C HA P T E R

4Film Processing

K E Y T E R M S

agitationarchival qualitydeveloperfixer

flood replenishmenthyporetentionlatent imagemanifest image

oxidation/reduction reactionsynergismvolume replenishment

O B J E C T I V E S

At the completion of this chapter the reader should be able to do the following:• Describe the main differences between manual and

automatic film processing• List the main components of the developer and fixer

solutions and state the function of each component• Explain the proper mixing procedure for developer and

fixer concentrate solutions• State the chemical safety procedures for the safe

handling of processing chemicals as described by the

Occupational Safety and Health Administration(OSHA)

• Describe the basic tests for determining the archivalquality of processed images

• List the six main systems of automatic film processorsand state the function of each system

• Describe the methods of installing film processors in adarkroom

O U T L I N E

Manual and Automatic FilmProcessing 50Manual Processing 50Procedure 50Automatic Processing 50

Processing Chemicals 50Developer 50

Developer Components 50Developer Activity 52Developer MixingProcedure 54

Fixer 54Fixer Ingredients 54Fixer Mixing Procedure 55

Washing 55Chemical Safety 56

Automatic Processor MainSystems 57Transport System 57

Roller Subsystem 57Transport RackSubsystem 58

Drive Subsystem 58Temperature Control System 59

Water-Controlled System 59Thermostatically ControlledSystem 59

Circulation System 60Replenishment System 60

Volume Replenishment 60Flood Replenishment 60

Dryer System 61

Electrical System 61Types of AutomaticProcessors 61Processor Size 61

Floor-Size Processor 62Intermediate-SizeProcessor 62

Tabletop-Size Processor 62Processor Location 62

Totally Inside 62Bulk Inside 62Bulk Outside 62Daylight Processing Systemsand Processors 62

Summary 62

After a film has been exposed to radiation, the image that itcontains is still invisible to the human eye and is called alatent image. For the film to be converted into a visibleimage, or manifest image, the silver contained in the filmmust be changed from an ionized state (Agþ) into a neutral,

or reduced, state (Ag�), in which the silver turns black. Thisrequires the film to be processed by various solutions thatconvert the latent image into a visible one and also preservethe image for permanent storage. The two basicmethods offilm processing are manual and automatic.

49

MANUAL AND AUTOMATIC FILMPROCESSING

Manual Processing

In the manual processing method, film is moved fromone solution to the next manually until processing iscomplete. This method requires more labor and timeand is more prone to variations than automatic proces-sing. For this reason, manual processing is seldom usedin diagnostic imaging today. For film to be processedmanually, several steps are required after the films arehung on special hangers.

Procedure

1. Wetting agent. The wetting agent is a chemical thatloosens the emulsion so that subsequent solutionscan reach all parts of the emulsion uniformly, whichreduces development time. This step is optionalbecause developer ingredients also soften the emul-sion. If a wetting agent is used, the film shouldremain in the solution for about 15 seconds.

2. Developer. The developer solution converts the latentimage into the visible image; therefore, this is themost important processing chemical. The filmremains in the developer for 3 to 5 minutes depend-ing on the temperature of the solution.

3. Stop bath or water rinse. The stop bath or water rinsestep stops the development process and removes excessdeveloper from the film. A stop bath is a 1% solution ofacetic acid that chemically neutralizes the developer(because it is an alkaline solution) and requires only5 to 10 seconds of film immersion time. A water rinserelies on water to remove the excess developer andrequires about 30 seconds of film immersion.

4. Fixer. The fixer solution removes the unexposed andundeveloped silver halide crystals from the film emul-sion and also hardens the emulsion so that the filmcan be permanently stored. The time of fixation var-ies with solution temperature, but the general rule formanual fixation is use of the following equation

Fixing time ¼ Clearing time þ Hardening time

The clearing time is the time necessary for the fixerto clear away the unexposed and undeveloped silverhalide crystals, which should be accomplished within5 minutes. The hardening time is the time it takes theemulsion to properly harden and is usually equal tothe clearing time; therefore, a film that requires 5 min-utes to clear requires another 5 minutes to harden,leaving a total fixing time of 10 minutes.

5. Washing. Excess fixer must be removed from filmbefore it is allowed to dry, or the fixer componentscrystallize onto the film surface, a process known ashyporetention. This white, powdery residue canimpair the diagnostic quality of the final image andmust therefore be avoided. An example of an image

with hyporetention can be found in Chapter 10. Thisstep may take up to 20 minutes.

6. Drying. Drying prepares the film for viewing and stor-age and can be accomplished either by an electric dryer,which works in less than a minute, or by exposure toroom air while the film is mounted on a special hanger,which may require an hour or more.

Automatic Processing

Automatic processing requires an electromechanical devicecalled an automatic film processor, which transports thefilm fromone solution to the nextwithout anymanual laborexcept for placing the film into the device. This shortens theoverall processing time, increases the number of films thatcan be processed in a given period, and ensures less variabil-ity of overall film quality than manually processed filmsbecause the processing time, solution temperature, andchemical replenishment are automatically controlled. Thedisadvantages of automatic processing include higher capi-tal and maintenance costs, increased chemical fog due tohigher processing temperatures, and transport problemsthat can damage or destroy images during processing. In adiagnostic imaging department that has not converted todigital imaging, the advantages far outweigh the disadvan-tages and automatic film processing is virtually exclusive.

PROCESSING CHEMICALS

Developer

As previously mentioned, the developer is the most impor-tant processing solution; it converts the latent image into amanifest image. This is accomplished by the developersolution carrying out an oxidation/reduction reaction,or redox. When a chemical is oxidized (broken down),it releases electrons. These electrons are then availableto convert another compound into a more simplified,or reduced, state (hence the term oxidation/reductionreaction). During film processing, the developer solutioningredients are oxidized and the silver halide crystal isreduced to black metallic silver. This chemical reactioncan be summarized by the following equations:

During exposure to radiation:

Agþ Br� þ radiation ! Ag� þ Br� þ Ag�

(5 atoms latent image)

During immersion in developer:

Agþ þ developerþ Ag�ð5 atoms latent imageÞ !Ag�ð108 atoms visible imageÞ þ oxidized developer

Developer Components. Developer is composed ofdeveloping or reducing agents, preservatives, accelera-tors or activators, restrainers, regulators, antifoggantsor starters, hardeners, solvents, and sequestering agents;all act on the film.

Developing or Reducing Agents. Developing orreducing agents carry out the oxidation/reduction reaction

50 CHAPTER 4 Film Processing

that converts the latent image into a manifest image. Twodifferent reducing agents are used in standard developersolutions: phenidone and hydroquinone.

Phenidone (Elon or Metol in Manual Developer).Phenidone is fast acting and produces the image opticaldensities of up to about 1.2. It is responsible for the mini-mum diameter (Dmin) and speed indicators used in sensito-metric testing (described in Chapter 5).

Hydroquinone. Because hydroquinone acts moreslowly than phenidone, the developmental process is com-pleted so that the image optical densities that are greaterthan 1.2 are visualized. Hydroquinone is responsible forthe maximum diameter (Dmax) and contrast indicatorsused in sensitometric testing. These indicators are the firstvariables to show an indication of developer failurebecause hydroquinone is the processing chemicalmost sen-sitive to changes in temperature, concentration, and pHand to exposure to light and heavy metals. Hydroquinonelevels should be maintained in the range of 20 to 25 g/L.

The overall optical density is created by the synergisticaction of the two reducing agents. Synergism means thatthe action of the two agents working together is greaterthan the sum of each agent working independently.Synergism is also known as superadditivity (Fig. 4-1).

Preservative. The preservative, or antioxidant, pro-tects the hydroquinone from both aerial oxidation (chem-ical reaction with air) and internal oxidation (chemicalreaction with other developer ingredients). If the hydro-quinone is oxidized, there is a decrease in the Dmax andcontrast indicators during a sensitometric test, along witha loss of the shoulder on the H and D curve. Oxidizeddeveloper causes the developer solution to turn from aclear, brown liquid into one that is dark and muddy. Ifstrongly oxidized, the solution also has the odor of ammo-nia because this is a byproduct of the oxidation chemicalreaction. Most developer replenishment tanks have afloating lid inside the tank in addition to the main lid onthe outside, to minimize contact with the outside air. Thechemicals sodium sulfite, potassium sulfite, and cyconcan be used as developer solution preservatives.

Accelerator, Activator, or Buffering Agent. Theaccelerator, activator, or buffering agent has two func-tions: to soften and swell the emulsion so that reducingagents can work on all of the emulsion and to provide

an alkaline medium for the reducing agents. The devel-oping agents must exist in an alkaline medium to have thefree electrons available to reduce the silver to Ag�.

An indicator known as pH is used to measure thealkalinity of a solution. Potential hydrogen (pH) refersto the exponential (p) value of hydrogen ions (Hþ) avail-able for a reaction. Those chemicals having a highhydrogen potential (Hþ) are called acids, and those hav-ing a high alkaline or hydroxide potential (OH�) (andtherefore, a low hydrogen potential) are called bases.The pH scale ranges from 0 to 7 (acids) and 7 to 14(bases) (Fig. 4-2). This scale is based on the concentra-tion of positively charged hydrogen ions (Hþ) in molesper liter. For example, a pH of 4 would mean that a par-ticular solution contains one ten-thousandth (10�4) of amole of hydrogen ions per liter. For this value to be con-verted to pH, the negative exponent (�4) is changed to apositive number (4). A solution with a Hþ concentrationof one ten-millionth (10�7) moles per liter would thenhave a pH of 7, and so on. Because the pH scale is loga-rithmic in nature, a change of one whole number on thepH scale can represent a tenfold change from the previ-ous concentration. A pH of 1 denotes 10 times more Hþ

ions than a pH of 2; a pH of 3 has 10 times fewerHþ ions than a pH of 2, and so on. Pure water is neutraland has a pH of 7. Fixer is an acid solution; therefore,care must be taken not to introduce it into developersolutions because only 0.1% contamination deterioratesthe developer activity enough to compromise image qual-ity. Chemicals that can be used as accelerators includesodium carbonate, sodium hydroxide, potassium carbon-ate, and potassium hydroxide.

0

Gastric juice (1.7)

Grapefruit juice (3.3)

1

2

3

4

5

6Rain water (5.5)

Milk (6.8)

7

Blood (7.4)Sea water (8.3)

8

9

10

11

12

Strong alkali

13

14

(7)

Developingsolution (9.5-10.7)

Strong acid

Neutral

Fixingsolution (4-5)

ALKALI

ACID

FIGURE 4-2 Potential hydrogen (pH) scale.

Phenidone � hydroquinone(simple addition of densities)

Den

sity

Development time

Phenidone� hydroquinone

(actual performance)

HydroquinonePhenidone

FIGURE 4-1 Graph demonstrating superadditivity effect ofphenidone and hydroquinone.

51CHAPTER 4 Film Processing