Ultraviolet light has a shorter wavelength than visible light. It is not visible to human eyes, but can affect human health, most notably by causing skin cancer. UV light can be useful to humans, as well. Reptile owners, for example, may use artificial UV bulbs to provide their reptiles with Vitamin D, and small amounts of UV light may help with seasonal affective disorder. Ultraviolet light comes in the form of UVA rays and UVB rays.
TL;DR (Too Long; Didn't Read)
Incandescent and fluorescent bulbs both emit UV radiation, but the levels are low when compared to reptile basking bulbs or tanning bulbs. The strongest source of UV radiation is the sun.
Incandescent light bulbs, the most commonly used light bulbs in homes, give off small amounts of UV light. The UV light emitted by these bulbs is so small that it's impossible for human health to be affected in any noticeable way. Incandescent bulbs won't cause sunburns and they won't help people or animals absorb Vitamin D. These bulbs emit only UVA rays.
Fluorescent bulbs are most commonly found in two varieties: compact fluorescent bulbs for use in homes and fluorescent tube lighting that is often used in offices and stores. Both bulbs emit more UV light than traditional incandescent bulbs. Some scientists have expressed concern about the impact these lights have on human health over a lifetime, but the UVA light emitted by these bulbs is too small to have an immediate effect, such as sunburn or eye pain.
UVB rays from the sun help organisms to absorb Vitamin D and may also help to prevent conditions like seasonal affective disorder. However, excessive UVB radiation can also cause skin damage. UVB bulbs, also known as retile basking bulbs, emit substantially more UV rays than fluorescent or incandescent light bulbs, and are usually found in pet stores. The most common use of these bulbs in the home is the same as it is in pets stores: a lighting supplement for reptiles and amphibians, who need UVB rays in order to metabolize calcium.
The lights used in tanning beds are typically long, tubular fluorescent bulbs that emit both UVA and UVB rays. These lights can cause skin damage and cancer, but may also help with Vitamin D production and seasonal affective disorder.
Light from the Sun
Sunlight is the strongest and best-known source of UVA and UVB light. This light is filtered through the earth's ozone, resulting in a much less potent light than the light at the original source. Sunlight is necessary for human life and health, but too much sunlight can cause skin cancer, genetic mutations and a host of other health problems. Holes in the ozone layer in recent years have increased the amount of UV light that hits the earth and the organisms that live here.