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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 by Brian Michael Weidemann (Member # 2243) on 11-08-2005, 02:54 AM:
 
That was very, very helpful!

The rope-on-a-wall example explains a lot. I'm having difficulty, however, trying to associate that information with what I know about quantum electrodynamics. A wave and something that behaves like a wave aren't quite the same.

For instance, sound can be represented by a linear wave, but I can't take that understanding and force it into a circular wave model, like the rope-to-a-wall demonstration, in a way that makes sense to me. What would that sound SOUND like? How can compressions/rarefactions be circularized. They probably can't, and I'm probably making a bad choice in attempting to understand light that way, since it's probably wrong to do so, plus it doesn't help.

The quarter-wave plate makes sense. And that definitely clears up the phenomenon where flipping the glasses around changes the behavior entirely.
 

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Posted by Mark J. Marshall (Member # 1409) on 11-08-2005, 07:32 AM:
 
What I'm fuzzy on is why does the quarter wave plate only seem to work in one direction? In other words, when light travels through the plate first and then the polarizer, the light is simply linearly polarized - vertically, if I understand my other linear polarizing glasses correctly. Maybe you can explain why the quarter wave plate doesn't slow the light down traveling in the other direction - or maybe it does, but for some reason, it doesn't result in the light being circularly polarized.
 

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Posted by Brian Michael Weidemann (Member # 2243) on 11-08-2005, 08:07 AM:
 
Technically, this might be misleading or inaccurate, so don't take it too literally, but my understanding is something like this: The linear polarization gives light its direction, the quarter-wave plate gives it its spin. If you spin something and THEN force it into a direction, it defeats the purpose of spinning it. So you spin it AFTER you've lined it up.

... Um, I may have just confused myself a bit.
 

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Posted by Phil Hill (Member # 371) on 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!)
 

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Posted by Stephen Furley (Member # 1277) on 11-08-2005, 05:34 PM:
 
Phil,

So what's the advantage of this, in this application, over a conventional polarised system?
 

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Posted by Phil Hill (Member # 371) on 11-08-2005, 05:48 PM:
 
Ya get to tilt your head and maintain the 3D effect.
 

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Posted by Hugh McCullough (Member # 1542) on 11-08-2005, 06:36 PM:
 
I dont know about the use of circular polarisers for film projection, having only used the linear type for twin projector 3D systems, but when using a polariser on a camera the following applies.

A linear polariser completely polarises the light.

This causes a problem with cameras that use a beam splitter either for auto focus, or auto exposure from the film plane. A beam splitter itself polarises the light, and it is possible for the polarised light from the filter to be cancelled out by the polarised light from the beam splitter. If this happens then no light reaches the AF sensor, or exposure cell.

Circular polarisers twist the plane of the polarised light,leaving the filter, so that when it hits the beam splitter it is not completely polarised and gets through to the AF.
 

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Posted by Phil Hill (Member # 371) on 11-08-2005, 06:46 PM:
 
Hmmm... never heard of that and have no experience with what you are talking about.

All my dealings with 3D production, including 16mm, 35mm, 65mm, and video has been with two separate cameras using a mirror or prism to separate the Left and Right image to each camera for recording.

The polarization comes into play during projection.
 

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Posted by Brian Michael Weidemann (Member # 2243) on 11-08-2005, 08:03 PM:
 
Yeah, I don't see how polarization would be an adequate system in the filming process. Definitely not ideal, anyway. Ideally, you would want each eye's image to be recorded with as much information as possible. Polarizing during filming would create a skewed color space, and a less dynamic image on film (or digital media, or whatever). Polarization just works well in getting each image to the correct eye.
 

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Posted by Mark J. Marshall (Member # 1409) on 11-08-2005, 10:34 PM:
 
Cross-eye ghosting seemed almost non-existant in Chicken Little, too. I'm not sure if the circular polarization does a better job of controlling that or not.
 

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Posted by Stephen Furley (Member # 1277) on 11-09-2005, 02:04 AM:
 
Polarising filters are used on cameras to darken skys, and reduce reflections from non-metalic surfaces, glass, water etc. Plane polarisers were traditionally used, but autofocus cameras need the circular type. This was the only use I had previously seen for circular polarisers. Since all of my cameras are manual focus, I've never needed one.
 

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Posted by Dick Vaughan (Member # 540) on 11-09-2005, 09:23 AM:
 
Okay now for the crunch,

is there any increase in cost using circular polarisation over linear polarisers?

If it's cost neutral why are we still using linear polarisers for IMAX 3D?
 

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Posted by Paul Mayer (Member # 355) on 11-09-2005, 10:10 AM:
 
Can't speak to the cost question, other than the circular polarizers I've used in photography were much more expensive than linear polarizers.

Circular polarizers were nothing new to us old Canon fans. Canon's 35mm still cameras all used a half-silvered prism to divide the light between the viewfinder and the metering optics. If one used a linear polarizer in the standard way on a Canon camera, there was a chance of significant exposure error due to the way the meter responded to polarized light.

The solution was to either use a circular polarizer, or use a two-step setting technique with a linear polarizer. The techinique was to rotate the polarizer through a complete revolution while observing the meter response. Find the polarizer angle that resulted in mid-scale deflection or swing, and then set the exposure to achieve a split needle with the polarizer in that position. Once the exposure was set, then the polarizer could be rotated to whatever angle was needed to achieve the desired photographic or artistic effect. This two-step process made using Canon cameras with polarizers a bit clumsy unless one sprung for the more expensive circular kind.

[ 11-09-2005, 01:25 PM: Message edited by: Paul Mayer ]
 

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Posted by Mark J. Marshall (Member # 1409) on 11-09-2005, 10:36 PM:
 
Soooo, I've been wondering, why is there so almost no ghosting in Chicken Little? IS it that the circular polarizers are that much better than linear polarizers? Or could it be that the two eyes aren't on the screen at the same time? Thoughts? Has anyone tried running double interlock film based 3D with circular polarizers to see if that solves the ghosting problem?
 

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Posted by Stephen Furley (Member # 1277) on 11-10-2005, 12:41 AM:
 

quote: Mark J. Marshall

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Or could it be that the two eyes aren't on the screen at the same time?

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