Safelight is light use 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. They emit nearly monochromatic light at 589nm, to which photographic materials are insensitive; as a result they can be extremely bright while still "safe".