Safelight is light used in a photographic darkroom and is designed to provide illumination without that part of the light spectrum to which the material in use is sensitive. A safelight usually consists of an ordinary lightbulb in a housing with a coloured filter, but it is possible to buy special bulbs and fluorescent tubes which are coated with a filter directly on the glass. Low-pressure sodium vapor lamps are also commonly used in larger darkrooms.http://www.kodak.com/US/en/motion/products/print/2383.jhtml?id=0.1.4.8.4.7&lc=en They emit nearly monochromatic light at 589nm, to which photographic materials are insensitive; as a result they can be extremely bright while still "safe".
Initially, monochrome photographic film was sensitive to blue and green light and could be handled under a deep red light. This material was called Orthochromatic film meaning that it theoretically correctly reproduced any colour as a shade of grey. However since it was designed to be handled in red light it actually meant that any red objects were rendered as black. Manufacturers then developed Panchromatic film but since this was sensitive to red light it has to be handled in either total darkness or in some instances a dark brown or violet light can be used. In either case these safelights are so dim as to be almost useless.
Until the middle of the 20th century, monochrome photographic paper was sensitive to blue and green light and required a deep red safelight. Photographic paper has been sensitive to just blue light for a great many years now, enabling yellow safelights to be used which provide much more light for the photographer to work in. Despite this, darkroom scenes in movies invariably show the use of a red light. In 2004, Lee Lighting introduced a theatrical lighting colour called "Rust" that closely approximates the colour of modern safelights.
safelight in German: Dunkelkammerleuchte
safelight in French: Lampe inactinique
safelight in Japanese: セーフライト
safelight in Polish: Lampa ciemniowa