Remember, the recommended maximum exposure time that is listed on your sunbeds’ labels refers to four MED (Minimal Erythemal Dose), which is the maximum amount of UV exposure a tanning system is allowed to deliver under FDA regulations. The exposure time is the result of testing against the erythemal (burning) effectiveness of the “system.” The sunbed manufacturer tests at distances and locations within the bed that approximate the tanner’s position during a session. Your UVA+B radiometer is testing total output (irradiance), which is mostly UVA. There are other affordable devices that can give you a better indication of the erythemal effectiveness that dictates the maximum exposure time, though they may not tell you as much about the output maintenance of your lamps.
Chances are your two sunbeds are not identical models. There are probably differences in bed geometry, lamp density, lamp-to-tanner distance and other variables aside from the differences between a reflector lamp and a non-reflector lamp. The UVA+B meter is best for monitoring total lamp output, not comparing different lamps or systems.
[callout]It is probably safe to assume that your “15 minute” non-reflector lamps employ a phosphor that is a bit more erythemally effective than the one that has reflector lamps. This will account for similar exposure times from dissimilar systems.[/callout]The non-reflector lamp operates with a specular (mirror-like) reflector positioned behind it, which redirects the UV radiating from the rear of the lamp back toward the tanner. If non-reflector lamps are positioned too close together, the efficiency of this specular reflector is actually diminished as some of the energy is blocked by the lamps themselves.
Reflector lamps do not radiate UV from the rear due to the reflective coating inside and on the rear of the lamp. The sunbed’s specular reflector does not have much, if any, work to do. In fact, using reflector lamps would allow the bed maker to eliminate the bed’s reflector without significantly affecting its efficiency. If both lamp styles employ the identical phosphor blend, then they will produce the same UV output. The reflector lamp, by itself, is more efficient at delivery of the output by 10-20% than the non-reflector lamp employed with an external reflector.
This is due to:
• The inherent efficiency of an internal (to the lamp) reflector versus external
• The fact that short-wavelength UV loses potency over distance, particularly UVB. The waves must travel further in a non reflector lamp (from the rear of the non-reflector lamp to the external reflector, then back to the tanner) than from a reflector lamp.
Sunbed manufacturers must take advantage of reflector efficiency, whether internal or external to the lamp. Beds equipped with reflector lamps often have greater lamp density (closer together) than a bed using non-reflector lamps. It is probably safe to assume that your “15-minute” non-reflector lamps employ a phosphor that is a bit more erythemally effective than the one that has reflector lamps. This will account for similar exposure times from dissimilar systems. Your reflector lamp bed shows higher total UV, and the one with non-reflector lamps is more erythemally effective.
