I'm using a hand-held UVA+B radiometer to register and track the output and service life of my lamps. The lamps that I'm using in my 15-minute beds are registering about the same as different lamps in my 20-minute beds. What causes this?
Before sunbed manufacturers market the equipment, they must test the bed with sophisticated spectroradiometry devices and determine the maximum exposure schedule in accordance with the regulations from FDA's Center for Devices and Radiological Health. The maximum exposure time is that period which allows 4 MEDs (Minimum Erythemal Dose) to the tanner. Regardless of the number of lamps, the character of the original equipment lamps, or the bed design, 4 MEDs is the maximum exposure.
The meter you are using is an excellent tool for tracking the output maintenance of a given set of lamps in a given bed. When you try to compare one set of bed/lamp data with another you are actually looking at results from many variables other than lamps, such as bed geometry, lamp density, distance from lamps to tanner, acrylic solarization, etc.
Remember also that commercial sunlamps emit almost all of their UV energy in the UVA region. Maybe 96% of the energy is UVA, while 4% is UVB. To make a lamp for shorter exposure times (a "hotter" lamp), lamp makers generally add more UVB-generating phosphor. If all other things are equal, to make a lamp with a 25% shorter exposure (20 minutes vs 15 minutes = 25%), one only needs to change the UVB a small amount. The result might only be the hotter lamp delivering 95% UVA and 5% UVB. Since most of the energy from both of these lamp styles is UVA, your UVA+B meter will give you very similar readings. After all, it is designed to detect total UV. There are other devices that actually "weight" the erythemal effectiveness of the output tested, which would tell you something about maximum exposure times.
The spectral character of different lamps can affect the readings drawn from any radiometer. This is another reason to avoid comparisons between different lamps and beds. Without changing the effectiveness, one lamp might have larger amounts of irradiance at wavelengths not well-detected while another lamp emits mostly at wavelengths easily detected by the radiometer.