If films are allowed to develop without any movement, there is a tendency for each area of the film to affect the
development of the areas immediately below it. This is because the products of development have a higher specific
gravity than the developer. As these products diffuse out of the emulsion layer, they flow downward over the film
surface and retard the development of the areas over which they pass. The greater the film density from which the
reaction products flow, the greater is the restraining action upon the development of lower portions of the film. Thus,
large lateral variations in film density will cause uneven development in the areas below, and this may show up in the
form of streaks.
It is recommended that radiographic inspection facilities use the replenishment
method while performing the manual film development process.
As films are developed without replenishment, the developing solution becomes exhausted chemically until no
developing action can take place. For a given quality of developer, without considering the effects of oxidation, levels
of bromide and hardener, and contamination; the development time must be increased for successive films to fully
develop them. It is estimated that a five-gallon tank of developer will develop 140 films, size 14 x 17, satisfactorily
without excessive increase in development time. It is convenient to divide the total number of films that can be
developed by 5 gallons of developer into seven groups of 20 films each. As each group of 20 films, 14 x 17, or
equivalent film area is developed, the development time must be increased 1/4 minute, assuming a normal time of 5
minutes at 20°C (68°F). Even when drained, each film carries about 1-1 /2 ounces of developer with it, so developer
must be added to keep the tank at the 5-gallon level. When the specified number of films have been developed, discard
the solution. This method is known is the exhaustion method of developing.
Another method of processing is the replenisher method. By adding replenisher solution periodically, the activity of
the developer is kept at the same level. In this method films must be removed from the tank quickly without allowing
the excess developer to drain off the film back into the tank. Approximately 1 gallon of replenisher should be added for
every 40 films, 14x 17, or equivalent film area (based on 5 gallons of developer). If this amount of developer cannot be
added at the specified time, too much developer is draining back into the tank. In this case enough developer must be
drained from the tank so the replenisher can be added. The developer solution shall be discarded when the replenisher
used equals four times the original quantity of developer solution, when it fails the process control requirements or at
each two month period, whichever occurs first. For dense radiographs it may be necessary to increase the quality of
replenisher added. In this case, it is also desirable to add replenisher at shorter intervals to keep the level of developer
activity more nearly constant.
Fresh developer is "wild" and will often result in excessive contrast on the first few films. This is apparently due to the
lack of equilibrium between the developer and the reaction products. It is sometimes recommended that a small
quantity of old developer be mixed with the fresh developer to temper the solution.
Testing Developer Activity.
The success of this method of compensating for the gradual decrease of developer activity will depend upon the use of
an adequate system for testing this activity. Since there is no simple direct physical or chemical test of developer
activity, the easiest way of making the test is to process, at frequent intervals, film strips exposed in some standard
manner, and to compare the densities obtained with a identical strip that had been processed in the fresh solution. The
standard strips are cut from a sheet film, 8 by 10 inches or larger, which has been exposed to direct x-rays through a
test object. The most suitable form of test object is a stepped wedge made up of a number of sheets of any convenient
metal. The wedge should have about 15 steps and be large enough to cover completely the largest cassette or film
holder used. When given the proper exposure this should produces series of densities extending over the density range
used in practice. It is essential that all strips used in testing a batch of developer receive identical exposures. For this