The principle of laser hair removal is still based on the theory of selective photothermal action. Hair follicles and hair shafts are rich in melanin. Melanin is distributed between the cells of the hair bulb matrix and can also be transferred to the structure of the hair shaft (such as medulla, cortex, and hair cuticle). Laser can precisely and selectively perform hair removal treatment with melanin as the target. After the melanin absorbs the energy of the laser, the temperature rises sharply, which causes the destruction of the surrounding hair follicle tissue and removes the hair. According to the selective photothermal theory, as long as the appropriate wavelength, pulse width and energy density are selected, the laser can accurately destroy the hair follicles without causing damage to the adjacent tissues. In order to achieve the effect, it is obvious that the melanin in the hair bulb and the carina above it (the outer root sheath of the hair follicle between the opening of the sebaceous gland and the attachment of the hair follicle of the erector pili muscle, which is the site where the hair follicle stem cells are located) must be treated with light. . ①Because the hair bulb is deeper, it is decided to use a light source with longer wavelength and deeper penetration for treatment. On the other hand, if the wavelength is too short, the melanin of the epidermis will compete with the melanin of the hair follicle to absorb more energy, which is likely to cause epidermal damage. Therefore, while ensuring that the light waves have better absorption of melanin, the wavelength of the hair removal laser should be 695-1 064 nm. ② To achieve a higher efficiency in the destruction of hair follicles-one solution is obviously to increase the laser irradiation time. Theoretically speaking, by lengthening the laser pulse width, the thermal relaxation time close to the hair follicle can improve the therapeutic effect. The longer the pulse width, the smaller the possibility of damage to the tiny structures in the epidermis such as melanosomes, while the relatively larger structures If the hair follicle is colored, the melanin in it is more susceptible to damage. The diameter of the hair follicle is about 200-300 μm, and the thermal relaxation time of the hair follicle is 40-100 ms. Therefore, the ideal laser pulse width should be between tens of milliseconds to 100 milliseconds. Recent studies have found that the use of lasers with a pulse width of more than 100ms for dark-skinned patients not only has better efficacy and fewer side effects.