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How does contact printing produce a positive photograph from a negative?

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Diagram showing the negative-positive photographic process: a scene is captured on film in a camera, developed into a negative, projected onto paper with an enlarger, and developed again to produce a positive print. — technology of photography Figure 1: Sequence of negative–positive process, from the photographing of the original scene to enlarged print (see text).

Contact printing is a fundamental technique in the technology of photography used to produce a positive image from a negative that is the same size as the original film.1

The process works by placing the negative film and a sheet of light-sensitive printing paper face-to-face in intimate contact, typically held together within a contact printing frame or a printing box. When exposed to a light source, the light passes through the negative onto the paper.12

Because the negative is tonally reversed—with dense, dark areas corresponding to bright parts of the original scene and thin, transparent areas corresponding to dark parts—the light exposure is modulated accordingly. The dense portions of the negative block the light, resulting in light areas on the print, while the thin portions allow more light to pass through, creating dark areas. After this exposure, the paper undergoes a development and processing sequence to reveal the final positive image.32