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step process. The electromagnetic radiation ejects an electron from the negatively charged bromine ion in the
crystalline structure, thus converting the ion into a bromine atom. The free electron can travel within the crystal to a
dislocation or other latent image site where it is trapped, establishing a negative electrical charge at that point. This
negative electrical charge attracts one of the positively charged interstitial silver ions to the latent image site. When the
silver ion reaches the image site, its positive charge is counteracted by the negative electron and it becomes neutralized
and exists as a silver atom. The latent image site is now electrically neutral. The process may be repeated several
times, adding silver atoms to the latent image site in the crystal. These few silver atoms act as a catalyst to the
reducing action of the developer, thus making the entire emulsion grain susceptible to conversion to metallic silver in
development.
6.4.1.2.3
Development.
The developing agent selectively reduces those crystals containing latent images into black metallic silver but has a
much smaller effect on those crystals that have not been exposed. The metallic silver is opaque and forms the
radiographic image.
6.4.1.2.4
Image Quality.
Microscopic variations in the response of film to the incident radiation produce effects of considerable practical
significance. The number of sites at which the silver atoms can respond to the radiation vary in location throughout the
emulsion and are inversely proportional to the size of the silver bromide grains. Thus, after exposure to radiation, the
density of the image will vary. The larger the number of sites activated by radiation, the larger the number of silver
atoms per unit area, and, from statistics, the smaller the density variations. The practical factors are:
a. Graininess. The graininess of the film is the visual impression of non-uniformity of density in a
radiographic image. In general, graininess increases with increasing film speed and with increasing
energy of the radiation. Apart from the visual appearance of graininess, the effect may be subjected to
physical measurements in which case the property measured is referred to as "granularity." This latter
term has been adopted as an expression for physical measurements of the statistical fluctuations of
density over the area of a photographic emulsion. Granularity measurements are obtained by scanning
a sample of emulsion by a small spot of light (diameter of the order of 0.08 mm) and recording the
resulting irregular fluctuations of the transmitted light. (See Reference 8).
b. Signal-to-Noise Ratio. The accidental variation in image density makes it more difficult to identify the
deliberate variation in image density that results from use of the film. The relationship between the two
density variations is known as the signal-to-noise ratio. For threshold visibility of detail, this ratio must
be at least 5.
6.4.1.3
Types Of X-Ray Films.
Various types of X-ray films are available that vary in signal-to-noise ratio, speed of response to radiation, and
graininess. It is most appropriate to classify X-ray film in relation to their signal-to-noise ratios. Very fine-grained
films have a very high signal-to-noise ratio, require comparatively large quantities of radiation for exposure and
produce images with excellent resolution of detail. In the choice of a particular film, a tradeoff must be made between
resolution and speed of exposure. The criticality of an inspection will determine this tradeoff. Some commonly used
X-ray films are classified as follows:
a. Class 1: This class has the highest signal-to-noise ratio and includes such films as Agfa D2, Kodak
Type R, and Fuji IX 25. These are, considered high detail resolution films and should be employed
when the most sensitive radiograph is desired.
b. Class 2: This class is considered as high in signal-to-noise ratio and includes such films as Agfa D4,
Kodak Type M, and Fuji IX 50.
c. Class 3: These films have a moderate signal-to-noise ratio and include Agfa D5, Kodak Type T, and
Fuji IX 59 with screen.
