Re: Rotating polarizing filters
- From: Charles <ckraft@xxxxxxxxxxxxxxxxx>
- Date: Fri, 15 Jun 2007 22:40:23 GMT
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.
.
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