Editorial Feature

A Guide to Ultraviolet Safety Considerations

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Aug 7 2007

Electromagnetic radiation with a wavelength between 10 and 400 nm is called ultraviolet radiation. Radiation below 180 nm is referred to as vacuum UV. Normally we use the term UVC for wavelengths to 290 nm, UVB for 290 – 320 nm, and UVA for 320 – 400 nm, although there is no agreement on the exact boundaries. Radiation below 320 nm is called ‘Deep UV’ in photolithography.

UV and Light Sources

Arc lamps and deuterium sources can produce UV radiation with wavelengths down to 180 nm and below. Most Tungsten Halogen lamps have UV stop envelope materials to absorb the shortest wavelength UV radiation produced by the lamp. This is usually for safety reasons as the UV intensity levels from some of these sources may be higher than those from the sun as well as some shorter wavelengths potentially being present.

This high-intensity UV and visible (VIS) radiation can permanently damage the cornea, lens, and retina of the eye, potentially causing blindness. Eye damage caused by UV sources may not be immediately apparent either. These wavelengths of light are dangerous for eye health as the deep UV is absorbed in the cornea or eye fluids and focused VIS and near-UV (NUV) can damage the retina. The normal blink reaction to visible light may not be adequate protection, as the blink response is not induced by invisible NUV radiation, produced by spectral filtering. It is not just eye health that is a concern when working with these wavelengths of light. UV radiation can also cause painful sunburn, and with prolonged exposure, serious burns.

Guidelines

It is recommended by the U.S. National Institute for Occupational Safety and Health (NIOSH) that the total intensity from 315 to 400 nm hitting unprotected skin should not exceed 10 W/m2 for periods longer than 1000 seconds. For shorter exposure times, the energy density should not exceed 104 J/m2.

See the fig below for wavelengths below 320 nm, which shows Threshold Limit Values for occupational exposure to ultraviolet radiation in an 8 hour period. These values for exposure of the eye or the skin apply to UV radiation from arcs, gas and vapor discharges, fluorescent and incandescent sources, and solar radiation, but they do not apply to UV lasers. (ACGIH ISBN: 0-9367-12-99-6).

Recommendations

The simplest approach to UV safety where the ultraviolet light is not required is to get rid of it at the source. Contact us for lenses or filters to accomplish this. If you require UV light, then there are several precautions you should take to minimize exposure and reduce the hazards.

Never look directly into the output beam even with safety glasses.
Do not look at the specular (mirror) reflection of the beam.
Always wear UV protective eyewear and gloves.
Do not view UV images without safety glasses. Looking at the big image of high wattage lamps on a distant wall or small image of low wattage lamps on a probe requires welding goggles!
Use a manual or electronic shutter to close the beam when the source is not in use.

Ozone

Shortwave ultraviolet light photolyzes molecular oxygen to produce ozone, O3. Lamps that have output below about 240 nm (continuous-wave or pulsed sources) produce ozone. This is emitted in the cooling air stream of the lamp housing. Ozone is a common pollutant at ground level in urban areas. Relatively low concentrations of ozone can cause nasal dryness and a burning sensation in the throat, headaches, nausea, and irritation of the mucous membranes.

There is no simple way of predicting the ozone concentration (or its impact on you) e.g., operation in a small enclosed area may lead to high concentrations Recommended maximum exposures are typically:

0.1 ppm for 8 hours exposure
2 ppm for a 2 hour exposure

A 150 W UV arc lamp can contribute more than 1 part ozone per million to the convective air stream. This may be of little consequence in a well-ventilated area but some people are particularly sensitive to the effects of ozone exposure and the long-term effects are not well documented. Noticeable symptoms for most people appear at 0.3 – 0.5 ppm.

A very sensitive nose can detect 0.015 ppm. 1 ppm produces a strong and obnoxious odor. As a rule of thumb, if you can easily smell ozone, the level is too high for prolonged exposure.

Recommendations

Use an ozone-free lamp unless you need the short-wave UV.
Operate the system in a large ventilated area.

Note: Ozone has an absorption in the UV. If ozone is created and built up in the optical path, particularly a long enclosed optical path, then the observed UV radiation level may change accordingly and lead to misinterpretation of lamp or sample performance.

UV Safety Glasses

Our UV protective eyewear is inexpensive and comfortable to wear. Both are made of green tinted plastic with UV blocking, polycarbonate lenses which offer high impact protection (see Fig. for transmittance curve).

Three types available

We offer spectacle and "Over the Glass" frame styles. Both are comfortable and lightweight. The "Over the Glass" model fits directly over prescription eyeglasses with no compromise in protection. Both models are made of hard, durable plastic, with close fitting side shields to prevent UV radiation from reaching the eyes. The sides are adjustable in length.

Adjusting the image of an arc or filament on a small probe or target is normally done by viewing the image. This requires welding goggles because of glare.

Protective Gloves

Hands working with fixtures in the beam are particularly prone to UV exposure. Even minimal exposure may cause burning of the skin. Protect your hands with gloves. Any glove type will offer some level of protection, though flammability may be a concern with some fiber-types.