Light fastness the sun-fading of dyed fabrics is a complex process. Under the action of sunlight, the dye absorbs light energy, and the molecules are in an excited state and become extremely unstable. It is easy to produce certain chemical reactions, which cause the dye to decompose and fade, resulting in a large fading phenomenon of the dyed fabric after sun exposure. The light fastness varies with the dyeing concentration, and the low concentration is inferior to the high light fastness. The light fastness of the same dye on different fibers is also quite different. For example, the light fastness of indigo on cellulose fiber is only grade 3, but on wool, it is grade 7-8, and the light fastness is also related to dye. It is related to factors such as aggregation state on the fiber and dyeing process.
The light fastness is divided into 8 levels, the 1st grade is the worst, and the 8th grade is the best.