The measurement of UVA (or total UV) irradiance is often part of a daily routine for indoor tanning salon operators. For reasons of practicality and cost, the UVA measuring instrument preferred for this purpose is generally the pocket-sized type.

We all understand that the UVA reading plays an important part in deciding when to replace tanning lamps. For financial reasons and customer satisfaction, there should be the best possible assurance that the salon owner can depend on the readout from his or her UV meter.

A UVA meter can be a great tool when used properly; however, all too often salon owners think it is an exact measurement for output, but instead it is relative. If salon owners choose to rely on the UV meter then they should be sure to follow the same format for measuring output including:

• Take an initial reading upon the installation of new lamps.
• Make sure the "measuring conditions" always are kept the same. Measure at the same location of your tanning unit, at the same distance, observe the same electrical conditions for each single measurement.
• Make sure that the acrylics and reflectors are clean. It is imperative that acrylics are changed according to the manufacturer's specifications.
• Wait with your measurement until your tanning unit is in a state of "thermal equilibrium," which takes about 5 to 10 minutes after turning it on.

If these steps are carried out, measurement readings for comparative purposes will be obtained. Pocket-sized meters are used to measure a variety of UV bandwidths. Even if your meter only registers UVA levels, it can be assumed that the UVB values will diminish by approximately the same relative amount.

You can check individual lamps at the acrylic, or take an average "body" reading in the middle of the bed. Body position is generally considered to be 25 cm above the bench, with meter pointed up at the closed canopy.

Accuracy Of Measurements
In general, the accuracy of UV readings on digital displays is sometimes overestimated. Renowned universities consider an absolute accuracy of between 5 percent and 10 percent as very good. Therefore, for some low-cost UV meters that operate under much less controlled conditions, considerably greater measuring errors could be expected. For example, it is known that with some low-cost UV meters, the readings for one and the same radiation source differ by up to 20 percent to 30 percent between identical products from the same manufacturer. Some manufacturers of UV meters do provide calibration accuracy of +/-5 to 10% reference NIST traceability, so it's important to look for that specification to minimize error potential.

UVB Measurement
It is more difficult for a hand-held meter to isolate the UVB bandwidth (280-320 nm) from the total UV bandwidth (280-400 nm). Hence UVB measurement can be more problematic. And since the primary purpose of using a UV meter is to tell the salon owner when to replace his or her lamps, the measurement of UVB values is not necessary for that purpose because UVA and UVB irradiance drop by about the same relative amount.

However, a very important usage of a UVB meter is to check acrylic transmission. Solarized, aged acrylic blocks a significant amount of UVB, while allowing most UVA to transmit through it. Only a UVB meter that has a selective 280-320 nm filter can display this problem. Another usage for a UVB meter is to divide the reading by UVA value and obtain an estimation of percent UVB.

Lamp Output
When examining the output performance of a lamp, as a rule, about 20 percent to 25 percent of the electrical input will be emitted as ultraviolet radiation. Yet, this only is true for new lamps that are in optimal operating conditions--meaning that the electrical conditions such as supplied voltage, lamp current and wattage meet the specifications of the lamp manufacturer.

Ballasts and starters--if used--also play an important role. The two major factors responsible for the actual performance of a tanning unit are the aging of the lamps over time and equipment characteristics of the tanning unit.

Any radiation source loses a certain degree of power the longer it is in operation. For example, the reduction in power hardly is noticed in the general lighting of private households. In general, incandescent or fluorescent lamps are not replaced until they stop functioning.

Unfortunately, this is not as simple with the UV lamps used for indoor tanning. Tanning units are expected to meet certain requirements with respect to their tanning effectiveness over a given period of time. With increased use, they are no longer able to meet such expectations because of a marked performance decline, even though they are still in full working order from an electrical point of view. With tanning lamps, the recommended "useful life" is considerably shorter than the electrical life.

Normally, manufacturers give recommendations on the "useful life" of their lamps, but these recommendations only can be used as a guide because there is no clear and official definition of the term "useful life." Each manufacturer can make its own definition. Additionally, different operating conditions, as well as equipment-related factors, have in certain cases a considerable effect on the actual useful life of a tanning lamp.

Although both of these points are not very helpful in answering the question, "When should lamps be replaced?", there are some guidelines which help determine the right point in time.

Measuring Different Types Of Lamps
Experience has proven that measuring different types of sunlamps with a UVA or total UV meter usually proves problematic in practice. The deviation of the UV meter from the actual measured value varies from spectrum to spectrum. As a rule, UVA readings obtained on different types of lamps are not comparable for effectiveness.

If lamp (A) shows a reading of 20 mW/cm, and lamp (B) shows 15 mW/cm, this does not generally mean that lamp (B) has a 25 percent lower effectiveness. If there is a clear distinction between the spectral characteristics of both lamps, it is perfectly possible that lamp (B) has higher UVB with lower UVA… and tans in a lower session time.

That is because the irradiance readings from UVA and UVB meters is unweighted and does not correlate to the erythemal action spectrum (EAS). The EAS weights 297 nm and below very high, and 298 out to 400 nm progressively lower.

For measuring different types of lamps' erythemal effectiveness, a MED/hr meter must be used. If it's sensor response curve accurately follows EAS weighting, it will yield Eeff in much the same manner as spectroradiometers do for FDA determination of Te maximum timer interval of (4) MEDs (minimum erythemal doseage).

Conclusion
This information is intended to emphasize the fact that low-cost, pocket-size UV meters are suitable for a variety of distinct purposes, depending on their specific response.

UVA (or total UV): Relative measurements for lamp aging.
VB: Acrylic transmission and %UVB.
MED: Erythemally weighted effectiveness.

Additionally, salon owners should not rely solely on customer response regarding the output of the lamps, due to the many and varied skin types and base tan levels. They should follow a typical lamp maintenance and replacement routine in order to provide their customers with consistent tans year-round.

NOTE: Special thanks to Steve Mackin of Solartech Inc for his knowledgeable input on metering techniques and parameters.