The Structure of Camera Film

The Structure of Camera Film

  • Black-and-white films render the naturally colored image that is projected upon them by the lens as a monochromatic (that is, one-color) image: one composed of an infinitely shaded series of grays whose lightness or darkness corresponds inversely to the brightness of the light forming the image in those areas. Such films consist of a sandwich of several layers bonded together. First there is the film base, or medium of support-usually a flexible plastic, but occasionally glass or a rigid plastic. The emulsion deposited upon this consists of light-sensitive silver halides suspended within a transparent, water-absorbent gelatin. Included in the sandwich are layers of dyes; one of these dyes (the anti-halation coating) absorbs the stray light scattered within the emulsion and film base, and other, light-filtering dyes determine the basic color sensitivity of the film.

  • Exposure to an image-that is, to light focused on the film by the camera lens-causes physical changes in the emulsion that establish a latent image, visible only after the film has been chemically developed. During development the emulsion absorbs the developing agents, and the portions of the light-sensitive compounds in the emulsion that were affected by exposure to light are chemically reduced to grains of metallic silver. These clump together roughly in proportion to the amount of light that struck their area of the film; in the aggregate, they make up the developed visible image. After development, the image is made permanent by chemical fixing, which also removes unexposed light-sensitive materials from the emulsion. At one stage or another, all this chemistry also removes the anti-halation layer and the color-sensitizing dyes. The result, after washing and drying, is a negative: a reversed-tone, translucent image on the film.

  • So-called color films are actually multiple-layered black-and-white films in which there are three or more layers of emulsion, each sensitized and filtered so as to record only certain wavelengths of light. In the course of processing, the silver grains forming the various layered images are dissolved out and are replaced by appropriately colored dyes. The result can be a color negative, with all colors the chromatic reverse of those that will appear on the color print; that is, blue color will appear yellow on the negative, green will appear magenta, red will appear cyan, and so on. Alternatively, reversal processing can provide a direct-positive transparency-what we call a color slide-with colors rendered quite naturally.

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