A safelight is a light source suitable for use in a photographic darkroom. It provides illumination only from parts of the visible spectrum to which the photographic material in use is nearly, or completely insensitive.
A safelight usually consists of an ordinary light bulb in a housing closed off by a coloured filter, but sometimes a special light bulb or fluorescent tube with suitable filter material coated directly on the glass is used in an ordinary fixture.
Differently sensitised materials require different safelights. In traditional black-and-white photographic printing, photographic papers normally are handled under an amber or red safelight, as such papers typically are sensitive only to blue and green light. Orthochromatic papers and films are also sensitive to yellow light and must be used only with a deep red safelight, not with an amber one. Panchromatic films and papers, nominally sensitive to the entire spectrum, sometimes have a region of minimum in their range of sensitivity that allows the careful use of safelight confined to that part of the spectrum, e.g. Kodak Panalure panchromatic paper, tolerant of limited exposure to light filtered through a Kodak 13 Safelight Filter. Other panchromatic materials must be handled only in total darkness.
Many photosensitive materials used in technical and industrial applications, such as photoresist, are sensitive only to blue, violet, and ultraviolet light and may be handled under a brighter yellow safelight. Low-pressure sodium vapour lamps sometimes are used in larger industrial darkrooms. They emit nearly monochromatic light at 589 nm (yellow), to which the materials are insensitive; as a result they can be extremely bright while still "safe".
The word "safe" in "safelight" is relative, as in most cases, a sensitised material eventually will be affected by its safelight if exposed to it for an extended length of time.
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