Just fell across this story (more available by following link):
"Light pollution forms 'eco-traps' By Mark Kinver Science and environment reporter, BBC News
An international team of researchers has found another form of light pollution that could have an adverse effect on wildlife.
The scientists showed that as well as direct light sources, polarised light also triggered potentially dangerous changes in many species' behaviour.
They added that road surfaces and glass buildings were among the main sources of this form of light pollution."
Irrespective of the merits of the story itself, it made me wonder, which artificial lights sources produce polarised light? Incandescent? Fluorescent? QH? LED? HID? etc. My initial thought is that incandescent, possibly including QH, are likely to be little polarised. The question is particularly string today as I noticed what strange effects the lighting in various shops had.
I would imagine that the light is not polarised, but the glass that surrounds it, filters it in a way that produces polarisation.
Also, I believe when light bounces off a surface such as a wet road it becomes polarised.
I recall reading of how bees navigate by patches of different polarisation in the sky: no doubt there are other ways in which polarisation is used.
I have, a couple of times tried to inquire, of motoring orgs and glass manufacturers, into the possibility of polarising car headlights and windscreens, so that light that travelled direct was blocked, but that which bounced off what one needs to see was allowed through. Thus greatly reducing glare. I have, of course, never received a reply.
Apparently one problem is that although such a polarised windscreen would heavily filter the unwanted out of plane component it would also filter out a significant proportion of the desired polarised component. To compensate for this the output power of your headlamps would need to be increased in order to provide you with enough illumination, this would create an unacceptable amount of dazzle for other road users not equipped with appropriately polarised screens.
You say 'apparently': does this mean you are aware of some actual discussion that has taken place on this idea 'in official circles'. If so, I would be interested to know more.
The 'headlight power wars' have already been raging for many years, and it is a very long time since anyone paid any attention to the old rule that limited glare by capping the (tungsten) wattage at 65: nobody seems to give a damn about dazzling those who do not have the latest/brightest equipment, so there would not really be anything differently 'unfair' about introducing windscreen technology into the mix. Today's lights are many times as bright as old illegal ones would have been, and far more intense and focussed to make matters magnitudes worse, but still they are getting more and more powerful, and the law never steps in to cap them to the equivalent of that old 65w.
The result is that for most of the time one can only see lights: the stuff they are supposed to illuminate is lost in all the glare. Essentially, your 'desired polarised component' is currently not getting through at all, because the straight line component is so much more focussed/intense and bright and is shining straight into our eyes. What we actually want is a diffused/scattered component to get through: for example, on a wet day focussed light would still bounce off the road into the eyes of oncoming drivers and be polarised in the bounce - whilst at the same time not scattering back to the driver they are supposed to be assisting (Which is why it always seems to be much darker when it is wet.) So really we need lights to be defocused *and* polarised to maximise the pass through of scattered light and minimise the effect of any that is directly transmitted.
Of course this is all speculation, and I'd like to see how it would turn out in practice/ experiment with angles of polarisation for both lights and screens etc. With lives at stake (assuming we still have any pedestrians and cyclists) it ought to be worth at least some trials.
Another is that you would be cutting down all other available light (i.e. not from headlamps) - by varying amounts, possibly by a very substantial proportion. This would of course, include sun and moon, and street lighting, torches carried by pedestrians, etc. Further, you might need to think very carefully about vehicle non-headlamp lighting such as brake and tail, indicator and other lights.
I first came across this idea as a child, 40 years ago. It was in a small book in my late Grandparents house. The book had probably been on the shelf for several decades.
Each chapter was a different idea, but the cross polarised windscreens/head- lights was the only one to catch my imagination. The author was a Communist, and each chapter ended in a political 'come the revolution' vain.
I would rather like to re-discover this work, does anyone know who might have wrote this slim volume?
In most circumstances, polarization results from either reflection or filtering. It's not generally a characteristic of lightsources. The light source of an LCD isn't the screen itself, it's the cold cathode tube that illuminates it - this definitely isn't polarized.
In summary the idea is just crap , which is why it is not considered.
You get very poor (light) efficiency at every stage so it is not really worth considering.
Read some basic optics books and you shoudl be able to work out most of it for yourself. If you have some good specific questions after that come back and ask here, if you like, or somewhere like sci.optics.
This is how polarised sunglasses allow you to see into water. The glasses remove the light reflected from the water, by filtering out any horizontally polarised light.
There's a critical angle, though, either side of which the reflected light isn't polarized. In practice, there's a gradual transition as you approach/depart the angle.
It really doesn't matter why the light is polarised (though it might well be interesting!), if the overall effect of the lamp and luminaire (or whatever) is to flood with polarised light.
I was trying to think if polarisation could be contributing to the strange perceptions in some shops. E.g. M&S - heavily lit by, I think, MH and fluoros, some focussed, some flooding. The lighting is making the shop look, well, like a model of a shop. Too perfect and unreal. (Sorry
- rubbish way of putting it - will try to come up with a better description.)
I would have thought that the light falling on any given surface in a store such as Marks & Spensive will be too diffuse, & from too many directions, for polarization to a significant factor in the overall appearance. The mostly downlit nature of the illumination will be a greater factor.
I think 'narrow spectral lines' is more the explanation.
E.g a colour which takes incident light and reflects at a very precise spectral line itself, will appear dark unless the light source emitter is broad spectrum. Fluorescent isnt. Its a series of narrow lines spaced across the spectrum. It also used to flicker at 100hz. Probably doesnt these days.
Good point, which I had overlooked since the line of questioning was about polarization (it's been a while since these were first-hand preoccupations). Also responsible for difficulties in getting a neutral balance with colour films, while retaining sufficient contrast. Mixing incandescent light with fluorescent doesn't solve that problem, although it does a mostly adequate job where the - highly adaptable - human eye is concerned.
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