Light-sensitive materials are composed of two basic parts: the emulsion and the base. The emulsion is the light-sensitive portion that records the image. The base is the support to which the emulsion is coated. Also, negative materials have additional layers that perform a special purpose. They are as follows: overcoating, antihalation backing, and noncurl coating (fig. 2-2). The purpose of the five parts of photographic film are as follows:

1. Overcoating The overcoating protects the film from friction, scratches, or abrasions before develop­ment. The overcoating is a clear, gelatin layer that is sometimes called the antiabrasion layer.

2. Emulsion thin layer of gelatin that suspends and supports the light-sensitive silver halides.

3. Base-This supports or holds the emulsion in place. The base may be transparent, translucent, or opaque, depending upon how the recorded image is to be used. The base is generally made of a cellulose acetate.

4. Antihalation backing-The antihalation backing prevents light from reflecting from the base back into the emulsion. The antihalation dye is sometimes incorporated in the anticurl backing. The dye used to eliminate halation is a color to which the emulsion is least sensitive. This dye is water soluble and is com­pletely dissolved during processing.

5. Noncurl coating-The noncurl coating is a hardened gelatin, about the same thickness as the

Figure 2-3.-Cross section of ordinary color negative film.

Figure 2-4. Modern high-speed color film.

emulsion, and is applied to the back of the film. A film emulsion swells when wet and shrinks when dry. This contraction produces a strain on a film base because it is highly flexible. The noncurl coating prevents the film from curling during the drying process.

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