The Photographic Process and Film Sensitivity

The Photographic Process 

and Film Sensitivity 

Thanks to the work of Dr. Perry 

Sprawls of Emory University and the 

Sprawls Educational Foundation, this 

material is available on-line. 

Lecture 7B Photographic Process 

1

Introduction and Overview 

• Radiographic images are recorded either in digital format 

on digital media or on photographic film. 

• Here we will be exploring the process of recording on 

film. 


2


• The active component of film is an emulsion of radiation 

sensitive crystals coated on a transparent base material. 

Image production is a two step process as seen above. 

• First the film is exposed to radiation, typically light that 

activates the emulsion but produces no visible change. 


3


• The exposure creates a so-called latent image. 

• Second, the films is processed in a series of chemical 

solutions that convert the invisible latent image into an 

image that is visible composed of different optical 

densities or shades of gray. 


4

Relation of Exposure to Film 

Density 

• The darkness or density of the film increases as 

the exposure is increased. 

• The specific relationship between the shades of 

gray and exposure depends upon the 

characteristics of the film emulsion and the 

processing conditions. 


5

Film Functions 

• Film performs several function in the 

imaging process. A knowledge of these 

function and how they are affected by the 

characteristics of different film types aids 

in the selecting of film for specific 

procedures and in optimizing technical 

factors. 


6

Film Functions: Image Recording 

• Image Recording: Film is an image converter 

changing radiation (light) into various shades 

of gray or optical density values. 

• Film retains the image produced by the 

exposure of a fraction of a second. 

• The amount of exposure required to produce 

the image depends upon the sensitivity or 

speed of the film and the way that it is 

processed. 

• The sensitivity of the film is selected to provide 

a compromise between patient exposure and 

image quality. 


7

Film Functions: Image Display 

• Image Display: Most images are recorded as 

transparencies so they can be easily viewed 

on a view box. 

• The overall appearance of the image depends 

upon a combination of factors, including the 

characteristics of the film used, the way it was 

exposed and processing conditions. 

• It is important that all factors associated with 

the image production be adjusted to achieve 

optimum image quality. 


8

Film Functions: Image Storage 

• Image Storage: Film has been the traditional 

medium for image storage If a film is properly 

processed, it will have a lifetime of many 

years. 

• The disadvantage of storing images on film is 

the bulk and inaccessibility. Much space is 

required for storage and retrieval requires a 

manual search. 

• Because of it’s limitations, film will eventually 

be replaced by digital imaging in many clinical 

applications. 


9

Optical Density 

• Optical density of film is the darkness or opacity 

of a transparency film that is produced by 

exposure and chemical processing. 

• An image contains areas with different densities 

that are viewed as various shades of gray. 


10

Light Penetration 

• The optical density of film is assigned numerical values 

related to the amount of light that penetrates the film. 

Increasing film density decreases light penetration. The 

relationship between density value and light penetration 

is exponential. 


11


• A clear piece of film allows 100% of light to 

penetrate has a density of 0. 

• X-ray film is never totally clear. The minimum 

density is usually in the range of 0.1 and 0.25 

density units. 


12


• This is designated as base plus fog density and 

is density of the film base and any inherent fog 

not associated with exposure. 


13


• Each unit of density decreases light penetration 

by a factor of 10. 

• A density of 1 allows 10% of the light to 

penetrate and generally appears as a medium 

gray on a convention view box with daylight light 

spectrum. 

• A film with a density of 2 allows 10% of 10% or 

1.0% of light to penetrate and is a relatively dark 

area when viewed on a viewbox. 


14


• With normal viewbox illumination, it is 

possible to see through areas of film with 

up to approximately 2 units of density. 

• A density value of 3 corresponds to the 

light penetration of 0.1% (10% of 10% of 

10%). Film with a density of 3 is 

essentially opaque when trans-illuminated 

with a conventional view box but may be 

seen through with a bright or hot light. 


15


• Radiographic film has a maximum density 

value of approximately 3 density units. 

This is designated as the D max of the film. 

• The maximum density that can be 

produced on a film depends upon the 

characteristics of the film and the 

processing conditions. 


16

Density Measurement 

Densitometer 

used to measure 

the amount of light 

passing through an 

exposed film. 

The primary use of 

the densitometer is 

monitoring 

processor 

performance. 


17

Film Structure 

• Conventional film is layered as shown. The active 

component is the emulsion layer coated onto a base 

material. 

• Most film used in radiography has emulsion on both 

sides of the base when used with two intensifying 

screens. 


18

Film Structure 

• Film used in cameras or for specialty procedures 

such as mammography has emulsion on one 

side and is called single emulsion film. 


19

Film Base 

• The base of the film is made of a clear polyester material 

about 150 µm thick. It provides physical support for the 

other film components and does not participate in image 

production. 

• In most film, it contains a blue dye to give the image a 

more pleasing appearance on the view box. 


20

Emulsion 

• The emulsion is the active component in which 

the image is formed and consists of many small 

silver halide crystals suspended in a clear 

gelatin. 

• The gelatin supports, separates and protects the 

crystals. The emulsion is about 10 µm thick. 


21

Emulsion 

• Several different silver halides have 

photographic properties, but silver bromide is 

the most common crystal or grain used in 

radiography. Each grain or crystal contains 10 9 

atoms. 

• There are two common grain shapes used in 

emulsion, cubic and tabular. The advantage of 

the tabular grains is that sensitizing dyes can be 

used more effectively to increase sensitivity and 

reduce crossover exposure. 


22


• The production of film 

density and visible 

image is a two step 

process. 

1. The film is exposed to 

light which forms an 

invisible or latent image. 

2. The chemical process 

that converts the latent 

image into a visible 

image with shade of 

gray or densities. 


23


• Film density is 

produced by 

converting silver 

ions into metallic 

silver which causes 

each processed 

grain to become 

black. 

• This complicated 

process is illustrated 

here. 


24


• Each film grain contains a large number of both silver 

and bromide ions. The silver ions have a one electron 

deficit and a positive charge. The bromide ions have a 

negative charge because they have one excess 

electron. 

• Each grain has a structural defect called the sensitive 

speck. A film grain in this condition is relatively 

transparent. 


25

Development 

• The invisible latent 

image is converted into 

a visible image by a 

chemical process called 

development. 

• The developer supplies 

electrons that migrate 

to the sensitized grains 

and convert them to 

specks of black metallic 

silver in the emulsion. 


26

Film processor 

• Today radiographic 

film is processed in 

an automatic film 

processor. 

• This illustration shows 

the four steps of 

processing. 

– Development 

– Fixing 

– Washing 

– Drying 


27

Film processor 

• When the film is inserted 

into the processor, it is 

transported by means of 

a roller system . 

• The film is usually in the 

developer for 20 to 25 

seconds. 

• Fixing and wash time are 

similar with the total 

processing time of 90 to 

120 seconds. 


28

Developer 

• There are minor differences in each 

manufacturer’s developing chemistry but 

most contain the same basic chemicals. 

Each chemical has a specific function in 

the process. 

• Reducer: Chemical reduction of the 

exposed silver bromide grains into visible 

metallic silver is the function of the 

reducer. 


29

Reducer 

• The reduction process is provided by two 

chemicals in the solution: phenidone and 

hydroquinone. 

• Phenidone is more active and primarily 

produces the mid to lower portion of the 

gray scale. 

• Hydroquinone produces the very dense 

or dark areas in the image. 


30

Activator and Restrainer 

• The activator causes the emulsion to 

soften and swell so the reducers can 

reach the exposed grains. 

• Sodium Carbonate is usually used as the 

activator. 

• Potassium bromide is used as a 

restrainer to moderate the rate of 

development. 


31

Preservative and Hardener 

• Sodium sulfite, a typical preservative, 

help protect the reducing agents from 

oxidation because of their contact with air. 

It also reacts with oxidation products to 

reduce their activity. 

• Glutaraldehyde is used as a hardener to 

retard the swelling of the emulsion to 

reduce the chance of film transport 

problems. 


32

Fixing 

• When the film is transported from the developer 

tank is goes into second tank containing the 

fixer solution. The chemicals in the fixer 

performs the following functions. 


33

Neutralizer 

• When the film is removed from the 

developer, the development process 

continues due to developer retained in the 

emulsion. 

• Continued development can result in overdevelopment 

and film fog. 

• Acetic acid is used to stop development 

and neutralize the reducers. 


34

Clearing 

• The fixer solution also clears the undeveloped 

silver halide grains from the film. The 

undeveloped grains leave the film and dissolve 

in the fixer solution. 

• Ammonium or sodium thiosulfate is the 

clearing agent. 

• The silver that accumulates in the fixer is toxic to 

aquatic life. The silver can be removed from the 

fixer by electroplate in a silver recovery unit. 

Often the fixer is retained and sent to recyclers 

for this process. 


35


• Again, sodium sulfite is used as a 

preservative in the fixer solution. 

• Aluminum chloride is typically used as a 

hardener. It shrinks and hardens the 

emulsion. 

• If the fixer goes bad, the film will be easy 

to scratch and more likely to jam in the 

processor 


36


• Again, sodium sulfite is used as a 

preservative in the fixer solution. 

• Aluminum chloride is typically used as a 

hardener. It shrinks and hardens the 

emulsion. 

• If the fixer goes bad, the film will be easy 

to scratch and more likely to jam in the 

processor 


37

Wash 

• Film is next passed 

through a water bath 

to wash any residual 

thiosulfate (hypo) 

from the film. 

• The amount of hypo 

retained will 

determine the useful 

lifetime of the film. 


38

Wash 

• Retained fixer will 

continue to react with 

the silver nitrate and 

air to form silver 

sulfate, a brown stain. 

• The maximum 

retention is 30 µg/in 2 . 


39

Dry 

• The final step in 

processing is to dry 

the film by passing it 

through a chamber in 

which hot dry air is 

circulating to remove 

the water from the 

film. 

• Some processors use 

IR to dry the film 

instead of hot air. 


40

Film Sensitivity 

• One of the most important characteristic of 

film is it’s sensitivity, often referred to as 

speed. 

• The sensitivity of the film determines the 

amount of exposure required to produce 

an image. 

• A film with high sensitivity requires less 

exposure than one with low sensitivity. 


41

Film Sensitivity 

• The sensitivities of films are compared by the 

amount of exposure required to produce an 

optical density of 1 unit above base plus fog. 

• Unlike photographic ASA numbers used for 

photographic film, radiographic films use rather 

general terms such as “half speed,’’ medium 

speed” or “high speed”. 

• Although it is possible to choose films with 

different sensitivities, the choice is generally 

limited to a range of not more than four to one by 

most manufacturers. 


42

Comparison of Film Speeds 

• This illustration show 

two film sensitivities. 

• Notice that a specific 

exposure, indicated 

by the relative 

exposure values, 

produces a higher 

density in the high 

speed film; therefore 

requiring less 

exposure. 


43

Comparison of Film Speeds 

• High speed films are 

chosen when the 

reduction of patient 

exposure or reduced 

tube loading are 

important factors. 

• Low speed films are 

used to reduce image 

noise. 


44

Factors that Affect Film Sensitivity 

• The sensitivity of film 

is determined by a 

number of factors that 

include design, 

exposure conditions 

and how it is 

processed. 

• Optimum speed is 

achieved only when 

all factors are correct. 


45

Film Composition 

• Film sensitivity is determined by the 

composition of the emulsion. 

• The size and shape of the crystals have 

some effect on film speed. 

– Larger crystals increase speed. 

– Tabular Grain is generally faster. 

• Most of the increase in speed is achieved 

by adding chemical sensitizers in the 

emulsion. 


46

Processing 

• The effective sensitivity of the film depends on 

factors associated with the development: 

– The type of developer 

– Developer concentration 

– Developer replenishment rates 

– Developer contamination 

– Development time 

– Development temperature. 

• The object is to get consistent and predictable 

sensitivity. 


47

Developer Composition 

• Processing chemistry supplied by different 

manufacturers is not the same. 

• It is usually possible to process film in a 

variety of developer solutions, but they 

may not yield the same sensitivity. 

• Variations are usually small but must be 

considered when changing developer 

providers or brands. 


48

Developer Concentration 

• Manufacturers supply chemistry in 

concentrate that must be diluted before it 

is pumped into the processor for use. 

• Mixing errors that result in an incorrect 

concentration can produce undesirable 

changes in film sensitivity. 


49

Developer Replenishment 

• The development process consumes some of 

the developer solution and that results in a 

decline in sensitivity. Unless the solution is 

replaced, film speed will gradually drop. 

• In radiographic film processors, replenishment is 

automatic. A film entering the processor will 

activate replenishment pumps. The rate can be 

monitored. The proper replenishment rate 

depends upon the size and volume of films 

being run. 


50

Developer Contamination 

• If the developer solution becomes 

contaminated with another chemical such 

as fixer, abrupt changes in film sensitivity 

can occur in the form of either an increase 

or decrease in sensitivity. 

• Contamination generally occurs when the 

roller racks are removed or replaced or 

during film transport problem. 


51

Development Time 

• The development process is not instantaneous 

but is a gradual process where more and more 

crystals are developed and more density is 

produced. 

• Development does not stop until it is terminated 

in the fixer tank. 

• To some extent, increasing development time 

increases speed so less exposure would be 

needed to produce the same density. 


52

Development Time 

• In most automatic processors, development time 

is fixed to 20 to 25 seconds with two exceptions. 

• Rapid access film is designed to be processing 

faster for specialized procedures. Total 

processing time is 30 to 45 seconds. 

• Some types of mammography film (not all) is 

designed to be processed for a longer time in 

extended processing. This improves density 

and contrast. 


53

Developer Temperature 

• Developer activity changes with temperature. An 

increase in temperature speeds up the 

development process and increases sensitivity 

because less exposure is required to produce a 

specific film density. 

• Developer temperature is thermostatically 

controlled in automatic film processors with a 

normal range of 90-95ºF. Specific processing 

temperature are specified by the manufacturer. 

• Developer temperature is monitors as a 

component of processing quality control. 


54

Light Color (Wavelength) 

• Film is not equally sensitive to all 

wavelengths (colors) of light. The spectral 

matching of film must be taken into 

account with selecting film for use with 

specific types of intensifying screens and 

cameras. 

• The film should be most sensitive to the 

color of light emitted by the intensifying 

screens, intensifier tubes, CRT or lasers. 


55

Blue Sensitivity 

• A basic silver bromide emulsion has its 

maximum sensitivity in the ultraviolet and 

blue region of the light spectrum. 

• For many years most intensifying screens 

contained calcium tungstate, which emits 

a blue light and was a good match. 

• Although calcium tungstate is no longer 

used widely, many contemporary screens 

emit blue light. 


56

Green Sensitivity 

• Many light sources including image 

intensifier tube, rare earth intensifying 

screens and CRT’s emit a green light. 

• Users must be careful to use green 

sensitive film with these products. 

• The use of blue sensitive film will result in 

dramatically reduced speed. 


57

Red Sensitivity 

• Many lasers produce a red light. Devices 

that transfer images to film by means of a 

laser must be supplies with red sensitive 

film. 

• The normal dark room safelight must be 

turned off if processing this type of film. 

• We will now discuss more about 

safelights. 


58

Safelights 

• Darkrooms in which the film is loaded into 

the cassettes and transferred to 

processors are usually illuminated with a 

safelight. A safelight emits a color of light 

that we can see but will not expose film. 

• Although film has a relatively low 

sensitivity to the light emitted by safelight, 

film fog can be produced under certain 

conditions. 


59

Safelights 

• The safelight should provide sufficient 

illumination for darkroom operations but not 

produce significant exposure to the film being 

handled. This is usually accomplished by 

controlling certain factors. The factors are: 

– Safelight color 

– Brightness of lamp 

– Location of safelight 

– Duration of exposure of film to the safelight. 


60

Safelight Color 

• The color of the safelight is controlled by the 

filter. 

• The filter must be selected in relation to the 

spectral sensitivity of the film being used. 

• An amber-brown safelight provides a relatively 

high level of illumination and protection for blue 

sensitive film. Type 6B filters are used for this 

purpose. However, this type of filter produces 

light that falls within the sensitivity range of 

green film. 

• A red safelight is used when working with green 

sensitive films. It is a type GBX. 


61

Other Safelight Factors 

• Selecting the correct safelight filter does not 

absolutely protect film because film has some 

sensitivity to the light emitted by most safelights. 

Therefore, the brightness (bulb wattage) and the 

distance of the light to the work surfaces must 

be selected as to minimize film exposure. 

• The bulbs are limited to 15 watts. The safelight 

should be mounted 48” to 60” from the work 

area. 


62

Other Safelight Factors 

• Film exposure is an accumulative effect, 

handling of film should be as short as 

possible to minimize exposure to the 

safelight. 

• Film is most sensitive to exposure after the 

latent image is produced but before it is 

processed. Safelight or light exposure to 

the film is referred to as fogging of the film. 


63

Safelight Testing 

• The performance of the safelight should 

be monitored as part of a quality 

assurance program. It should be tested 

when the lamp is replaced and semiannually. 


64

Darkroom Safelight Testing 

• Procedure: 

• Place stepwedge in 

the center of 8” x 10” 

extremity cassette. 

• Exposed the film. 

• Take exposed film 

into the darkroom. 


65

20.2 Darkroom Safelight Testing 


• Turn off the safelight 

• Remove exposed film 

from cassette. 

• Cover half of film with 

opaque material. 

• Turn safelight on. 


66



• Leave film on counter 

for two minutes. 

• Process the film. 

• Read densities with 

Densitometer. 


67


• Standards: 

• At mid density, there 

should be less than 

0.10 optical density 

change between both 

sides of the film. 

• Exposed film is more 

sensitive to fog. 


68


• Example of faulty 

safelight and light 

leak. 

• Fog destroys contrast 

• Fog increased the 

speed so the image is 

flat and dark. 


69


• Solutions: 

– Check wattage of installed bulb. Max: 15W 

– Check height from filter to work area. Must 

be at least 48”. 

– Make sure filter matches the film. 

Orthographic film is blue/green sensitive 

while other film is blue sensitive. 

Orthographic film will be fogged by wrong 

filter. 


70


• Solutions: 

• After all of the above fails, try 

relocating the light farther away from 

work surface. 

• The filters will not last forever, replace 

biannually. 

• A rare problem is after glow of 

fluorescent lights. 


71

Radiographic Exposure Time 

• In radiography it is usually possible to deliver a 

given exposure to film by using many different 

combinations of radiation intensity 

(exposure rate) and exposure time. 

• Radiation intensity is proportional to the tube MA 

and time, this is the same as saying a given 

exposure (in milliampere-seconds) can be 

produced with many combinations of mA and 

time. 

• This is known as the law of reciprocity. 


72


• The law of reciprocity hold true for direct 

exposure to film so the intensity and be changed 

as long as the product of the mA and time is the 

same. 

• 100 mAs can be made with 1000 mA and 

0.01second exposure or 10 mA and 10 seconds. 

• However when the film exposure but by light as 

with intensifying screens or image intensifiers 

the reciprocity law does not hold true. 


73


• With light exposure instead of x-ray exposure, a 

single silver halide grain must absorb more than 

one photon before it can be developed and 

contribute to film density. 

• The sensitivity of the film is dependent upon the 

intensity of the exposing light. The significance is 

that MAS values that give correct density with a 

short exposure time may not do so with a long 

exposure time. 


74

Radiographic Exposure 

• With exposure times of 1 msec. and 

exposure time longer than 1 second, the 

light from the intensifier screens will not 

produce the same speed as exposures 

within that range. 


75

Processor Quality Control 

• There are many variable, such as 

temperature and chemical activity, that 

can affect the level of processing that a 

film receives. 

• Each type of film is designed and 

manufactured to have a specified 

sensitivity (speed) and contrast 

characteristics. 


76


• Under Processing 

– If a film is under developed, speed and contrast will 

be reduced below specified values. The operator can 

increase exposure to replace speed but the loss of 

contrast can not be recovered. Increased exposure is 

bad for the patient. 

• Over Processing 

– Over processing can increase speed or sensitivity. 

With some film types contrast may increase to a point 

before dropping due to increased fogging of the film. 


77


• Processing Accuracy 

– The first step in processing quality control is 

to set up the correct processing conditions 

and then verify that the film is being properly 

processed. 

• Processing Conditions 

– A specification of recommended processing 

conditions (temperature, time, type of 

chemistry, replenishment rates, etc) should be 

obtained from the film manufacturer. 


78


• Processing Verification 

– After the recommended processing conditions 

are established for each type of film, a test 

should be performed to verify that the film is 

producing the design sensitivity and contrast. 

These specifications are usually provided in 

the form of a film characteristic curve that can 

be compared to one produced by the 

processor. 


79


• Processing Consistency 

– The second step is to reduce the variability 

over time in the level of processing. 

– Variations in processing conditions can 

produce significant differences in film 

sensitivity. 

– The objective of processor quality control is to 

assure that the processor will never be the 

cause of over or under exposed films. 


80


• Processing Consistency 

– Daily when patient x-rays are scheduled, 

processor quality control should be performed 

before exposing the first patient. 

– After verifying that the processor is warmed 

up to operating temperatures, a film is the 

type normally used is exposed to a 

reproducible light in a sensitometer. 

– After the film is processed, the densities are 

read using a densitometer. 


81

Sensitometry 

• Besides monitoring 

the developer 

temperature only a 

few steps on the 

exposure steps are 

needed to monitor the 

performance of the 

film processor. 


82


• The densities values are 

graphed on a chart so 

that fluctuations can be 

easily detected. 

• Base + Fog Density 

– One density is read in an 

area of no exposure to 

measure processing fog. A 

low base + fog is desired. 

– A high number would 

indicate over development, 

increased fog or a 

darkroom problem. 


83


• The densities values are 

graphed on a chart so 

that fluctuations can be 

easily detected. 

• Base + Fog Density 

– One density is read in an 

area of no exposure to 

measure processing fog. A 

low base + fog is desired. 

– A high number would 

indicate over development, 

increased fog or a 

darkroom problem. 


84

Densitometry 

• Speed 

– A single exposure step 

that produces a film density 

of about 1.0 density unit 

above the base + fog level 

is designated as the “speed 

step” 

– The density of this step is 

an indication of the film 

speed. 

– Any factor that impacts 

development can show a 

variation in speed. 


85

Densitometry 

• Contrast 

– Two steps are selected , 

and a difference between 

them is used to measure 

film contrast. 

– This is called the contrast 

index. 

– If the two steps represent a 

two-to-one exposure ratio 

(50% contrast, the index is 

the same as the contrast 

factor often called the 

average gradient. 


86


• If abnormal variations in 

film densities are 

observed, all possible 

causes must, such as: 

– Developer temperature 

– Solution replenishment 

– Developer concentration 

– Contamination. 

• If more than one 

processor is used, they 

should be matched. 


87

Artifacts 

• A variety of image artifacts can be produced 

during the storage, handling and processing of 

the film. 

• Bending or rough handling of unprocessed film 

will produce artifacts or “kink marks or crinkle 

marks” which appear black on the film. 

• Handling film in a dry environment, can produce 

static electricity build-up that upon discharge will 

produce dark spots or streaks like lightning. 


88

Artifacts 

• Artifacts can be produced during 

processing by factors such as uneven 

roller pressure or accumulation of a 

substance on the rollers. 

• This type of artifact is often repeated at 

intervals corresponding to the 

circumference of the rollers. This is often 

referred to as pi marks. 


89

The End 

Thanks to the work of Dr. Perry 

Sprawls of Emory University and the 

Sprawls Educational Foundation, this 

material is available on-line. 


90

The Photographic Process and Film Sensitivity

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