The UV subject is interesting--and a bit complicated and at times abstract.
The two most important parts of the subject revolve around the difference between UV Fluorescence and Refleced UV light.
Maybe you don't want to get this technical. But a lot of people are talking about it. And a lot of new fly tying materials are attempting to jump on this new marketing band wagon.
1) UV Fluorescence is easy enough. That's the stuff that glows (visibly to humans) when you shine UV light onto it. Daylight contains UV. So fluorescent materials do glow a little, all the time. The ratio of UV to other parts of the light spectrum is greatest at dawn and at dusk. So fluorescent materials (and UV reflective materials both) stand out even more at those times. Dawn and dusk often turn out to be prime times for fish feeding activity.
Most parts of the humanly visible light spectrum fade to shades of gray as they travel through water. Reds and yellows fade to gray the fastest. Greens and chartreuses penetrate deeper. Blues and violets penetrate deeper yet. UV penetrates the deepest of all. In otherwords a red streamer might appear as light gray ten feet down, while a chartreuse streamer might still appear as chartreuse. And because UV light penetrates so deeply, fluorescent colors will still glow a bit, even at depth. Fluorescence happens in those rare materials that ABSORB UV light, then undergo a chemical reaction and emit new photons that are not UV anymore. Those new photons appear as parts of the humanly visible spectrum. Like hot pink, glowing chartreuse, etc.
2) Some materials reflect UV light (rather than absorb it). Reflected UV light (coming off a reflective surface) is no different than the UV before it gets reflected. It's just UV light. Most humans cannot see UV. A few can see it. Those humans who can see UV light usually see it as a light violet color.
Fish and birds typically can see UV light. Because UV light penetrates so deeply, it is particularly important to fish. UV light allows fish to distinguish UV-reflective outlines at depth. Some biologists theorize fish have evolved an optimized ability to see UV light because it is so powerful and useful at depth.
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Finding out what materials fluoresce is easy. Shine a UV flashlight on it and notice if it fluoresces or not.
Finding out what materials are highly UV reflective is difficult for humans because we cannot see it.
You can expose UV-sensitive film with a film camera, develop the film and then examine the prints. The brightest subjects will be highly UV reflective. But that's not so easy to do. You can also purchase a UV light meter, and then walk around inspecting test surfaces in daylight, or in the presence of a UV flashlight. But UV light meters are not cheap.
The recently published "New Scientific Angling" displays pages and pages of photos exposed with UV-sensitive film. But the book has so many images it's hard to remember which image is what. The author never did publish a simple bullet list of the most highly UV reflective fly tying materials.
Finally:
Is anybody still reading? This far down?
The author of the New Scientific Angling dismissed interest in fluorescent materials as "not interesting" because "they are not natural." His interest and discussion was limited entirely to UV reflectance.
Natural or not, I'm interested in fluorescent fly tying materials--if they help me to catch a fish. And it turns out they do. Big time.
But I think the New Scientific Angling author is wrong about the "unnatural" claim in any case. Some parrots (just one example) have highly fluorescent cheek feathers. Many night crawler gatherers use UV flashlights to find worms in dark grassy meadows because night time night crawlers (are said to) glow brightly when exposed to a UV light beam. I have yet to test that worm-gathering technique because I'm not a worm gatherer. But the claim is all over the internet.
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