Danish physician Nils Finsen is believed to be the father of modern phototherapy. He developed the first artificial light source for this purpose, and used his invention to treat lupus erythematosus. He received the Nobel Prize in Physiology or Medicine in 1903.

Sunlight was long known to improve acne, and this was thought to be due to antibacterial and other effects of the ultraviolet spectrum which cannot be used as a long-term treatment due to the likelihood of skin damage.

It was found that some of the visible violet light present in sunlight (in the range 405–420 nm) activates a porphyrin (Coproporphyrin III) in Propionibacterium acnes which damages and ultimately kills the bacteria by releasing singlet oxygen. A total of 320 J/cm2 of light within this range renders the bacteria non-viable.

Deep penetrating light therapy for 3 consecutive days has been shown to reduce the bacteria in the pores by 99.9%.[citation needed] Since there are few porphyrins naturally found in the skin, the treatment is believed safe except in patients with porphyria; although eye protection is used due to light-sensitive chemicals in the retina. The light is usually created by fluorescent lamps, or very bright LEDs.

Overall improvements of on average 76% for 80% of patients occurs over 3 months; most studies show that it performs better than benzoyl peroxide and the treatment is far better tolerated. However, approximately 10% of users see no improvement.

A feature of psoriasis is localized inflammation mediated by the immune system. UV radiation is known to suppress the immune system and reduce inflammatory responses. Light therapy for skin conditions like psoriasis or eczema use UVA (315-400 nm wavelength) or UVB (280-315 nm wavelength) light waves. UVA, combined with a drug taken orally, is known as PUVA treatment. Narrow band UVB is the 310 nm wavelength and is given as a light therapy treatment rather than full spectrum UVB.

Tanning is caused by the effects of two different spectrums of ultraviolet radiation: UVA and UVB.

Light therapy has been suggested for use in healing of wounds. Some say that low-level laser therapy does not appear to be effective, while others find that it can be effective. LLLT is used clinically in many areas outside the United States including Canada, Europe and Asia.

Visible blue light is used with aminolevulinic acid for the treatment of actinic keratosis. This is not a U.S. FDA-approved treatment for acne vulgaris.

The production of the hormone melatonin, a sleep regulator, is inhibited by light and permitted by darkness as registered by photosensitive ganglion cells in the retina. To some degree, the reverse is true for serotonin, which has been linked to mood disorders. Hence, for the purpose of manipulating melatonin levels or timing, light boxes providing very specific types of artificial illumination to the retina of the eye are effective.[citation needed]