The How and Why of Photo Film

Photo film paper is used for making photographic prints and consists of specially made paper, which is exposed to light in a controlled way after being coated with light-sensitive chemicals. A gelatin-silver process is then used to develop the film paper and this creates the image that is visible on the paper.

There are three categories of photo film paper: two for the two positive-positive photographic processes and one for the negative-positive process. One of the positive-positive processes is the film-to-paper process where, after it has been enlarged, the positive image is copied onto the photo film. The other is where the photo film itself is the surface on which the final image appears. The sole negative-positive process uses black and white film paper and chromogenic film paper and consists of silver images being formed and then transformed into colored dye images.

Photo film is made of a light-sensitive emulsion mix, which is usually but not always made of silver halide salts, suspended in a colloidal material like gelatin and spread on a specific type of paper or other support. Color photo film usually uses a solid polyester or resin-coated paper as a support and there are usually three layers of emulsion, being magenta, cyan and yellow, as well as further supporting layers. The black and white papers usually use either paper coated with resin or baryta (barium hydroxide), or polyester, although it is not unknown for linen to be used. Black and white resin-coated paper is sealed with two impenetrable layers of polyethylene while fibre- or baryta-based paper is sealed with a layer of hardened gelatin that lies above the emulsion mix and is known as a supercoating.

There are three basic types of emulsions that are used to make photo film. One of which is the pure silver bromide emulsion that produce images with neutral or cold tones. Another is a mix of silver bromide salts and silver chloride, which produces images with warm-black to neutral image tones. The final, and least used, is the pure chloride emulsion mix, which produces warm-toned images. The film papers created through the use of these emulsions are called Bromide Papers, Chlorobromide Papers and Chloride Papers respectively.

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