a darkroom light with a filter that transmits only those rays of the spectrum to which films, printing paper, etc., are not sensitive.
Origin of safelight
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
photog a light that can be used in a room in which photographic material is handled, transmitting only those colours to which a particular type of film, plate, or paper is relatively insensitive
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012