Psoriasis is a common and chronic skin disorder. Plaque psoriasis is the most common type of psoriasis and is characterized by red skin covered with silvery scales and inflammation. Patches of circular to oval shaped red plaques that itch or burn are typical of plaque psoriasis. The patches are usually found on the arms, legs, trunk, or scalp but may be found on any part of the skin. The most typical areas are the knees and elbows.

Sunlight therapy is particularly effective in cases of psoriasis, a benign but chronic inflammatory skin condition. During the last 30 years tens of thousands of patients mainly from Western Europe, have been given sunlight therapy for psoriasis. In one recent study, published in the British journal of Dermatology in 1998, some 46 Finnish patients received 4 weeks of heliotherapy treatment in the Candy islands Spain. They were sent abroad because in Finland solar radiation is too weak and sunny days are too infrequent to have any real impact. For psoriatic patients exposure to sun remains an effective alternative to systemic drugs.