FILMS, FILM HOLDERS AND SCREENS

Films

Function. Films can be used as a recording medium because their emulsions are sensitive to the quantity and the energy of electromagnetic radiation over a wide spectral range. In the photographic process, the electromagnetic radiation of the visible spectrum is focused by a lens upon the film surface to record the variations of light intensities and form an image. In radiographic applications, the radiation is of such high energies they cannot be focused by a lens. In radiography, a shadowgraph of the test object is formed by recording the variations in radiation quantities caused by absorption and scattering by the test specimen. After final processing, film that has been exposed with X or gamma rays is called a radiograph; film exposed by using a radioisotope may be called a gammagraph. The term radiograph is used throughout this chapter. Films are an excellent recording medium with a very high signal-to-noise ratio and high amplification. Real-time, radioscopic electronic devices cannot match the excellent recording characteristics of film. This section describes how films work, reviews how films respond to radiation, and discusses radiographic paper.

General Theory of Industrial Radiographic Film. Films consist of a base material coated with an emulsion containing the radiation-sensitive silver-halide crystals, which are usually silver bromide. Most modern films have a polyester base which is either transparent or has a slightly blue tint. The polyester is very durable, does not absorb water or processing chemicals, and is dimensionally stable dries easily, and will not support combustion.

Sketch of Cross Section of X-Ray Film

Emulsion. The emulsion consists of a gelatin material containing an even distribution of the radiation-sensitive silver bromide crystals or grains. This emulsion is coated on the polyester base in very thin layers, usually about 0.001 inch in thickness. Most X-ray films have double emulsions, i.e., are coated on both sides of the base material. Since the thin support material offers very little absorption to the X-rays normally used for industrial applications, the double emulsions essentially reduce exposure requirements to one-half that required for a single emulsion. However, some films, intended for radiography in which visibility of the smallest detail is required, have emulsion only on one side.Latent Image. The latent image is formed by interactions of the electromagnetic radiation with the silver bromide crystals. When solid silver bromide is formed in the manufacture of film, the silver atoms give up an orbital electron to a bromine atom. Since the silver atoms have given up an electron, they have a positive electrical charge and are silver ions (Ag+). The bromine atoms have acquired this negative electron and have become bromide ions (Br-). The silver bromide crystal is a cubical array of the silver and bromine ions. The cubical crystalline structure of the silver bromide crystal is not perfect; if it were, the photographic process could not exist. Within the crystal lattice structure are extra silver ions called interstitial ions; these do not occupy a lattice position in the crystal. There are also foreign molecules or dislocations (distortions) of the crystal array within the crystal, all of which form latent image sites.

The accepted theory of the formation of the latent image (that is, an image which may be revealed by development) in a photosensitive emulsion is based upon the Gumey-Mott concept of exposure. It is theorized that the formation is a two­step 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.

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.

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:

1. 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.
2. 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.

Source: KARTA TECHNOLOGY, INC

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