Ultra-violet light is the name given to the invisible part of light spectrum, whose frequency is higher than the violet band of the visible light spectrum, but lower than the x-rays. This light has a shorter wavelength and high energy with a bigger potential for deep penetration in the skin layers.
The full spectrum of UV light has been subdivided into many bands like UVA, UVB, UVC etc. This division is based on their wavelengths (and hence energy).

UVA (Wavelength 400-315 nm)

(Also Known as Black light)

treating Vitiligo with UVAUVA is abundantly present in sunlight. It has longest wavelength of the various UV lights, with least energy. It is hence least harmful to man. It can tan the human skin very rapidly without causing significant sunburn, and most of the tanning potential of sunlight is basically due to the UVA present in sunlight.
UVA has been used as a healing tool in various conditions. Here are a few examples
UVA is used in Woods lamp to aid the diagnosis of Vitiligo in difficult cases
The tanning potential of UVA is used for repigmentation of the white skin patches of Vitiligo. UVA may be extremely useful when it is used on a skin that has already been sensitized by oral/topical use of psoralens. This is referred to as PUVA, which has been found effective in various dermatologic conditions, especially psoriasis and Vitiligo and eczema.
UVA is also being used in electronic insect killers which attract the insect to a UVA lamp, where they are burned with high voltage current. Such devices have helped in the control of diseases like malaria, dengue fever and yellow fever.
Most of the UVA passes through ordinary glass, but less easily through plastic materials, a fact that may be considered in choosing reading and sunglasses.

UVB (Wavelength 315-280 nm)

UVB is the medium wave length, medium energy ultraviolet radiation whichTreating Vitiligo with UVB can hence bring about dramatic changes to human skin.
Phototherapy with UVB is now considered an effective longterm remedy for disorders like Vitiligo, psoriasis and eczema. Additional topical or oral medicines are generally not required for the effectivity of UVB radiation. UVB has however been effectively used in combination with psoralens (PUVB) and other medicines like coal tar, vitamin A&D, Methotrexate and soriatane.
Phototherapy with UVB is usually advised 3 to 5 times a week, and the benefits are usually perceptible in 6-8 months. The diseases amenable to phototherapy with UVB are however chronic and continued phototherapy may be needed to maintain remissions. Home devices for UVB are now available, which can be very economical for protracted usage.
UVB has a higher energy radiation than UVA, and hence a higher potential for side effects, if used without discretion. Itching, redness and sunburns are possible, and cataracts can develop if eyes are not adequately protected during phototherapy sessions.