Last Modified: March 20, 2000.
If you don't care about the details, here are the important points:
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 completely (or 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 from 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 slightly 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 traveling down a rope where you hold one end and the other end is tied to a wall. If you shake your hand up and down, the waves will travel in a plane perpendicular to the ground. If you shake the rope side to side, the waves will travel in a plane parallel to the ground. In general, if you move your hand back and forth along any line, the waves will be in a plane, and are equivalent to linearly polarized light (polarized in different directions).
But if you 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 should 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 light 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.
Some manufacturers (B+W and Heliopan, for example) sell a so-called Kaesemann polarizer which is even more expensive. A Kaesmann type has the foil stretched and held under constant tension in all directions. To do this it is necessary to totally edge seal the filter in glass rather than just bind the glasses and foil with an adhesive. This type of polarizer is available in linear, circular and in warmtone types. Its advantages are that the polarizing effect is slightly greater, the filter is "tropicalized" so it is immune to moisture, fungus, etc and it is very, very flat, so it will not adversely effect the sharpness of longer lenses. For this reason Heliopan only supplies Kaesmann type polarizers in sizes from 82mm up.
I suppose a little fungus between the polarizer and quarter-wave plate might make for a couple of artistic photos, but you'll probably be upset if that's the filter you happen to have on when you get the once in a lifetime shot of the harpy eagle fighting with the jaguar.