lightfast

Lightfastness is the resistance of a pigment, dye, or other colorant to fading or color change when exposed to light. It is a critical property for artworks, commercial prints, coatings, and textiles because prolonged light exposure can alter the appearance of colors.

Assessment typically uses standardized laboratory tests that expose samples to controlled light sources, such as xenon arc lamps that simulate sunlight. The color change is usually measured with a spectrophotometer and reported as a fade rating or color-change value. Ratings are presented on scales that may be numerical (for example 1–8 or I–VIII) or descriptive (excellent, very good, good, fair, poor); higher ratings generally indicate greater lightfastness.

Lightfastness depends on pigment chemistry, binder and film formation, pigment dispersion, and additives, as well as exposure duration, light spectrum, and the presence of UV blocking materials. In practice, inorganic pigments and stable organics tend to be more lightfast than many natural or highly transparent organic dyes, though there are exceptions.

Because lightfastness varies with application, manufacturers label paints, inks, and coatings with their rated performance to guide users for outdoor or archival use. For textiles and plastics, standardized tests often rely on separate blue-wool or similar scales, but the underlying goal is the same: to predict how color will endure when illuminated over time.