The complaints of poor color fastness when sports dress loses color and form, the seat fabric inside of a car or the carpet fades too fast are very common. All dyes will fade when exposed to sunlight and it is possible for a dyed article to fade noticeably during drying on a washing line in bright sunlight or, more importantly, under display lighting conditions in a retail outlet. Any reputable retailer would, however, regard such a level of performance as quite unacceptable. Testing for light fastness provides an objective framework for what is acceptable.

Fastness to light is basically a feature of the actual colorant molecule itself; it is, in fact, a measure of the ability of the molecule to absorb radiation without being destroyed, the absorbed radiation being dissipated in other ways. In a dye with poor light fastness the molecule will be broken down bFastness to light is basically a feature of the actual colorant molecule itself; it is, in fact, a measure of the ability of the molecule to absorb radiation without being destroyed, the absorbed radiation being dissipated in other ways. In a dye with poor light fastness the molecule will be broken down by the absorbed radiation. No colorant is completely fast to light, but it should be emphasized that this is not of real importance. All that matter is the colorant should not fade appreciably during the life of the article.

 Light fastness is the degree to which a colorant resists fading due to light exposure. Different dyes have different degrees of resistance to fading by light. All colorants have some susceptibility to light damage, simply because their colors are indications that they absorb the wavelengths that they do not reflect. Light is energy, and the energy that is absorbed by the colorant compounds degrade them or nearby molecules.