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:
- trout (at least juveniles) have the mechanism to see UV in addition to the three colour components that we see*.
- fluorescent colours appear to have some use in trout lures.
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 -
- what materials reflect (unconverted) UV light and might be attractive to a (juvenile) trout?**
- under what conditions can fluoro colours be effective?
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.
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.
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.
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.
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",
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: