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film OD. If the screen is made thicker so that 10% more x-ray photons are detectedwith no change in the conversion efficiency, then a reduction in the incident x-raybeam of 10% is required to deliver the same degree of film darkening. The fractionof x-ray photons that are detected increases, but the number of incident x-ray photonsmust be reduced by that same fraction, so the total number of detected x-rayphotons remains the same. Consequently, the noise in the image is not affected bychanging the absorption efficiency. However, spatial resolution suffers when thescreen is made thicker.6.5 CHARACTERISTICS OF FILMComposition and FunctionUnexposed film consists of one or two layers of film emulsion coated onto a flexiblesheet made of Mylar (a type of plastic). Tabular grain emulsions (Fig. 6-13A) are usedin modern radiographic film; laser cameras and older radiographic films use cubicgrain film (see Fig. 6-13B). The grains of silver halide (AgBr and AgI) are boundin a gelatin base and together comprise the film emulsion. A typical sheet of dual-FIGURE 6-13. Scanning electron micrographs (SEM) are shown. A: A top-down SEM image of theemulsion layer of T grain emulsion is shown. B: A cross section of the T grain emulsion film is illustrated,showing the grains in the gelatin layer, supported by the polymer film base below. C: Cubicgrain film emulsion is shown in a top-down SEM view. D: A cross sectional SEM image of the cubicgrain film is illustrated. SEM photographs courtesy of Drs. Bernard Apple and John Sabol.