kw: observations, fluorescence
I got a check in the mail yesterday, one of those rather official looking ones. On the back, in barely-visible blue ink, where you are supposed to endorse it, it states that security features include a diamond pattern visible when viewed at a low angle, and "invisible fibers". I tried looking at a low angle, and I could indeed just barely see some letters in a diamond pattern. The text also warns not to accept a check that lacks these features. I reckoned that the bank needs to have a way to see the "invisible fibers", so they must be fluorescent, visible by ultraviolet illumination.
A note about UV wavelengths. Until recently, all UV sources were mercury vapor tubes with filters. An ordinary "black light" tube such as those used in rock concerts is long wave UV, at a wavelength of 365nm, one of the strong "lines" in the mercury spectrum.
Visible light is officially in the range 400-700nm, though most people can see a little beyond these limits if the source is bright enough. In particular, the purple glow you see when you look at a black light tube is the 365nm line, plus fluorescence in the range 360-380nm due to a phosphor that converts short wave to long wave, to make the tube more efficient. Don't look too long, it promotes cataract development. Rock hound long wave UV lights do not usually have the extra fluorescence, just using the 365nm line through a filter. The more recently-developed UV LED flashlights use a wavelength of 375nm, though this is likely to
change as technology continues.
Short wave UV, for rock hounding purposes, is 254nm, a much more energetic light. It can also damage your eyes a lot faster. Glass blocks it, so the special filters used on such light sources use quartz with a filter that blocks visible and long wave UV. Short wave UV makes some substances fluoresce that long wave will not affect, and will also make most (but not all) long-wave-fluorescent substances glow. It sure did the trick for the security features in the check.