How the film is made?
Approximately half the world’s production of silver is used by the photographic industry.
Bars of the metal is dissolved in nitric acid to form silver nitrate, and this is reacted with a halogen element (typically iodine, bromine, or chlorine in the form of alkali salts or halides such as potassium iodide, potassium bromide, or potassium chloride). After by-products have been removed, the resulting compound consists of silver halide crystals, which are light-sensitive. So that these finely divided silver halides can be coated evenly onto the film base they are mixed with gelatin to form a creamy silver halide emulsion.
Gelatin is used because it is highly transparent and has no visible texture of its own. It becomes a liquid when heated – ideal for coating – but also sets (‘gels’) when chilled or dried. It holds the silver halides in a firm, even coating across the film surface, yet swells just sufficiently in processing solutions to allow chemicals to enter and affect the halide crystals, without disturbing their positions.
In detail, modern film manufacturing is very complex and demanding. Mixed emulsions are given additives and held for fixed periods at a controlled temperature to ‘ripen’. This makes some crystals (‘grains’) grow larger, giving increased light sensitivity (greater ‘speed’) and producing less extreme contrast. Film contrast changes because, when first formed, crystals are all very small and not particularly sensitive. They are affected by light equally. When an emulsion contains a mixture of different-sized grains (mixed sensitivity), however, low-intensity light affects large crystals only, more light affects large and medium sizes, and the brightest light affects all crystals, even the smallest. When the film is developed, these variations in light intensity are therefore recorded as various grey tones rather than simply extremes of black or white.
Farther additions to the emulsion alter its sensitivity to colored light. In its raw state emulsion responds to blue and ultraviolet (UV) only, but this can be extended to farther bands or the whole visible spectrum. Meanwhile the film’s base (most often polyester or tri-acetate) receives several preparatory coatings, including an anti-curl gelatin layer applied to the back to prevent the film from shrinking or curling when the emulsion is coated on the front. Another layer of dark ‘anti-halation’ dye prevents light from reflecting from the base and forming ‘halos’ around the images of bright highlights.
This layer may be between the emulsion and base or coating on the back of the film. In addition, 35 mm film has grey dye in the base to prevent light from passing into the cassette, piped along the thickness of the film like a fiber-optic. Like the anti-halation material, it disappears during processing and is sometimes seen as a darkening of the used chemicals.
Emulsion coating itself is extremely critical and carried out in ultra-clean conditions. Black and white films may need between one and four layers, while most color films have more than ten layers of several different color sensitivities. The final layer is clear protective gelatin. (For details on the structure of instant-picture materials, see Advanced Photography.) The film is coated in large rolls, typically 1.5 m wide by 900 m long. After drying, this is cut down to the various standard film sizes, and edge printed with frame numbers and other information.