Lightfastness refers to the ability of a dyed material to keep its original shade under sunlight. By general convention, lightfastness is measured against sunlight as the standard. In the laboratory, artificial light sources are usually used for control and corrected when necessary; the most common is the xenon arc lamp, and carbon arc lamps are also used. Under light, the dye absorbs light energy, its energy level rises, and the molecules enter an excited state. The chromophoric system of the dye molecule changes or is destroyed, so the dye decomposes, causing shade change or fading.

1. Effect of light on dyes: when a dye molecule absorbs the energy of one photon, its outer valence electrons jump from the ground state to the excited state. Depending on the structure, different excitation processes occur under light of different wavelengths - pi to pi*, n to pi*, charge transfer (CT), singlet-singlet, singlet-triplet, ground to first excited state, and ground to second excited state transitions.

2. Factors affecting lightfastness:

(1) Dye structure: dyes with stable chromophoric systems generally have better lightfastness.

(2) Fiber type: the same dye has different lightfastness on different fibers.

(3) Concentration: higher dye concentration generally gives better lightfastness.

(4) Auxiliaries: some finishing agents such as softeners and UV absorbers change lightfastness; UV absorbers can improve it.

(5) Humidity and temperature: high humidity accelerates fading of some dyes.

Selection of dyes with good lightfastness, adequate dyeing depth, and proper use of UV absorbers and antioxidants in finishing are the main ways to improve the lightfastness of dyed goods.