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Essentials of dental radiography and radiology
Characteristic curve The characteristic curve is a graph showing the variation in optical density (degree of blackening) with different exposures. Typical characteristic curves for direct-action (non-screen) and indirect-action (screen) film are shown in Figure 5.13. This curve describes several of the film's properties. Background fog density
Fig. 5.12 Graph showing the relative spectral sensitivity of standard silver halide (BLUE), modified silver halide (ULTRAVIOLET), orthochromatic (GREEN) and panchromatic (RED) film emulsions.
This is the small degree of blackening evident even with zero exposure. This is due to:
Characteristics of radiographic film
• The colour/density of the plastic base • The development of some unexposed silver halide crystals.
This section summarizes the more important theoretical terms and definitions used to describe how radiographic film responds to exposure to X-rays.
If the film has been stored correctly (see later), this background fog density should be less than 0.2 (see Fig. 5.13). Film speed
Optical density (OD) OD = log
Incident light intensity Transmitted light intensity
Optical density is the term used for describing the degree of film blackening and can be measured directly using a densitometer. In diagnostic radiology the range of optical densities is usually 0.25-2.5.There are no units for optical density.
This is the exposure required to produce an optical density of 1.0 above background fog (see Fig. 5.14). Thus, the faster the film, the less the exposure required for a given film blackening and the lower the radiation dose to the patient. Film speed is a function of the number and size of the silver halide crystals in the emulsion. The larger the crystals, the faster the film but the poorer the image quality.
Fig. 5.13A A typical characteristic curve of indirect-action radiographic film, showing the main regions of the curve including background fog density, toe and shoulder. B A typical characteristic curve for direct-action film.