 It depends upon what the definition of is, is - LOL
When "watts" is used, you know that the term is being used to describe a "frequency" measurement for a given area. The usual term we see is W/cm^2/nm (Watts per centimeter squared per nanometer).
When "joules" is used, you know that the element of "time" has entered the equation. One joule is the amount of work that can be done by 1 watt in 1 second.
What we have to start focusing on if we want to (1) better understand the dynamics of sunlamps/sunbeds, and, (2) be better able to explain "tanning" to our clients is photon density. We have learned how to convert "frequency" measurements to "photon density", i.e., photons/s/cm^2/nm (photons per second per centimeter squared per nanometer) and this allows us to compare sunlamps and/or sunbeds (or sunlight) regarding their photon density, i.e., the number of photons in a given UVR segment that strike a given area of the skin.
[Note: A cm^2 (centimeter squared) is about the size of a . that is square.]
Once we all get used to talking about photon density instead of frequency, we can get away from arbitrary (and confusing) measurements like UVB% and begin looking at the the actual number of photons in a given UVR segment that strike a known area of the skin. I'll post some examples when I get time.
The "bottom line" is that while "wattage" is important (because "more wattage" USUALLY - not always! - means more photons) other factors (the phosphors in LP sunbeds and the absorption filter/sunlamp combination in HID/hp sunbeds) can (and does) affect photon density.
Don |