Q:I have two 15-minute beds in my salon and my customers like tanning in both of them. One bed is equipped with regular 100W lamps, the other has 100W reflector lamps. The UV readings with my UVA+B meter show a pretty big difference between the two beds, with the reflectors metering higher. How can they both be 15-minute beds?
A:Remember, the maximum exposure time listed on your sunbeds’ labeling refers to four (4) MED (Minimum Erythemal Dose), the maximum exposure allowed under FDA regulations. The exposure time is the result of testing against the erythemal (burning) effectiveness of the “system”. The sunbed manufacturer tests UV emission 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 idea of the erythemal effectiveness which dictates the maximum exposure time, though they may not tell you as much about the output maintenance of your lamps.
Chances are, your two beds are not identical; there are probably differences in geometry, lamp density, lamp-to-tanner distance and other influential 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 for comparing different lamps or systems.
The non-reflector lamp operates with a specular (like a mirror) reflector that is positioned behind the lamps (away from the tanner) which redirects the UV radiating from the rear of the lamp back toward the tanner. If non-reflector lamps are 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 from behind, due to the reflective coating inside and on the rear of the lamp. The specular reflector in the bed does not have much, if any, work to do. In fact, using reflector lamps would allow the bed maker to eliminate the reflector without significantly affecting the system’s efficiency. If both lamp styles employ the identical phosphor blend, they will produce the same UV output. The reflector lamp, by itself, is more efficient at delivery of the output by 10 to 20 percent 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, and
- Short wavelength UV loses potency over distance, particularly in the UVB region. It is simply further from the rear of the non-reflector lamp to the external reflector, then back to the tanner, than from a reflector lamp without the detour.
Of course, the sunbed manufacturer must take advantage of the reflector efficiency, whether internal to the lamp or external. Equipment using reflector lamps often has 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 that employed in your reflector lamps. This will account for similar exposure times from dissimilar systems. Your reflector lamp system shows higher total UV, and the non-reflector lamp system is more erythemally effective.
