Treatment of certain skin diseases using ultra-violet light is called phototherapy. Two types of UV rays are used in phototherapy, namely UVB and UVA. Treatment with UVB does not require intake of any sensitizing chemicals whereas treatment with UVA requires intake or application of photosensitizing chemicals called psoralens. Treatment can be done either using sunlight or artificial chambers that emit either UVB or UVA rays. Sunlight is the cheapest source of UV light that is accessible to all. But the disadvantage of using sunlight is that the UV irradiance varies with the time of the day, season of the year and the place, in addition to cloud and pollution effects. However, treatment using sunlight is still possible in India as UV irradiance does not vary much through out the year in most parts of India, provided the irradiance at different time of the day is known [See Sunlight In India].

Chambers emitting specific wavelengths of UV light are available in well established dermatology clinics and teaching hospitals. These are expensive but effective in delivering the required dose accurately, in addition to providing privacy.

The important parameters to be known before starting the treatment are the irradiance of the source (milliwatts or microwatts/cm²/sec for UVA and UVB respectively), the dose to be delivered (joules or millijoules/cm² for UVA and UVB respectively) which in turn depends on the type of the skin and the  duration of exposure to the source. The treatment is done 2-3 times per week. The number of treatments required depend on the underlying disease.

Certain precautions are to be taken:

• Phototherapy using the artificial chamber is preferably done in the evenings, after 5 p.m.
• When using sunlight as the source, eyes and other exposed parts of the body should be protected for the rest of the day.
• Sun screens should be used over the untreated areas during treatment with either sunlight or artificial chambers.