Incandescent UV (was fluorescent UV)

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

The "torchiere"(sp?) halogen floor lamp that I bought within the past year or
so has a warning label prominently fastened to the reflector surface:
  "CAUTION: RISK OF EXPOSURE TO ULTRAVIOLET RADIATION. DO NOT OPERATE
  WITHOUT COMPLETE BULB ENCLOSURE AND PROTECTIVE GLASS IN PLACE OR IF LENS
  IS DAMAGED."

Regarding "black body" radiation in general (of which incandescent lighting
might be considered an approximation, over a certain temperature range),
for a given temperature there will be radiation over a range of wavelengths,
but there will be some particular wavelength (characteristic of that
temperature) for which the intensity of radiation is greater than for any
longer or shorter wavelength. As the temperature of the black body increases,
the intensity of radiation increases across the spectrum, but the wavelength
of peak intensity shifts toward the shorter wavelengths.

A regular incandescent filament may have only a tiny fraction of its light 
output in the ultraviolet range (most of which is absorbed by the glass bulb),
and a lot of its output in the infrared range, so it's not particularly
efficient at producing visible light, and the UV output in proportion to the
visible light output will be very small. A filament in a halogen lamp runs
hotter than a filament in a regular incandescent lamp, so while the infrared
emission per unit area of the filament is increased, the visible light
emission increases even more, so a halogen lamp under normal operating
conditions produces more visible light per watt than a regular incandescent.
The percentage of UV (relative to visible light) also increases, and the
quartz bulb is not as effective at absorbing UV as regular glass, so proper
UV shielding becomes a matter of some interest when using bright halogen
lamps.

If you go beyond the operating temperature of existing incandescent lamps,
the percentage of UV continues to increase. As the temperature approaches
"infinity", the apparent color of a black body emitter will never go beyond
bluish-white, and while the visible light output continues to increase, the
ratio of UV to visible will also increase, so even if distance or a neutral
density filter cuts down the light intensity to bearable levels, the
percentage of UV could become very high - you could have a radiator for which
more than half of the output is beyond the visible range.

That being said, my Red button viewer has halogen lighting. :-)

John R


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