IPA: //ˈseɪf.laɪt//
KK: /ˈseɪfˌlaɪt/
A type of lamp used in darkrooms that has special filters to provide light without damaging light-sensitive materials like film or paper.
The photographer used a safelight to work on his prints without ruining them.
Safelight → It is formed from "safe" (from Old English *sæf*, meaning secure or protected) and "light" (from Old English *leoht*, meaning illumination or brightness). The word "safelight" refers to a type of light that is safe to use in certain conditions, particularly in photography, where it does not expose sensitive materials.
Think of a light that is 'secure' ('safe') and provides 'illumination' ('light') without causing harm — that's what a safelight does.