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Question:
Why do blacklights glow a purple blue color instead of black?



Replies:
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.

Vince Calder


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.

Robert Avakian



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