# Circular polarized glasses, how it works

1. Jun 29, 2009

### DeepSeeded

I learned how circular polarization works way back in electricity and magnetism, I also learned how gratings worked.

I recently watched a movie through circular polarized glasses which I had never heard of, nor can I find any information online including RealD's website (the company that uses them).

It is the symmetry that bothers me, even if you could figure out how to block out polarized light changing in one direction it would work against you at the other end of the circle.

Can someone explain or link a white paper?

2. Jun 29, 2009

### mgb_phys

Normally circular polarisers (eg filters for a camera) are simply a linear polariser followed by a quarter wave plate to generate both polarisations. You need to do this because otherwise the polarised light wouldn't be transmitted through other polarisation sensitive components in the camera (especialy the autofocus and metering system).
I can't see why you would need to use them with your eyes.

3. Jun 29, 2009

### DeepSeeded

Ok ill explain. It is a stereoscopic movie. One eye receives one image, the other receives another. The glasses need to make sure this happens. So the light is projected off a semi-conductive screen in order to preserve polarization. One image in the movies is projected in clockwise circular polarized light, the other anti-clockwise.

One lens in the glasses allows only clockwise circular polarized light through while the other does the opposite.

I understand how everything works except the glasses. I can't understand how you would allow only clockwise polarized light through a lens.

4. Jun 29, 2009

### mgb_phys

Yes that's how it's normally done with linear polarizers.
But since a circular polarizer is simply a linear polarizer with a quarter wave plate behind it I can't see the advantage for a visual system.

5. Jun 29, 2009

### DeepSeeded

A circular polarizer is changing its angle over time, it is projecting in all polarization angles.

A regular polarizer is just a conductive grating that only allows light polarized in one direction including the components of light in other directions non-orthogonal.

This is why I can't figure out how those glasses work.

The advantage is that the movie apears 3 dimensional. It is called Real D technology.

6. Jun 29, 2009

### Andy Resnick

Let's back up and first ask: why does the technology not simply use horizontal and vertical linear polarization states? The answer is that the transmission of the glasses (and the quality of the steroescopic images) will then depend on the orientation of the glasses- tilt your head, the transmission changes. Using circular polarization states will prevent that.

It's been pointed out how to *generate* circular polarization states: a polarizer + retarder. That's not a filter. And the Real D website seems to indicate the glasses work by stroboscopic alternation between left and right- the glasses are powered, and may switch between eyes at 120 Hz. The projection devices claim not to need "special' display surfaces, and operate at 120 Hz.

My suspicion is that the system does not use polarization at all (except as how the LCD elements function), but two sets of images (see the special 3D camera system) which are alternately passed to the left and right eye. One way to check is to place 2 glasses back-to-back or face-to-face, and see what happens- if they are stroboscopic, there should be no image at all.

7. Jun 29, 2009

### DeepSeeded

"The projector alternately projects the right-eye frame and left-eye frame, and circularly polarizes these frames, clockwise for the right eye and counterclockwise for the left eye."

In order for the left eye to always see only counterclockwise images projected, it must block out clockwise images at all times and vice versa.

also: " A special cinema screen is used to minimize depolarization "

8. Jun 29, 2009

### clem

A quarter wave plate will convert the circular polarization to linear polarization.
Following this with plain plane polaroid bonded to the quarter wave plate will do the job.

9. Jun 29, 2009

### DeepSeeded

No, it would start blocking light after 180 degrees since it reverses direction. And I might add it would let the wrong image through after 180 degrees. The components of light is coming in every direction, its only the direction that it is changing in that matters.

10. Jun 30, 2009

### clem

I'm not sure what you mean about 180 degrees. The quarter wave plate and polaroid are on the glasses you wear. The light only lpassess through once.