A paper property that measures the extent to which a paper will retain its original color, brightness, and whiteness with exposure to light. Various chemical constituents in paper pulp reduce its lightfastness, in particular the presence of lignin, a naturally-occurring chemical found in wood that binds fibers of cellulose together, and is responsible for paper darkening and yellowing upon exposure to light. It is the goal of many pulping systems to remove as much lignin as possible from the wood pulp before papermaking. Various chemical pulping procedures, and the use of bleaching agents remove nearly all lignin from wood pulp, but groundwood pulp still contains a high amount of lignin, so paper composed of some quantity of it yellows very quickly (such as newsprint). Bleaching and the use of fast-to-light dyes help increase a paper's lightfastness, but no paper can be completely lightfast.
Tests to gauge lightfastness involve exposing a paper sample to a carefully-controlled light source for a set period of time using a Fade-Ometer.
The term lightfastness also refers to a printing ink's ability to retain its color strength and resist fading upon exposure to light. Tests to gauge an ink's lightfastness are performed in a manner similar to measures of a paper's lightfastness. Lightfastness is also referred to as colorfastness.