Archives - Photographs - Types

Types of Photographs
A photographic image is formed by the action of light falling onto a base material which has been coated with a light-sensitive emulsion. This latent image may then be "developed" and "fixed" by further chemical processing. The emulsion coating in most modern photographs is usually gelatin, which incorporates either silver halide salts to form black and white images, or dyes or pigments to form colour images. The base can be made from any material that can be coated with an emulsion, but the most common bases are paper and film.

• Paper based prints – either color or black and white with a variety of emulsion types, depending on the age of the material. These can be simple "snapshots" to mounted presentation photographs to very large aerial survey shots.
• Negatives – reversed polarity images from which positive prints are produced which can be colour or black and white, and in a range of sizes. Early negatives were on glass, but for most of the twentieth century, flexible film bases have been used. Film-based negatives can be in either sheet or roll form.
• Transparencies or slides – positive images on a transparent film base, therefore requiring viewing with a back light source or projection system. The most common are 35mm slides, but other sizes are also used.

Types of Materials
Historical photographs may be daguerreotypes, using copper sheets and mercury vapors, or albumen prints made on very thin papers with an emulsion derived from egg whites. Early photographs were also made with supports of glass and metal and collodion emulsions. In the late 1880s glass negatives were replaced by a clear plastic film made from cellulose nitrate. This was superseded in the 1930s by "safety film" on cellulose triacetate. Triacetate is still used as a film base today in conjunction with polyester film which was introduced in 1960.

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