posted 11-07-2005 11:00 PM                   

quote:

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A circular polarizer is just a linear polarizer followed by a quarter-wave plate set at 45 degrees to the axis of polarization.
A quarter-wave plate is made of a material in which light polarized in one particular direction travels more slowly than light polarized in the perpendicular direction. A quarter-wave plate is just thick enough that after passing through it, light polarized in one direction is delayed 90 degrees (or one-quarter wavelength) relative to light polarized in the other direction.
Since the quarter-wave plate is set at 45 degrees to the polarization, you can think of the incoming light as having two equal components in the principal directions of the quarter-wave plate. After passing through the plate, one component is delayed 90 degrees, and the resulting light is circularly polarized.
The idea is to use a linear polarizer up front to get rid of some linearly polarized light you don't want (glare off shiny surfaces, for example, will have a large linearly polarized component), and then it "stirs up" the result so you don't have linearly polarized light bouncing around in the camera.
A problem with linearly polarized light in your camera, for example, is that when you bounce it off a mirror at (near) Brewster's angle, it may be (nearly) completely eliminated. If the light meter measures the light after it bounces off a mirror, the amount of light arriving at the meter may be drastically different than the amount of light that will arrive at the film with no bounce, since the mirror has flipped out of the way.
Of course, a quarter-wave plate is only exactly a quarter wave for one frequency of light. That frequency is usually chosen to be a yellow in about the middle of the visible spectrum so that on the average, the light will be circularly polarized with various degrees of elliptical polarization mixed in. I suppose if you were photographing something that was primarily red, or primarily violet, your metering might be slightly off, even using a circular polarizer.
And of course, since there's another chunk of material in the way (the quarter-wave plate), there will be slighly more degradation of the image with a circular than with a linear polarizer.
Another nice way to think of circular polarization is to imagine a wave travelling down a rope where you hold one end and the other end is tied to a wall. If you shake your end back and forth along a line, the waves will all lie in a plane. You can shake your end in any direction perpendicular to the rope, and the only change will be in the direction of the polarization. Now start moving your end around in a circle, and circular waves will move down the rope. This corresponds to circular polarization. If you move your hand in an ellipse with various eccentricities, you'll get the equivalent of elliptical polarization (with various eccentricities).
If you're wondering whether your polarizer is circular or not, look through your polarizer at a mirror and look at how dark the polarizer is that the guy in the mirror is holding. Reverse the polarizer in your hand so the other side of the glass is pointing toward the mirror. With a circular polarizer, one direction will be significantly darker than the other. With a linear polarizer, both sould be the same. The reason is that linearly polarized light will still be linearly polarized in the same direction after bouncing off the mirror. Clockwise circularly polarized will be counter-clockwise after bouncing off a mirror, and will be cancelled when it comes back.
So if you hold a circular polarizer as if your eye is the camera (with the side that's normally screwed into the camera nearest your eye), it'll appear light in the mirror. If you flip it over it should appear almost black.

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posted 11-08-2005 05:14 PM                 

quote: Brian Michael Weidemann

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So you spin it AFTER you've lined it up.

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You got it Brian. AND for viewing, if you use a filter with the opposite "spin" (i.e. flipping the quarter-wave plate over), it blocks the light.

Thus by using "opposite" plates for each eye, you can assure that the intended light (image) gets to the intended eye.

(NOW, ***I've*** confused myself!)