Thursday, November 14, 2013

Goodbye Battery Powered Sunglasses

It's been a while since my last post mainly due to my lack of time to work on personal projects due to school. I did manage do something noteworthy this weekend though, and now I have a passive 3D projector to show for it. I had the idea a while ago and realized that I had everything I needed to complete it already lying around from a high school 3D projector project. The projector I have now has 120hz DLP link 3D support. Out of the box it works with some $20 3D shutter glasses on I found on Amazon. If you don't know what shutter glasses are or how they work, I think it's easiest just to show you:

As you can see, the projector is actually switching between two images very quickly, the one for your left eye and the one for your right. The shutter glasses "close" and "open" in sync with the projector electronically so each eye only sees the image it's supposed to. This all happens so fast (120 times per second) that you can't see the switching with the naked eye. This is how most consumer 3D TV's and projectors work. These glasses in particular have a relatively small viewing window and they're somewhat bulky because of the contained battery, IR receiver and accompanying circuitry, which is why passive 3D is preferable. That's the kind in movie theaters where you use the disposable plastic glasses and the switching is done on the projector.

As it turns out, there are devices available to convert an active 3D projector into a passive one, but they all had features and price tags geared towards movie theaters that I wasn't interested in. The cheapest I could find started at $1500. But what exactly is this magical filter doing? It actually works just like the screen on your phone or computer and the $20 glasses work in the same way. The lenses are actually two big LCD "pixels". The liquid crystals just polarize the incoming light vertically or horizontally depending on the electric field applied. The glasses are blacked out when the liquid crystals in them polarize the light in one direction, and clear when the polarization is rotated 90 degrees. This means there is a polarizing filter on the front of the lens that polarizes the light initially so that the LCD "pixel" can block out the light, as our eyes can't perceive polarization on their own (OK, well almost can't http://xkcd.com/1080/).

















My idea was simply to peel off the first polarizing filter and put the glasses in front of the projector lens. This would leave the LCD portion to switch the polarization of the projector in sync with the frames it was showing. Disassembling the glasses showed that there was in fact a plastic layer on top of the glass lenses and with some time, a knife, and some methanol I was able to remove it. The biggest challenge was the adhesive left behind after peeling the filter off, which is where the methanol came in. I used Rain-X which has methanol in it to dissolve the adhesive because it's what I had, but rubbing alcohol may work as well.


After reassembly you can clearly see the one lens is a bit lighter with the polarization filter removed. It's worth noting at this point that there are a few caveats to passive polarized 3D. Because it relies on switching the polarization of the image the light coming out of the projector can't be polarized already or it would be blocked out. Most three LCD projectors have one color polarized oppositely from the rest which wouldn't bode well for this method of 3D. The DLP chip in my projector requires no polarization so it will work great. In order for the polarization to make it from the lens, to the screen, and then back to your glasses requires a special type of screen. Normal white fabric or paint destroys polarization by absorbing light and re-emitting it with random polarization. A reflective material is required, like sanded aluminum, to maintain the polarization yet still diffuse the light for viewing.
I tested the system with an old window shade painted with silver paint that I had from the high school project mentioned earlier. The screen is very wrinkled and uneven so it won't work too well for movies, but it did allow me to test my hacked together filter and it worked far better than I expected. There was almost zero cross-talk, or bleeding between the left and right images, when using leftover movie theater passive glasses (linear IMAX ones, as opposed to the circularly polarized RealD ones) that I had laying around. Soon I hope to build a collapsible frame with silver fabric stretched across it to make this a usable system for 3D movies and games. I'll post more about the whole setup when that happens. Back to school work for now.

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