How to build 2D glasses

It’s the week-end, which is the perfect time for a slightly off-topic post. It’s still engineering of sorts though in that it provides what I think is an original and cheap solution to a real problem.

3D movies are all the rage recently. But they are not comfortable for everyone. A friend of mine recently went with her family to see Avatar in 3D and instead of enjoying this rather good movie experience, she had to leave the theater after 20 minutes, suffering from a terrible headache. Of course, removing the glasses is not a solution because you then see both of the images –the ones destined for your left and right eyes– at the same time, blurring most of the screen.

Before I explain my solution to this problem, let me explain how modern 3D movies work.

Most 3D viewing technologies rely on providing a different image to the right and left eye. They are not reproducing an actual 3D structure like holography does. Rather, they feed later into the series of events that end up with perceived volumes. It works fine for movies, but you will never be able to walk into or around a scene with any of these techniques. Holodeck they are not.

The problem is to project both images on the same screen but still allow goggles to separate them. Several approaches exist, the most simple being the infamous red and blue glasses that use the plasticity of the brain when perceiving colors. Another interesting approach is to re-use retinal persistence not just to make what is a series of static images look like something that moves, but to multiplex two versions of the same movie into one. In other words, slice time, alternate left and right images and synchronize that with shutters on each eye. This has the advantage that it can work without special screens or projectors, only special glasses and a feed into the sync signal of the screen or of its video source.

What is used in movie theaters is quite different though and relies on a subtle quality of light that our eyes are completely blind to. Our visual sensors (a.k.a. eyes) are fantastic devices but are quite limited in a number of ways: they only perceive three color ranges out of the infinitely fine light spectrum, they frequently go out of tune and require correction and surgical intervention, and they do not see polarization at all. That last quality opens a uniquely neat way of creating stereoscopic vision.

Polarization is a quality of all transverse waves (of which light waves are one example and sound waves are not). Transverse waves are the propagation of a displacement that is orthogonal to the direction of propagation. Because we live in a three-dimensional space, that leaves two directions for the wave to wiggle in addition to the propagation direction. A light ray aimed directly at you may vibrate horizontally or vertically, or in a combination of both, but you won’t see the difference because the eye only sees the amplitude and some frequency information, not that directionality. This means that you can have two completely independent signals at any given frequency (which for light means color) simultaneously propagating in the same direction. You can see where this is leading: you can have the left and right images coexist in the same beam. All you need is to separate those images with glasses that see only one direction to create the illusion of volume.

Early polarized movies were using the linear polarization that I just described in a technique that is older than you may realize. For example, when I was a kid, I saw Hitchcock’s Dial M for Murder in 3D using linearly polarized glasses.

Modern 3D movies use a variation of polarization called circular polarization where light is split not in horizontal and vertical components but in clockwise and counter-clockwise rotating components. This has the advantage of better maintaining the illusion when you rotate your head. It’s not perfect because the images are still shot with a horizontal offset but it definitely helps.

In both cases, the left and right images travel on the two polarized components of light and they are split by the glasses before they reach the eye. As you can see, it’s a good thing for all this to work that we only have two eyes…

So what do we do for my poor friend who can’t watch those movies for any prolonged period of time? Well, that’s fairly easy, we suppress the 3D effect by feeding her only one of the two images. We do that by building her a pair of 2D glasses that filter out and send the same image to both eyes.

To build that pair of glasses, I bought two pairs of circularly polarized glasses from e-Bay (they are quite easy to find and go for a dollar or two) and broke them open to extract a left filter out of the first pair and a right filter out of the second pair. I then exchanged these filters and glued the frames back together. The result is two pairs of glasses, one of which will see only the right image, the other seeing the left image. In effect, my friend can now enjoy the same movie as the rest of her family in the same theater, except that to her and to her only it just looks like a plain old 2D movie. It’s just as comfortable as seeing the movie in a regular movie theater except for the weight of the glasses. No movement of the head affects the experience.

A small note on polarized sunglasses and why they wouldn’t work here. First, wearing sunglasses in a movie theater would further obscure even David Lynch’s Inland Empire. More importantly, polarized sunglasses use linear polarization because they are designed to eliminate specular reflection from the sun off water or ice and to eliminate part of the sunlight scattered by the atmosphere, both of which are linearly polarized.

Understanding polarization:
http://en.wikipedia.org/wiki/Polarization_(waves)

The RealD Cinema technology:
http://en.wikipedia.org/wiki/RealD

Note: feel free to build your own 2D glasses for your own personal use, but please contact me for any bigger-scale use of the idea.