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constant difference, in these cases, 0.30 and 1.00 respectively. In other words, a constant increase in the logarithm of a
number means a constant percentage increase in the number itself.
6.7.5
Industrial Radiographic Film Characteristics.
6.7.5.1
Background.
Historically, when faced with the necessity for film substitution, the radiographer would examine manufacturer's
literature and then perform trial exposures with a new film. Using the first radiograph as a basis, the radiographer
would modify his exposure parameters and try again. Often this procedure would have to be repeated several times,
depending on the experience of the radiographer and difficulty of subject, before an acceptable radiograph was
produced. This iterative process involves considerable expenditure of time and the now significant cost of film.
Available manufacturer's literature generally provides speed, contrast and processing data pertinent to only the films
and chemicals they produce. Presentation of the data differs greatly between manufacturers, as do their methods for
developing this data. Contact the engineering authority for your weapon system for current information on how each
manufactures film works for your specific application.
6.7.6
Developing Theory.
The purpose of a developing solution or developer is threefold. First, it blackens those parts of the emulsion that have
received exposure. When a crystal of the film's silver bromide emulsion has been exposed to X-ray radiation and is put
into a developing solution, the developer takes the bromide away from the silver and leaves black metallic silver in the
gelatin. Thus, where full exposure has occurred, a maximum number of crystals are affected and almost all of them are
reduced by the developing solution to metallic silver.
6.7.6.1
The second purpose of a developer is to produce various shades of gray where the film has been only partially exposed.
These grays are the result of partial removal of bromide. The concentration of black metallic silver per unit area of the
film is dependent upon the amount of exposure received and determines the factor known as film density. The image of
the object radiographed consists of varying densities spread over the film, corresponding to the varying amounts of
exposure received by the film.
6.7.6.1.1
The third and equally important function of the developer is its effect on those parts of the film that have received no
exposure. Since no crystals have been affected, the developer should leave these parts unchanged. Thus we see that a
developing solution should remove bromide from the film emulsion where exposure has occurred, but should produce
no effect on unexposed areas of the film.
6.7.6.1.1.1
A very limited number of chemicals possess the ability to distinguish between exposed and unexposed crystals and
therefore only a few are suitable for use in developers. No chemical known will leave an unexposed area indefinitely
unchanged. All will begin to develop unexposed parts after a period of time, producing a condition called chemical
fog. All developing agents have a definite fogging time beyond which bromide will be freed in unexposed areas.
6.7.7
Developing Solutions.
NOTE
Follow manufactures data sheet for use of developers.
Three reducing agents commonly used in radiographic developers are metol, phenodone and hydroquinone. A
combination of these ingredients produces all of the steps of grays and jet black, bringing out the best possible results.
Developers are made up, by the manufacturer in standard powder and liquid forms. The temperature of the
development chemicals has a direct effect on their activity and therefore, the time it takes for film to achieve a specific
density. The higher the development solution temperature is above 68°F (20°C), the lower the sensitivity, resolution
and contrast of the developed film will be. With the reduction of these preferred qualities, latitude and fog level will
