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UV radiation

G’day; another $0.02 bit coming. Ordinary filament lamps give
out negligible UV. Fluorescents give more, but it’s nothing much
to worry about. Halogen lamps give out very little UV, especially
if there’s a glass shield in front of them. You see, ordinary
glass is quite good at filtering out UV (it converts it to a tiny
bit more heat, if you wondered where it went) The halogen lamps
are in fact very compact low voltage, filament lamps, similar to
the ordinary household bulbs. The transparent envelope containing
the filament is made of pure quartz which melts or deforms only at
very high temperatures, so can be very close to the white-hot
tungsten filament. Inside the quartz bulb is an atmosphere of
nitrogen under a bit of pressure plus a small amount of pure iodine
or bromine which are called halogens. The secret is so simple
it’s very elegant. The very high temperature keeps the halogens
off the tungsten - which they like very much. So much that as the
tungsten evaporates at the high temperature, it is instantly jumped
on by the halogen atoms. But the high temperature makes them soon
jump apart again, then instantly reform. Again and again.

But when the lamp is switched off, the tungsten cools very
quickly; much slower than the quartz. So the tungsten halide
settles back on the relatively cool filament and not the envelope
(as you must have seen it do on ordinary lamp bulbs). Then when
the lamp goes on again, the tungsten halide instantly evaporates
and swirls about, dissociating and re-associating, waiting to do
it’s trick many more times. You see, if it wasn’t for the
halogens, the tungsten would itself evaporate and burn out fast
when so hot, so this recycling business deposits it right back
where it belongs as the bulb cools. So, the halogen lamp doesn’t
get really hot enough to give off much UV; you need the temperature
of an electric arc or at the very least, a big welding torch for
that. What little UV the halogen lamp does radiate is soon mopped
up by a bit of glass. Is that clear as mud? I do come all over
a bit academic at times if unrestrained - I slipped my leash

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/ /___| \ @John_Burgess2
(______ )
At sunny Nelson NZ But it is only 8C outside my window tonight.

John— Thank you for you clear explanation of both “UV
radiation” and “Cleaning” . Next time I see my physicist friend
I’ll have something I CAN TALK ABOUT! Dolores—New Jersey