Sizes

Various sizes of film are available, although only three are usually used routinely (see Fig. 4.1).

The film packet contents

The contents of a film packet are shown in Fig. 4.2. It is worth noting that:

The radiographic film

The cross-sectional structure and components of the radiographic film are shown in Fig. 4.3. It comprises four basic components:

Film orientation

The film has an embossed dot on one corner that is used to help orientation. Its position is marked on the back of the packet or can be felt as a raised dot on the front. The side of the film on which the dot is raised is always placed towards the X-ray beam. When the films are mounted, this raised dot is towards the operator and the films are then arranged anatomically and viewed as if the operator were facing the patient.

Indirect-action film construction

This type of film is similar in construction to direct-action film described above. However, the following important points should be noted:

The relative spectral sensitivity of these four different film emulsions is shown in Fig. 4.4.

Optical density (OD)

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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.

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 Fig. 4.5. This curve describes several of the film’s properties.

Background fog density

This is the small degree of blackening evident even with zero exposure. This is due to:

If the film has been stored correctly (see later), this background fog density should be less than 0.2 (see Fig. 4.5).

Film speed

This is the exposure required to produce an optical density of 1.0 above background fog (see Fig. 4.6). Thus, the faster the film, the less the exposure required for a given film blackening and the lower the radiation dose to the patient.