Re: Rotating polarizing filters

Charles wrote:
On Fri, 15 Jun 2007 14:58:25 -0400, Phil Hobbs
<pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Charles wrote:
On Tue, 12 Jun 2007 15:02:48 +0200, "toto" <toto@xxxxxxx> wrote:

Hello,
Sorry for my bad english.
I'm looking for information about rotating polarizing filters, same as the ones used in photography.
I wish to use such filters but I've found different filters giving different results :
the most commons have a very little polarizing effect;
others are very efficient : in a position the filter is transparent and after a rotation, it become opaque.
Does it exist different gradation for this filters ?
Thank you for your help (and your comprehension for my "english")

Polarizing filters used for photography come in two different kinds,
one labeled linear, one labeled circular.

The linear ones behave as you expect.

The ones labeled circular have the effect of a linear polarizer on the
side away from the camera, the polarization is converted to circular
on the side towards the camera.

If you hold the circular pol. away from your eye, with the camera side
toward your eye, hold a linear pol. between the circular pol. and your
eye, then rotate either pol filter, you should see no change

If you turn the circular pol. around so that the camera side is away
from your eye, then when you rotate either filter you should see
extinction.

Lots of things to try.
The difference is due to the 'circular polarizer' not actually passing circularly polarized light. Circular polarizers work by using a quarter-wave plate to convert incoming converting circularly polarized light to linear polarization. Left and right circular turn into two orthogonal linear polarizations, and one of these is selected by a bit of linear polarizer film. Properly speaking, there ought to be another quarter-wave plate after the linear polarizer, to turn the light back into circular polarization. However, since this makes very little difference to images, nobody ever does this, so circular polarizers that you buy actually turn circular to linear and pass the linear. That's why they behave so differently when you turn them back to front.

The reason circular polarizers are useful is that when circularly polarized light bounces off a surface, the helicity is inverted--left-circular becomes right-circular and right-circular becomes left, just like screw threads in the mirror. (On reflection, the direction of rotation of the fields stays the same, but the propagation direction is reversed.) Diffuse scatterers, such as skin, fabric, plants, rocks, and so forth, generally scramble the polarization.

If you put a circular polarizer on your flash, and another one on your camera, the combination will greatly reduce the effects of glints, while letting through more of the diffusely scattered light.

Cheers,

Phil Hobbs


I think it's the other way around in the camera usage. The light
outside Has a linear component that we wish to discriminate,
reflections off glass, light from the sky.

A linear polarizer works fine for that, but ran into trouble when the
insides of cameras got tricky with partially silvered mirrors and
things used for exposure and focus settings. By converting the light
behind the polarizing filter to circular it reduced the interaction of
the filter with the internal workings of the camera.

Interesting...I may be mildly out of date here--im my day, SLRs had real mirrors ;)

Cheers,

Phil Hobbs
.

Relevant Pages

  • Re: "Sports" Polarizer
    ... You can get two basic types: one just is a simple linear polarizer, and the other first does the linear polarization for the effect that you want and then converts that linearally-polarized light into circularly-polarized light. ... The circular filter is used for cameras with internatl reflections that could potentially interact with linerally polarized light and perhaps throw off light metering. ...
    (rec.photo.digital)
  • Re: Which Polarizer?
    ... However I'm trying to decide on a circular polarizer filter ... A linear polarizer is better. ... Note also that the effect of the filter will depend on the relative position ...
    (rec.photo.digital.slr-systems)
  • Re: Rotating polarizing filters
    ... I'm looking for information about rotating polarizing filters, same as the ones used in photography. ... The difference is due to the 'circular polarizer' not actually passing circularly polarized light. ... Left and right circular turn into two orthogonal linear polarizations, and one of these is selected by a bit of linear polarizer film. ... However, since this makes very little difference to images, nobody ever does this, so circular polarizers that you buy actually turn circular to linear and pass the linear. ...
    (sci.optics)
  • Re: Mass Transit
    ... except to the extent that the filter is imperfect. ... I then attached my ancient linear polarizer to the front ... the reflection, which makes it parallel to the windshield. ...
    (rec.arts.sf.fandom)
  • Re: Mass Transit
    ... except to the extent that the filter is imperfect. ... I then attached my ancient linear polarizer to the front ... is polarized in the same plane as before ...
    (rec.arts.sf.fandom)