Hi fellow listers. If you will allow me a comment, I would be grateful. Regarding taking UV flashlights to test materials in a fly shop, JWK is pretty much right on. There is UV fluorescence and reflectance. Jack is testing for fluorescence. My comment is that the UV flashlights that are used for UV acrylics such as Clear Cure Goo, are really near UV lights in the upper UV A spectrum. The are about 400 nanometers and 400 nanometers can be seen by the human eye. Indeed, we can see the light from these flashlights because they are just above the 390 nm limit of the human visible spectrum. As a practical matter, if we didn't see them we couldn't tell if the flashlights were on or off. http://en.wikipedia.org/wiki/Visible_spectrum Birds, insects and fish can see further into this near UV spectrum. Near UltravioletNUV400 – 300 nm3.10 – 4.13 eVvisible to birds, insects and fishes http://en.wikipedia.org/wiki/Ultraviolet#Subtypes What the UV flashlights cannot test for is fluorescent material that responds to a wavelength that is shorter and a higher energy than the near UV flashlight emits. So although the flashlight can confirm that a material fluoresces, it cannot confirm the opposite. A material that does not fluoresce with the light may fluoresce at a shorter wavelength. For example, it may absorb UV at 360 nm and emit in the 370 nm spectrum, both of which are invisible to us but may be visible to trout (see discussion below). Secondly, the UV flashlight cannot test for UV reflectance in the invisible UV range. It can only test for reflectance in the near UV range that it emits and that we can see. Regarding trout color vision. They have 4 cones to our 3. Our "blue" cone response peak is at 440 nm. Trout also have a 440 nm cone but they have an extra cone that peaks at 355 nm, well into the UV A spectrum. This is the cone that allows trout to see well into the invisible (to us) UV range. This cone regresses as the fish ages (thought to disappear at about 2 yo) but then thought by some researches to regain function during spawning. So smaller trout can see deep into the UV range but for most of the year, large trout cannot see anymore UV that we can. For spawning runs such as Salmon and Steelhead, UV regains its importance.