Why are my sunglasses polarized diagonally?

July 18, 2006 7:01 AM

Why are my sunglasses polarized diagonally?

I always imagined that polarized sunglasses were polarized vertically, in order to block out reflections from the ground around you.

However, mine are polarized neither horizontally or vertically, but at 45 degrees - both lenses at the same angle. Why?

posted by Mwongozi to science & nature (24 comments total)

I'd also expect them to be vertically polarised. I'm pretty sure mine are.

The only purpose I can think of for putting them at 45 degrees is that if you are trying to read an LCD display that is vertically polarised you will have trouble reading it. 45 degrees will let you have some chance of reading the display and still cut out some glare.
posted by edd at 7:08 AM on July 18, 2006

Does one lens point 45 degrees left and the other 45 degrees right?
posted by zek at 7:09 AM on July 18, 2006

No, they're both "left". (I assume.) If I tilt them right, my LCD display goes dark. If I tilt them left, it's visible. Horizontally, it's just slightly dimmed.
posted by Mwongozi at 7:10 AM on July 18, 2006

Are you sure it's not the polarizer in your laptop's display that's at 45 degrees?
posted by buxtonbluecat at 7:13 AM on July 18, 2006

My guess is that it's a manufacturing mistake.
posted by Steven C. Den Beste at 7:16 AM on July 18, 2006

I'll trade you - my car stereo LCD is unreadable with my sunglasses on. I have to tilt my head 45 degrees to read it, and it makes me look like a dog that just heard a weird noise. So maybe that's why...
posted by cebailey at 7:17 AM on July 18, 2006

It's not my laptop, I also tested it with light reflecting off my car, and I don't think it's a manufacturing mistake, as a friend with a completely different brand (though still UK-originated) has exactly the same angle of polarization on his glasses.
posted by Mwongozi at 7:19 AM on July 18, 2006

Someone needs to make polarized sunglasses like Circular Polarizing Filters for digital SLR cameras. You can adjust the polarization by rotating the filter ring. It'd be cool to have sunglasses with a tiny little slider on the frame that adjusts the polarization. I don't know if adjustable polarization works only on purely circular lenses though. If so, then you wouldn't really have a lot of styles to choose from ;)
posted by sprocket87 at 7:20 AM on July 18, 2006

First, for light from the sky, it doesn't really matter which way you polarize the glasses. It's polarized some way, and putting any polarizer on the glasses will filter out a lot of light.

People use vertical polarization because, as you pointed out, light reflecting from the ground will be horizontally polarized.

I'd expect the diagonal polarization is due to somebody clever enough to realize that people live in cities, with vertical buildings that can get very bright.

Also, your laptop probably has it's own polarizing filter on it, are you sure you know which way that's pointing?
posted by dsword at 7:22 AM on July 18, 2006

As above - I don't know about my laptop (It's a 15" PowerBook G4, with the newest high-res screen, in case anyone else knows), but I did test it with light reflecting off a horizontal surface (my car) and had the same effect.
posted by Mwongozi at 7:26 AM on July 18, 2006

Your SLR filters are not circularly polarising in that they filter out light of a given helicity, but are rather linear polarisers that then change your linearly polarised light to circularly polarised light. This is because your autofocus uses polarising filters too, and if you passed it linearly polarised light it wouldn't work.

Light from the sky is slightly linearly polarised, so it does matter which way the glasses are polarised for that too.
posted by edd at 7:28 AM on July 18, 2006

It doesn't matter which direction your glasses are polarised in. Reflected light striking a surface at a certain angle (the Brewster angle) is polarised.

At this angle all polarisations (except diagonal in your case) will be blocked. This is why sunglasses (and lenses) are often sold as 'anti-glare'.

It doesn't matter what angle they are polarised at, providing that they are polarised.
posted by alby at 8:23 AM on July 18, 2006

Try looking at reflections off of the water. When light reflects off of water it is horizontally polarized. A vertically oriented polarizer, such as proper sunglasses, blocks out the reflection. Reflections off of windshields also tend to be horizontally polarized. Either your glasses are defective (they should be vertically polarized) or your monitor's polarization is on an angle.
posted by caddis at 8:26 AM on July 18, 2006

hey! i've wondered about this too.

in the end i decided that there must be some kind of de-facto standard for this, or else no one would be able to read their laptop screens or iPods with sunglasses on. so i think its on purpose that its 45 degrees.

i dont think the purpose of polarized glasses is to block glare from the ground; its just to cut out some percentage of the light through means other than darkening the lenses.
posted by joeblough at 8:28 AM on July 18, 2006

alby: erm. What?

Yes, the reflected light is polarised. No, that doesn't mean it's blocked by polarising filters at all other angles. The transmission is cos² of the difference in angle between the filter and the incident polarised light (IIRC).

It very much matters what angle they are polarised at. Put a pair on and this becomes apparent.
posted by edd at 8:30 AM on July 18, 2006

My guess is that "general use" polarized sunglasses use a 45 degree angle and that only those which are expressly designed for water-based use use vertical polorisation. I can't find any evidence to back this up/ Conceivably this could be a tactic by manufacturers to avoid being sued by car drivers who fail to see things properly.
posted by rongorongo at 9:07 AM on July 18, 2006

Reflections come off of the ground and off of buildings. In order to minimise the transmission of both of these kinds of refelctions, you want a 45 degree angle.

If you only wanted to prevent ground originating reflection, then you want both lens to be horizontally polarised.

I don't think that the direction matters as long as both lens are the same. If they are 90 degrees different, then things would look 'weird'.
posted by dantodd at 9:16 AM on July 18, 2006

General use sunglasses are polarized vertically, not at a 45 degree angle. That way they are most effective against glare from windshields and water.
posted by caddis at 10:29 AM on July 18, 2006

You have a pair of quantum sunglasses.
They will work perfectly provided you don't look through them.
posted by weapons-grade pandemonium at 11:56 AM on July 18, 2006

Sprocket87: A circular polarizer is two layers, one which is a regular polarizing filter and one wich basically mixes up the polarization of whatever light gets through (because AF and metering systems use partially silvered mirrors which are sensitive to polarized light).

If you aren't using autofocus or built in metering, you can use a linear polarizing filter which is just one layer, and is cheaper and a little more efficient (more light gets through).
posted by aubilenon at 12:00 PM on July 18, 2006

My sunglasses do the same thing with an LCD display, so it's not likely a manufacturing defect.
posted by cardboard at 12:45 PM on July 18, 2006

I just checked my polarized sunglasses. They also do the same. They also work properly with water and window reflections; they are properly vertically oriented to block horizontally polarized glare from water and windshields. I guess the LCD is polarized at an angle. It is a no name cheapo. Perhaps I should try it the rest of my LCDs. Here is how LCDs work. Are they really all at an angle?
posted by caddis at 3:14 PM on July 18, 2006

I want 'em. Peugeot trip computer is polarised, so I have to do the head-tilting thing to see the damn thing.
posted by obiwanwasabi at 5:25 PM on July 18, 2006

After much research, it seems I was incorrect about my sunglasses. They do seem to work normally on reflections, but not on my LCD, which therefore must be polarized at 45 degrees - rather than the glasses!

For bonus fun: Try looking at Nintendo DS Lite screens through polarized glasses, and rotating the DS. Pretty colours!
posted by Mwongozi at 5:17 AM on July 19, 2006