Safelight
A darkroom lamp emitting only wavelengths to which the material in use is insensitive.
Measured in
nm
In depth
Most black and white papers are blue-sensitive, so an amber or red safelight can be used freely. Panchromatic film is sensitive to everything and permits no safelight at all. Safelights are only relatively safe — prolonged exposure will fog paper, which is why a coin test on a sheet of paper is worth running periodically.
Examples in practice
Laying a coin on a sheet of paper under the safelight for five minutes reveals fogging if its outline appears after development.