In 1981, two foreigners (Parrish and Jaenike) found that the treatment of psoriasis (psoriasis) and vitiligo with ultraviolet light at 311 nm to 313 nm wavelengths was more effective than ordinary ultraviolet light and comparable to complementary long-wave ultraviolet light, and had fast onset of action and few side effects. Subsequently, in-depth research was conducted on UV light of 311 nm to 313 nm wavelengths, gradually understanding its mechanism of action and refining the related treatment methods. For the first treatment, the initial dose was decided according to the patient’s skin type and treatment experience, and treatment was given three times a week. Depending on the patient’s erythematous response to irradiation, the dose is increased in increments of ten to twenty percent. In the course of treatment, if mild erythema occurs, the dose should be suspended; if moderate erythema occurs, the dose should be reduced by 10 to 20 percent of the previous dose; if painful erythema or blistering occurs, treatment should be suspended and treated accordingly. When the patient’s skin lesions have subsided more than ninety percent, extend the treatment interval and enter maintenance treatment, which usually takes three months or longer. Indications: Currently, it is mainly used for the treatment of psoriasis (psoriasis), vitiligo and atopic dermatitis, but also for subangular pustulosis, mycosis fungoides, acute and chronic graft-versus-host disease, granuloma annulare, herpes zoster, palmoplantar pustulosis, red furunculosis, and pigmentary urticaria. Compared with ordinary UV light, no photosensitizer is used during the treatment, no adverse reactions caused by photosensitizer, and no special protection of eyes is needed after the treatment. It is also safe for pregnant women and children and has not been reported to cause cancer.