The ABCs of UV
The ABCs of UV
Ultraviolet radiation (UV), including that from electric lighting, always has been a concern for art galleries, museums, etc., because of its harmful impact on artifacts. Even though indoor lighting emits much lower levels of UV than normal sunlight does, the strong association found between skin cancer and extensive exposure to sunlight seems to have raised concerns about health effects of UV from electric light sources.
To address these concerns, the US-based E Source recently published a report for its members entitled "Ultraviolet Radiation from Electric Lighting". The report concludes that health effects of UV from lighting should continue to be studied, but that normal levels of fluorescent lighting yield so little UV that this is not a valid reason for fearing or avoiding such sources. Where sensitive individuals or artifacts require it, UV filtering is available. Visible light is electromagnetic radiation with wavelengths between 380 and 700 nanometers (nm). The wavelengths of UV range from 100 to 400 nm. Every light source produces some UV, and UV is inherent to the operation of most discharge light sources.
UV radiation can be described in two ways:
-
by intensity or concentration, as watts/m^2 (W/m^2) or as watts per lumen (W/lm). The former is a measure of the amount of UV impinging on a surface, while the latter is a measure of the amount of UV per unit of visible light.
- by wavelength band: UV-A (400 to 315 nm), UV-B (315 to 280 nm), and UV-C (280 to 100 nm).
UV in sunlight can be a thousand-times more intense than typical indoor lighting, but the most dangerous wavelengths (shorter than 290 nm) are screened out by the atmosphere.
HEALTH STANDARDS
International health standards (IRPA-International Radiations Protection Association) are based on the damage potential of the most potent wavelengths, between 270 and 290 nm. Lamps emit a mix of wavelengths, so to estimate the impact requires that each wavelength's intensity be multiplied by a factor that adjusts for its damage potential. The result is the "effective irradiance" (in W/m^2) of an equivalent amount of UV at 270 nm. Health standards are then typically based on how long (e.g., an eight-hour working day) the skin in sensitive individuals can be exposed to light of a given intensity without harmful effects.
LAMPS AND UV(lamps intended to emit UV are not considered here):
- High-output (HO) and very-high output (VHO/SHO) fluorescent lamps emit high levels of the most biologically damaging wavelengths: UV-C and short wavelength UV-B. The risk of developing the least harmful types of skin cancer may increase by 20% in sensitive individuals exposed to light from bare lamps for eight hours a day. Standard acrylic lenses can reduce UV exposure to levels safe even for sensitive individuals.
- Earlier high color-rendering-index T-12 lamps emit UV that is slightly above the IRPA standards. UV emissions of most other T-12 lamps and all efficient T-8 lamps are below the levels considered potentially harmful.
- Bare metal-halide lamps emit high UV-A and UV-B levels, but an acrylic lens (or tempered UV glass lens) can reduce UV levels.
- Halogen lamps produce both high UV and visible light levels in the center of their beams, but the concentration of UV from bare lamps is not much higher than that produced by other incandescents. Halogen-infrared lamps reduce UV-B and UV-C by 50-fold, compared with the emissions of a standard halogen lamp of equivalent light output. Lenses reduce all UV to safe levels.
MUSEUM CONCERNS
Articles circulated among preservationists show that UV levels emitted by compact fluorescent lamps (CFLs) are four-times higher than than those of incandescents. However, closer examination has shown that some of these data are erroneous. According to the report, a British UV-meter produced most of the faulty results. The meter "sees" a portion of the visible spectrum as UV, thus skewing its results against any light source rich in violet-blue, such as fluorescent and metal-halide. Moreover, the meter "sees" some of the infrared radiation (i. e. heat) as visible light, favoring the most inefficient incandescents.
Better measurements found that UV levels from bare CFLs ranged between 50 and 140 microwatts per lumen (µW/lm) (the US standard, e.g., allows up to 75 µW/lm), while several incandescents exceeded 100 µW/lm. UV levels from CFL fixtures with acrylic lenses were as low as 13 µW/lm, and adding a UV filter cut emissions to less than 2 µW/lm.
For comments see letters.
Nils Borg
Note: Lindsay Audin, author of the E Source report, kindly provided background material for part of this article.