Light fastness refers to the ability of dyed products to maintain their original color under sunlight. According to general regulations, the determination of fastness to sunlight is based on sunlight. In order to facilitate control in the laboratory, artificial light sources are generally used, and corrections are made when necessary. The most commonly used artificial light source is a hernia lamp, but also a carbon arc lamp. When the dyed product is irradiated by light, the dye absorbs light energy, the energy level is increased, and the molecules are in an excited state, and the color system of the dye molecules changes or is destroyed, causing the dye to decompose and cause discoloration or fading.