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.