A sticky subject...

UV and Fluorescence

(UV = ultraviolet "light", i.e. radiation just beyond the short wave limit of our vision)

People get confused about a couple of details. These are: Then, thinking about this, "reflected UV" immediately becomes confused with fluorescence.

Let's get this straight - we can not see UV. That's why we call it "ultra". We do see fluoro colours, therefore this does not require ability to see UV. Some materials take in UV radiation, convert its energy to a different wavelength and throw it back out. So a wavelength we can not see is converted to one which we can see. Since this converted energy is added to the normal colour reflection which we see, it appears to be brighter than "natural". We and the trout see it.

When we talk about the ability to see UV, this means UV directly reflected without conversion to a different wavelength. We can not see it, the trout maybe can. Some fly-tying materials have recently had "UV" added to their description. This is rubbish. They are not fluorescent, and nobody has assessed their UV reflectance. It is just a fashionable sales gimmick.

So there are two issues now to examine -

Reflection of UV:

The UV that is of interest to us is the "near" UV, i.e. just out of our range. Bearing that in mind, consider that a "white" surface is one which reflects all the visible light which falls upon it. Is it reasonable to suppose that the reflective properties of a white surface stop dead right at the limit of our sensitivity? No it is not. It is more reasonable to suppose that any white surface is likely to reflect UV also.
Furthermore, that all surfaces are likely to reflect UV roughly in proportion to their general reflectivity. Can this be confirmed? Given a UV lamp, it is a simple matter to show that all pale surfaces and shiny ones (e.g. silver tinsel) do in fact reflect UV strongly.
So you may confidently forget about the possible significance of UV reflection. It occurs for much of the ordinary material that we use, but what it means to the trout is uncertain.

Incidentally, it is often supposed that the spots on jungle cock are fluorescent. This is not true. Perhaps it has escalated from an understanding that they simply reflect UV. No doubt true, but not very significant as stated above.

Fluorescent colours:

Long light wavelengths (red) are absorbed very quickly in water. Short (blue) travel farthest of the visible light, and near UV travels even farther, even to 500 feet. BUT ONLY IN VERY CLEAR WATER.
Peat colour is the result of all short wavelengths having been absorbed, and that includes UV.
Fluoro colours are only effective where UV is present. In the absence of UV, some fluoro colours have almost no colour, so in peaty water ORDINARY COLOURS are likely to be more effective. Perhaps a mixture of both gives the best of both worlds.
In clear water fluoro red, orange and yellow can be seen at depths beyond which these wavelengths have been absorbed and ordinary colours can not be seen.

* I have carefully avoided saying "trout see UV" because some clever-clever people maintain that we can not know what trout see. It is a fact, however, that they have the mechanism. I think no sensible person woud not assume that the mechanism is of some use.
** It is not established what significance UV vision has for the trout. It has been suggested that it relate to identifying small food for the young trout; it has been suspected that it might be seasonal in older trout and that it is related to migration.

What use are fluorescent materials in water? or in other words, how far into water does ultraviolet light penetrate?

It was recently stated in a letter to one of the fly fishing magazines that "UV does not penetrate water and that should put paid to the nonsense about UV materials", or words to that effect.

I have lost track of the source of this statement, but let us put the record straight - that statement was wrong. Of that portion of the electro-magnetic spectrum which is visible to humans, and which we therefore call "light", absorption by water is highest at the long wavelength end (red) and lowest at the short end (violet). It is lower still just beyond our short wavelength limit, which is what we call "ultraviolet".

It is true that absorption increases rapidly with the "shorter-than-ultraviolet" wavelengths, because at that level different mechanisms come into play, but we are only interested in solar UV, i.e. 290 to 400 nm. where nm stands for nanometre, of which there are 100,000,000 in a metre.

A search of the internet reveals that there have been several investigations into the effects of UV penetration into lake waters. Quoted figures are: It can be deduced from the above that peat stain seriously reduces UV penetration and that the Canadian lake is the only one with water which was anywhere near clear.

Of course, we don't know just how clear that Canadian lake was and it is elsewhere claimed that in CLEAR water, UV penetrates to 500ft.

  Absolute proof of UV penetration to at least 120ft is given by this writer's discussion with divers who use ropes with yellow fluorescent threads which can be seen as yellow at the above depth which is well beyond the penetration limit of the yellow wavelengths of sunlight. It is difficult to obtain scientific evidence of water penetration of all light wavelengths ('colours'), imcluding UV, and difficult to apply this to real situations, since objective scientific research would necessarily involve pure water which never occurs in nature; and in natural bodies of water, conditions vary enormously.

The best evidence that the writer has found in an extensive search suggests that maximum penetration is as follows:

Colour   Wavelength   Max depth Metrs
red   700 - 600nm   2
orange   600 - 580   6
yellow   580 - 560   15
green   560 - 500   30
blue   500 - 450   100
violet   450 - 400   150
UV   400   200
UV   300   40

Note that the near UV, i.e. 400nm just off the end of the visible violet, has maximum penetration but moving farther into the UV range, penetration becomes rapidly reduced. At shorter than 200nm penetration is almost zero, possibly giving rise to the supposition that UV radiation does not penetrate water.

So we can be assured that our fluorescent materials are significant at the depths we normally fish in rivers, unless the water is peat stained.
(Peat colour is the result of short wavelengths having been filtered out - UV being the first). In a very clear water lake, 10m is probably nowhere near the limit and it would be unusual for a fly fisher to reach such a depth. This is relevant to trolling and since "very clear" water rarely occurs it would be more effective to use luminescent trolling lures rather than fluorescent.