic film. It has been defined bythe American National Standards Institute(ANSI) as those storage conditions suitablefor the preservation of photographic filmhaving permanent value. The AmericanNational Standards Institute does notdefine archival storage in years but interms of the thiosulfate content (residual

ographic Testing

fixer) permissible for storage ofradiographs.

Although many factors affect thestorage life of radiographs, one of themost important is the residual thiosulfateleft in the radiograph after processing anddrying. Determined by the methyleneblue test, the maximum level is 2 mg·cm–2

on each side of coarse grain radiographicfilms. For short term storagerequirements, the residual thiosulfatecontent can be at a higher level but thislevel is not specified by the AmericanNational Standards Institute.

Washing of the film after developmentand fixing, therefore, is very important.The methylene blue test and silverdensitometric test are laboratoryprocedures performed on clear areas ofthe processed film.

Temperature and humidity should becarefully controlled. Radiographic filmshould be stored with precautionsspecified in ASTM E 1254.10

Storage SuggestionsRegardless of the length of time aradiograph is to be kept, these suggestionsshould be followed to provide formaximum stability of the radiographicimage.

1. Avoid storage in the presence ofchemical fumes.

2. Avoid short term cycling oftemperature and humidity.

3. Place each radiograph in its ownfolder to prevent possible chemicalcontamination by the glue used inmaking the storage envelope (negativepreserver). Several radiographs may bestored in a single storage envelope ifeach is in its own interleaving folder.

4. Never store unprotected radiographsin bright light or sunlight.

5. Avoid pressure damage caused bystacking a large number of radiographshorizontally in a single pile or byforcing more radiographs than cancomfortably fit into a single filedrawer or shelf.

Radiographic film offers a means ofprecise discontinuity detection anddocumentation. Despite the introductionof digital means of image capture, displayand storage, film radiography willcontinue to be an important part ofnondestructive testing well into thetwenty first century.

MicrofilmRadiographic film images can be copied tomicrofilm and microfiche for storage. Amicrofiche the size of a postcard can storemore than a hundred radiographic

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images. Like film negatives, microfilm andmicrofiche require climate control toprevent degradation of the medium whenstored for years.

In the twenty-first century,microfilming services offer imagedigitization and will provide the imageson compact disks or digital video disks.Film digitization is discussed below.


180 Radio

PART 7. Film Digitization

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Film digitization is the conversion ofexisting radiographic film images to adigital format for electronic imageanalysis, management, transmission andstorage. Film digitization also permitsdisposal of film that would degrade overtime.

In film digitization, film densities areconverted into digital values by measuringlight transmitted through the film andassigning each measurement a digitalvalue and a particular location. Threeprimary parameters affect the resultingimage quality of film digitization: densityrange, density resolution and spatialresolution (pixel size).

Optical density (OD) is a logarithmicfunction. This means that at opticaldensity 0, 100 percent of the incidentphotons are transmitted through the film;at 1.0 optical density, only 10 percentmake it through; at 2.0 optical density,only 1 percent; and at 3.0 optical density,only 0.1 percent. It is difficult to measurehigher densities with the same accuracyand precision as lower densities.

There are two basic methods ofdigitizing film. The first method makesthe measurement using a diffuse lightsource and a charge coupled device (CCD)whereas the second uses a combination ofa laser and a photomultiplier tube(PMT).11,12

Charge Coupled DeviceFilm Digitization SystemsA charge coupled device film digitizerilluminates the full width of the film witha diffuse light source and then uses a lenssystem to focus the light down to the sizeof the charge coupled device elements.The charge coupled device is a siliconsemiconductor device consisting of a largenumber of gridlike elements sensitive tolight. When light energy impinges on thecharge coupled device elements, thephotons generate a charge within eachelement. Periodically, the element isdischarged and the amplitude of thecharge is measured. In this way, lightamplitude can be converted to aproportionate electrical signal related tothe density at any given point.

In film digitization systems, a lineararray of charge coupled device elements isused with optics to focus the film image

graphic Testing

onto the much smaller charge coupledelement. A narrow line of diffuse light ispassed through the film and thetransmitted light is focused onto thecharge coupled device array, one line at atime. Once one line of data is collected, asecond line is then scanned.

The total density range of chargecoupled devices can be affected if, whenthere is a rapid and drastic change in lightlevel, the charge coupled devicemomentarily becomes saturated. Theimage may be corrected by changing thesampling time (integration period). Athigh light levels, the integration period isreduced to avoid saturation of the chargecoupled device whereas at low light levels,the integration period is increased toachieve an adequate ratio of signal tonoise. To obtain optical density dynamicranges up to five, multiple scans may beperformed at varying charge coupleddevice integration periods and scanspeeds.

The density resolution of chargecoupled device digitizers is determined bythe conversion of the logarithmic densityscale to a linear voltage scale. Forexample, at an optical density of zero,maximum light passes through the film,so the charge coupled device elementproduces the maximum voltage. Becauseoptical density is a logarithmic function,only 10 percent of the light transmitted atan optical density of 0 will be detected atan optical density of 1. If the chargecoupled device’s maximum voltage iscalibrated to be 10 V, then the voltageoutput at an optical density of 1 is 1.0 V,at 2 it is 0.1 V, at 3 it is 0.01 V and at 4 itis 0.001 V. Therefore, if the chargecoupled device output of 10 V is digitizedat 12 bits or 4096 density levels, then anoptical density from 0 to 1 will producean output of 9 V, which equates to3686 density levels (90 percent of 4096).The output of densities from 1 to 2 willequate to 369 levels; densities from 2 to 3,to 37 levels; and densities from 3 to 4,4 levels. While this results in a nonlineardensity resolution, it is similar to theoriginal image and as a result is adequatefor many purposes. It is important thatthe application and image analysisrequirements are thoroughly understoodbefore a particular digitizer is used.

Another aspect of charge coupleddevices is spatial resolution. The elements

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can be arrayed along one or twodimensions. The array’s resolution issometimes given as the element sizebecause all charge coupled device chipshave a large number of elements (forexample, 4000 or 6000) that in theory canbe digitized into a like number of pixels.For example, the 1 cm2 chip in a videocamera is said to have a resolution ofabout 20 µm. Once the various focusinglens aberrations are coupled together, thetrue resolving capability of the chargecoupled device chip is low. A home videocamera is surely not able to discernobjects 20 µm apart.

Therefore, it is important todifferentiate between the chipspecifications versus those of the imagingsystem. The resolution of the imagingsystem depends in part on the quality ofthe focusing optics and in part on thecross talk between charge coupledelements (as when one photon activatesmore than one element).

Laser Film DigitizationSystemsLaser scanners use a nonimagingphotomultiplier tube to detect lighttransmitted through the film. The laserbeam is a focused beam of coherent lightof known value and is transmittedthrough the film at one discreet point.The transmitted light is then detected bythe photomultiplier tube and digitizedinto a value directly proportional to thedensity of the film at that point.

The photomultiplier tube has a widedynamic range, a good ratio of signal tonoise and uses a log amplification processsuch that a uniform density resolution ismaintained over the entire range. The logamplifier normalizes the extremely highnumber of photons detected at lowoptical densities versus the relatively lownumber of photons detected at highoptical densities. For example, if a laserfilm digitizer converts each optical densitymeasurement into a pixel value 1000× theoptical density at that point, then thereare 1000 levels from 0 to 1 optical density,from 1 to 2 optical density, from 2 to 3optical density, and so on. This wouldprovide a density resolution of 0.001optical density at all levels.

The spatial resolution of a laser scanneris determined by the point of laser lightthat impinges on the film. Because thereis only a single beam, there is no crosstalk between pixels and a true limitingresolution equal to the laser spot size canbe achieved. Typical laser spot sizes are100 µm (0.004 in.) and 50 µm (0.002 in.).However, the actual resolution will

depend on overall laser beam quality,detector noise and electronic noise.

It is important that the application andimage analysis requirements of aparticular digitizer are thoroughlyunderstood before it is used. There areseveral ways to determine experimentallythe performance of a film digitizer. Onemethod is to scan a modulation transferfunction (MTF) pattern to validateperformance. Another, simpler method, isto place a strip of cellophane tape overthe image of a step wedge. Thistranslucent tape will produce a densitydifference of about 0.03 optical density.The idea here is to demonstrate (1) thedynamic range of the system (when thesteps begin to be difficult to differentiate)and (2) the density resolution (at whatdensity the tape can no longer be seen).

Other ConsiderationsBeyond the technical issues of thescanning scheme, other issues are relatedto image size relative to the display andstorage medium. All the information maybe captured in memory, but both theprocessor size and monitor size limit whatcan be worked with or displayed.

For example, let us assume that eachpixel contains 12 bits of grayscaleinformation plus 4 bits of headerinformation. This equates to 16 bits or2 bytes. The image size then, is the totalnumber of pixels times two bytes. If thescan resolution is 100 µm (0.004 in.),then a 14 × 17 in. image would contain3500 pixels (14 in. ÷ 0.004 in.) ×4300 pixels (17 in. ÷ 0.004 in.) or a totalof about 1.5 × 107 pixels. Having 2 bytesof information per pixel results in animage size of 30 megabytes. If the scanresolution were increased to 50 µm(0.002 in.), then the image size wouldincrease by 4× to 120 megabytes.

Some monitors may not be able todisplay the entire image. For example,cathode ray tubes may have displayresolutions of 1200 × 1600 or 2000 ×2500. Therefore, it is important toremember that, depending on themagnification of the image on themonitor, there may actually be more rawdata available than are displayed. Todisplay an image that has either more orfewer data displayed than are in the rawimage, pixel mapping techniques areused. Pixel replication or pixelinterpolation are used when magnifyingbeyond the image resolution. Whenreducing the image size, pixel averaging isused.

Digital images may contain 12-bit (ormore) digital data that must be displayedon a monitor that only displays 8 bits(about what the human eye can discern).


182 Ra

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A method must then be employed to mapthe original 4096 gray scale levels of dataonto the available 256 display levels. Thisis commonly done either (1) by selectingwhich 256 levels of the original 4096 aredisplayed or (2) by equally dividing the4096 levels over the available 256.

diographic Testing

1.Quinn, R.A. and C.C. Sigl, eds.Radiography in Modern Industry, fourthedition. Rochester, NY: EastmanKodak Company (1980).

2.Nondestructive Testing Handbook,second edition: Vol. 3, Radiography andRadiation Testing. Columbus, OH:American Society for NondestructiveTesting (1985).

3.Nondestructive Testing Handbook,second edition: Vol. 10, NondestructiveTesting Overview. Columbus, OH:American Society for NondestructiveTesting (1996).

4.ASTM E 1025, Standard Practice forDesign, Manufacture, and MaterialGrouping Classification of Hole-TypeImage Quality Indicators (IQI) Used forRadiology. Philadelphia, PA: AmericanSociety for Testing and Materials.

5.ASME Boiler and Pressure Vessel Code.New York, NY: American Society ofMechanical Engineers.

6.ASTM E 747, Standard Practice forDesign, Manufacture and MaterialGrouping Classification of Wire ImageQuality Indicators (IQI) Used forRadiology. West Conshohocken, PA:ASTM International.

7.DIN 54109. Non-Destructive Testing;Image Quality of Radiographs;Recommended Practice for DeterminingImage Quality Values and Image QualityClasses. Berlin, Germany: DeutscheInstitut für Normung [GermanInstitute for Standardization] (1989).Superseded by EN 462 (DIN).

8.EN 462 P1 (DIN), Non-DestructiveTesting — Image Quality of Radiographs— Image Quality Indicators (Wire Type)and Determination of Image QualityValue. Brussels, Belgium: EuropeanCommittee for Standardization(1994).

9.ASTM E 801, Standard Practice forControlling Quality of RadiologicalExamination of Electronic Devices. WestConshohocken, PA: ASTMInternational (2001).

10.ASTM E 1254, Standard Guide forStorage of Radiographs and UnexposedRadiographic Film. WestConshohocken, PA: ASTMInternational (1998).

11.“CCD Versus Laser Film DigitizationSystems.” Liberty Technologies,Incorporated, Imaging SystemsDivision.

12.Soltani, P.K., C.R. Chittick, T. Chuang,M.J. Dowling, G.R. Kahley andT.E. Kinsella. “Advances in 2DRadiography for IndustrialInspection.” Presented at theInternational Conference on QualityControl by Artificial Vision. Le Creusot,France: Institut Universitaire deTechnologie (May 1997).


References

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Engineering