Physics 110: Answer
Fall Term, 2001
In a tanning booth, how is the UV light produced? How did
scientists come to discover that the sun's effects could be duplicated
in an artificial product? And how is it different from that of the
natural sun?
-- Danielle Gower
Having never been to a tanning booth (I get sunburnt if there is a full moon!) I can't be too sure about the exact details but I assume they produce their UV light in much the same way as regular black lights work, they probably just have more tubes or higher power tubes in order to make the process quicker. Black lights work in very similar way to regular fluorescent tubes so let me first describe those.
A fluorescent tube contains a gas (usually argon) at low pressure and a very small amount of mercury. When a current is passed through the tube the mercury is ionized and emits UV radiation. In regular light tubes this UV radiation is absorbed by a phosphorus coating on the inside of the tube and is emitted as visible light. Any UV that isn't absorbed by the coating is usually blocked by the glass of the tube. So to make a black light we make two adaptations of the fluorescent tube: we remove the phosphorus coating and we use quartz glass rather than regular amorphous glass. And I think they use these things in tanning booths.
I'm not sure of the exact history of the development of artificial light sources but I will at least give you an idea as to some of the thought processes that may have gone on. Newton and others noticed that by using a prism we can split sunlight into the primary colours that make a rainbow. After this we never looked back and now this science of spectroscopy is used all the time, not just with our sun. People noticed that if you put a thermometer just beyond the red line in the spectrum it heated up---the discovery of the infrared part of the spectrum. It turns out that visible light is just a small part of the electromagnetic spectrum and the sun actually emits in more than visible. It also has a lot of emission in the infrared and some in the UV. Remember, above, I said we had to change the glass to quartz glass, well you may have noticed that you don't get a tan when sun light passes through glass to get to you. And yet the visible portion isn't altered at all. So this means that the tanning part of the sun's spectrum is not in the visible (or for similar reasons in the IR) and so it must be in the UV. By looking at the emission of mercury vapour scientists realized that they could artificially produce UV light and I imagine it wasn't long before some enterprising sun starved scientist realized they could get a tan from mercury vapour emission.
Finally, the ways in which the UV from lamps is different from the sun may be clear by now. The sun emits a very broad spectrum of electromagnetic waves, form the IR to the UV and a tanning lamp only emits at certain specific frequencies (low pressure mercury lamp emits at 254 nm). So the sun's spectrum is a "fuller" spectrum.
-- P. Fox