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I was under the assumption that a circular polarizing filter would filter circularly polarized light. I now understand that what is commonly called a circular polarizing filter only filters based on linear polarization - and then makes the linearly polarized light circularly polarized.

Surely it must also be possible to build a filter that removes light polarized with a certain spin direction. Does such a thing exist for photographic use (or other use) and what would be the effect of using one?

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This might get better answers on the Physics site.... –  mattdm Feb 26 '12 at 14:26
    
Very interesting question. What type of application did you have in mind? –  dpollitt Feb 26 '12 at 17:30
    
@mattdm, I'm interested in potential photographic applications though, not electromagnetic theory for its own sake. –  SoftMemes Feb 26 '12 at 18:34
    
@dpollitt, I'm not sure, but normal polarization filters can be used for so much more than "removing reflections from sunlight", for example removing (or boosting) direct reflections from a specific polarized light source such as a flash. Maybe there's something that can be accomplished with non linear polarized light that cannot be done with "normal" polarized light? –  SoftMemes Feb 26 '12 at 18:37
    
I think the physics people do applied physics as well. I'm not suggesting that this is off-topic, just that here you'll have to wait for a physicist who is into photography to come along. –  mattdm Feb 26 '12 at 18:48

1 Answer 1

up vote 2 down vote accepted

As the photographic circular polarization filter is a polarization filter combined with a λ/4-plate, which converts the linearly polarized light to circularly polarized light, you can simply turn the filter around and you'll get a filter that filters for one direction of circularly polarized light and makes the result linearly polarized.

However, as the light that is reflected from an appropriate surface (like glass or water) is linearly polarized, there wouldn't be much use for such a filter, except maybe to take pictures of a 3D movie in a cinema (which uses circular polarization), of which the cinema guys probably wouldn't approve.

In addition, you'd have to put another filter (in the "normal" direction) behind it to convert it back to cirularly polarized light because the linearly polarized light confuses the autofocus sensors of some cameras.

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Is it really at simple as turning a circular polarizing filter around would make it filter based on circular polarization? Would this not, rather than filtering based on rotational direction arbitrarily filter based on the phase the photo happened to be in when it hit the first layer? –  SoftMemes Feb 26 '12 at 18:34
    
Yes, it's that simple, see upload.wikimedia.org/wikipedia/commons/thumb/0/09/… And in general, don't try to look at polarization as a movement that can be frozen by having a very short shutter speed, it just doesn't work that way. :) –  AndreKR Feb 26 '12 at 18:42
    
That picture shows light of specific polarization being converted from circularly polarized light and back, it does however say nothing about what the quarter wave plat would do to light of other polarizations, can you please elaborate? –  SoftMemes Feb 26 '12 at 18:54
    
As the plate works both ways, any linear polarized light will be converted to circular polarized light, which is not affected by the linear polarization filter in any angle-dependent way. –  AndreKR Feb 26 '12 at 19:10
    
but light isn't necessarily one or the other. Linearly polarized is just one extreme example of polarization. –  SoftMemes Feb 26 '12 at 20:21

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