A safelight is a lamp with a colored filter used to light a traditional darkroom while you work. For black-and-white printing it is usually red or amber.
Most black-and-white photographic paper is sensitive mainly to blue and green light, so it does not react to red or amber light for a limited time. This lets you see what you are doing while you expose, develop and fix prints. Very long exposure to a safelight, or a light that is too strong, can still fog the paper. Most films, especially panchromatic black-and-white and color films, must be handled in total darkness.
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