Exposure unit

A contact copier (also known as contact printer), is a device used to copy an image by illuminating a film negative with the image in direct contact with a photosensitive surface (film, paper, plate, etc.). The more common processes are negative, where clear areas in the original produce an opaque or hardened photosensitive surface, but positive processes are available. The light source is usually an actínic bulb internal or external to the device

Commercial contact printers or process cameras usually use a pump-operated vacuum frame where the original and the photosensitive surface are pressed together against a flat glass by a grooved rubber mat connected to the vacuum source. A timer-controlled mercury vapor (arc or fluorescent) light source is on the other side of the glass.

The contact copying process was used in the early days of photography and sunlight-exposed blueprints; it is still used in amateur photography, silkscreen printing, offset printing, and , such as the manufacture of Printed circuit boards. By the early 20th century, blueprinting (producing white lines) or diazo blue line printing used contact-rollers rather than flat-glass exposure.

Silkscreen printing and photochemical machining originally were based on gum bichromate photosensitive materials, where exposure to intense ultraviolet light made previously-soluble gum or gelatin colloids insoluble; after exposure, the exposed surface was washed in water and the unexposed coating dissolved, leaving the hardened gum or gelatin to resist the passage of the silk-screen ink or the metal-etching solution. Offset printing can use either a negative plate, where the hardened, exposed photosensitive coating attracts ink and repels water, or a positive plate, where the exposed photosensitive coating decomposes or exposes the metal, water-attracting surface.

The contact copier is used for duplication of negative or positive prints obtaining what are called contact prints, that is, to reproduce on paper or film, a photographic negative or positive of exactly the same size of the original. (With normal photographic non inverting processes, black generates white on the target while white generates black). It was the common mode to make prints until it began the use of the alternative photographic enlarger. There are some models with internal light source constructed as a closed box, in which one or more lamps illuminate the negative through an opal or frosted glass.

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