Pho­tother­a­py uses light­waves to treat cer­tain skin con­di­tions. The skin is exposed to an ultra­vi­o­let (UV) light for a set amount of time. Pho­tother­a­py uses a man-made source of UV light. UV light shuts down immune sys­tem cells in the skin. It can help in skin con­di­tions that are caused by an over­re­ac­tion of the immune sys­tem. Skin con­di­tions that are treat­ed with pho­tother­a­py include Pso­ri­a­sis, Atopic Der­mati­tis, Vitili­go, Myco­sis Fun­goides and many oth­er conditions.