(light fastness, color fastness) The ability of a plastic material to resist fading, darkening, or degradation upon exposure to sunlight or ultraviolet light. Nearly all plastics tend to change color under outdoor conditions, due to characteristics of the polymeric material and/or pigments incorporated therein. Tests for light resistance are made by exposing specimens to natural sunlight or to artificial light sources such as the carbon arc, mercury lamp, germicidal lamp, or xenon arc lamp. ASTM D 4459 and D 4674 describe procedures for testing light fastness of plastics for indoor applications. (Degradation and stabilization of polymers. Zaiko GE (ed), Nova Science, New York, 1995) See also Artificial Weathering.