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 actinic 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.
Uses
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 photochemical machining, 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.
Photography
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.
The reproduction is done by placing the negative on top of the glass and then the photo paper with the emulsion in contact with the negative. The negative role fits the body, the lid is closed applying some pressure against the glass to prevent blur; then proceed to impress photographic paper by turning on the interior light (contact copier) or an external focus, in the last case, usually from a photographic enlarger. Exposure time can be controlled "manually" or using a timer that controls the light source for a preset time and with greater precision.
Whiteprint
In the process of whiteprint copying , (essentially a diazotype process), a dynamic contact copier is used (similar to the manual action of exposing both sheets strongly bonded directly to the sunlight.). The exposition is made progressively as the twin rollers pull the two papers together (original and copy) against a source of ultraviolet light, typically a powerful black light lamp . The original plan (on a transparent support) and the diazo paper are introduced, in perfect contact, within the pinch rollers of the contact copier. The sensitized paper, has a photosensitive coating -an impregnation of diazo - covering the surface of the paper. Once exposed, the copied paper is immersed in a developer solution made from ammonia (or ammonia vapor) converting the parts of the paper not exposed to the light source to a characteristic dark violet colour (blue-line).
Silkscreen
In screenprinting it is normally use a contact copier box with several fluorescent tubes close to the silk frame. The emulsion reacts depending on the amount of light received, for that reason it is important to make some tests for determining the exposure time. Usually for a "box type" contact copier, exposure is usually not more than one minute. In the industrial type contact copiers with actinic lamp placed at a distance, exposure time can be about 20 minutes.