Safelight

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 or phosphor (in fluorescent tubes) coated directly on the glass is used in an ordinary fixture.

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.

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.

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".