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RonD
With regard to polarizing headlights and windshields, I just sent the following to another site:
Mr. XxXxxx-
I don't know whether anyone ever seriously tried the concept of polarizing the lights and windshields, but the idea has certainly been around long enough! I believe I saw it in Popular Science, probably in the "New Ideas From The Inventors" feature, around forty years ago. I think I even saw an article calling for the use of ±45 degrees positions because specular reflections from water surfaces are already partially polarized in one of the "cardinal" planes, and this would cut that glare, too. (The headlights and windshields of a given car need to be polarized in the SAME plane unless you wish to eliminate light from returning; some glare will actually get through, but not the "real" light!)
The most obvious drawback is the loss of total light at the outgoing interface (polarizer over the headlight). Starting with completely randomly-polarized light, and using a lossless polarizer couldn't offer more than 50% of the original energy, if my recollection of optics is correct. Of course, there's no such thing as a 100% efficient polarizer, and there will be additional losses at each surface in the beam, though modern thin-film coatings are a huge help in that regard.
Some materials will modify the polarization of reflected light, further cutting the amount that could return through a windshield. On the other hand, using less than perfect polarizers will be a necessity, as this will help drivers see the oncoming cars. A "perfect" set of polarizers would mean that the lights of oncoming traffic couldn't be seen through the windshield! Those of us who routinely dip our lights just before cresting a hill [and it's still done around here in rural Wisconsin when we observe oncoming traffic about to do the same] won't have the option. I suppose that some automatic-control system would have to be required - I don't recall hearing much about GM's Autronic Eye or whatever it was called, lately.
If we choose a number, say 80% polarization, for the outgoing beam, the initial energy supplied to the lamp would need to be 25% greater than an ordinary non-polarized unit to get equal amounts to the road and other objects illuminated. That's not good enough, because the windshield is going to "lose" some more for us. We need to put out MORE light than a non-polarized source to have as much perceived return. Now for the $64 question: how can you transition from existing systems to the new one? "Old" cars' drivers will see a brighter-than-normal light, more glaring and blinding than the conventional ones, coming at them, and "new" cars' drivers will see much less. At present, we have a mix of conventional tungsten, tungsten-halide, and arc-discharge headlights on most roads. Even with better optics and edge cut-off control, the arc-discharge ones can be downright annoying; imagine needing to have even brighter ones out there! At least we don't have to contend with kerosene and carbide ("acetylene") headlights in the mix.
Finally, note that the use of a polarized windshield means having less light to the driver's eyes in daylight, too. On a bright, sunny day this is probably good, but for an overcast dusk or dawn period it is not. Daylight-adjusted eyes are not going to be helped much by turning on the headlights at those times.
Do I sound like a pessimist? I just think there are better ways to handle the complex problem than this flawed method. Rant mode off.
RonD
[Aug 29 2001] |
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