Film-Tech Forum ARCHIVE: Real-D glasses

This is topic Real-D glasses in forum Film-Yak at Film-Tech Forum ARCHIVE.

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Posted by Thomas Pitt (Member # 4322) on 11-26-2007, 05:17 PM:

After the fiasco of watching half of Beowulf in 3D (see my other post about how the sound went to white noise), I got to keep the Real D glasses afterwards.

At home, I noticed something odd about them. Look through one of the lenses at any LCD monitor (such as a computer screen) and rotate the lens - the color temperature changes.

However, if you look through the lens the wrong way round, the LCD screen will black out and brighten as you rotate it!

Sometimes, if the LCD polarizer is not perfectly flat, you'll see interesting patterns as you rotate the glasses - these are interference patterns. An iPod screen and a digital wristwatch produce good patterns [Big Grin]

In the cinema though, unlike IMAX 3D glasses, I noticed that the 'double vision' effect didn't occur if I tilted my head to one side. Clearly these glasses work in a different way to IMAX 3D glasses.

Anyone know why this effect happens? Or indeed, why looking through the glasses the other way causes different effects? Logically, a polaroid piece of plastic should work the same no matter which way you look through it...

Posted by Jon P. Inghram (Member # 4125) on 11-26-2007, 06:36 PM:

They're circular polarizers, which use a regular linear polarizer in front and a quarter-wave plate behind it to convert the linearly polarized light to circular.

Besides 3D glasses, they're also used with SLR cameras (both digital and film) due to the mirrors used to get the light to the exposure and focus sensors.

Posted by Bobby Henderson (Member # 840) on 11-26-2007, 07:32 PM:

Circular polarizing filters are accessories for cameras, not something built into just any camera lens.

Many photographers use circular polarizer filters to increase brilliance and contrast of certain colors and cut down on certain kinds of glare and reflections. They're great to use if you want to take a picture of a fish swimming below the water surface. Depending on the angle you turn the filter, it can work like a gradient filter. A good circular polarizer filter for a good D-SLR lens will often cost $100 or more (B+W's 77mm circular polarizer costs $140 at B&H Photo).

Linear polarizer filters also exist, but you're stuck using manual focus with the things since they do funky things with auto focus functions.

Good camera filters are made of glass and held in metal frames. The Real D eyeglass lenses are merely thin plastic held in a thicker plastic frame. They're a decent step up from folding cardboard 3D glasses.

Posted by Mark J. Marshall (Member # 1409) on 11-26-2007, 11:35 PM:

Looking through the glasses backwards means the light is traveling through the quarter wave plate, then through the linear polarizer. So in effect, you're simply looking at linearly polarized light instead of circularly polarized light. Also, since the linear polarizers in both lenses are polarized at the same angle, the light going through the glasses when you look through them backwards is also polarized the same way. To verify this, wear linear polarizers while looking backwards through circular polarizers. Close one eye, and rotate the circular polarized glasses. Eventually both eyes will go black.

Another interesting trick is to put the circular glasses on and look in a mirror. With linearly polarized glasses, if you close one eye, you can see the open eye in the mirror, and not the closed eye. This is because the linearly polarized light waves keep their polarity when bouncing off the mirror. Wear the circular polarized glasses, look in a mirror and close one eye, and you can see the closed eye and not the open one. This is because the circularly polarized light is spinning in the opposite direction on its way back from the mirror.

Tilting your head doesn't impact the 3D effect much when looking through the circular polarizers because light spinning clockwise is always spinning clockwise no matter how far your head is tilted. However if you tilt it too far, the 3D effect will fall apart completely because even though you can still see the two images in the proper eyes, your eyes are no longer on a relatively parallel plane with the images. So your brain loses the effect. Sort of like trying to look at one of those "Magic Eye" things while tilting your head. Tilt it too far, and you can't see it anymore.