Black light is a type of Ultraviolet light that is not part of the visible light spectrum. It produces invisible ultraviolet rays of a certain wavelength. (Incidentally, most black lights also produce “blue” or “violet” light which is visible so you can tell when your light is on). Many objects, including American Brilliant Cut Glass (ABCG), contain phosphors. Phosphors are substances that emit visible light when exposed to various types of radiation. So, when an item containing phosphors is exposed to a black light, that object will “glow in the dark” or “fluoresce”.
When a black light is used on a piece of American Brilliant Cut Glass, there are phosphors in the glass that respond to the radiation by producing a yellow-green light. Almost all authentic American Brilliant Cut Glass will fluoresce under black light, but sometimes it’s very difficult to detect so black lighting should never be the only method for deciding whether a piece of cut glass is authentic American Brilliant Cut Glass.
For best results when using a black light there are a couple of details to be aware of. First, the room must be completely dark. Outside sources of visible light will interfere with the ultraviolet radiation. In addition, keep in mind that sometimes a piece of ABCG has so many pattern cuts that it will reflect the visible “blue” or “violet” light coming from the black light. This makes it especially difficult to distinguish any fluorescing. Finding an area with less pattern, such as near the teeth, can help with this problem.
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Periodic, short-term use of a black light is perfectly safe since the rays are longwave (which is close to the wavelength of the visible light spectrum), but prolonged periods of exposure to a black light are not recommended.