As mentioned previously, radiographic film consists of a transparent, blue-tinted base coated on both sides with an emulsion. The emulsion consists of gelatin containing microscopic, radiation sensitive silver halide crystals, such as silver bromide and silver chloride. When x-rays, gamma rays or light rays strike the the crystals or grains, some of the Br- ions are liberated and captured by the Ag+ ions. In this condition, the radiograph is said to contain a latent (hidden) image because the change in the grains is virtually undetectable, but the exposed grains are now more sensitive to reaction with the developer.
When the film is processed, it is exposed to several different chemicals solutions for controlled periods of time. Processing film basically involves the following five steps.
- The developing agent gives up electrons to convert the silver
halide grains to metallic silver. Grains that have been exposed to the
radiation develop more rapidly, but given enough time the developer will
convert all the silver ions into silver metal. Proper temperature control is needed to convert exposed grains to pure silver while
keeping unexposed grains as silver halide crystals.
- Stopping the development - The stop bath simply stops the development process by diluting and washing the developer away with water.
- Fixing - Unexposed silver halide crystals are removed by the fixing bath. The fixer dissolves only silver halide crystals, leaving the silver metal behind.
- Washing - The film is washed with water to remove all the processing chemicals.
- Drying - The film is dried for viewing.
Processing film is a strict science governed by
rigid rules of chemical concentration, temperature, time, and
physical movement. Whether processing is done by hand or automatically
by machine, excellent radiographs require a high
degree of consistency and quality control.
Manual Processing &
Manual processing begins with the darkroom. The darkroom should
be located in a central location, adjacent to the reading room
and a reasonable distance from the exposure area. For portability,
darkrooms are often mounted on pickups or trailers.
Film should be located in a light, tight compartment, which is
most often a metal bin that is used to store and protect the film.
An area next to the film bin that is dry and free of dust and
dirt should be used to load and unload the film. Another
area, the wet side, should be used to process the film. This method protects the film from any water or chemicals that may be located on the
surface of the wet side.
Each of step in the film processing must be excited properly to develop
the image, wash out residual processing chemicals, and to provide
adequate shelf life of the radiograph. The objective of processing
is two fold: first, to produce a radiograph adequate for viewing,
and second, to prepare the radiograph for archival storage. Radiographs are often stored for 20 years or more as a record of the inspection.
Automatic Processor Evaluation
The automatic processor is the essential piece of equipment in
every x-ray department. The automatic processor will reduce film
processing time when compared to manual development by a factor
of four. To monitor the performance of a processor, apart from
optimum temperature and mechanical checks, chemical and sensitometric
checks should be performed for developer and fixer. Chemical checks
involve measuring the pH values of the developer and fixer as well as both replenishers. Also, the specific gravity and fixer silver levels must be measured. Ideally, pH should be measured daily and it is important
to record these measurements, as regular logging provides very
useful information. The daily measurements of pH values for the developer
and fixer can then be plotted to observe the trend of variations
in these values compared to the normal pH operating levels to identify
Sensitometric checks may be carried out to evaluate if the performance
of films in the automatic processors is being maximized. These
checks involve measurement of basic fog level, speed and average
gradient made at 1° C intervals of temperature. The range
of temperature measurement depends on the type of chemistry in
use, whether cold or hot developer. These three measurements:
fog level, speed, and average gradient, should then be plotted
against temperature and compared with the manufacturer's supplied