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The ultraviolet (UV) light spectrum is divided into ultraviolet A (UVA) and ultraviolet B (UVB) light. Broadband UVB has been used to treat skin conditions for many years. It may be used alone, but in many cases, a moisturizing agent or coal tar preparation is applied in a thin layer to the affected area before UVB exposure.

Within the past few decades, the specific spectrum of UVB that affects most skin conditions has been detected. This narrowband UVB is much more effective than broadband UVB for a variety of skin conditions (e.g., dermatitis, psoriasis).

The spectrum of UVA has been subdivided into UVA-1 and UVA-2. UVA-2 is very similar to UVB and is generally not specifically used in phototherapy. Because of its longer wavelength, UVA-1 penetrates more deeply into the skin, affecting mid-and deep-dermal components (including blood vessels) as well as epidermal structures.

Grenz therapy is another form of phototherapy. Other forms of therapy have largely replaced this, making it quite rare today. Instead of ultraviolet light, grenz therapy uses black light to treat minor skin conditions.

Long-wave UVA may be combined with psoralens (PUVA), drugs that increase the sensitivity of the skin to light. This results in therapeutic effects that are not produced with psoralens or UVA alone. Psoralens may be administered orally (e.g., capsules, pills), applied topically (e.g., creams, ointments, lotions) or through bath water. Delivery through bath water provides uniform distribution of the psoralens over the skin surface, very low levels of psoralens in the blood and quick elimination of psoralens from the skin.