ASNT NDT Handbook Volume 4 Infrared and Thermal Testing

artifacts from static electricity, handling or diffuser and wash it. Then paint the
moisture. In addition, adequate inside with a good, durable white enamel
ventilation is needed to keep harmful finish.
chemical fumes from accumulating and
affecting either the darkroom personnel Avoiding Static
or the radiographic film.
There are two ways to avoid markings on
Cleaning Tanks X-ray films. One is to prevent the
generation of static electricity; the other is
Corrosion seldom occurs when the tanks to cause such charges, once generated, to
are full of normal chemical solutions and leak off gradually rather than to discharge
are kept clean. rapidly, which is what causes damage.

Deposits often form on the walls of the The most successful procedure is to
developer tanks because of the action keep a high relative humidity in the
between mineral salts dissolved in the surrounding atmosphere. An accurate
water and carbonate in the developing instrument for measuring relative
solutions. These deposits can be removed humidity, called a psychrometer, is a
by using commercially prepared stainless valuable addition to any radiographic
steel tank cleaner. Follow the directions of darkroom. Periodic checks on prevailing
the manufacturer, being sure to rinse the darkroom humidity enable one to take
tank walls with fresh water. Wipe the tank special precautions necessary to minimize
out with a clean cloth or cellulose sponge. the generation and discharge of static
electricity. The relative humidity in the
Clean the exterior stainless steel before darkroom should be between 40 and
any deposits can attack the surface. Wipe 60 percent.
with a cloth and warm soapy water and
then rinse, making sure no soap deposits The following precautions will be of
get into the chemistry. Once a week, use a assistance in overcoming the most
stainless steel cleaner according to the common causes of static:
directions on the label.
1. If using X-ray film that has
Always give special attention when interleaving paper, handle film gently.
cleaning welds and corners where deposits Let the interleaving paper fall away
can cling. If deposits are heavy, remove from the film and place film in the
the worst of them with fiber brush or cassette gently, without sliding it over
plastic cleaning pad, then polish with a the screen.
stainless steel cleaner. If an abrasive is
required, use a very fine sandpaper. 2. Following X-ray exposure, the cassette
or holder should be opened slowly
Use caution never to use metallic and the film removed carefully. The
abrasives, steel wool or wire brushes, as reason for careful handling is that the
they can contaminate the surface of film is more than twice as susceptible
stainless steel. Any foreign metal particles to an energy source once it is exposed.
will cause corrosion and may contaminate Thus, a film will react to much smaller
chemicals. Do not use commercial steel electrical discharges after exposure.
wool pads or strong detergents, because
these are hard on the stainless steel and 3. Move slowly when handling the film.
could react unfavorably with the 4. Make sure everything is grounded.
chemistry. 5. Use X-ray antistatic salt screen cleaner

Cleaning Illuminators regularly.
6. Avoid static generating synthetic
Quite frequently, good radiographs will
appear dull because they are viewed on clothing.
faulty illuminators. Illuminators are faulty
when the glass plate is dirty or bulbs of Grounding. Electrically ground the metal
different wattage, age, color or size are top of the film loading bench, film bin,
used. Old interior paint that is dull or X-ray table, pass boxes and other
dusty will cause the radiograph to appear equipment such as processors. In the
dull. Use a regular photographic exposure darkroom, avoid nonconductive floor
meter to test the illuminator. Identical covering (rubber tile), hard floor waxes on
radiographic studies should be viewed at concrete, rubber soled and plastic soled
the same intensity. An amperage meter shoes, intensifying salt screens with worn
can be installed to control uniform surfaces and using a dry cloth to clean
output. intensifying screens. A camel hair brush
or vacuum cleaner should be used for
Wash the outside of the viewing glass dusting and a lintfree cloth and screen
plate every day. Once a month wash out cleaner with antistatic solution should be
the inside of the view box. Always use used for washing intensifying salt screen
bulbs with similar intensities. When surfaces. In periods of low ambient
conditions indicate paint deterioration, humidity (winter time or northern
unplug the illuminator, remove the front climates), when static is prone to occur,
antistatic solution can be applied to

Radiographic Film Development 239

intensifying screens as added protection people use is to write on each box the
against static discharges. date when it is received. Whatever system
is used, be sure to keep records and always
Common Static Marks. Three kinds of use the oldest film first. Film must be
static markings are illustrated in Fig. 15. stored and inventoried by expiration date,
Crown and tree are considered to be results film brand, type (class) and speed.
of heavier electric discharges. They can be
generated by very rapid motions, such as Film boxes should be stored on their
occur when film is removed from edges. This distributes the weight and
interleaving paper, when interleaving helps protect the film from pressure
paper breaks contact with the film or marks.
when the film is touched by fingers.
Smudge static markings may result from When using one box at a time or when
photographic exposure to visible light there is no film bin, always be sure to fold
produced by sparks in air next to the film. over or close the bag and to replace the
Smudge static is produced when relatively lid after each film is removed. Film is
low potential discharges occur over a large packaged in hermetically sealed light tight
area. and moisture proof pouches — for
example, black polyethylene or
Color Conditioning aluminized plastic. Once the pouch is
opened to expose the fresh film there is
Surfaces above the working area should be still enough bag remaining to fold over to
finished with a dark matte paint to make the bag light tight again. This
minimize reflected light. feature is not to obviate the box top or
film bin but is an added safeguard.
Storing X-Ray Film
Cassettes should not be stored for
Recommended storage conditions for all prolonged periods of time loaded with
types of X-ray films are temperatures film. Load cassettes and holders with fresh
between 18 and 24 °C (65 and 75 °F) and film before each use.
40 to 60 percent relative humidity.

Usually most radiographic facilities will
have two storage areas. One area is for
long term supplies and another, usually
the darkroom, is for short term needs. In
either case, it is required that a stock
rotation plan be instituted. The plan is
quite simple in that as new film arrives, it
is placed on the right side of the supply.
As the film is needed, it is removed from
the left side. This is called a FIFO system:
first in, first out. To assist in rotating film
boxes, remember that all film boxes have
an emulsion number and an expiration
date on the end label. A system many

FIGURE 15. Static electricity markings:
(a) smudge; (b) tree; (c) crown.
(a)
(b)

(c)
240 Radiographic Testing

PART 4. Processing Technique3

Principle thoroughly but not too violently or too
long.
Processing completes what the exposure
starts. After the exposure is made, the film If the processing solutions do not seem
is removed from the cassette and placed to be working well enough, answer these
on a hanger. The film is processed. This is four questions.
the simplicity of radiographic processing
but there is obviously more to forming a 1. Are the tanks really clean?
visible image radiograph. 2. Are solutions properly mixed? Were

There is indeed an exactness to manual they mixed in the right sequence?
processing. This is a controlled scientific Were they overagitated?
process in which something in one form 3. Are temperature and timer accurate?
(the latent image) is converted into 4. Are the exposure techniques accurate?
another form (the visible image). No
matter how superior the exposure Water for Processing
techniques are, if anything less than
optimal processing technique is permitted The water temperature for mixing should
then an image less than optimal will be ± 3 °C (±5 °F) of the manufacturer’s
result. recommendation.

Equipment and Practice The rule of thumb is to use water of
drinking quality. In spite of the variety of
Thermometers impurities in water supplies, most city
water in the United States is pure enough
In developing X-ray films by the time and for photographic processes.
temperature system, an accurate
thermometer is of the utmost importance. Time and Temperature Technique
Service thermometers should be checked
at regular intervals against a thermometer The time and temperature technique
whose accuracy is known. They should be provides the controlled basis for obtaining
graduated in degrees or, better, in half consistent, optimal radiographic
degrees. information. This scientific technique also
permits alteration of the processing cycle
A thermometer should be read while it to suit specific needs and requirements.
is inserted in the thoroughly mixed
solution. To avoid parallax, the Regardless of the optimal time and
thermometer should be held so that an temperature recommendations for a given
imaginary line from the eye forms a right chemistry and film, it must be
angle to the axis of the thermometer. To remembered that everyone has different
further avoid this problem, a bimetallic or likes and desires. With this in mind,
electronic thermometer might be consider that faster processing, higher or
considered. lower contrast, greater speed, less density
and other considerations can be altered
Safety Warning. Never use glass through the judicious use of the time and
thermometers containing mercury or temperature technique. At a given
iodine. If the glass breaks, the mercury or temperature, longer times will
iodine can be hazardous to personnel and overdevelop the film, increasing density.
developer. At a given time, higher temperatures will
cause overdevelopment fogging (increased
Mixing Solutions noninformational density) of the film.
This does not mean that sight
Make sure the solution tanks are clean. development is advocated. No one should
Carefully read the instructions on the use anything but the specific time and
chemical container. Mix as recommended temperature technique.
at the suggested temperature to ensure
satisfactory performance. Avoid high Developers
temperatures, inclusion of air and
contamination during preparation. Stir Optimal radiographs require correct
development. The developing time to be
followed differs with the processed film
type, the developer type and the
processing solution temperature.

Radiographic Film Development 241

Radiographs require just the right Processing quality control is a procedure
amount of contrast. Too much reduces of monitoring to see if, and to what
the range of densities covered by a single extent, there is consistency. Consistency is
exposure; the thinner parts become too necessary before quality can be improved,
dark and the thickness ones too light. because the variables of processing must
Conversely, insufficient contrast, though be identified, their degree of fluctuation
it affords more latitude in exposure, and cycling patterns noted and the limits
lessens the total differentiation, thereby of acceptability established. Once the
obscuring fine details. To achieve an ideal uncontrolled variables are identified, the
contrast, a given type of X-ray film should best control measure will be more
be developed in the same brand of apparent. Sometimes, the variable may
solutions using the time and temperature have to be compensated for, minimized or
and replenishment characteristics eliminated. Also, in this process of
supplied by the manufacturer. identifying and controlling the variables,
indications are often provided as to the
Compensating for Developer best way to consistently achieve optimum
Exhaustion quality.

Replenisher Technique. Replenishers Sensitometry
replace the reducing agents as they are
exhausted and, if added correctly, obviate The study and measurement of
adjustment of developing time to relationships between exposure,
maintain constant density and contrast processing conditions and film response
over the useful life of the developer. to exposure are known as sensitometry. The
Replenishers should be mixed as directed properties of a film that affect or govern
on the label and should be added the relationships are known as
frequently and in small amounts to sensitometric properties. Quality is defined
maintain the developer level; if an by the sensitometry of the visible image.
amount greater than 10 percent of the Processing is the vital link between the
developer tank volume is added at one latent image and the visible image
time, fog may increase. The remaining radiograph; careful control of the many
replenisher should be kept in a tightly factors involved is essential. Establishing
stoppered bottle or in a plastic jug or tank processor quality control to maximize
with a floating lid and dust cover. uniformity is desirable.

Because developer exhaustion depends Control Strips. Each day control strips are
on the type of film emulsion and the film processed periodically and read on a
density as well as the film area, the densitometer. The changes in density
quantity of replenisher per 355 × 430 mm levels of the exposure are plotted and the
(14 × 17 in.) film will not be constant. As characteristic curve is generated. Speed
a rule, about 90 cm3 (3 oz) of replenisher and contrast can be determined. This
are required for each 355 × 430 mm system is the professional approach to
(14 × 17 in.) film processed. quality control. The matched exposure
radiographs are easily duplicated from day
Quality Control of to day. Check the processor, note the
Processing conditions mentioned below and process
the control strip. This control strip
Maintenance of the time and temperature becomes the master and subsequent daily
technique of processing depends on some strips are simply matched to it. Deviations
system of quality control. Quality control from the master strip necessitate
kits are available to give experience to interpretation, investigation, and
operators and help them set up their own corrective actions. The master test strip
program. A simple quality control test is should be processed under optimum
to use the same cassette or film holder conditions of chemistry and processor
and a step wedge with known exposure, performance.
film and processing techniques. Establish
a control film and routinely make test Density. Density and adequate differences
films that can be compared to the control in density (contrast) are considered the
film. Keep exact records as a personal most important of all properties in the
teaching and record file. radiograph. Proper densities and adequate
contrast make visible the structural details
Processing of a radiograph is done in within the image of the object.
an automated processor to achieve Radiographic density has been defined as
consistent quality. The processing system the amount of film blackening that is the
is a chemical process with specific result of metallic silver deposits remaining
conditions of time and temperature based on the film after exposure and processing.
on a given chemical activity. Good A useful way to measure the amount of
processing makes good radiographs. film blackening is to measure its
interference with a beam of light passing
through a radiograph. The amount of

242 Radiographic Testing

light absorbed by the film is measured of radiation by the subject. Although
with a densitometer. radiographic contrast can be altered by
changing one or both of these factors, it is
Sensitometrically, density is defined as good practice to standardize the film and
the common logarithm of the ratio of the processing procedure and to control
amount of light striking one side of the radiographic contrast by changing subject
radiograph compared to the amount of contrast. Contrast can be changed easily
light that passes out the other side. When by adjusting the kilovoltage or general
the metallic silver in the emulsion allows exposure technique to alter the quality of
one tenth of the light to pass through, radiation. Sensitometrically, contrast refers
this ratio is 10:1. The common logarithm to the slope or steepness of the
of 10 equals 1 and the silver deposit is characteristic curve of the film; this is also
said to have a density of 1. Density can called gamma.
now be defined by the equation:
Gamma. Gamma is the slope or steepness
(8) D = log Ii of the straight line portion of the
It characteristic curve. In plotting a
characteristic curve, density (a logarithmic
where D is density, Ii is intensity of light value) is most often plotted against log
incident to the film and It is intensity of relative exposure. Exposure is defined as
light output transmitted through the film. the intensity multiplied by the time; it
can be expressed in exposure units, such
The data obtained by sensitometric as J·cm–2 (107 erg·cm–2) of X-radiation.
procedures are usually plotted in the form Relative exposure is much more
of graphs. Figure 16 shows a typical convenient and equally useful.
characteristic curve of an X-ray film X-radiography exposure is expressed in
exposed with intensifying salt screens. terms of milliampere seconds (mA·s) or
The portion of the curve designated as the milliampere minutes (mA·min). Then if
toe demonstrates the nonlinear response the amperage is doubled, the exposure is
of the emulsion to relatively small doubled, kilovoltage peak remaining
amounts of radiant energy. With uniform constant. If the kilovoltage remains
increases in exposure, the density builds constant, the ratios of the exposure
up slowly until the linear response part of reaching the film through two different
the curve. Along this straight line the regions of the subject are always the same,
density increases uniformly with the no matter what the values of
logarithm of the exposure until the milliamperage, time or distance from focal
nonlinear shoulder of the curve is reached. point to film. For example, two exposures,
The shoulder is produced when slat one of which is twice the other, will
screens are used. Additional exposure always be separated by 0.3 on the
results in smaller increases in density to a logarithmic exposure scale (the logarithm
point where additional exposure does not of 2 being 0.3).
produce greater density.
Speed. It has been determined that the
Contrast. Contrast by definition is the contrast of a film is indicated by the shape
difference between two densities. As a of the characteristic curve. Speed is
radiograph is viewed on an illuminator, indicated by the location of the curve
the difference in brightness of the various along the exposure axis. The faster film
parts of the image is called radiographic will lie toward the left of the graph. In
contrast. This is the product of two Fig. 17, film A is faster than film B but has
distinct factors: (1) film contrast, inherent
in the film and influenced by the FIGURE 17. Speed shifts.
developing process; and (2) subject
contrast, a result of differential absorption

FIGURE 16. Characteristic curves. 5.0 Class II film

5.0 Class IV film Class II film 4.0
salt screens direct exposure
Density
Density4.03.0 Class IV Class I film
with B
Shoulder
salt screens
3.0 2.0

2.0 Straight line 1.0 A
portion

1.0 Toe

12345 1 234 5
Relative log E Relative log E

Radiographic Film Development 243

the same contrast. The separation of the then proceeds to form soluble compounds
films is a measure of the speed difference. that are removed by washing.
Film A requires less exposure to achieve a
speed point of density 2.00 above base Inadequate fixing leaves small amounts
plus fog. Film B requires more exposure, of these complexes that cannot be
so it is said to respond slower or be less removed by normal washing. In time,
fast. The convenience of using relative these residues break down and react with
exposure also applies to speed. The speed the silver image to form silver sulfide
of one film can be expressed on a basis (Ag2S). This sulfide is usually yellow but
relative to another film when one is made may range from pink to brown. It cannot
the standard of comparison. This be easily removed from the radiograph.
reference film can be assigned any Extremely minute amounts of residual
arbitrary speed value, such as 100. If thiosulfates are sufficient to cause serious
another film requires only half the deterioration. Therefore, the radiograph
exposure to reach the same density as the may appear transparent and at the same
reference film, the faster film will have a time be inadequately fixed. For this
relative speed of 200. A density of 2.00 reason, close attention to recommended
above base plus fog has been designated fixing technique is mandatory.
as the density to compute film speed. This
density has been chosen because it Washing
represents the minimum useful density
range for much of radiography. For lower Radiographs must be washed thoroughly
density films, speeds are often calculated to prevent discoloration with age and to
at the lower density of 1.00. ensure preservation of the image.

Fixer Drying

Failure to fix a film sufficiently results in Avoid temperatures above 49 °C (120 °F)
its discoloring with age. A good rule of in drying and maintain a steady volume
thumb for determining the minimum of air across the film surface. An
fixing time is as follows. After a film has excessively low relative humidity within
cleared, leave it in the fixer an additional the dryer may cause water spot drying
two times as long as it took to clear. For marks and streaks on the film surface.
example, if it takes 1 min for a film to
clear, then it should be left in the fixer at Safety
least an additional 2 min or a total period
of 3 min. Developer, with its hydroquinone and
alkalinity, forms a very hazardous
To prepare fixing baths from liquid solution. Always use good ventilation
concentrate fixer, it is essential that when mixing chemicals. A nose filter or
directions on the container be followed. respirator is suggested when powdered
Attention is paid only to the temperature chemicals are mixed. Goggles are required
of the developer, because fixer by the United States Occupational Safety
temperature is less critical; however, at and Health Administration to protect the
high temperatures, it is important that the eyes.10 In addition an eye wash station is
processing baths be maintained at about required. If developer gets into the eye the
the same temperature. Changes in worker must begin washing within 15 s
temperature cause the gelatin of the and continue washing for 15 min
emulsion to swell and contract. When the minimum. Washing for 1 h is preferred.
temperatures of two baths differ
excessively, this change in the gelatin The developer is most hazardous but
takes place so abruptly that unevenness is care must be taken when working with
likely to result. The effect produced on fixers also. In addition to the above,
the film is known as reticulation — the rubber gloves will protect the skin and
gelatin breaks, producing fine cracks. keep chemicals out of cuts; rubberized or
plastic aprons will protect the worker and
Archival Quality. The importance of clothing.
adequate fixing to archival keeping
quality cannot be overemphasized. Poorly Clearing Film Base
fixed films will not deteriorate until many
months after processing. The lighter Liquid laundry bleach will dissolve the
portion of a radiograph may become gelatin and produce a clean, clear sheet of
yellow and the image may tend to fade. base plastic. Warm solutions work faster.
This delayed action may be traced back to Enzymes could be used but are very
the nature of the fixing process, which is hazardous.
believed to consist of two steps.
Undeveloped silver halide is first
converted to silver thiosulfate complexes
that are only slightly soluble. The reaction

244 Radiographic Testing

Removing Developer Stains from chemicals to various locations and to
Hands return spent fixer to central collection and
recovery area.
Developer solutions should be considered
hazardous and hands should not be Daylight Installation. In daylight, a film
routinely submerged in them. Rubber dispenser dispenses films into hard
gloves should be worn. When manually cassettes. After exposure the film is fed
processing or working on automatic directly into a processor, sitting in a
processor rack components where contact lighted area, by means of an unloading,
is prevalent, be sure to rinse hands feeding device. Film is never touched by
frequently in fresh water. In manual humans until after processing. Processors
processing make sure hands are rinsed can be placed virtually anywhere there are
while rinsing or fixing the film. utilities. There is no darkroom or
associated personnel.
Automatic Processing
Darkroom Installation. In a conventional
Automatic processing is a chemical darkroom, the processor may be installed
reaction performed in a machine; it is totally inside the darkroom. The best way
often spoken of as if the machine were is to put the bulk of the unit outside the
the important part. However, since the darkroom to reduce heat inside. Some
introduction in 1957 of a roller transport people put the bulk of the unit inside the
processor, processors have provided darkroom so that any jams can be cleared
improved consistency over manual in the dark. The disadvantage of this
processing. But humans control the system is that all service requiring white
machine as they control manual light necessitates closing down the
processing, only less frequently. Keep in darkroom. An alternative would be to put
mind that automatic processing produces the bulk of the unit in the outer room but
consistent quality; the quality will be construct the room so that it can become
consistently good or bad depending on a darkroom.
the operator.
All processors should have a minimum
There are three distinct advantages to of 600 mm (24 in.) access on a side. If this
automatic processing: consistent quality, is not possible the processor can be made
improved quality and economy of time portable on wheels or skids with quick
and labor. Given a well functioning, disconnects to allow easy service. Near the
properly adjusted machine, film after film processor there should be a floor sink for
will be of better quality from the machine washing racks or the entire processor may
than from a person sight developing. As a be placed over a large grill work with a
result, processors have found their way drain below.
into small and large laboratories, trailers
and the backs of pickup trucks. Automatic Feeders. Automatic feeders are
available for some makes of processors,
Automatic processing developer which allow a stack of films to be placed
chemistry has a hardener that is in the feeder and the lid closed; the
extremely important in controlling the operator can do other jobs as the feeder
amount of emulsion swell. An automatically feeds films. Feeders require
overswollen film can result in adjustment and periodic monitoring. In
overdevelopment fog, uncleared films, addition, it may take twice as long to feed
poorer archival quality, wet films and one sheet of film automatically as it
increased transport problems. Automatic would manually, because of the cycle time
chemistry is usually replenished and delay of the mechanism, especially at
automatically to sustain volume and the faster processing times (in excess of
activity. 7 min).

In an institution or laboratory all the Darkroom Workflow
processors may be located in one central
darkroom and processing area or in An automatic processing darkroom shares
several areas. The major advantage of many of the features of a conventional
centralization is that one darkroom darkroom but there are significant
person can feed several processors, saving differences. No provision need be made
space and manpower. The main advantage for hangers or hanger storage. Deep tanks
of dispersed or decentralized processing is for developing and fixing solutions and
that the processor can be placed in for washing and drying may be
different areas, such as production, eliminated, because these operations take
quality control and research, to reduce place within the automatic processor;
down time and the confusion of however, deep tanks may be retained for
intermixed films. The major disadvantage training and emergency use. Only the
is increased space and manpower. It is a input end of the processor needs to be
good idea, with dispersed processing, to located within the darkroom. Dual
centralize the mixing and distribution of processor installation is convenient. To
handle peak loads, both machines are

Radiographic Film Development 245

operated simultaneously; one suffices at
other times.

Processing Chemicals

Processing chemicals for automatic
equipment differ from deep tank
chemicals. Never attempt to use
conventional chemicals in automatic
processors. Although the basic chemical
reactions are similar, automatic processing
chemicals are especially formulated for
high speed roller operation. Modern
automatic processing chemicals make
possible maximum ease of use and
uniformity of finished radiographs,
regardless of the make of processor.

Although all chemicals operate
efficiently with any make or speed of film,
they generally provide best results with
the same brand of film. In addition, using
companion products causes fewer
variables and is better understood by a
manufacturer.

All ingredients, plus full instructions,
are included in each package of developer,
fixer and starter solution. These chemicals
are prepared in strict conformity with
basic formulas known to produce
excellent results. When properly mixed
and cared for, they operate efficiently over
long periods of time.

Finally, clean the work area and
equipment and put the equipment away.
Then install the crossovers and prepare
the processor for operation. This
installation of fresh chemistry should be
entered in the processor log book.

Cleanup and Inspection

Operating instructions for each processor
also include suggestions for cleaning and
inspection. In general, these procedures
require little time and trouble but this
does not minimize their importance. They
contribute substantially to efficient
operation and to maintenance of
optimum film quality.

Solution Services

In many localities, specialized
organizations handle the mixing and
maintenance of chemicals for automatic
processing equipment. These services
include routine inspection, cleaning and
refilling. Although the services are
provided by professionals, it is still good
policy for each X-ray department to have
its own in-house specialists and to
perform routine, daily inspection.

246 Radiographic Testing

PART 5. Silver Recovery3

The scarcity and high price of silver make safe but require constant pH adjustment
its recovery from fixing baths important by a technician.
for ecological, environmental and
economic reasons. Fixer contains about Electrolytic Recovery. Silver is recovered
40 percent of the original silver in the by passing an electric current (direct
film. Laws and standards in the United current) through the silver laden fixer.
States establish limits for dissolved silver Electrolytic recovery systems (called cells)
and forbid its disposal in drains.10,11 This are classified as agitated or nonagitated,
consideration is in addition to silver’s high or low current density units. The two
scarcity and value. Even the smallest user terminals (electrodes) are a positively
should treat the used fixer to remove charged anode (usually graphite) and a
silver — the money recovered will pay for negatively charged cathode of stainless
the effort. steel. Positively charged silver ions (Ag+),
are attracted to the cathode, where they
Recovery Techniques plate out as metallic silver (Ag°), called
flake. This is the most efficient technique
The techniques of recovery are chemical for medium or larger installations.
(which includes precipitation and metallic
replacement) and electrolytic. Metallic Efficiency of Silver Recovery
replacement is simplest but requires a low Equipment
volume continuously. Electrolysis is
recommended for higher volume During processing, the developer converts
processors. the latent image bearing silver halide
crystals into a visible image — black
Metallic Replacement. If a fixer solution metallic silver. Those crystals exposed but
containing silver ions is brought into not developed and those not exposed are
contact with a metal, the less noble metal washed out in the fixer. In industrial
(such as steel wool, zinc, copper or steel radiography the silver is dissolved into the
turnings) is replaced by the silver. The fixer in a ratio of more than 6 g·L–1
silverless fixer cannot be reused. (1 troy oz·gal–1) of fixer. Silver recovery or
reclamation is the process of converting
Metallic replacement units, also called the silver to metallic silver. Understanding
buckets or cartridges, contain steel wool the factors that control the efficiency of
or zinc screen and are usually used in this operation will help in understanding
tandem. As the acid fixer breaks down the and upgrading existing systems or in
less noble steel, the more noble silver generating specifications for new systems.
metal precipitates as metallic silver. A
sludge of iron oxide and silver forms in Dwell Time. Sufficient time is required for
the bottom of the container. the reaction to occur. Electrolytic units
(cells) are rated in troy ounce per hour
This technique is both inexpensive and capacity. Buckets are rated in cubic
efficient. It can remove 60 to 95 percent centimeters per minute or gallons per
of the silver: 1 kg (2.2 lbm) of steel wool hour of fixer flow. Exceeding these limits
will collect 3 to 4 kg (6 to 9 lbm) of silver. will result in silver going through the unit
However, efficiency is based on a slow, and down the drain.
steady, continuous flow of silver laden
fixer. Efficiency is about perfect for the Agitation. Buckets provide agitation by
first 25 percent of the unit’s life flowing the fixer over the many wire
expectancy, 400 to 800 L (100 to 200 gal) filaments. Electrolytic cells use pumps or
of silver laden fixer and then often impellers. Greater physical agitation
becomes only 30 percent effective. Also, increases the unit’s efficiency in
the sludge produced by this technique is producing metallic silver and allows
expensive to ship and refine. higher plating currents to be used.
However, agitation should not be so
Chemical Precipitation. Precipitation is a violent as to cause splatter, spillover or
chemical reaction that separates the silver excess evaporation.
from the solution in an insoluble, solid
form. This type of unit, particularly those Surface Area. The larger the surface area
that use sodium sulfide and zinc chloride, the higher the plating current can be in a
produces toxic and volatile fumes and so cell. In any unit, increased area increases
should be avoided. The units using recovery rate.
sodium borohydride are very efficient and

Radiographic Film Development 247

Edge Effect. The edge effect is related to reduces fixer consumption and silver lost
surface area and electron flow efficiency; into the wash. The major disadvantages
the more edges or surface area, the better are increased cost of the cell and
the efficiency. potentially higher hypo retention levels
because of reduced fixer efficiency. Fixer
Electrolysis. As the acid fixer enters the can only be recirculated from cells —
bucket the steel wool is attacked, never from buckets or precipitation.
producing chemical electrolysis. The steel The following precautions should be
becomes oxidized to iron oxide and the observed when installing or operating any
silver in solution becomes metal. Cells silver recovery units:
contain an anode and a cathode, a
rectifier and a transformer to pass direct 1. Make sure than fixer overflow to the
current through the solution. The higher silver recovery unit is a continuous
the current, the greater the efficiency. Too downward flow.
high a current (usually when little silver is
present) can break down the fixer, this is 2. Clean the standpipe on metal
called sulfurization and is to be avoided. exchange units regularly.

Maintenance. All units require records, 3. Ensure against air locks in electrolytic
regular inspection and regular units.
maintenance to ensure proper use.
Buckets can clog, back up or leak. Cells 4. Be sure there is an air break
can become too loaded with silver, short (electrolytic break) between the
out, blow a fuse or burn up an agitation solution in the electrolytic recovery
pump. The amperage should be unit and the incoming fixer. Without
automatically or manually adjusted one, there is danger of plating silver in
according to the film (silver) volume the processor fixer tank.
during the day.
5. Use the highest amperage possible for
Centralization. Centralized recovery is the optimal recovery — but keep it short
most efficient system where three or more of sulphurization (characterized by
units are involved. A holding tank feeds a yellow color and smell of rotten eggs).
single cell a continuous supply for High amperage produces soft, black
optimal efficiency. silver; low amperage produces hard,
shiny silver but of approximately
Silver Estimation equal quality. The higher amperage
helps to ensure removal of most of the
Silver estimating paper, which indicates silver.
the relative amount of silver per gallon of
used fixer, can be readily purchased from Scrap Film
most silver reclaimers. The test strips are
used just like pH paper strips. Industrial Films that are to be discarded also have
radiography usually operates at a level of value for their silver content. About
about 10 g·L–1 (1.2 ± 0.2 troy oz·gal–1). 60 percent of the original silver remains
in the film to form the visible image. Both
An even simpler technique of waste film and outdated records are
determining if there is any silver in the valuable and should be sold for their
solution is to put a brightly polished silver content.
copper tube in the solution. Any silver in
the used fixer will quickly adhere to the Security and Selling Recovered
copper tube and give it a gray color. Silver

Purchase tailing or central electrolytic In radiography, business economics
units according to the calculated capacity revolve around the cost of producing a
in gram per second. Collect the fixer in visible image versus the value or price of
the processor at cleaning time. If a silver the product. The single largest budget
recovery unit malfunctions, disconnect it item is manpower, which must be used
or isolate it so that it cannot ruin efficiently.
radiographs being processed.
Radiographic film is expensive. It must
Silver Recovery Installations be kept in the best possible condition and
protected against abuse and theft,
Tailing units may be buckets or cells whether in its fresh or used form, through
placed individually or in tandem on each inventory controls such as records, policy
processor. Usually buckets are used in procedures and security. Such programs
tandem, with the second becoming the will pay for themselves in improved
first after every 400 to 1300 L (100 to earnings.
300 gal) of fixer, depending on brand and
size. Properly sized cells should not Silver should be recovered from used
require a tailing unit. fixer and used films to reduce the cost of
the original fresh stock. Because a
Recirculation cells take the fixer from substantial value is represented by
the processor, remove most of the silver recovered silver, it is important to impose
and return the fixer to the processor. This inventory and security controls.

248 Radiographic Testing

Fresh green (unexposed) film and black
or scrap (processed or discarded green)
film represent money that is easily
transported or lost. Both conditions of
X-ray film require inventory controls
including records, proper storage
conditions and security.

Of all the silver in the film, 40 percent
goes into the fixer and 60 percent remains
on the film. Ninety percent of the silver
from the fixer and 70 percent of the film
silver may be recovered, giving a total of
of about 75 percent of the original silver
that is recoverable. Considering that the
film costs are about five times the price of
silver, only 10 to 15 percent of the film’s
retail price may be recovered.

Specific Suggestions

Silver flake is derived from scraping off the
collection plate of an electrolytic silver
recovery unit (cell). There is little
significance whether it is silver colored
and hard (result of low current levels) or
black and soft (result of high current
levels). A properly sized cell will collect 90
to 95 percent of the silver, which will be
95 percent pure. Dry the silver before
weighing. (If weighed wet, deduct five
percent of the weight for trapped
moisture).

Silver sludge from buckets should be
shipped in solution in the bucket.
Draining the fixer exposes the sludge to
air and an exothermic reaction produces
heat. If possible, the sludge should be
dried in a large open pan and then
shipped. The sludge damages the refiner’s
crucibles and the refiner charges more to
handle sludge than flake.

Radiographic Film Development 249

References

1. Rivkin, W.B. and G. Wicks. Sec. 4, 11. 2001 TLVs® and BEIs®. Cincinnati,
“Radiation Detection and Recording.” OH: American Conference of
Nondestructive Testing Handbook, Governmental Industrial Hygienists
second edition: Vol. 3, Radiography and (2001).
Radiation Testing. Columbus, OH:
American Society for Nondestructive Bibliography
Testing (1985): p 174-185.
Books
2. Quinn, R.A. and C.C. Sigl, eds.
Radiography in Modern Industry, Bruce, H.F. Your Guide to Photography,
fourth edition. Rochester, NY: Eastman second edition. New York, NY: Barnes
Kodak Company (1980). and Noble (1974).

3. McKinney, W.E.J. Sec. 7, “Radiographic Bunting, R.K. The Chemistry of
Latent Image Processing.” Photography. Normal, IL: Photoglass
Nondestructive Testing Handbook, Press (1987).
second edition: Vol. 3, Radiography and
Radiation Testing. Columbus, OH: Control Techniques in Film Processing. New
American Society for Nondestructive York, NY: Society of Motion Picture
Testing (1985): p 299-376. and Television Engineers (1960).

4. McKinney, W.E.J. Radiographic Eaton, G.T. Photographic Chemistry in
Processing and Quality Control. Black-and-White and Color Photography,
Philadelphia, PA: J.B. Lippincott fourth edition, revised. Dobbs Ferry,
Company (1988). NY: Morgan and Morgan (1986).

5. Gurney, R.W. and N.F. Mott. Feininger, A. Darkroom Techniques. Garden
Proceedings of the Royal Society of City, NY: Amphoto (1974).
London: Series A, Mathematical and
Physical Sciences. Vol. 164. London, Folts, J.A., R.P. Lovell and F.C. Zwahlen, Jr.
United Kingdom: Royal Society (1938): Handbook of Photography, fifth edition.
p 151. Albany, NY: Delmar Thomson
Learning (2001).
6. Gurney, R.W. and N.F. Mott. Electronic
Processes in Ionic Crystals. Oxford, Gray, J.E. Photographic Processing, Quality
United Kingdom: Clarendon Press Control and the Evaluation of
(1940). Photographic Materials, Vol. 2. HEW
Publication 77-8018. Rockville MD:
7. Marstbloom, K., R. Kochakian, Bureau of Radiologic Health (1977).
B. Vaessen and P. Willems. “Analog
Film Radiography Technology Gray, R.H., ed. Applied Processing: Practice
Advancements.” ASNT Fall Conference and Techniques. Washington, DC:
and Quality Testing Show 2001 Paper Society of Photographic Science and
Summaries Book. Conference [Columbus, Engineering (1968).
OH, October 2001]. Columbus, OH:
American Society for Nondestructive Gregg, D.C. Principles of Chemistry, third
Testing (2001): p 165-167. edition. Boston, MA: Allyn and Bacon
(1968).
8. ASTM E 1815-96, Standard Test Method
for Classification of Film Systems for Haist, G. Modern Photographic Processing,
Industrial Radiography. West Vols. 1 and 2. New York, NY: Wiley
Conshohocken, PA: ASTM Interscience (1979).
International (2001).
Herz, R.H. The Photographic Action of
9. ISO 11699-1, Non-Destructive Testing — Ionizing Radiations in Dosimetry and
Industrial Radiographic Films. Geneva, Medical, Industrial, Neutron, Auto- and
Switzerland: International Microradiography. New York, NY: Wiley
Organization for Standardization Interscience (1969).
(1998).
James, T.H., ed. The Theory of the
10. 29 CFR 1210, Occupational Safety and Photographic Process, fourth edition.
Health Standards. Code of Federal New York, NY: Macmillan (1977).
Regulations: Title 29, Labor.
Washington, DC: United States James, T.H. and G.C. Higgins.
Department of Labor, Occupational Fundamentals of Photographic Theory,
Safety and Health Administration; second edition. New York, NY: Morgan
Government Printing Office. and Morgan (1960).

250 Radiographic Testing

John, D.H.O. Radiographic Processing in ANSI IT9.9-1990, American National
Medicine and Industry. London, United Standards Institute. Standard for Imaging
Kingdom: Focal Press (1967). Media (Film) — Stability of Color
Photographic Methods. New York, NY:
Marcus, A. Basic Electricity. Upper Saddle American National Standards Institute
River, NJ: Prentice-Hall (1958). (1990).

Mason, L.F.A. Photographic Processing ANSI PH1.28-1984, Specifications for
Chemistry. New York, NY: Focal Library Photographic Film for Archival Records,
(1975). Silver Gelatin Type on Cellulose Ester
Base. New York, NY: American
Mitchell, J.W., ed. Fundamental National Standards Institute (1984).
Mechanisms of Photographic Sensitivity
[Bristol, United Kingdom, March ANSI PH4.32-1986, American National
1950]. London, United Kingdom: Standard for Photography (Processing) —
Butterworths Scientific Publications Methods for Evaluating Processing with
(1951). Respect to the Stability of the Resultant
Image — Black-and-White Papers. New
Pauling, L. College Chemistry: An York, NY: American National
Introductory Textbook of General Standards Institute (1986).
Chemistry, fourth edition. San
Francisco, CA: W.H. Freeman (1964). ANSI PH1.41-1984, Photographic Film for
Archival Records, Silver Gelatin Type on
Silver Recovery for Hospitals. Chicago, IL: Polyester Base, Specifications for; When
American Hospital Association (1980). Tested by ANSI Standard PH4.8-1984,
Methylene Blue Method for Measuring
Stiles, E. Handbook for Total Quality Thiosulfate and Silver Densitometric
Assurance. Waterford, CT: National Method for Measuring Residual
Foreman’s Institute (1965). Chemicals in Films, Plates, and Papers;
and Stored in Accordance with ANSI
Todd, H.N. Photographic Sensitometry: A Standard PH1.43-1983, Storage of
Self-Teaching Text. New York, NY: Processed Safety Photographic Film,
Wiley-Interscience (1976). Practices for. New York, NY: American
National Standards Institute (1984).
Todd, H.N. and R.D. Zakia. Photographic
Sensitometry: The Study of Tone ANSI PH4.8-1985, American National
Reproduction. New York, NY: Morgan Standard for Photography (Chemicals) —
and Morgan (1969). Residual Thiosulfate and Other
Chemicals in Films, Plates, and Papers —
Standards Determination and Measurement. New
York, NY: American National
ANSI IT1.15, Photography (Films) — Standards Institute (1985).
Industrial Radiographic Film (Roll and
Sheet) and Metal Intensifying Screens — ANSI/AIIM MS26-1990, American National
Dimensions. New York, NY: American Standard for Determining Illumination
National Standards Institute (1994). Uniformity. New York, NY: American
National Standards Institute (1990).
ANSI IT1.48, Photography (Films) —
Medical Hard Copy Imaging Film — ASTM E 999-99, Standard Guide for
Dimensions and Specifications. New Controlling the Quality of Industrial
York, NY: American National Radiographic Film Processing. West
Standards Institute (1997). Conshohocken, PA: ASTM
International (2002).
ANSI IT2.46, Photography — Industrial
Radiographic Film — Determination of ASTM E 1254-98, Standard Guide for Storage
ISO Speed and Average Gradient When of Radiographs and Unexposed Industrial
Exposed to X- and γ-Radiation. New Radiographic Films. West
York, NY: American National Conshohocken, PA: ASTM
Standards Institute (1997). International (2002).

ANSI IT9.1-1988, American National FED-STD-125D, Film, Photographic and
Standard for Imaging Media (Film) — Film, Photographic Processed (for
Silver Gelatin Type — Specification for Permanent Records Use). Federal
Stability. New York, NY: American Standard. Washington, DC: General
National Standards Institute (1988). Services Administration (1977).

ANSI IT9.18, Imaging Materials — Processed
Photographic Plates — Storage Practices.
New York, NY: American National
Standards Institute (1996).

ANSI IT9.2-1991, American National
Standard for Photography (Processing) —
Processing Films, Plates, and Papers —
Filing Enclosures and Containers for
Storage. New York, NY: American
National Standards Institute (1991).

Radiographic Film Development 251

10

CHAPTER

Radioscopy1

Kenneth W. Dolan, Lawrence Livermore National
Laboratory, Livermore, California
Jerry J. Haskins, Lawrence Livermore National
Laboratory, Livermore, California

PART 1. Fundamentals of Radioscopic Imaging

Principles efficiency is generally lower for higher
energy radiation.
Radioscopy is a nondestructive testing
technique, a subset of the radiographic Light from fluorescent screens may be
testing method, that uses penetrating viewed directly by the human eye,
radiation to produce images viewed amplified in an image intensifier tube
during irradiation. Radioscopic imaging and with video output, or imaged directly by a
real time radiography mean the same as low light level video camera. Low
radioscopy. brightness level, operator dependent
vision acuity and radiation safety are
The arrangement of radiation source, significant disadvantages for viewing by
object and image plane in radioscopic the human eye. Electronic imaging by
imaging are similar to film radiography. image intensifiers and low light level
Whereas traditional radiography uses film video provides both remote viewing for
as the imaging medium, radioscopy uses a radiation safety and improved signal.
fluorescent screen to convert radiation to Image intensifier tubes amplify the light
light for direct viewing or electronic signal by converting the light to electrons
imaging. With electronic imaging systems, in a photocathode layer adjacent to the
the image signal is amplified and input phosphor layer, accelerating the
presented as an analog signal for viewing electrons and then converting back to
on a television monitor, video recording, light in an output phosphor layer. A video
analog processing or for converting to camera provides output signal by imaging
digital for computer display, storage and the output phosphor. Low light level
analysis. video cameras are more recent
developments that provide additional
Radioscopy is a powerful and versatile options for imaging higher output
technique for rapid testing of objects and phosphor screens.
structures and for imaging dynamic
events. For assembly line inspection, Many of the principles of film
robotics and remote positioning allow radiography apply directly to radioscopic
rapid testing at many locations or imaging. The geometric rules for
orientations of an object and provide an unsharpness and magnification are
inspector the freedom to review details of examples. Radioscopic imaging
interest or to move on to other locations. technology, however, is significantly
Accept or reject decisions may be made different from film based imaging and
immediately without the delay or expense therefore the emphasis in some of the
that would be incurred in film processing. radiographic principles is different.
The same applies to large structures such Dynamic range and sensitivity, for
as aircraft or pipelines where robotics instance, are examples of differences
positioning systems can be applied to between these techniques. This chapter
testing of large areas. For dynamic events, presents radioscopic imaging based image
radioscopic imaging is typically used at acquisition by fluoroscopic screen and
television frame rates of 30 frames per video technology. Digital imaging systems
second, providing time resolution in a that use discrete detectors and image
fraction of a second. High rate events can analysis are discussed elsewhere.
be captured with high speed cameras that
operate at tens of thousands of frames per Background
second.
Radioscopic imaging has it roots in the
Fluorescent screens consist of a discovery of X-rays. Roentgen used
phosphor material deposited on a phosphorescent (or fluorescent) screens
substrate. Radiation interacts with the for X-ray detection. The earliest form of
phosphor to produce light. The thickness the radioscopic imaging technique was
of the phosphor coating and the called fluoroscopy. With this technique,
coarseness of the grains affect both light X-rays from a source pass through an
conversion efficiency and resolution. Fine object and strike a fluorescent screen. The
grain, thinner coatings provide higher screen emits light observed by the human
resolution but lower light output. Screen eye. Human vision acuity and dark
brightness or light conversion efficiency adaptation were very important for
also depends on the energy of the
incident radiation. Light conversion

254 Radiographic Testing

optimum interpretation. Modern electronics industry. This chapter
phosphors developed for fluoroscopy discusses the fluorescent screens for
systems have changed dramatically over conversion of X-rays to light, image
the years, resulting in improvements in intensifier tubes and video camera
both quantum efficiency and resolution. technologies and how they can be
effectively used for radioscopic imaging.
The advent of image intensifiers, video
cameras and television systems in the
1950s provided dramatic improvements in
the radioscopic imaging technique and
improved safety by allowing the operator
to be remote from the X-ray source.
Digital imaging and computer systems
have further increased the possibilities of
image acquisition, enhancement,
information storage and display. Robotic
systems, automated data acquisition
systems and both online and offline
image processing have made radioscopic
imaging a very versatile, sensitive, rapid
and safe technique for radiographic
testing in both industry and medicine.

An industrial radioscopic test system is
shown in Fig. 1. The source (X-ray tube),
part manipulator and detector (image
intensifier tube) are located in a shielded
radiation chamber. The parts, already on a
tray, are introduced into the chamber by
conveyor belt and positioned by robotics.
The source and detector system are
located on radial arms and are
repositioned in fixed orientation to each
other to provide different angular views
including different geometric
magnifications. The controls for the X-ray
source, part manipulator, image
processing and image display are located
at a computer workstation (not shown).
Variations in the size of the radiation
chamber make it possible to image smaller
or larger items such as such as castings,
transmission cases and wheels for the
automotive industry, munitions and
rocket motors for the defense and
aerospace industries and electronic
components and assemblies for the

FIGURE 1. Radioscopic imaging system.

Radioscopy 255

PART 2. Light Conversion

Fluorescent Screens which the light photon originates
(dependent on the X-ray stopping power
Principles of Operation of the phosphor); and (4) the spatial
orientation of the free path lengths
Fluorescent screens consist of phosphor between collisions.
particles dispersed in a binder and coated
on a reflecting, supporting base. The basic Light emitted simultaneously with the
function of the fluorescent screen is to excitation energy (X-ray absorption) is
convert X-ray energy to light. This called fluorescence. By contrast, light that
happens in three steps. persists after the excitation source is
removed is called phosphorescence.
1. Absorbed X-ray energy is converted to
high energy electrons. Screen Types and Construction

2. Part of the kinetic energy of the high Fluorescent screens use a variety of
energy electrons is used to produce supports, depending on the application.
excited states within the phosphor Typical screens have a white plastic or
material. cardboard base, about 0.4 mm (0.02 in.)
thick, as a support for the phosphor
3. Light emission occurs when the material layer. The screen base must be
excited states return to their normal chemically inert so as not to react
state. unfavorably with the luminescent
material. It must be uniformly radiolucent
As shown in Fig. 2, an X-ray or gamma and cannot contain radiopaque
ray photon striking the screen deposits inclusions, which might cause shadows
energy in a grain of phosphor at the point on the fluorescent image. The support
z releasing high energy electrons, which must also be durable enough for use in
cause formation of excited states and the radiation fields to be encountered.
emission of light photons. Typical paths
followed by the photons are illustrated. The phosphor material is in the form
of small grains. This material is combined
A light photon generated in the screen with a suitable binder and is coated on
phosphor has a probability of leaving the the support in a uniform layer. Grain size
emitting surface, depending on (1) the and thickness of the phosphor layer are
number of scattering events; (2) the construction parameters affecting
probability of absorption at each collision performance. The final packing density of
with phosphor particles; (3) the depth at the phosphor particles is usually on the
order of 50 percent. A protective surface is
FIGURE 2. Structure of typical X-ray intensifying screen and often added to the screen to help it resist
typical paths followed by light photons. markings and abrasive wear and to permit
cleaning.
X-rays
In X-ray image intensifier tubes, the
z Base fluorescent layer is deposited on the
X-ray photon Reflective layer inside of a vacuum tube envelope. In this
case, phosphors whose physical properties
z Phosphor are unacceptable for use in air, such as
Protective coating hydroscopic sodium activated cesium
Light emission iodide (CsI[Na]), can be used. The
Legend depositing technique is often proprietary.

= Excited grain Screen Characteristics
= Absorption of light photon
Fluorescent screens are characterized by
their efficiency, spectral emission,
persistence, unsharpness and gamma.
Table 1 contains parameters of typical
screen phosphors.

256 Radiographic Testing

Efficiency Spectral Emission

The overall efficiency of the fluorescent Although the spectral emission of
screen in converting X-rays to light, is phosphors is broad band the emission
composed of three terms: (1) the incident spectra are characterized by a maximum
X-ray absorption efficiency na; (2) the intensity at a characteristic wavelength.
intrinsic absorbed X-ray to light Spectral emission is used to match the
conversion efficiency nc; and (3) the light phosphor to application, whether for
transmission efficiency nτ determined by human eye in a direct viewing
the path length of light in the screen fluoroscopy system, a low light level
coatings. camera in a camera based fluoroscopy
system or photocathode for an image
Hence the overall efficiency can be intensifier tube. Figure 3a is a plot of the
expressed in Eq. 1 as: spectral emission of four types of screens:
calcium tungstate (CaWO4), lanthanum
(1) n = na × nc × nτ oxybromide (LaOBr), gadolinium
oxysulfide (Gd2O2S) and zinc cadmium
The absorption efficiency na, which is sulfide (ZnCdS).
the fraction of the incident flux absorbed
by the screen, can be calculated when the The effect of spectral emission is
incident X-ray spectrum and the demonstrated in Table 2 where the
composition of the screen are known. At relative light yield for different phosphors
low energy, where X-ray absorption at 140 kVp X-ray energy are compared for
predominates over scatter, the absorption phototopic response (the human eye) and
efficiency is about equal to the a multialkali photocathode. When
attenuation of the radiation beam. A high measured with a multialkali
absorption efficiency is important for photocathode, the calcium tungstate and
maximizing the signal-to-noise ratio in lanthanum oxybromide screens (which
the detection process. emit in the blue) show an increase in
response over the photopic response,
The conversion efficiency nc is about which has a maximum in the green.
equal to the phosphor efficiency under
cathode ray excitation, which can be Persistence
measured separately. The transmission
efficiency nτ can be estimated if the The persistence of a fluorescent screen is
scattering and absorption parameters of the time over which it continues to emit
the screen are measured and if surface light following excitation. Persistence
conditions are known. Experimental curves are a characteristic of phosphors.
measurements of light photon output for Some curves have an exponential decay,
X-ray photon input with typical whereas others have long decay tails. An
fluorescent screen materials results in an estimate of the persistence can be made
energy efficiency in the range of 1 to 7 by assuming an exponential decay and
percent at low X-ray energies (20 to 100 then assigning a decay constant (time for
kV peak).2

TABLE 1. Phosphor parameters. Chemical Density Emission Decay
Phosphor Symbol
(g·cm–3) Peak (nm) Constant (µs)

Zinc sulfide ZnS (Ag) 4.1 450 0.060

Zinc cadmium sulfide ZnCdS (Ag) 4.5 550 0.085

Sodium activated cesium iodide CsI(Na) 4.5 420 0.650

Calcium tungstate CaWO4 6.1 430 6.00

Terbium activated gadolinium oxysulfide Gd2O2S(Tb) 7.3 544 480.0

TABLE 2. Relative light yield as function of detector measurement.

Chemical Emission Photopic Multialkali Photocathode
Symbol Color
Screen (relative yield) (relative yield)

Zinc cadmium sulfide ZnCdS green 100 100
Gadolinium oxysulfide yellow green 50 50
Calcium tungstate Gd2O2S violet 7 32
Lanthanum bromide CaWO4 blue 4 25
LaOBr

Radioscopy 257

FIGURE 3. Fluorescent screen phosphors: (a) spectral the phosphor to decay by a factor of e–1,
emission; (b) mass absorption coefficient as function of where e is the natural logarithmic base). A
energy. few typical decay constants are listed in
Table 1. The persistence, particularly with
(a) rapid decay phosphors, can vary
significantly depending on the purity and
Radiation intensity (µW·cm–2·nm) 6 x 10–5 the manufacturing process.

4 x 10–5 Unsharpness

2 x 10–5 Unsharpness in images formed by
fluorescent screens is primarily a function
0 400 450 500 550 of the grain size of the phosphor and the
360 Wavelength (nm) screen thickness, increasing as the
parameters increase. Light transmission
(b) characteristics of the screen can also affect
the unsharpness. Figure 4a demonstrates
100 how unsharpness can affect the detection
of a sharp edged discontinuity by
spreading the edge shape. Here, C
represents the contrast in percentage of
brightness change, d represents width of
discontinuity and U represents screen
unsharpness. For a fixed value of U, a
change in contrast C produces a change in
the slope of the unsharp edge. It can be
seen from Fig. 4b that when d is smaller
than 2U, the discontinuity will vanish

Mass absorption coefficient (cm2·gm–1) FIGURE 4. Effect of unsharpness on
discontinuity detection: (a) spread of edge
shape; (b) discontinuity above minimum
contrast level.

10 (a)

Radiation

Object

1 d C Image
U
0.1 U
10 30 50 70 90 110 130 150 C
Energy (keV) (b)

Legend d
Cesium iodide C1
Gadolinium(III) oxysulfide
High speed, calcium tungstate U
Lanthanum(I) hypobromite
Zinc cadmium sulfide Legend

C = Contrast
C1 = Minimum observable brightness difference
d = Dimension
U = Screen unsharpness

258 Radiographic Testing

unless Cl is above the minimum (7) na = 1 – exp − µ 
observable contrast level. Equations 2 to 4  ρ TP

show the relationships among variables in 

Fig. 4: where µ·ρ–1 is the mass absorption
coefficient, T the phosphor thickness and
(2) C1 ≤ C P the packed phosphor density.

(3) d ≤ 2C1U The energy spectrum of the source,
C such as the continuum spectrum of an
X-ray machine, must be considered when
d determining screen efficiency. The total
absorption efficiency na is found by
(4) 2 =U integrating the absorption efficiency and
C1 C photon intensity over energy.

The following relationship may be The X-ray energy spectrum incident on
obtained from Fig. 4b: the screen will be changed by the
presence of an object; the direct
(5) d = 2C1U for C1 ≤ C transmitted beam is hardened. At the
C same time, lower energy scattered
radiation is generated in the object for
which screens generally have higher
absorption efficiency.

Typical values of screen unsharpness Special Screens
for commercially available screens vary
from U = 0.50 mm (0.02 in.) to Neutron Sensitive Screens
U = 1.0 mm (0.04 in.).
Real time radiography may be performed
Screen Gamma using neutron beams when the
fluorescent screen is a good neutron
The fluorescent screen gamma γ is a absorber. Elements with high thermal
measure of the contrast ratios between the neutron cross sections, such as lithium-6,
output screen image brightness B and the boron-10 and gadolinium are used in
input X-radiation intensity I: neutron sensitive screens. Plastic
scintillation materials can be used for
(6) ∆ B = γ ∆ I radioscopy with fast neutrons.
BI
The characteristics of screen
As in film radiography, the output composition and construction are more
image must have a minimum brightness important in neutron imaging than in
ratio between adjacent image areas for X-ray imaging because the intensity of
detection by an observer or an image available neutron sources is generally
intensifying component. For most lower than for X-ray sources. It is
fluorescent screens at industrial important that the screen absorb a
radiography energies, the screen gamma is sufficient quantity of neutrons to obtain
very close to 1.0. So the fluorescent screen an acceptable light yield for adequate
itself is very seldom the limiting factor as contrast.
far as the total imaging system gamma is
concerned. High Energy Screens

Radiation Energy Effects Some materials, when absorbing X-rays,
emit secondary electrons copiously. The
The efficiency of fluorescent screens is a phosphor materials used in fluorescent
function of the energy of the radiation. screens are generally more sensitive to
This is shown in Fig. 3b where the mass electrons than to primary X-rays. At high
absorption coefficient is plotted as a X-ray energies, the electrons from suitable
function of X-ray energy for different secondary electron emitters can be used to
phosphor materials. These data are enhance the imaging process. Heavy
calculated from X-ray cross section metals such as lead, tungsten or tantalum
tabulations.3 The K absorption edge of the are often used in MeV radiography,
heaviest element in each phosphor is serving both as secondary electron
indicated by the step in the coefficient emitters and low energy X-ray filters.
plot. Other than at the K absorption edge, Along with the production of secondary
the absorption coefficient (and hence electrons to increase the absorbed energy
efficiency) decreases with increasing in the adjacent fluorescent screen, the
photon energy. heavy metal will shield the screen from

The absorption efficiency is given by
Eq. 7:

Radioscopy 259

TABLE 3. Scintillator materials and some of their properties.

Scintillator Chemical Density Emission Light Yield Primary Afterglow
Symbol (g·cm–3) Maximum (103 photons × Decay (percent)
(ns)
(nm) MeV–1·g–1) —
< 0.3 after 6 ms
Bismuth germinate (BGO) Bi4Ge3O12 7.13 480 8 to 10 300
Europium activated calcium fluoride CaF2(Eu) 3.18 435 19 0.94 0.1 after 3 ms
Cadmium tungstate CdWO4 7.9 470/450 12 to 15 0.5 to 5.0 after 6 ms
Thalium activated cesium iodidea CsI(Tl) 4.51 550 52 to 56 20/5
Thalium activated sodium iodideb 3.67 415 38 1 —
Terbium activated high density glass NaI(Tl) 3.5 to 3.8 543 <0.5 after 300 ms
— 250
a. Slightly hygroscopic. — 3400
b. Hygroscopic.

low energy scattered X-rays. Both of these thicknesses of 1 to 2 mm (0.04 to
processes improve the contrast sensitivity. 0.08 in.). It provides an efficient imaging
medium at high resolution for small fields
Scintillator Plates of view. Features as small as 4 µm have
been resolved with this material using a
Scintillators are optically clear, generally plate thickness of 0.7 mm (0.03 in.).4
single-crystal, phosphor materials that
produce very short pulses of light as a The terbium activated high density
result of X-ray interactions. In addition, glass is an innovation that provides a
the amount of light emitted is generally large area imaging, 250 to 300 mm (10 to
proportional to energy deposited in the 12 in.) diameter, and thicknesses from
X-ray interaction. Because scintillators are 2 mm (0.08 in.) to 12 mm (0.5 in.) and
optically clear (that is, transparent to their greater. This is available as a monolithic
own emission wavelengths) and plate and as a fiber optic plate with fiber
single-crystal components, they can be diameter as small as 10 µm. Spatial
used in thicknesses not possible with resolution with these materials has been
polycrystalline phosphor materials that demonstrated in excess of 20 line pairs
form the light emitting coatings for per millimeter for low X-ray energies (less
fluorescent screens. Scintillators can be than 120 kV).5
made as single crystals, monolithic plates
or fiber optic plates. The increased
thickness provides a more efficient means
for absorbing X-ray energy, which is
particularly important for high energy
(megavolt) X-rays. In addition,
scintillators are not limited in spatial
resolution by material grain size as are
fluorescent screens. Some scintillator
types are available in sizes up to 0.30 m
(12 in.) diameter or more, with
thicknesses ranging from 2 mm (0.1 in.)
to 100 mm (4.0 in.) or more. Typical
scintillator materials and their properties
are given in Table 3.

To be useful for radioscopic imaging,
the scintillator must be nearly flawless
and must be large enough in area to
provide the desired field of view. The
scintillator plate must be thin enough to
allow lens focusing at limited depth of
field associated with fast lenses and thick
enough for adequate conversion of X-ray
energy to light. Light emission must be in
the wavelength compatible with camera
imaging and afterglow must be low.

One of the highest density scintillators
with emission in the green is cadmium
tungstate (CdWO4). This scintillator is
available nearly flaw free in diameters of
10 to 20 mm (0.4 to 0.8 in.) and

260 Radiographic Testing

PART 3. Image Quality

The factors that limit resolution in (8) B = mI
radioscopic imaging are similar to those in
normal film radiography. Some aspects of or
radioscopic image quality are given in (9) ∆ B = m ∆ I
Table 4.
where m is the proportionality constant.
In radioscopic imaging only the X-ray The absorption law for monochromatic
energy absorbed during the scan time of radiation is:
the image pickup system will add to each
single image. For a direct viewing [ ](10) I = I0 exp −µx
fluoroscopic system, this is the image
summation time of the human eye (0.2 s). Differentiating Eq. 10 gives:
With the increased sophistication of (11) ∆ I = −µ I ∆ x
radioscopic imaging systems, the limits of
object thickness penetrated and contrast where µ is the linear absorption
of details detected are only restricted by coefficient and x is the absorber thickness.
the quantum structure of the radiation Combining the above equation gives
and the noise introduced by each stage of Eq. 12:
the imaging process.
(12) ∆B = −µ∆x = C
Each detail, as defined by size and B
contrast, can be described by a number of
radiation quanta that is proportional to where C is contrast.
the intensity of the radiation. The relative The effect of X-ray energy on subject
statistical fluctuation of radiation
intensity is proportional to the reciprocal contrast C manifests itself through the
of the square root of intensity. In general, absorption coefficient µ, which varies
radiation contrast in the image element with X-ray energy. The efficiency of the
must exceed the value of this fluctuation fluorescent screen affects subject contrast
to yield detail. by its ability to convert the incoming
X-ray photons to light and manifests itself
Contrast through the screen brightness response B.

Subject Contrast Observed Contrast

Subject contrast for fluorescent screens is The observed contrast in radioscopic
defined as the fractional change in imaging is affected by several factors
brightness resulting from a change ∆x in beyond the screen response. The effects of
absorber thickness. The nearly linear all system components must be included.
relationship between screen brightness B This is done by defining proportionality
in candela per square meter (or factor γ for the contrast ratio of output
millilambert) and X-ray intensity I on the intensity B to the input intensity I (see
screen in gray (or roentgen) per minute also Eq. 6):
may be written as:

TABLE 4. Image quality. Factor ∆B
Aspect
B
(13) γ = ∆I

Subject contrast thickness, scatter, radiation hardening I

System contrast screen gamma, intensifier, camera,
monitor

Definition, geometric source focal spot size, source-to-object or
distance, object-to-screen distance,
screen thickness, motion (14) ∆ B = − γ µ ∆ x
B
Definition, mottle quantum fluctuations, screen gain, raster
scan of monitor

Radioscopy 261

where µ is the linear absorption Control of Scatter
coefficient and ∆x is the change in the
thickness of the absorber material. The Scattered radiation comes from many
combined system gamma is the product sources in a radioscopic imaging setup.
of the individual components in the There is scatter from the room, object,
imaging chain. In a vidicon television fixtures and air path in the primary
chain, for example, the system gamma radiation beam and scatter from objects
would be given by: placed in the path of the beam. The
control of scatter for radioscopic imaging
(15) γ = γa γc γ k γ s is the same as for normal radiography.
Specific techniques to reduce scatter are
where γ = overall system gamma, listed below.
γa = electron amplifier chain gamma,
γc = vidicon tube gamma, γk = television 1. Collimate the primary beam to the
picture tube gamma and γs = fluorescent minimum viewing area necessary.
input screen gamma.
2. Shield the setup to reduce room
For a typical case, the fluorescent scatter from walls, ceiling and floor.
screen gamma is taken as γs = 1.0, a
conventional closed circuit television 3. Filter the primary beam to remove the
amplifier chain provides γa = 1.0 low energy portion of the spectrum.
(maximum), the vidicon tube provides a
γc = 0.9 and the television picture tube 4. Filter the radiation beam between the
gamma is typically γk = 3.0.6 Although it object and the fluorescent screen
appears that considerable contrast gain is because many filters preferentially
remove the lower energy scattered
possible in a television chain, the final radiation.

imaging element, the human eye, must 5. Use antiscatter grids, both fixed and
moving, between the object and the
also be considered. The human eye fluorescent screen.

gamma is nonlinear and is less than one. 6. Use projection magnification to
increase the distance of the fluorescent
A typical value of the human eye gamma screen from the object scatter.

is 0.3. With increased viewing room In general, it is very important in
radioscopic imaging to consider all areas
brightness or glare, this value drops where scattered radiation can be
introduced and to attempt to eliminate or
rapidly. reduce their effect to improve image
contrast.
Effects of Scatter
Definition
Scattered radiation affects contrast in
fluorescent screens by effectively raising Unsharpness and Optimum
the background brightness level. The Magnification
scattered radiation affects only the
primary imaging component of a The same rules that apply for unsharpness
radioscopic system, the fluorescent screen. and optimum magnification in film
Here, the contrast is defined as: radiography apply to radioscopic imaging.
Unsharpness and optimum magnification
(16) C = ∆B = ∆I are discussed elsewhere.1
B I
Motion Blur
If however, scattering is not eliminated,
the equation for contrast becomes In radioscopic imaging, unsharpness due
to object movement can limit image
(17) C = ∆I definition. Determining factors for this
I + Is are the X-ray excitation rate, the decay
time of the fluorescent screen phosphor
where Is is the scattered radiation and the delay time or scan time of the
intensity. Now, if Is = KI, where K is the imaging system components. Typically,
scattering factor, the equation for screen radioscopic systems are used with
continuous or rapid pulse rate X-ray
contrast becomes excitation (120 pulses per second or
greater), rapid decay phosphor (on the
(18) C = ∆I order of milliseconds or faster) and frame
rates of 30 frames per second.
I
K +1

As this equation illustrates, it is important
to keep the scattered radiation incident
on the fluorescent screen to a minimum
to keep the contrast at an acceptable level.

262 Radiographic Testing

Quantum Mottle number of photons arriving per second,
times the storage time of the detector t.
Quantum mottle is the statistical Therefore:
fluctuation of brightness on fluorescent
screens and is due to the randomness of (25) nrt = n
X-ray production and absorption and
needs to be considered in any radioscopic where:
imaging. The following list shows the
numerous sources of this fluctuation: (26) nr = πd2n0
(1) X-ray photon production, (2) X-ray 4
photon absorption in the object, (3) X-ray
photon absorption in the screen, Here d is the diameter of the object under
(4) conversion of X-ray photons to light
photons, (5) fraction of the light photons observation and n0 is the number of
reaching the eye after traversing the photons reaching the detector per unit
imaging system and (6) light photon area of the fluorescent screen per second.7
absorption in the retina.
Now contrast is defined again:
These fluctuation obey poisson
statistics so that the standard deviation σ 2k g
is equal to the square root of the (27) C =
intensity n:
d πtn0
(19) σ = n
or
Because radioscopic imaging involves a
series of processes, the standard deviation (28) d = 2k g
for the total sequence is: C πtn0

(20) σ2 = σ12 + σ 2 + … + σ 2 The smallest discernible object size that
2 n can be detected is seen from Eq. 28 to
improve with increasing contrast and
Certain stages in the process will cause number of stimulating photons. Detail
either an increase or decrease in the sensitivity in an image will be limited by
intensity. The relationship can be shown statistical fluctuations as long as d is
to a first approximation: greater than the unsharpness of the
system.
(21) σ2 = g n
The statistical fluctuations of
where g is the amplification of the process fluorescent screen brightness, which are
(from the state of lowest intensity to the due to the randomness of the process, are
final observation by the retina) and n is important at low brightness levels, which
the final intensity.7 usually occur with low intensity sources.
Most industrial X-ray machines produce
To observe detail in the light image sufficient intensity to render the statistical
reaching the retina, there must be a detail fluctuation unimportant for most
intensity difference greater than the applications. Much of the effect of
standard deviation: quantum fluctuation in cases where it is
important can be removed in near real
(22) ∆n = kσ time by video frame averaging or
summing.

where ∆n is the smallest difference in the Radiation Sources
number of detectable photons in the
retinal image and k is a constant termed The radiation source plays an important
the threshold contrast to standard role in radioscopic imaging. In choosing a
deviation ratio. radiation source, consideration must be
given to the types of materials to be
Contrast is defined as: tested, densities, thicknesses, smallest
feature size to be resolved, smallest
(23) C = ∆n thickness change to be detected, response
n of the radioscopic imaging system and
rate of image acquisition. High output,
Then: high stability, constant potential X-ray
systems are available for these
(24) C = k g applications. Minifocus and microfocus
n X-ray tubes expand the possibilities of
radioscopic imaging to include high
Because n is the total photon intensity in spatial resolution applications.
the retinal camera image, it is equal to the

Radioscopy 263

Limitations of generator wave form are no ( )(29) I = I0 exp −µx
longer a consideration with the
availability of high frequency generators. or

Projection magnification is one of the (30) ∆ I = −µ∆ x = −µ x ⋅ ∆ x
most advantageous applications of Ix
radioscopic systems. A small focal spot
permits the object to be moved away from Table 5 presents this result in tabular
the detector, allowing easy robotic form. The image contrast, or detection
manipulation. The projection increases percentage, given in this table is based on
magnification so that fine detail can be a subject contrast of ∆x·x–1 = 2 percent.
imaged with a detector whose resolution The image contrast is seen to increase
is relatively poor. Magnifications of 2× to considerably for larger thicknesses
50× are common in microfocus expressed as µx. This improvement is only
radioscopic systems. realized in practice if the transmitted
intensity is large enough to produce a
Linear accelerators for high energy statistically accurate measurement.
applications, for example, greater than
1 MeV, use resonant waveguides to TABLE 5. Half value layer thicknesses
accelerate electrons to a target. The associated with different values of µx.
radiation is produced in pulses typically of
3 to 4 µs width and at repetition rates of Thickness Value Contrast Transmission
60 to 420 Hz. Radioscopic imaging
components may need shielding from (half value of ∆I·I–1 I0·I–1
radiofrequency fields produced by
accelerator operation and from direct high layers) µx (percent) (percent)
intensity X-rays. Setup configuration,
synchronization of the imaging system to 1.45 1.0 2 36
the accelerator pulse rate and image frame 22
averaging can be used to reduce or 2.17 1.5 3 13
eliminate these effects.
2.89 2.0 4 8
Neutron radioscopic imaging is 5
accomplished with portable accelerator 3.62 2.5 5 3
neutron sources and neutron reactors 1.8
with beam ports suited for radioscopic 4.33 3.0 6 1.1
imaging. The effective focal spot of 0.67
neutron sources is typically large. For 5.05 3.5 7 0.41
most neutron sources, whether accelerator 0.25
type or reactor type, the focal spot is 5.78 4.0 8 0.15
defined by the collimator opening at the 0.09
neutron source. To transport a reasonable 6.50 4.5 9
number of neutrons down the beam tube,
these collimator openings are necessarily 7.22 5.0 10
larger than the focal spots possible with
X-ray machines and linear accelerators. 7.94 5.5 11

8.66 6.0 12

9.40 6.5 13

10.00 7.0 14

Radiation Energy

The size of the object to be imaged and
the material type will determine the
radiation type and energy to be used. As
a general rule, the radiation type and
energy should be selected so that the
object thickness to be penetrated is three
to five half value layers. One half value
layer reduces the transmitted radiation
intensity by 50 percent. Satisfactory
results can also be obtained if deviation
from the rule is no greater than a factor of
2 (that is, in the range from 1.5 to 10 half
value layers). Configuration of the part is
of some importance because scattering
from the part will reduce contrast.

The half value layer (HVL) can be
equated with values of µx from the
absorption law stated in Eqs. 10 and 11:

264 Radiographic Testing

PART 4. Imaging Systems

Introduction systems and very important in high
energy operations. Shielding for personnel
Imaging systems with video pickup for protection is a necessary feature.
remote viewing offer advantages of
radiation shielding for operator Image Intensifier Tubes
protection, display monitor with lighting
adjusted for vision acuity, video signal for The image intensifier tube converts
image enhancement and computer photons to electrons, accelerates the
analysis. The fluorescent screen converts electrons and then reconverts them to
radiation to light and either an image light. Figure 5 shows a generalized
intensifier is used to boost light intensity diagram of an intensifier tube. Intensifiers
to a level suitable for pickup by a solid typically operate in the range of 30 to
state or television camera or a low light 10 000 light amplification factors. The
level camera is used to image the screen intensification is not necessarily solely
directly. The signal from the camera is electronic but may also include a
sent to a television monitor or computer reduction in image area where electrons
video card and monitor for viewing. A from a large area input screen are focused
video or digital recorder is used to provide on a small area output screen.
permanent record of the test.
The earliest type of image intensifiers
At X-ray energies above 1 MeV, for X-ray applications used a zinc
shielding is used to protect the electronic cadmium sulfide (ZnCdS) layer inside the
components from radiation with camera glass envelope to convert the X-rays to
placed out of the direct line of radiation light. The photocathode adjacent to the
by use of folding mirrors. Below 300 kV, fluorescent layer converted the light to
the intensifier and camera can be placed electrons. The original X-ray tube used a
in the direct line of the radiation. 25 kV potential between the
Typically, radiation damage occurs in photocathode and output phosphor. Even
electronics such as transistors at 103 Gy though only 10 percent of the light
(105 rad) and will discolor optical photons from the fluorescent screen
components at 103 to 104 Gy (105 to would generate electrons at the
106 rad). Table 6 lists radiation damage photocathode and only 10 percent of the
thresholds for a few components. Given accelerated electrons would produce light
time outside the radiation field, most at the output phosphor, a 10× to 15×
materials will recover from the damage. increase in luminous flux was generated
Specialized equipment such as camera by the acceleration. The tube had a
tubes can be made to tolerate up to curved input screen with a 130 mm (5 in.)
106 Gy (108 rad).
FIGURE 5. X-ray image intensifier tube design.
Glass may fluoresce under strong
irradiation resulting in undesired light Protective
signals. Noise may also be generated in vacuum
the electronics, increasing with the envelope
radiation intensity. Good shielding
practice is desirable for medium energy Focusing electrodes

X-ray window

TABLE 6. Radiation damage thresholds. Output phosphor

Component __________T_h__r_e_sh__o_ld___________ Intensified
kGy (Mrad) light
output
Silicon semiconductor 70.0 (7.00) Input
Germanium semiconductor 0.50 (0.05) X-rays
Capacitors (300 to 700)
Resistors 3000 to 7000 (200 to 500) Input Photocathode layer
Ceramics, glass, optical 2000 to 5000 (0.02) conversion
Plastics (0.02)
0.20 screen
0.20

Radioscopy 265

diameter and an output screen with a resolution in the large format is specified
15 mm (0.6 in.) diameter. The ninefold at 3.6 line pairs per 1 mm (91 line pairs
reduction in diameter from the per 1.0 in.). Tubes as large as 400 mm
fluorescent screen to the viewing screen (16 in.) diameter have also been
provided an additional factor of 80 in marketed.
brightness gain. The total gain was
between 800 and 1200.8 Channel Electron
Multiplier
Technical improvements in electronic
gain, fluorescent and photocathode layer The channel electron multiplier or
efficiencies and electron optics have made microchannel plate (MCP) is an assembly of
modern X-ray image intensifiers very small tubes for amplifying an electron
useful in both medical and industrial signal using secondary emission. The
applications.9-13 Sodium activated cesium channels are glass coated or ceramic
iodide (CsI[Na]) is now commonly used as coated with a high resistance material on
the fluorescent layer because it has twice the inside. A potential difference of 500 to
the X-ray absorption of zinc cadmium 1000 V is applied across the channel
sulfide and because its crystalline plate. An electron entering the channel
structure minimizes lateral light diffusion. will strike a wall causing one or more
Rare earth phosphors such as gadolinium secondary electrons to be released. These
oxysulfide (Gd2O2S) are also found to be will continue to strike the channel wall
superior to zinc cadmium sulfide. At X-ray yielding more electrons as they are
energies below 100 kV, sodium activated accelerated by the electric field along the
cesium iodide is very good; at higher channel. The gain of the channel
X-ray energies, the rare earths are more multiplier depends on the applied voltage
useful.13 and the ratio of length to diameter.

Modern tubes are available with 100 to Channel electron multipliers are
400 mm (4 to 16 in.) input diameters, limited only by the technology for
multiple modes that electronically select fabricating small diameter channels that
variable field size of the input and do not break down in the electron field.
fiberoptic output for direct camera With 10 kV and a length-to-diameter ratio
coupling. A typical 210 mm (8 in.) tube of 50, a typical gain is about 104. The
performs with resolution on the order of resolution of the device is limited by the
4 line pairs per millimeter and gains on size of the channel spacing.15,16 Figure 6
the order of 10 000. Resolution is at a shows a diagram for a microchannel
maximum at the center of these electron multiplier.
intensifiers and decreases somewhat at the
edges. The channel electron multiplier is used
in conjunction with an electrostatic image
The advantages of these tubes are the intensifier tube to form what is called a
relatively low cost, generally compact size second generation or third generation image
and high resolution and contrast. A converter. In these devices, a photocathode
disadvantage is that minification will is coated on the inside surface of the
increase image unsharpness. Also, because input window. A voltage applied between
a ratio of length to diameter of from the photocathode and the microchannel
1.0:1.0 to 1.5:1.0 is required for the plate accelerates the photoelectrons to the
electron optics, large diameter inputs input surface of the small channels that
require large tubes. This requirement not make up the electron multiplier. Electron
only increases bulk but creates a potential multiplication occurs as described above.
implosion hazard. The curved input A high voltage applied between the
screens in these tubes cause distortion. output of the microchannel plate and a
The tubes are sensitive to voltage drifts, phosphor screen coated and the inside
stray magnetic fields and space charge surface of the output window accelerates
defocusing at high dose levels.12 Electron the electrons between the two. The
scattering, thermionic emission and light electron image is converted back into a
reflection on interior surfaces are causes photonic image at the output window.
for loss of contrast from intensifiers.
Fabrication techniques in the latest The simplest electron optics for
generation tubes minimize these focusing electrons in second generation
problems. and third generation microchannel
electron multiplier image intensifiers is
Tubes with 360 mm (14 in.) input have based on proximity planar electrode
been manufactured.14 The advanced design. The proximity image intensifier
vacuum tube technology requires a metal consists of an input window
tube body. A titanium membrane is used (photocathode surface), microchannel
for the entrance window to withstand plate and the output window (phosphor
atmospheric pressure and maintain surface), all parallel and in close relative
transparency to X-rays. The titanium position to each other. Appropriate
produces less scatter than a glass window
that improves contrast. An acceleration
voltage of 35 kV is used. Limiting

266 Radiographic Testing

voltages are applied between the elements typical photocathodes. Desirable
to provide minimum electron spread for a characteristics include high efficiency at
distortion free image at the output the wavelength of light being observed
window. Proximity focused image and a low dark current (the signal level
intensifiers offer small size with little when no light is falling on the
increase in length as the diameter is made photocathode).
larger.
The light emitted from the intensifier
Direct X-ray sensitive microchannel is generated by the action of electrons on
plates can be made. A metallic converter a phosphor. The spectral emission
that emits secondary electrons following a characteristics of some common
high energy photon excitation (200 kV phosphors are shown in Fig. 8.
and above) serves as the input. These
electrons are amplified in the channels.17 FIGURE 7. Photocathode response spectrum.

Spectral Matching S-20

Image intensifiers rely on a photocathode Sensitivity (relative unit) S-25
to convert input light radiation to Violet
electrons. The X-ray image intensifiers Blue S-1
have a fluorescent screen ahead of the Green
photocathode to convert X-rays to light. YellowS-11
The spectral response and sensitivity
varies among photocathode materials. See Red
Fig. 7 for the spectral response of several 0 200 400 600 800 1000 1200

FIGURE 6. Microchannel plate.

Channel plate

Photocathode Wavelength (nm)

Phosphor Legend
S-1 = Ag+ Cs2O, 10–10 to 10–13 A·cm–2

S-11 = Cs3Sb, 10–14 to 10–15 A·cm–2
S-20 = Na2KSb+Cs, 10–15 A·cm–2
S-25 = Na2KSb+Cs3Sb, 10–15 A·cm–2

FIGURE 8. Phosphor spectrum.

P-11 P-20

Primary electrons Spectral efficiency
(relative unit)

P-22B P-24 P-4
600 700
300 400 500

V Violet
Blue

Green
Yellow

Red

Secondary electrons

Legend Wavelength (nm)

P-4 = ZnS:Ag + ZnCdS:Ag
P-11 = ZnS:Ag (Ni)
P-20 = ZnCdS:Ag
P-22B = ZnS:Ag
P-24 = ZnO:Zn

Radioscopy 267

Statistics

The image intensifier system can improve
imaging by boosting the light output so
that the statistical limitation in the image
process is not at the eye but at the input
fluorescent screen. The intensifier itself
operates on a statistical process for the
generation of electrons and the
regeneration of light. The sources of
fluctuation are essentially independent, so
the Eqs. 19 and 20 apply.

Amplification g (where g = σ2·n–1, per
Eq. 20) may be used for improving detail
sensitivity in an intensifier system. This
improvement is accomplished by
choosing that amplification that makes
the number of light quanta (used by the
observer’s eye) equal to the number of
radiation quanta used by the input
fluorescent screen.

268 Radiographic Testing

PART 5. Cameras

Up until the last few years, real time X-ray devices and related solid state devices
imaging systems (fluoroscopic or have advantages over image tubes in
radioscopic systems) typically used a stability, geometric accuracy, signal
television camera in combination with a uniformity and size. Conversion of
device (such as an image intensifier tube X-radiation to visible light image is
or phosphor screen) to convert incident provided by a phosphor screen or
X-rays into visible light at wavelengths scintillator plate. The light image is
compatible with the response of the collected by the charge coupled device
camera. Television cameras with image camera by either lens coupling (with or
tubes were the common implementation. without folding mirrors) or by fiber optic
Systems introduced at the beginning of bundle coupling (Fig. 9b). A more recent
the twenty-first century, however, use advance in the field of radioscopic X-ray
charge coupled device (CCD) cameras imaging is flat panel solid state arrays.
almost exclusively. Charge coupled

FIGURE 9. Charge coupled device: (a) array schematic; (b) intensified camera.
(a) Sensor array

Vertical scan generator
Video coupling circuits

Video Video coupling circuits Charge
output Horizontal scan generator coupled
device
(b) Microchannel plate
Output phosphor
Photocathode

Photons

Fiberoptic lens Intensified photons
Relay optics

Radioscopy 269

Sensitivity, (µA·µW–1)These devices are discussed in with digitalCharge coupled devices work like
imaging elsewhere in this volume. photodiodes.19-21 A photon, incident on
the depletion region of a charge coupled
Charge Coupled Devices device, will create an electron hole pair if
absorbed. This creates a current flow that,
Charge coupled devices and related solid in a charge coupled device, is stored in
state cameras use an array of photodiodes the potential well of the device. The
or charged coupled devices as the amount of charge collected at the
sensitive layer. These arrays may be linear potential well is in direct proportion to
or area arrays of individually addressable the amount of local light intensity.
elements. The solid state cameras are
small and have wide spectral response The charge coupled device is fabricated
(Fig. 10), reduced lag, higher quantum with a combination of thin film
yields (50 percent) and (depending on the technology and silicon technology. Arrays
application) may have equivalent are available in a variety of
resolution capabilities when compared to configurations, such as 768 × 512, 1536 ×
commercially available vidicon cameras. 1472 and 2184 × 1472 with photoelement
Solid state cameras are rugged, are not center spacing typically ranging from 9 to
damaged by intense light images and do 23 µm. More recently, diode arrays as
not require the scanning electron beam large as 4000 × 4000 and even 8000 ×
found in vidicons. 8000 have become available. A dynamic
range of 1000:1 (10 bits) is typical but
The photodiode arrays in solid state 4000:1 (12 bits), 16 000:1 (14 bits) and
cameras are simple photon detectors, 64 000:1 (16 bits) are common.
typically reverse biased silicon
photodiodes, that absorb incident The image on the solid state array is
photons and liberate current carriers. This coupled to video circuitry by horizontal
gives rise to a current referred to as the and vertical scan generators that read the
photocurrent signal, proportional to the charge level at the detector elements.
arrival rate of the incident photon. The Figure 9 shows the schematic of a charge
efficiency of the photodiode strongly coupled device array. The output of the
depends on its material and construction video can be specified to fit a particular
as well as the wavelength of the incident video format. The clock sequence, which
photons. The diodes consist of p type sequentially reads the charge level on
islands in an n type substrate. Standard each device, is started after a suitable
arrays are available with 128 × 1024 image integration time. This integration
diodes with a center spacing as small as time can be adjusted, making solid state
25 µm. A dynamic range of 100:1 is cameras useful for low light level
typical.18 applications, provided the detection
element can retain the charge over the
FIGURE 10. Sensitivity of a photodiode array integration period.
(solid state) camera versus vidicon camera.
The interesting feature of solid state
10 cameras is that individual pixel elements
may be addressed and the signals
Silicon photodiode array processed digitally. With individually
addressable elements, the integration time
Vidicon and the video output format may be
0.1 simply specified. The cost of solid state
cameras increases significantly with
0.01 increasing array size and the electronic
0.4 0.5 0.6 0.7 0.8 0.9 10 1.1 circuit complexity required to scan large
arrays. Each element must be individually
Optical wavelength (µm) calibrated for uniform response
throughout the field. This feature can be
used to correct nonuniform fields in
radiography. Because each element is
independent, blooming can be controlled;
a bright element does not spill over into a
neighboring dark element.

The imaging in real time radiographic
applications may be accomplished with
optical focusing of the light from a
scintillation screen onto the solid state
detector. In this case, the image format
and resolution are similar to conventional
television cameras. Also, scintillation
materials may be deposited directly on the
array. Each element becomes an
independent radiation detector and the

270 Radiographic Testing

resolution is dependent on the element Simple lenses are not often used in
spacing. coupling because of the low optical
efficiency. For example, if a reduction in
Intensified Charge image size by a factor of 2 is required
Coupled Device Cameras from the coupling, the distance from
phosphor to lens must be twice the
Intensified charge coupled device cameras distance from lens to pickup surface. From
consist of a microchannel plate image the lens formula, the distance from
intensifier tube with relay optics to a phosphor to lens will be three times the
charge coupled device incorporated into a focal length of the lens, giving poor light
single camera body. Proximity focused collection efficiency. With a coupling of
microchannel plate image intensifiers one to one, this distance is still twice the
offer smallest camera body size as focal length.
described above. Relay optics transmits
the image by exiting the intensifier to the Collimated optics are superior because
charge coupled device image array as the objective lens, focused at infinity, is
shown in Fig. 9. Typical luminance gain is located at its focal length from the
18 000 at 20 µlx (2 × 10–6 ftc) input. The phosphor. Collection efficiency is nine
limiting resolution of the intensifier is and four times greater, respectively, for
typically 30 line pairs per millimeter. the two magnifications discussed above
(2×, 1×). The second lens will determine
Automatic brightness control can be the image size at the pickup surface by
built into the electronics to automatically the ratio of its focal length to the focal
limit the maximum output of the length of the objective lens. Vignetting, a
intensifier to prevent saturating the reduction in light intensity at the edges,
charge coupled device array. The output does occur in collimated optics. This
can be limited by sensing the current in reduction is minimized when the lenses
the microchannel plate and adjusting the are close to each other. Some real time
microchannel plate voltage accordingly. imaging systems in the 1980s used
Automatic brightness control is used not specialized optics such as
only to protect the charge coupled device bouwers-schmidt lenses, where concentric
from saturation but also allows intrascene mirrors provide a low f number22 — for
dynamics (very dark and bright areas in example, 0.65. Since then, the light
the same image) to be viewed with good collection efficiency of cameras have
contrast at both extremes. obviated extreme lens coupling systems.

Optical Coupling Fiber optics may also be used for
coupling. Intensifier tubes often have
To couple optical signals between fiber optic input and output surfaces.
components, real time imaging systems These may be coupled directly to other
use fluorescent screens for conversion components with similar surfaces by
from X-radiation to light and systems use using an optical gel. Fiber optic light
chains connecting intensifiers to guides can be considered for moving light
television camera. Mirrors or lenses are from one location to another (that is,
the most common means of coupling. from the fluorescent or phosphor screen
to the camera). The potential advantages
Front faced silver mirrors must be used of this are greater retention of the light,
to avoid ghost images caused by multiple one-to-one size transfer, improved
reflections in back faced mirrors. Optical contrast by suppression of undesirable
lenses provide good coupling, depending reflections and shortening of the system
on the f number and transmission dimensions. Fiber optic connections can
characteristics. The illuminance E on a also be used over considerable distance or
pickup surface, coupled by a lens in a around unusual obstructions.
simple optical system, is given by:
Fibers for fiber optic systems are glass
E= πBT or plastic with diameters of 1 or 2 µm to
4f 2 1 + M 2 50 µm. The fiber optic array operates on
( )(31) total internal reflection. To reduce leakage
each fiber is coated with a material having
where B is the luminance of the output a lower index of refraction. Losses in the
phosphor, T is the lens transmission, M is fiber optic system are due mainly to the
the magnification and f is the f number, opacity of the fibers in long systems or
or relative aperture of the lens — that is, the acceptance angle of the light in short
the ratio of focal distance to aperture lengths.
diameter. The lower the f number, the
more light is collected for imaging. Image Tubes

A wide variety of television cameras and
image tubes are available for use on real
time imaging systems. Many different

Radioscopy 271

camera configurations can be used to uses cadmium and zinc telluride and the
accommodate test requirements. On one silicon diode tube uses a silicon diode
extreme is the small compact camera with array target structure.
no user adjustments. At the other extreme
is the larger, two-piece camera, with many Silicon Intensifier Targets
controls for optimizing image quality. In
addition to solid state cameras as Another type of tube called the silicon
discussed above, a variety of image tubes intensifier target (SIT) uses a photocathode
are also available. The most common as an image sensor and focuses the
types for radioscopic applications are photoelectrons onto a silicon mosaic
(1) vidicons, (2) silicon intensifier targets, diode target. Readout is similar to the
(3) image isocons and (4) X-ray sensitive vidicon. The design allows for very high
tubes. light gains in the pickup by accelerating
the photoelectrons to high energies
Vidicons (perhaps 10 keV) before they strike the
target. The silicon intensifier target tube
The vidicon is a small, rugged and simple and intensified silicon intensifier tubes (ISIT)
tube. An electron beam scans a light are used extensively for low light level
sensitive photoconductive target. A signal applications.
electrode of transparent material is coated
onto the front of the photoconductor. Image Isocons
The scanning electron beam charges the
target to the cathode potential. When The image isocon tube (Fig. 12) was widely
light is focused on the photoconductor, used in radioscopic applications in the
the target conductivity increases, 1980s. The image on its photocathode
changing the charge to more positive forms a photoelectron pattern focused by
values. The signal is read by the electron an axial magnetic field onto a thin,
beam that deposits electrons on the moderately insulating target. The
positively charged areas, causing a photoelectrons striking the target cause
capacitively coupled signal at the signal secondary emission electrons collected in
electrode (Fig. 11). a nearby mesh, leaving a net positive
charge on the target. The beam from an
The vidicon has a number of electron gun scans the target, depositing
variations, depending on the selection of electrons on the positively charged areas.
the photoconductive material. The The scattered and reflected components in
standard vidicon uses an antimony the return beam are separated. Only the
trisulphide layer. The plumbicon uses a scattered component enters the electron
lead oxide junction layer. The newvicon multiplier surrounding the electron gun.

FIGURE 11. Vidicon television camera.

Semitransparent Photoconductor Cathode
conducting coating 0 V in dark Electron gun

on glass Electron beam
(+20 V direct current)
Fine mesh screen
Glass

Photons

Focused image of Electron beam +– 0V – +
scene viewed R 20 V 300 V

Connected to semitransparent
conduction coating on glass

Video signal

272 Radiographic Testing

This signal is amplified to become the quality indicator sensitivities of two
video output. percent have been obtained. The cameras
have experienced problems with
X-Ray Sensitive Cameras deterioration, possibly due to local
overheating in the target layer, poor
Although the usual input to a television bonding to the heat sink layer, substrate
camera is a light signal, for radioscopic irregularities or incompatibility between
purposes it is possible to make the camera beryllium and target materials.
sensitive directly to X-radiation. The
vidicon camera in particular may be Camera System
modified for X-ray sensitivity and has Characteristics
been found useful for obtaining direct real
time radiographic images. Two alterations The performance criteria for camera tubes
of the vidicon are needed for good results: are based on the sensitivity, dynamic
an X-ray window and an efficient target. range, resolution, dark current and lag. A
Thin glass or beryllium X-ray windows plot of signal output versus faceplate
located close to the target replace the illuminance for some typical camera tubes
heavy optical glass windows in is shown in Fig. 13 and the slope of these
conventional tubes. Although the normal curves is called the tube gamma. The light
vidicon photoconductive targets will source for illuminance is important in the
respond to X-ray, they are so thin that response characteristics of the tubes.
absorption of radiation is minimal.
Suitable thick targets must be used. In X-ray imaging applications, image
Selenium has been found to be very isocon television tubes are commonly
effective, having adequate response and used because of their low light level
low lag.6 Lead oxide targets are more sensitivity and high dynamic range;
common, having a high density for good unfortunately, isocons are very expensive.
X-ray absorption and resulting Vidicons are often used in combination
sensitivity.23 with X-ray sensitive image intensifier
tubes; vidicons are simple as well as
The X-ray sensitive vidicon is an inexpensive. The newvicon is more
imaging system for small objects and low sensitive than the plumbicon or the
kilovoltages, 150 kV or less. The X-ray antimony trisulfide vidicon. silicon
intensity must be high, in the range of intensifier target tubes are used with low
8 to 80 mSv·s–1 (50 to 500 R·min–1). The light level systems when high dynamic
vidicon tube typically has a sensing area range is not required. Table 7 lists
of only 9.5 × 12.5 mm (0.37 × 0.5 in.). characteristics for television tubes used in
Presentation of the image on a 480 mm real time radiographic applications. Lag is
(19 in.) television screen results in better given as the percentage of the original
than 30 times magnification. With a signal present after 50 ms.
525-line scan rate, the resolution in the
object is better than 0.02 mm (0.0008 in.). FIGURE 13. Television camera output versus light input with
The X-ray sensitive vidicon camera has a 2856 K tungsten source.
gamma on the order of 0.7 to 1.0. Image

FIGURE 12. Image isocon television camera.

Focusing
coil

Deflection Signal output (nA)104 Image isocon Newvicon Vidicon
yokes 103
Silicon diode array102
Scattered Plumbicon10 SIT
return beam

Photocathode Signal output

Field mesh Scanning Electron 1 10–4 10–3 10–2 10–1 1 10 100
Target beam Electron gun 10–5 10–5 10–4 10–3 10–2 1–1 1 10
multiplier 10–6

Photoelectrons Faceplate illuminance lx (ftc)

Reflected
beam

Radioscopy 273

TABLE 7. Typical characteristics of television camera tubes.

Tube Type Dynamic Typical Resolution Current Lag Gamma
Range (television lines) (nA) (percent)

Image Isocon 2000 1000 07 1.0
Antimony trisulfide vidicon 300 900 20 20 0.65
Newvicon 100 800 1.0
Lead oxide vidicon 300 700 8 20 0.95
Silicon intensifier target 100 700 34 1.0
8 12

Camera Matching TABLE 8. Common adjustments for television cameras.

The standard scanning rate for camera Control Effects
tubes is 30 frames per second. Each frame
image is created in a 33 ms exposure time. Beam current controls electron beam in image tube; usually
The frame is composed of two fields in set only to discharge picture highlights
which the electronically scanned 525
vertical lines are interlaced. The first field Focus electrostatic focus adjustment for electron
in 60–1 s contains the odd numbered lines beam
and the second field contains the even
numbered lines. Gamma correction electronic change of slope characteristic
(gamma) of tube
In some applications, where very low
radiation intensities are experienced, it is Pedestal level voltage adjust for black level of picture
necessary to use a slower scanning rate.
The target of the television camera can be Polarity reverse inverts black and white areas of image
made to integrate the incoming signal for
several minutes and then scan it to Target voltage sets positive potential on image tube target
provide one frame of information. Faster fixed voltage for most tubes; is variable on
scanning rates may be used to image rapid sulfide vidicons and controls sensitivity
dynamic systems, provided a sufficient
light intensity is present and the lag
features are acceptable.

Television cameras require electrical
adjustments to set up the operating
parameters of the image tube and signal
processing electronics. Cameras may be
one-piece or two-piece systems. The
one-piece camera is usually self-contained
with few (or no) user adjustable controls.
The two-piece camera is much more
versatile. The camera head can be made
much smaller, decreasing bulk and weight
for restricted mounting requirements. The
camera control unit will have all controls
readily accessible to the user for
optimizing image quality. Common
adjustable controls or switches found in
most cameras are described in Table 8.

The beam, target and focus controls
optimize the image tube’s performance.
Other control features (such as pedestal,
gamma correction and polarity reversal)
enhance image quality and ease of
operation. The polarity reverse feature, for
example, is a comparatively simple and
advantageous option. Small detail against
a bright background is difficult to detect;
however, inverting the polarity and
having details appear against a dark
background makes them more visible to
the observer.

274 Radiographic Testing

PART 6. Viewing and Recording

Monitors bandwidth. The result is that the standard
number of scan lines is often as good as
Television monitors for observing video or superior to higher scan line
signals are cathode ray tubes using a systems.22,24
modulated electron beam to write on the
output phosphor. In North America the Many cameras and television monitors
standard format is 525 lines, interlaced. are designed for higher bandwidth
Interlacing means that the total picture operation (10 MHz and greater). This
frame is composed of two fields: the first provides greater resolution horizontally
field uses every other line on the screen; (800 lines or more) than vertically.
the second writes between the lines of the Because the 525-line vertical is standard,
first. The television camera provides the the resolution of video systems is often
monitor with the appropriate operating quoted by the horizontal resolution value,
format in the video signal. a function of bandwidth.

Other systems are used routinely in Recording Equipment
Europe and have various numbers of lines
that can be as high as 1200, triple Recording the video signal provides a
interlaced. The tubes vary in size, permanent record of the radioscopic
deflection system and component design. results. Technology in this area changes
Contrast controls can be adjusted to rapidly with new options appearing
increase or decrease the gamma over a frequently.
range of values, typically 0 to 10.
Recording of video data generally
To see an object, there must be a occurs in either of two ways. It can be
sufficient number of scan lines in the recorded as an analog signal on some
television image; following is one form of video tape or it can be recorded
description of scan line requirements.22 To digitally. When the output of the video
visualize n objects there must be 2n lines. system is analog the signal is first
Allowing for random orientation, this converted to digital form and then
should be increased by a factor of recorded on some form of digital media
2 percent. To see a mesh with 5 holes per such as digital audio tape (DAT), compact
1 mm (125 holes per 1.0 in.) would disk (CD), digital video disk (DVD) or
require 14 lines per 1 mm (350 holes per hard disk.
1.0 in.) scan rate. Horizontally, the
resolution is determined by the Cassette video tape recorders are still
bandwidth of the signal. One cycle of the most common means of saving video
bandwidth is required to see the mesh information. They are easily operated but
(half cycle for the holes and half cycle for are limited to about 2 h of playback. The
the spacing between holes). In a ratio of signal to noise in the record
conventional system, the viewing matrix improves with tape width, which varies
uses a 3 × 4 aspect ratio, the horizontal from 13 to 51 mm (0.5 to 2.0 in.).
being larger than the vertical. If 525 scan
lines are used, then the horizontal will Video tape recorders may be equipped
require 4 × 525 × (√2)–1 half cycles, or with pause and slow motion modes. The
about 250 cycles, to maintain the same pause mode, however, shows only one
resolution. Using 30 frames per second field rather than the full two-field frame.
scanning and a factor of 1.2 for retrace This limitation results in a reduction of
time, the required bandwidth is on the information by half.
order of 525 × 250 × 30 × 1.2 = 4.7 MHz.
In place of the second field, some
Although it may appear that resolution expensive recording equipment repeats
could be improved by increasing the the first field of information in the pause
number of scan lines, two problems result: mode. This improves the visual display
(1) the charge capacity on the camera’s but still represents a reduction in
target elements will be reduced in information. The reason for showing only
proportion to the area change that may one field is to eliminate interfield jitter
reduce the sensitivity and (2) an increase caused by movement between field scans
in bandwidth will be required that of the camera.
increases the noise in the electronics in
proportion to the square root of the Slow motion modes are quite useful for
replaying rapid events. Unfortunately in
slow motion replay a broadband

Radioscopy 275

synchronizing signal is reproduced on the captured image can then be displayed on
video screen, sweeping by at each frame. the computer monitor for image
This signal is not easily removed and can enhancement. It can be processed into a
be very distracting when evaluating number of digital formats (see Table 9)
images; removal of the synchronizing that can be output to any printer.
signal noise is possible with further
investment in video replay equipment. The images can also be stored or
transmitted as electronic files. Multiple
All video recording devices alter the frame averaging with video capture can
video signal in some way. When linking improve the image quality. Noise is
the output of the recording camera to reduced by the square root of the number
monitors and recorders, the strength of of frames averaged.
the signal is reduced in proportion to the
number of devices. This results in a The frame averaging technique is
weaker signal at the recorder and a poorer commonly a running average:
record. Repeated copying results in a
degradation of the image as the signal ( )(32) F = Fc + Fp n − 1
becomes successively reduced. Playback nn
from recorded images will be inferior to
the original image. It is also important to where F is the displayed image frame, Fc is
consider the bandwidth capability of the current row frame, Fp is the previous
recording equipment. To maintain as averaged frame and n is the number of
much information as possible, bandwidth frames in the running average. When
should be matched to the bandwidth viewed in real time, running averages
required in the original image. The reduce noise but create image lag or blur
standard television broadcast is 4.2 MHz, for moving objects, depending on the
to which many video recording devices number of frames averaged.
are matched.
TABLE 9. File formats widely used for bitmapped graphics.
Digital recording of the video signal is
generally more expensive but has the Abbreviation Format
potential of providing a high quality
record if the digital signal has a large BMP bit mapped picture
dynamic range. Digital video disk (DVD) DIB device independent bitmap
technology offers a high quality recording GIF graphics interchange formata
in a compact storage medium capable of JFF JPEG file formatb
holding large amounts of data (4 to JIF JPEG image formatb
10 gigabytes). JPEG Joint Photographic Experts Group
JPG JPEGb
The technology of digital storage media PIC PICSb
in the two decades from 1982 to 2002 has PICS platform for internet content selection
undergone the following developments. PICT picture
PNG portable network graphics
1. Optical media have been RLE run length encoding
supplementing or replacing magnetic TIF TIFFb
media. TIFF tagged image file format

2. Media have become more economical. a. GIFSM is a service mark of CompuServe Incorporated.
3. Media have become able to store more b. For compatibility with the disk operating system (DOS) many file names

bytes of data. end with a file extension consisting of a period followed by three letters.
4. Media have become more compact. For this reason, some format abbreviations consist of three letters or
5. Platforms have become mutually have been shortened to three letters so the format abbreviation can be
appended as an extension to the file name.
intelligible, so media formatted by one
computer or program are more likely
to be able to be read by another
computer.

Similar trends affect instruments as well as
media. These trends seem likely to
continue in the twenty-first century.

Electronic Soft and Hard
Copy

Hard copy of a single frame image is often
desirable and a radioscopic system should
be capable of generating hard copy.
Photographic imagers that capture a
single frame from a video input are
available. Using a video capture card or
similar hardware enhancements in a
computer, however, is very useful. The

276 Radiographic Testing

PART 7. System Considerations

System Evaluation (33) R(ω) = I (ω)
O(ω)
Image Quality Indicators
where I(ω) is the image amplitude, O(ω)
The most common means for evaluation the object amplitude and R(ω) is the sine
of radiographic sensitivity is to use a wave response.
penetrameter, or image quality indicator.
The penetrameter is a thin plaque of the This approach to system evaluation can
same material as the object. The plaque be understood by imagining a bar pattern.
thickness is a certain percentage of the As the pattern becomes finer and finer,
object thickness. The penetrameter has the image response begins to lose
holes with diameters one, two and four contrast. A plot of this response is called
times its thickness. The radiographic the square wave response (when a bar
quality of the imaging system is then pattern is used) and is very similar to the
listed as the smallest percentage modulation transfer function. Square
penetrameter detectable and the smallest wave response factors can be used to
hole detectable. Other types of evaluate imaging systems and under
penetrameters are in use, such as the certain conditions may be corrected to the
German Industry Standard, Deutsche sine wave response or modulation transfer
Industrie Norm (DIN), penetrameter in function equivalence.27,28
Europe.26 This penetrameter uses wires of
graded diameters, made of the same The true modulation transfer function,
material as the object. The smallest wire generated by an object having sinusoidal
detected is the quality level. variations in intensity, is measured
routinely for optical components such as
Many experimenters use their own lenses, intensifiers and cameras. Normally,
measurement systems for evaluating the in radiography, determining the
quality of an imaging system. These are modulation transfer function for such an
generally called an image quality indicator object is prohibitively difficult. Instead,
(IQI). Image quality indicators vary in the modulation transfer function is
design but commonly contain a step derived by generating the edge spread
wedge of material from which the function, differentiating to obtain the line
smallest percentage change (in material or
contrast level) can be determined. FIGURE 14. Modulation transfer function of system.
Resolution may be measured by the
smallest hole size detectable. Because the Response (percent) 100 Optical coupling
ability to observe a certain diameter hole Image orthicon
is a function of the depth of the hole, it is 80
common to specify resolution using a Total system
high contrast object. 60
Fluorescent
Wire meshes are commonly used to 40 screen
evaluate real time imaging systems. The
wire mesh is an object whose fine 20 0.1 1 10
structure is repetitive. Wire meshes are (2.5) (25) (250)
often used to indicate the quality level of 0
inspection at certain speeds of movement. 0.2
(5.0)
Modulation Transfer Function
Scan density, lines per 1 mm
Another technique of system evaluation, (lines per 1.0 in.)
more rigorous in its approach, is the
modulation transfer function (MTF). The
modulation transfer function is the ratio
of the image amplitude to the object
amplitude, as a function of sinusoidal
frequency variation in the object:

Radioscopy 277

spread function and fourier transforming modulation transfer functions of the
to yield the modulation transfer function. components:
Derivations of this and examples may be
found in the literature.29-33 (34) MTFsystem = MTF1 × MTF2 × MTF3

The importance of the modulation This is shown in Fig. 14, where the
transfer function in evaluating systems is modulation transfer functions for the
that the total system modulation transfer
function is the product of the individual

FIGURE 15. Assembly line radioscopy for automotive parts using automated part positioning:
(a) top view; (b) front view.

(a) Maintenance door

Offloading Test piece Detector Loading
door positioning door
apparatus

Transport
shuttle

Transport Conveyor
shuttle
Test
piece Test piece

Conveyor
Front door

(b)

Test
piece

Radiation
source

278 Radiographic Testing

fluorescent screen, optical coupling, isocon camera systems may also be used
image orthicon camera and total system for X-ray or neutron detection. Both
are shown.34,35 The components of a systems can perform in the 1.5 to
system are analyzed quantitatively and 2.0 percent sensitivity range. The
the poorest is easily determined. This resolution limit will generally depend on
technique allows for the prediction of a the input field size, the video bandwidth
proposed system’s performance from data used and the input phosphor.
on the individual components.
The digital flat panel detectors are
System Design generally more expensive than traditional
radioscopic imaging components. They
General purpose remote reviewing are expected, however, to command a
systems are usually of three types: X-ray large share of the market because of their
image intensifier with charge coupled performance advantages.
device or vidicon camera; fluorescent
screen with intensified charge coupled A cabinet system incorporates many of
device or isocon camera; and scintillator the modern features of radioscopic
plate with charge coupled device camera. imaging, using an image intensifier tube
Amorphous silicon digital flat panels with remote viewing and radiation
(discussed elsewhere in this volume) have protection for the operator. Assembly line
been implemented in remote viewing radioscopy in a larger enclosure using
radioscopic systems. Additional robotics to position automotive parts is
equipment should include a high quality shown in Fig. 15 and a radioscopy system
video monitor, a video tape recorder, a for automotive wheel inspection is shown
video disk for dynamic recording and a in Fig. 16.
computer based frame digitizing and
image enhancement system for real time
summing, running average or contrast
adjustments plus edge enhancement and
subtraction techniques. The computer
data acquisition system provides digital
data storage and hard copy output
capabilities.

The X-ray image intensifier systems are
usually less expensive. They operate best
at the low and intermediate X-ray
energies for which they are designed.
Scintillator plates are appropriate for both
low to intermediate range and megavolt
X-ray energies but may not provide
enough sensitivity for video framing rates.

The intensified charge coupled device
and isocon camera systems are more
versatile. Because the fluorescent screen is
changeable, the system can be adjusted
for optimum performance at any energy.
Intensified charge coupled device and

MOVIE. FIGURE 16. Radioscopy systems for
Automated automotive wheel inspection.
wheel
inspection.

Radioscopy 279

References

1. Bossi, R.H., C. Oien and P. Mengers. 10. Diakides, N.A. “Phosphors.” SPIE
Sec. 14, “Real-Time Radiography.” Proceedings, Vol. 42. Bellingham, WA:
Nondestructive Testing Handbook, International Society for Optical
second edition: Vol. 3, Radiography and Engineering (1973): p 83-92.
Radiation Testing. Columbus, OH:
American Society for Nondestructive 11. Schagen, P. “X-Ray Imaging Tubes.”
Testing (1985): p 593-640. NDT International. Vol. 14, No. 1.
Guildford, United Kingdom:
2. Ludwig, G.W. and J.S. Prener. Butterworth-Heinemann (1981):
“Evaluation of Gd2O2S:Tb as a p 9-14.
Phosphor for the Input of X-Ray
Image Intensifier.” Schenectady, NY: 12. Wang, S.P., D.C. Robbins and
General Electric Corporate Research C.W. Bates, Jr. “A Novel Proximity
and Development (1972). X-Ray Image Intensifier Tube.” SPIE
Proceedings, Vol. 127: Optical
3. Storm, E. and H.E. Israel. “Photon Instruments in Medicine VI. Bellingham,
Cross Sections from 0.001 to 100 MeV WA: International Society for Optical
for Elements 1 through 100.” Report Engineering (1977): p 188-194.
LA-3753. Los Alamos, NM: Los Alamos
National Laboratory (1967). 13. Vosburg, K.G., R.K. Swank and
J.M. Houston. “X-Ray Image
4. Kinney, J.H. and M.C. Nichols. “X-Ray Intensifiers.” Advances in Electronics
Microscopy (XTM) Using Synchrotron and Electron Physics. Vol. 43. Orlando,
Radiation.” Annual Review of Materials FL: Academic Press (1977).
Science. Vol. 22. Palo Alto, CA: Annual
Reviews (2002): p 121-152. 14. Kuhl, W. and J.E. Schrijvers. “A New
14-Inch X-Ray Image Intensifier Tube.”
5. Placious, R.C., D. Polansky, H. Berger, Medicamundi. Vol. 22. Eindhoven,
C. Bueno, C.L. Vosberg, R.A. Betz and Netherlands: Philips Electronics
D.J. Rogerson. “High-Density Glass (March 1977).
Scintillator for Real-Time X-ray
Inspection.” Materials Evaluation. 15. Fink, D.G. and D. Christiansen, eds.
Vol. 49, No. 11. Columbus, OH: Electronic Engineers’ Handbook, third
American Society for Nondestructive edition. New York, NY: McGraw-Hill
Testing (November 1991): (1989).
p 1419-1421.
16. Woodhead, A.W. and G. Eschard.
6. McMaster, R.C., L.R. Merle and “Microchannel Plates and Their
J.P. Mitchell. “The X-Ray Vidicon Applications.” Acta Electronica. Vol. 14,
Television Image System.” Materials No. 2. Limeil Brévannes, France:
Evaluation. Vol. 25, No. 3. Columbus, Laboratoires d’Électronique et de
OH: American Society for Physique Appliquée (1971): p 181-200.
Nondestructive Testing (March 1967):
p 46-52. 17. Chalmetron, V. “Microchannel X-Ray
Image Intensifiers.” Real-Time
7. Sturm, R.E. and R.H. Morgan. “Screen Radiologic Imaging: Medical and
Intensification Systems and Their Industrial Applications. Special
Limitations.” American Journal of Technical Publication 716. West
Roentgenology and Radium Therapy. Conshohocken, PA: ASTM
Vol. 62, No. 5. Leesburg, VA: American International (1980): p 66-89.
Roentgen Ray Society (1949).
18. Hobson, G.S. Charged Transfer Devices.
8. Teves, M.C. and T. Tol. “Electronic New York, NY: Holsted Press (1978).
Intensification of Fluoroscopic
Images.” Philips Technical Review. 19. Biberman, L.M. and S. Nudelman, eds.
Vol. 14, No. 2. Eindhoven, Photoelectric Imaging Devices. New York,
Netherlands: Philips Research NY: Plenum Press. Vol. 2 (1971).
Laboratory (1952): p 33-43.
20. Howe, M.J. and D.V. Morgan. Charge
9. Bates, C.W., Jr. “Concepts and Couple Devices and Systems. New York,
Implementation in X-Ray Image NY: Interscience (1979).
Intensification.” Real-Time Radiologic
Imaging; Medical and Industrial 21. Titus, J. “How Do CCDs Capture
Applications. Special Technical Images?” Test and Measurement World.
Publication 716. West Conshohocken, Newton, MA: Cahners Business
PA: ASTM International (1980): Information (April 1999).
p 45-65.

280 Radiographic Testing

22. Siedband, M. “Electronic Imaging 33. Smith, F.D. “Optical Image Evaluation
Devices II.” Physics of Diagnostic and the Transfer Function.” Applied
Radiology. USDHEW Publication Optics. Vol. 2, No. 4. Washington, DC:
No. (FDA) 74-8006. Washington, DC: Optical Society of America (1963):
United States Department of Health, p 335.
Education, and Welfare [DHEW]
(1971). 34. Halmshaw, R. “Direct-View
Radiological Systems.” Research
23. Jacobs, J.E. “X-Ray–Sensitive Television Techniques in Nondestructive Testing.
Camera Tubes.” Real-Time Radiologic Vol. 1. Orlando, FL: Academic Press
Imaging: Medical and Industrial (1970): p 241-268.
Applications. Special Technical
Publication 716. West Conshohocken, 35. Halmshaw, R. “Fundamentals of
PA: ASTM International (1980): Radiographic Imaging.” Real-Time
p 90-97. Radiologic Imaging: Medical and
Industrial Applications. Special
24. Webster, E.W. “Electronic Imaging Technical Publication 716. West
Devices I.” Physics of Diagnostic Conshohocken, PA: ASTM
Radiology. USDHEW Publications International (1980): p 5-21.
No. (FDA) 74-8006. Washington, DC:
United States Department of Health, Bibliography
Education, and Welfare [DHEW]
(1971). Bates, C.W., Jr. “New Trends in X-Ray
Image Intensification.” Application of
25. Webster, E.W., R. Wipfelder and Optical Instrumentation in Medicine III.
H.P. Prendergrass. “High Definition SPIE Proceedings, Vol. 47. Bellingham,
versus Standard Television in Televised WA: International Society for Optical
Fluoroscopy.” Radiology. Vol. 88, No. 2. Engineering (1974): p 152-158.
Easton, PA: Radiological Society of
North America (1967): p 355-357. Cassen, B. and R.C. McMaster.
“Fluoroscopic Methods of Inspection
26. ASTM E 1647-98a, Standard Practice for of Metallic Materials, Part VII.” WPB
Determining Contrast Sensitivity in Contract W-138. Pasadena, CA:
Radioscopy. West Conshohocken, PA: California Institute of Technology
ASTM International (1998). (1945).

27. Bossi, R.H., J.L. Cason and Chamberlain, W.E. “Fluoroscopes and
C.N. Jackson, Jr. “The Modulation Fluoroscopy.” Radiology. Vol. 38.
Transfer Function and Effective Focal Easton, PA: Radiological Society of
Spot As Related to Neutron North America (1942): p 383-412.
Radiography.” Materials Evaluation.
Vol. 30, No. 5. Columbus, OH: Coltman, J.W. “Fluoroscopic Image
American Society for Nondestructive Brightening by Electronic Means.”
Testing (May 1972): p 103-108, 112. Radiology. Vol. 51. Easton, PA:
Radiological Society of North America
28. Coltman, J.W. “The Specification of (1948): p 359-367.
Imaging Properties by Response to a
Sine Wave Input.” Journal of the Optical Criscuolo, E.L. and D. Polansky.
Society of America. Vol. 44. “Improvements in High Sensitivity
Washington, DC: Optical Society of Fluoroscopic Technique.”
America (June 1954): p 468. Nondestructive Testing. Vol. 14, No. 1.
Columbus, OH: American Society for
29. Klingman, E. “Theory and Application Nondestructive Testing (January
of an Edge Gradient System for 1956): p 30-31, 40.
Generating Optical Transfer
Functions.” NASA TN D-6424. Csorba, I.P. Image Tubes. Indianapolis, IN:
Washington, DC: National Aeronautics Howard W. Sams and Company
and Space Administration (1971). (1985).

30. Mees, C.K. Theory of the Photographic Curtis, L.R. “Fluoroscopic Inspection of
Process. New York, NY: MacMillan Aluminum and Magnesium Castings.”
Company (1942). Nondestructive Testing. Vol. 7, No. 4.
Columbus, OH: American Society for
31. Morgan, R.H. “The Frequency Nondestructive Testing (Spring 1949):
Response Function.” American Journal p 24-27.
of Roentgenology and Radium Therapy.
Vol. 88. Leesburg, VA: American Dalberg, R.C. “Real-Time Digital Image
Roentgen Ray Society (January 1962): Filtering and Shading Correction.”
p 175. Applications of Digital Image
Processing III. SPIE Proceedings,
32. Morgan, R.H., L.M. Bates, Vol. 207. Bellingham, WA:
U.V. Gopalarao and A. Marinaro. “The International Society for Optical
Frequency Response Characteristics of Engineering (1979)
X-Ray Films and Screens.” American
Journal of Roentgenology, Radium
Therapy and Nuclear Medicine. Vol. 92.
Leesburg, VA: American Roentgen Ray
Society (February 1964): p 426.

Radioscopy 281

Halmshaw, R. Physics of Industrial
Radiography. London, United
Kingdom: Heywood Books. New York,
NY: American Elsevier Publishing
Company (1966).

Hampe, W.R. “Modern Fluoroscopy
Practices.” Nondestructive Testing.
Vol. 14, No. 1. Columbus, OH:
American Society for Nondestructive
Testing (January 1956): p 36-40.

Hecht, S. and Yun Hsia. “Dark Adaptation
Following Light Adaptation to Red and
White Light.” Journal of the Optical
Society of America. Vol. 35.
Washington, DC: Optical Society of
America (1945): p 261.

Klasens, H.A. “Measurement and
Calculation of Unsharpness
Combinations in X-Ray Photography.”
Philips Research Reports. Eindhoven,
Netherlands: Philips Research
Laboratories (1946): p 241.

O’Connor, D.T. “Industrial Fluoroscopy.”
Nondestructive Testing. Vol. 11, No. 2.
Columbus, OH: American Society for
Nondestructive Testing (Fall 1952):
p 11-22.

Rossi, R.D. and W.R. Hendree. “Some
Physical Characteristics of Rare-Earth
Imaging Systems.” Application of
Optical Instrumentation in Medicine IV.
SPIE Proceedings, Vol. 70. Bellingham,
WA: International Society for Optical
Engineering (1975): p 224.

Smith, W.J. Modern Optical Engineering: The
Design of Optical Systems. New York,
NY: McGraw-Hill (1976).

Wagner, R.E. “Noise Equivalent
Parameters in General Medical
Radiography: The Present Picture and
Future Pictures.” Photographic Science
and Engineering. Vol. 21, No. 5.
Springfield, VA: Society for Imaging
Science and Technology (1977).

282 Radiographic Testing

11

CHAPTER

Digital Radiographic
Imaging

Clifford Bueno, General Electric Company, Niskayuna,
New York

PART 1. Overview of Digital Imaging

Definition transported, stored and displayed with
relatively inexpensive computer systems.
The digital imaging chapter of this
volume presents approaches available to The medical community has led the
obtain digital radiographs by electronic development of digital X-ray imaging,
means. The discussion and examples in where the demand for imaging systems
the present chapter include techniques of allows significant investment in the
conversion of X-rays to light and then to development of the tools. Spinoff from
electronic images, photoconductive the medical community has occurred,
conversion of X-rays to electronic images, allowing the introduction of digital
photostimulable phosphors, array imaging technology for the industrial
detectors, line scan imaging and scanning radiography community.
electron beams.
In the early 1980s, digital imaging for
Radioscopic digital imaging is related radiographic purposes was primarily done
to radioscopy. In radioscopic imaging the by electronic digitization of the video
major emphasis is on the conversion of signal from a radioscopic system.1 Charge
X-rays to analog electronic data that are coupled device cameras were available but
viewed as video signals in real time. the most common application was as a
Digitization of these analog signals is a video output camera. Developments in
technique of digital imaging. Many of the direct digital image output for these
principles for X-ray detection are cameras resulted in charge coupled device
identical, particularly where digital based arrays in the 1990s that consisted of
cameras such as charge coupled device millions of pixels.
cameras are used. The present chapter, on
digital radiographic imaging, differs from Also developed in the 1970s and 1980s
radioscopic imaging in that the systems were digital imaging systems using line
are not video based (although in some scan detector arrays. To form the image,
cases video could be output). Rather, either the part or the detector array was
digital systems use discrete sensors with physically scanned in the dimension
the data from each detection pixel being perpendicular to the array. In the late
read out into a file structure to form the 1970s to early 1980s the photostimulable
pixels of the digital image file. phosphor array was developed for medical
use and was used in industry in the 1990s.
An exception to the discrete sensor
based systems discussed in this chapter is In the 1990s the development of large
the photostimulable phosphor system thin film transistor arrays provided the
that forms a latent image (similar to film) tool that could make large area X-ray
on a storage phosphor imaging plate. The imagers using either amorphous silicon or
screen is read out electronically using a amorphous selenium possible.
special laser scanner. The pixelization in
this case is based not on the X-ray Detectors for Digital
sensitive phosphor but in the laser Imaging
scanner process.
Digital radiographic detectors are used in
Development numerous industries from airport baggage
scanning to medical diagnosis. In
The ability to develop digital imaging addition to these widely used
technology that would be useful for applications, digital radiography is finding
radiographic testing is due in large part to an increasing role for inservice
the growth in the speed and memory of nondestructive testing, as a diagnostic
computer systems. In the 1980s images of tool in the manufacturing process, for
512 × 512 pixels 8 bits deep of data online production line testing and with
(256 kilobytes) were considered large and conveyer handling systems. Digital
created storage and display problems for radiographic detectors are also being used
the computer systems at that time. By the as hand held devices for pipeline
twenty-first century, image files of inspections, as film replacement devices,
1500 × 2000 with 16 bits of data in industrial and medical computed
(6 megabytes) are common and can be tomography systems and as part of large
robotic scanning systems for coverage of
large structures.

284 Radiographic Testing

The digital image by its nature will
provide numerical results important for
metrology and thickness measurements.
The development of a wide range of
digital X-ray imaging products
complements the recent digital revolution
and provides digital image data and
results that can be incorporated into the
massive digital manufacturing and
services databases that have emerged to
help manage the life cycles of products
and structures.

In the field of industrial digital
radiography, there is really no single
standard X-ray system to address all
applications. Economics, speed, quality
and the impact on the overall
manufacturing or service processes are key
in designing and building digital
radiographic systems. A large aspect of
that design is the consideration of the
digital X-ray detection device itself. For
this selection, there are almost as many
choices of detectors as there are ways to
configure the overall test system. The
different digital detector technologies
available are discussed below.

Digital Radiographic Imaging 285

PART 2. Principles of Digital X-Ray Detectors

The detection devices that support the amorphous silicon, amorphous selenium
larger imaging systems already mentioned and charge coupled device technologies
are the following: (1) phosphors deposited are described below.
on amorphous silicon thin film transistor
diodes; (2) photoconductors such as Each of these devices can be used to
amorphous selenium deposited on thin replace film radiographic techniques
film transistors; (3) phosphors deposited depending on the size of the application
or coupled through fiber optic lenses onto and on the spatial resolution, image
charge coupled device based detectors and contrast and speed required. As noted in
complementary metal oxide silicon based Table 1, the detectors have variable modes
detectors; (4) photostimulable storage of operation or are available in different
phosphors; (5) phosphors deposited on architectures to address diverse
linear array systems; and (6) X-ray applications. There are numerous pixel
scanning source reversed geometry architectures of amorphous silicon
detectors. Further details are provided in detectors but it is important to note that
Table 1. currently not all detector choices allow
real time operation of 30 frames per
Each of these devices has an X-ray second.
capture material as its primary means for
detecting X-rays. This material is either an Charge Coupled Devices
X-ray phosphor material combined with a
photoelectric device (diode, Scientific charge coupled devices,
photomultiplier tube or charge coupled although they are typically small in size,
device) or is an X-ray photoconductor have been made with high pixel densities.
material that is then followed by an The fields of photography, astronomy and
electronic readout device. The most microscopy have demanded this and the
common of these detection systems in nondestructive testing industry has been a
operation today are the flat panel beneficiary of these developments. Table 1
detection systems based on amorphous illustrates these small pixel dimensions
silicon and amorphous selenium (9 to 50 µm pixels).
structures, the camera systems based on
charge coupled device technology and the Charge coupled devices have not been
storage phosphor systems. The fabricated into larger arrays because the

TABLE 1. Properties of digital radiographic detectors.

Detector Size Range Pixels Square Pixel Image Conversion Atomic Number
(mm) Size (µm) Acquistion Material Z of Conversion
Speed (min)
Material

Amorphous 200 × 200, 1024 x 1024, 100, 127 or 200 < 1 (real time) thallium activated cesium 55/53 or 64
silicon 230 × 190, 2304 × 1920, iodide or terbium
410 × 410 or 2048 × 2048 or activated gadolinium 34
280 × 410 2304 × 3200 oxysulfide 55/53 or 64
56/35
Amorphous 350 × 430 to 2560 × 3072 139 < 1.0 to 1.5 selenium 55/53 or 64
selenium 11/126
thallium activated cesium
Charge coupled small to 100 × 100; to 4096 × 4096 9 to 50; effectively < 1 (real time) iodide or terbium activated
larger with other gadolinium oxysulfide
devices larger with lenses optics
europium activated
Storage small to 1550 × 430 to 15 500 × 4300 25 to 250 < 1 to 4 barium fluorobromide
phosphors
thallium activated cesium
Linear arrays small to 500 to 4096 10 to 200 < 1 (real time) iodide or terbium activated
gadolinium oxysulfide
Reversed small to 450 × 450 to 2048 × 2048 25 to 200 < 1 (real time)
geometry thallium activated sodium
iodide

286 Radiographic Testing

charge coupled device is based on Radiation Conversion Material
crystalline silicon, which has traditionally
been cut from silicon wafers available in The amorphous selenium device is similar
sizes only as large as 100 to 150 mm (4.0 to the amorphous silicon based detector.
to 6.0 in.) in diameter or less. A larger They both use thin film transistor readout
field of view can be accomplished with circuitry. The difference lies in the X-ray
charge coupled devices through tiling of conversion material. The amorphous
the charge coupled devices or through a selenium detector relies on the selenium
lens or a fiber optic transfer device to view photoconductive material (not a
an X-ray conversion (phosphor) screen. phosphor layer) as a means to detect
The downside of the lens approach is that X-rays. The selenium converts X-rays to
it has very poor light collection efficiency. electron hole pairs that then get separated
Fiber optics or tiling do not provide large by the internal bias of the device and
fields of view but will result in more captured by an electrode structure. The
efficient light collection. A more detailed amorphous silicon thin film transistor
discussion of charge coupled device circuitry beneath the selenium layer
technology may be found elsewhere.2 provides readout of the charge with the
aid of field effect transistors (FETs) in a
Thin Film Transistor similar manner to that of the amorphous
silicon detectors. The selenium layer is
Larger amorphous silicon and amorphous typically 500 µm (0.02 in.) thick.3
selenium detectors based on thin film
transistor technology have been made For applications with large fields of
commercially available with a pixel pitch view, amorphous selenium offers direct
smaller than 75 µm. Amorphous silicon X-ray collection efficiency in a compact,
through large area amorphous silicon robust package.
deposition and processing/etching
techniques offers a solution to the size Storage Phosphors
constraints of charge coupled devices
while maintaining good light collection Storage phosphors trap X-ray induced
efficiency from the phosphor or charge carriers in the color centers of such
photoconductor (selenium) material. phosphor materials as europium activated
Because the phosphor layer is typically barium fluorobromide (BaFBr:Eu).4
deposited directly onto the silicon, Although prompt phosphorescence occurs
efficient light transfer is easily obtained. during X-ray exposure, some of the charge
However, the readout circuitry (described trapped in the phosphor material is stored
elsewhere) in these devices requires a large in these discontinuity color centers in the
pixel space to accommodate the thin film crystalline structure. The carriers stored at
transistor (TFT) and data lines and scan these discontinuity centers can be released
(gate) lines required for operation, thus when stimulated by infrared or red laser
limiting how small a pixel this device can light. The rerelease of trapped carriers
permit. subsequently creates photostimulated
luminescence of the same emission
Light Collection Technology wavelength that the prompt emission
process produces.
The amorphous silicon thin film
transistor circuitry has a fill factor of A photomultiplier tube converts the
active photodiode ranging from 65 to emitted photostimulated luminescence to
90 percent. Charge coupled devices use a an electrical signal that is then amplified
transparent polysilicon gate structure for and sampled.5 These systems have a
reading out the device and have a fill practical spatial resolution and contrast
factor of close to 100 percent.2 On a per sensitivity and have been widely used in
pixel basis, the charge coupled device is production radiography. Additionally,
therefore more efficient in collecting the they are used like film and are somewhat
light produced from the phosphor flexible (moldable about parts), portable
material. For small field of view like film in the field and fully reusable.
applications, the directly coupled charge Similarly these screens have to be
coupled device approach will provide transferred to a laser processor before they
high spatial resolution and high light can be interpreted. This removal process
collection efficiency. For large field of step is where this technology departs from
view applications, the amorphous silicon the other digital approaches.
approach offers excellent light collection Photostimulable luminescence techniques
efficiency (no lenses), in a thin, compact, can be more productive when imaging
robust package. plates can be used in the field in a
collection or batch that covers large areas
for each exposure.

The main advantage of phosphor
screens over film is the reduction of film
use, the ability to digitally acquire a film

Digital Radiographic Imaging 287

quality image, the dynamic range and the imaging detector and the source is about
corresponding benefits of that digital the size of a point source. The data
image file, such as easy archival and acquisition computer also controls the
retrieval. rastering of the electron beam. By
acquiring the output of the detector as a
Linear Arrays function of electron beam position, the
computer can generate a real time
Linear array detectors are much like radiograph of the specimen under
charge coupled devices, except that they examination.
typically only have pixels in 1 dimension
or they may be composed of a small Because a single small area detector is
rectangular array such as a 32 × 1024 pixel used and the object is placed at the
array. The advantage of linear arrays is source, not at the detector, the X-ray
their intrinsic scatter rejection capability. scatter from the object is essentially nil.
X-ray scatter exiting a specimen under The disadvantage of this approach is that,
examination can be a large contribution because it is reversed geometry, the
to the degradation of the contrast in the effective focal spot size is that of the
image. The linear array system acquires its detector size. The detector size is typically
image by being scanned one line (or a much larger than a typical industrial
group of lines) at a time across an X-ray focal spot. So that any specimen
object.6,7 The key is that the radiation that has some thickness will show
beam is masked or collimated to match significant unsharpness as the feature of
the size of the detector. This dramatically interest moves away from the X-ray
decreases the object’s scatter field. The source.
scatter detected at each of those lines is
substantially less than that of individual Detection Efficiency
lines in an area array. Linear arrays have
been successfully used in computed With the exception of the
tomography applications and have also photoconductive selenium based detector,
been found to be effective for digital all detectors listed use a phosphor layer of
radiographs. one sort or another to capture and
convert the X-ray intensity. The selection
Scanning Beam, Reversed of the phosphor or photoconductive
Geometry material, its thickness and effective
atomic number will impact the total
The reversed geometry system8-10 goes one number of X-rays absorbed in the
step further in reducing X-ray scatter in conversion material. Once energy is
the examination. In this case the data are absorbed each material, phosphor or
acquired with a small thallium activated photoconductor, has its own efficiencies
sodium iodide (NaI:Tl) scintillator coupled for conversion of this energy into either
to a photomultiplier tube. A large scanned light or charge carriers. There are other
X-ray source with a target diameter of coupling steps following this to transfer
about 250 mm (10 in.) is used to define the signal onto the pixelized readout
the image. The X-ray source operates in a circuitry. The performance of the X-ray
manner similar to a video monitor. An detector to convey the information in the
electron beam is electronically rastered radiation beam is then dependent on the
over the inner surface of the front of the efficiency of each step in the X-ray
X-ray source. Where the electrons collide conversion process leading to an
with the inner surface of the tube, X-rays electronic signal. The signal-to-noise ratio
are generated. By electronically scanning of the detector and thus the image
the electron beam, the instantaneous contrast are therefore dependent on the
position of the X-ray source is scanned transfer of information along the imaging
over an area of the front surface of the chain. Digital imaging chain statistics and
tube. The size and location of the scanned the relation with image contrast are
region is user definable, variable from discussed immediately below.
0.25 to 16 s. The acceleration voltage is
also user definable from 55 to 160 kV
with an electron beam current up to
about 0.5 mA. The diameter of the
electron beam spot at the inner surface of
the tube is about 25 µm (0.001 in.).

The specimen under examination is
placed on top of the X-ray source. This is
the opposite of conventional radiography
where the object is placed near the

288 Radiographic Testing

PART 3. Image Contrast and Signal Statistics

The transmitted X-ray beam signal phosphor layer typically creates a large
propagates through various energy gain factor at this point. Following this,
conversion stages of an imaging system, any subsequent inefficiencies in emitting
as discussed elsewhere.11,12 In Fig. 1, N0 the light and capturing it by the
quanta are incident on a specified area of photodiode will result in losses and
the detector surface (stage 0). A fraction of additional sources of noise. If the number
these, given by the absorption efficiency of quanta falls below the primary
(quantum efficiency) of the phosphor quantum sink, then a secondary quantum
material, interact (stage 1). Here it is sink will be formed and becomes an
important that the absorption efficiency additional important noise source.
be high, or a larger X-ray dose would be
needed to arrive at a desired signal level. For most detection systems discussed
here, where the phosphor is in direct
The mean number N1 of quanta contact with the diode as in the flat panel
interacting represents the primary detectors, the limiting source of noise is
quantum sink of the detector. The the quantum efficiency of the X-ray
fluctuation about N1 is σN1 = √(N1). This conversion material. Noise characteristics
defines the signal-to-noise ratio of the of digital detectors are discussed
imaging system, which increases as the elsewhere.12
square root of the number of quanta
interacting with the detector. Regardless In efficient systems, because the noise
of the value of the X-ray quantum is related to the square root of the number
efficiency, the maximum signal-to-noise of X-ray quanta absorbed, it is crucial to
ratio of the system will occur at this have a sufficient signal level to avoid
point. If the signal-to-noise ratio of the quantum mottling. Quantum mottling
imaging system is essentially determined makes detection of smaller features more
there, the system is said to be X-ray difficult. In medical imaging, regulations
quantum limited in performance. The allow a certain maximum dose to the
patient and optimal signal levels may not
FIGURE 1. Quantum statistics of X-ray imager. be obtainable. In this scenario, it is critical
to absorb as many X-ray photons as
Quantity of quanta or electrons 105 Scintillating gain to light photons possible and not to allow secondary
104 quantum sinks. In nondestructive testing,
103 Exitance efficiency it may be possible to increase signal levels
by selecting any or all of the following:
N0 g2 a longer exposure time, a higher beam
102 Optical efficiency flux, a higher radiation beam energy
(assuming absorption is still high at those
Quantum energies) or a closer working distance
efficiency of between source and detector. These
diode or charge techniques will provide improved image
g1 coupled device contrast throughout the spatial frequency
spectrum of the device. Some of these
N1 Absorption techniques, however, may not meet other
goals, such as throughput or allowable
10 Poor quantum space needed for a specimen between the
efficiency, secondary detector and the X-ray tube and tradeoffs
Incident radiation must be made.

quantum sink As just discussed, the phosphor is
therefore an important component of the
01 23 4 5 system. For the amorphous silicon
Stage detector, the phosphor of choice has been
cesium iodide with thallium as the
luminescent activator in the material (see
Fig. 2).13 This phosphor is ideal because it
has the following beneficial properties.

Legend

g = gain
N = quanta

Digital Radiographic Imaging 289