Colour fastness describes the binding force between photochromic dyes and fibres. Textiles are exposed to numerous external factors such as light, washing, ironing, sweat, friction, and chemical agents. The fastness grade evaluates to what extent these external factors result in discolouration. Wash fastness, light fastness and rub fastness are the principal forms of colour fastness that are tested. The wash fastness of textile dyes is divided into categories from one to five and light fastness from one to eight, with a higher number indicating better fastness.