A "Black Light" is one of those light bulbs that is "purple colored". This dates me a bit, but in the 1960's they used Black Lights to make the pshycodelic Hippy posters glow from the bright fluoresent paint used.
The theory with black lights is that they did not use synthetic thread back in WW2, and the natural material fiber threads they used back then will not "glow" when looked at under a black light bulb. I may need to be corrected a little, but this is the general idea behind "Black Lights"
Regards, Steve
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