View Full Version : double polarization

DJ Erock
07-11-06, 02:25 PM
I recently went out and bought myself a pair of new sunglasses for the summer. I was sure to get a pair with polarized lenses for the benefit of complete UV protection. When driving home, I noticed that the tint on the windows of my car must be polarized as well. When wearing my sunglasses and looking through the tinted windows, I see a distortion that features all the colors of the visible spectrum. I was wondering what it was about polarization that made this distortion when you look through two lenses that have it. Any ideas?

07-11-06, 03:37 PM
That just happens when two polarization filters are not perfectly lined up. The window tint is often not very uniformly polarized so you can somewhat splotchy effects. The same thing happens with LCD displays (that is how they turn on and off pixels). Try looking at a LCD display with your sunglasses on and tilt your head slowly left and right to align and mis-align the various filters.


DJ Erock
07-11-06, 03:42 PM
What exactly is a polarization filter? What does it do to the light coming through that blocks UV light?

07-11-06, 06:03 PM
Polarized lenses are neither necessary nor sufficient to block UV light.

A polarization filter will block all radiation (in a particular range of the EMR spectrum) that is polarized perpendicularly to the filter's orientation. Generally, this is exactly half the incoming light, but it could be up to 100% if the incoming light is polarized.

To get an idea of what's happening, try looking at a calculator screen or digital watch with your sunglasses. Rotate the calculator/watch and see what happens.

Polarized lenses are often not recommended for driving or flying, specifically because of their interaction with windscreens, windows, and LCD instruments.

07-11-06, 06:35 PM
Generally, this is exactly half the incoming light, but it could be up to 100% if the incoming light is polarized.

Which is often the case with glare (i.e., light reflected off of flat surfaces such as water, automobiles, etc). In situations with glare, non-polarized sunglasses need to be very dark to sufficiently reduce glare, but this makes it hard to see everything else. Since polarized lenses reduce glare much more than other light, the lenses need not be as dark, and you can see much better. I wear polarized lenses every day, and gladly pay the extra cost.

Most people I know strongly prefer polarized lenses for driving, since they want to avoid being blinded by glare from other cars. The rainbow artifacts are not much of a nuicance, and unless they prevent you from reading your dials they shouldn't pose any problem for driving with polarized lenses on.

07-11-06, 07:17 PM
and kick ass for fishing =]

07-16-06, 01:08 AM
Having had the advantage of actual practical experience of driving many, many miles in sunlight, ( I stopped counting at a half million miles ) I will allow you ( all ) to have the benefit of my humble opinion.

I strongly prefer polarized sunglasses. I have never experienced any difficulty in the nature of being unable to see my road, my instruments, my radio dial, etc.

Sometimes people who never have had actual experience will freely give advice from some written source which they are unable to personally verify.

07-16-06, 04:17 AM
Yes... and sometimes people with comprehension problems will mistake factoids for advice.

07-23-06, 02:55 AM