• Nws

Ultraviolet

UV itself can be subdivided into near UV (380-200 nm wavelength) and extreme or vacuum UV (200-10 nm). When considering the effects of UV radiation on human health and the environment, the range of UV wavelengths is often subdivided into UV-A (380-315 nm), UV-B (315-280 nm), and UV-C (280-10 nm). See 1 E-7 m for a list of objects of comparable sizes.

Ordinary glass is transparent to UV-A but is opaque to shorter wavelengths. Quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths.

The sun emits ultraviolet light in the UV-A, UV-B, and UV-C bands, but because of absorption in the atmosphere's ozone layer, 99% of the ultraviolet light that reaches the Earth's surface is UV-A. (Some of the UV-C light is responsible for the generation of the ozone.)

Table of contents

1 Health effects 2 Astronomy 3 Uses 3.1 Fluorescent lamps 3.2 Disinfecting drinking water 3.3 Analyzing minerals 3.4 Sterilization 3.5 Resolution 3.6 Spectroscopy 3.7 Photolithography 3.8 Other 4 History 5 References

Health effects

In general, UV-A is the least harmful, but can contribute to the aging of skin, DNA damage and possibly skin cancer. It penetrates deeply and does not cause sun burn. Because it does not cause reddenning of the skin (erythema) it can not be measured in the SPF testing. There is no good clinical measurement of the blocking of UVA radiation, but it is important that your sunscreen block both UVA and UVB.

High intensities of UV-B light are hazardous to the eyes, and exposure can cause welder's flash (photokeratitis or arc eye).

UVA, UV-B and UV-C all can damage collagen fibers and thereby accelerate aging of the skin.

Tungsten-Halogen lamps have bulbs made of quartz, not of ordinary glass. Tungsten-Halogen lamps that are not filtered by an additional layer of ordinary glass are a common, useful, and possibly dangerous, source of UV-B light.

UV-B light in particular has been linked to skin cancers such as melanoma. The radiation ionizes DNA molecules in skin cells, causing covalent bonds to form between adjacent thymine bases, producing thymidine dimers. Thymidine dimers do not base pair normally, which can cause distortion of the DNA helix, stalled replication, gaps, and misincorporation. These can lead to mutations, which can result in cancerous growths. The mutagenicity of UV radiation can be easily observed in bacteria cultures.

This cancer connection is the reason for concern about ozone depletion and the ozone hole.

UV-C rays are the strongest, most dangerous type of ultraviolet light. Little attention has been given to UV-C rays in the past since they are normally filtered out by the ozone layer and do not reach the Earth. Thinning of the ozone layer and holes in the ozone layer are causing increased concern about the potential for UVC light exposure, however.

As a defense against UV light, the body tans when exposed to moderate (depending on skin type) levels of radiation by releasing the brown pigment melanin. This helps to block UV penetration and prevent damage to the vulnerable skin tissues deeper down. Suntan lotion that partly blocks UV is widely available (often referred to as "sun block" or "sunscreen"). Most of these products contain an "SPF rating" that describes the amount of protection given. This protection applies only to UV-B light. In any case, most dermatologists recommend against prolonged sunbathing.

A positive effect of UV light is that it induces the production of vitamin D in the skin. Grant (2002) claims tens of thousands of premature deaths occur in the U.S. annually from cancer due to insufficient UV-B exposures (apparently via vitamin D deficiency).

Astronomy

In astronomy, very hot objects preferentially emit UV light (see Wien's law). However, the same ozone layer that protects us causes difficulties for astronomers observing from the earth, so most UV observations are made from space. (See UV astronomy, space observatory).

Uses

Fluorescent lamps

The main mercury emission wavelength is in the UV-C range. Unshielded exposure of the skin or eyes to mercury arc lamps that do not have a conversion phosphor is quite dangerous.

The light from a mercury lamp is predominantly at discrete wavelengths. Other practical UV light sources with more continuous emission spectra include xenon arc lamps (commonly used as sunlight simulators), deuterium arc lamps, mercury-xenon arc lamps, metal-halide arc lamps, and tungsten-halogen incandescent lamps.

Disinfecting drinking water

Analyzing minerals

Sterilization

Resolution

Spectroscopy

Photolithography

It is photolithography which is primarily used to create integrated circuit components and printed circuit boards.

Other

It is advisable to use protective eyewear when working with ultraviolet light, especially short wave ultraviolet. Ordinary eyeglasses give some protection. Most plastic lenses give more protection than glass lenses. Some plastic lens materials, such as polycarbonate, block most UV. There are protective treatments available for eyeglass lenses that need it to give better protection. The most important reason that ordinary eyeglasses only give limited protection, however, is that light can reach the eye without going through the lens. Full coverage is important if the risk from exposure is high. Full coverage eye protection is usually recommeded for high altitude mountaineering, for instance. Mountaineers are exposed to higher than ordinary levels of UV light, both because there is less atmospheric filtering and because of reflection from snow and ice.

Some insects, such as bees, can see into the near ultraviolet, and flowers often have markings visible to such pollinators.

History

References

• Grant, William B. (2002). An estimate of premature cancer mortality in the U.S. due to inadequate doses of solar ultraviolet-B radiation. Cancer 94 (6), 1867–1875.