Black Light Visibility
Why do blacklights glow a purple blue color instead of black?
You put your finger on a paradox -- if a lamp glows "black" you cannot see it!!
Blacklights have a transparent film that is purple in order to mask the intense
blue lines in the mercury spectrum. If that were not done, these intense lines
would obscure the much less intense fluorescent emission lines.
Two safety points: Do not look directly into a "blacklight" the ultraviolet light
could cause eye damage.
Blacklights contain mercury, so treat them as hazardous waste. You do not want to
get mercury into solid municipal waste.
Black light is really an accurate name for these lamps but, "near visible ultra
violet light sources" is a lot harder to say. Both "long wave" and "short wave"
lights used by mineral collectors give off ultraviolet (shorter wavelength than
violet or blue) light. They use gas filled tubes that give off more light in the
ultraviolet range, (they aren't perfect by any means) and a filter to cut out
visible light. But some of the visible violet light leaks through the filters
and is seen. A quick search in the Wikipedia or Google domains should provide you
with far more detailed information including why the same material glows different
colors under different frequencies of ultraviolet.
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Update: June 2012