A black light lamp emits long-wave UVA radiation and little visible light. Fluorescent black light lamps use a phosphor on the inner tube surface, which emits UVA light instead of visible light. Some lamps use a deep-bluish-purple Wood's glass optical filter that blocks almost all visible light with wavelengths longer than 400 nanometers.
Others use plain glass instead of the more expensive Wood's glass, so they appear light-blue to the eye when operating. A black light may also be formed, very inefficiently, by using a layer of Wood's glass in the envelope for an incandescent bulb.
Though cheaper than fluorescent UV lamps, only 0.1% of the input power is converted to usable ultraviolet radiation. Mercury-vapor black lights in ratings up to 1 kW with UV-emitting phosphor and an envelope of Wood's glass are used for theatrical and concert displays. UVA/UVB emitting bulbs are also sold for other special purposes, such as tanning lamps and reptile-keeping.https://en.wikipedia.org/wi...
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