Contrast in Film

CONTRAST TRANSFER	CONTENTS
The ability of a film to produce contrast can be determined by observing the difference in density between two areas receiving a specified difference in exposure, as shown in the figure above. However, since the amount of contrast is affected by the level of exposure, a range of exposure values must be delivered to a film to demonstrate fully its contrast characteristics.    One method of doing this is illustrated in the following figure; this type of exposure pattern is usually produced by a device known as a sensitometer. In this method, a strip of film is divided into a number of individual areas, and each area is exposed to a different level of radiation. In this particular illustration, the exposure is changed by a factor of 2 (50% contrast) between adjacent areas. When considering contrast characteristics, we are usually not interested in the actual exposure to a film but rather the relative exposure among different areas of film. In the figure below the exposures to the different areas are given relative to the center area, which has been assigned a relative exposure value of 1. We will use this relative exposure scale throughout our discussion of film contrast characteristics. Note that each interval on the scale represents a 2:1 ratio. This is a characteristic of a logarithmic scale. When the film is processed, each area will have density values, as shown directly below the area. The amount of contrast between any two adjacent areas is the difference in density, as shown. In this illustration we can observe one of the very important characteristics of film contrast. Notice how the contrast is not the same between each pair of adjacent areas throughout the exposure range: there is no contrast between the first two areas, but the contrast gradually increases with exposure, reaches a maximum, and then decreases for the higher exposure levels. In other words, a specific type of film does not produce the same amount of contrast at all levels of exposure. This important characteristic must be considered when using film to record medical images. The Variation in Contrast with Exposure    All films have a limited exposure range in which they can produce contrast: if areas of a film receive exposures either below or above the useful exposure range, contrast will be diminished, or perhaps absent. Image contrast is reduced when a film is either underexposed or overexposed.