In the very early days of indoor sun tanning, there was only one choice of tanning lamp strength; the BL series (or blacklight type) of tanning lamp was all that was available. As years went by, many new phosphor coatings were developed for tanning lamps. These made it possible to offer lamp types and strengths to satisfy practically any customer. As a direct result of these many new tanning lamp strengths, it became necessary to have a simple way to define the strength of each lamp type; the UVB/UVA ratio came into popular use. This was simply a method of dividing the total ultraviolet light emitted in the UVB region by the total ultraviolet light in the UVA region, and expressing the fraction as a percentage. While this gave the industry an index of lamp performance, the index was somewhat inaccurate. For example, if we took a “2.6” lamp, doubled the amount of UVB it emitted and doubled the amount of UVA it emitted, what would we get? We would have a much faster tanning lamp than, say the “5.0” lamp—but since the ratio would be unchanged, it could be perceived as a weaker lamp.
The Food and Drug Administration (FDA), which regulates the manufacture of indoor tanning equipment and some tanning components, defines tanning lamps through a complex calculation of Te and Tm ratings for these lamps. Te refers to the time required to produce erythema; Tm refers to the time necessary to produce melanogenesis, or the formation of melanin in the skin.
The rating 1 MED (minimal erythemal dose) is the dose necessary to produce a faint reddening in the skin. Te, which is equal to 4 MED, is the maximum dose allowed per session. The rating 1 MMD (minimum melanogenetic dose) is the UV dose necessary to cause a faint tan in previously unexposed skin. Tm, which is equal to 4 MMD is the maximum weekly UV dose allowed. At first the Te and Tm concepts may be hard to grasp. Once the reader understands them, a logical question might be, “Why aren’t they in common use?” If every lamp type has its own Te and Tm ratings, it should be simple to use these ratings to select the best tanning lamps for your equipment. Further, they should make it simple for the salon operator to accurately calculate the proper exposure times for his customers, but this is not necessarily correct.