Hello everyone.

I'm wondering has anyone heard of a rig using one projector? I saw one time an article of an 80's style projection system that utilizes one projector and mirrors. I actually want to replicate this for today's projectors. Any help is appreciated.

I think the RealD movie theaters use only one projector for passive 3d. Not sure how to DIY that rig, but its technically possible.

What I've seen is it's a series of mirrors, much like two small periscopes, angled to meet both sides over each other on the screen. The digital picture carries both sets of the movie (in an over/under fashion). The hard part is positioning the mirrors so they line up. I have an idea there, making a lineup dot/line format. Reason to do this: Play full 3D from a projector via DVD, watchable by standard polarized lenses. I have seen this setup from the 80's, so it has been around.

Do feel the latest 3D movie wasn't that great.

I am a bit surprised that this is not the typical method, and passive glasses are not the norm. The reason is that a projector system tends to be viewed by many people, so the cost of the glasses is an issue. But, I must be missing some technical details that prevent this from being as cost effective as using the the active glasses.

A DLP projector should be able to incorporate a rotating polarizer wheel. It could use two circularly polarized filters, with a clockwise filter on one half and the counter-clockwise filter on the other half. Then, as this set of filters rotate in front of the light path, the image alternates between the two stereo images.

Granted, it would cut light output, and so would be less than desirable for when the display was being used for non-3d work.

Joe Dunfee

nice little read about the real d cinema tech on 3d . http://en.wikipedia.org/wiki/Real_D_Cinema

I thought about this too but since circular polarization is less desirable than linear polarization for passive projection the rotation would affect the polarization. Also, the projection lens would screw with the light polarization as well so it would have to sit outside the projector. (which could be a simple solution for those who want to try it)

I'm working on a rotating cylinder of two different linear polarizations that rotates matched with the vsync / 2 (two frames per rotation), and it sits just outside the projection lens with a 45 degree front surface mirror to minimize the surface area required by the rotating cylindrical polarizer.

Jack

Why is circular polarization less desirable? I know it is used in some of the movie theaters.

Joe Dunfee

I have practical experience of linear vs circular regarding passive projection. The advantage with circular is that you should be able to tilt your head. This can only be done to a small amount more than with linear. At that time linear and circular ghosts more or less equally.
Advantages with linear is way better ghosting rejection, maybe even half the amont of ghosting you have with circular.
Now: Why not going linear since they're also generarily cheaper?
To maintain a proper impression with a stereoscopic view you still need to have your head horisontal. If you turn your head the image get skewed. The point of circular filters in one single projector is because the color/polarization wheel inside the projector rotates. It's a great idea though.

Edit: Later tests when trying standard high quality circular polarizers as well as homemade circular ones proved me that even if i rotate the filters all the way round the image is still separated, however with some small coloranomalies. I wonder what kind of polarizers the cameras were since they didn't have linear properties either.
Still: Linear ghosts half as much as circular. To maintain good stereopsis the brain tries to keep the head in a perfect horisontal alignment with the screen. So there's no practical use for circular polarization in a dual pj-rig anyway.

Regarding ghosting with Circular polarizers; I am sure this is an issue with linear polarizers as well... it is perhaps an issue regarding the quality of the polarizer.

Since I have no experience to compare the various sources of polarizers, I can't comment on availability or cost of good quality filters. Are circular polarizers available that don't have a noticeable ghosting effect? Is it just a matter of cost?

Joe Dunfee

I went to see "my bloody valentine 3D" at a RealD theater last week. They use circular filters and I did not notice any ghosting in the movie. I'd go with the cheaper filters.

At the testing stage for the passive projection rig i tried circular/linear polarized filters for cameras at first (i know a friendly cameraboutique owner who helped me get a hold on these very cheap. For free actually. ). The polarizers were of high quality though. The only problem i encountered was that i after some time fried them.

I burned some cheap theme park style linear polarized glasses when I tried to use them as a temporary filter. They were this kind of glasses.

Besides the clip-ons i have i also have these kind of glasses (enst from silverfabric). Quality of the filters in those are very good actually. I assume it's the same ones as silverfabric because they look exactly the same. However iz3d ones are more popular because of their more stylish approach.

I know this is a late post to the subject but for any future readers.

I've tried using one projector and mirrors to make passive 3d with no luck. Now I have a dual projector set-up, but before I did, I tried for quite some time. The only way I think it will work is if you can "freehand adjust" the angle of each stream of video. Be it side by side, or top on bottom.
I was able to get both streams of video to the screen and they were polarized in opposite angles, but the angle that the video images itself showed up on the screen was waaaay far off. I adjusted the mirrors a hundred times over with no luck on changing the angles of the video stream back to straight up and down on the screen. So side by side I dont think would ever work.
The problem with top and bottom is spacing. The space between the video streams must be far enough away that you can put two "parascopes" in front of the projector without cutting off any of the picture. Also allowing both images to go through opposite "parascopes". This greatly reduces resolution, which was ALREADY chopped in half because of the use of one projector putting out two images. Now that there was space inbetween the videos involved, the quality was just terrible.
In the long run, it's not worth it. Too much hassle, I gave up, and got another projector.

Anyone else ever try?

The problem with circular polarizers is that the lens is layered different than linear. So projecting through a circular polarizer wont work, however circular polarized glasses are fine. The issue is how it handles the light. A circular polarizer is meant to work for a camera with autofocus. What a polarizer does is block wavelengths of light that are going in all directions but one. When you rotate a lens, you are changing which direction of wavelength gets through. Linear polarizers were designed for manual focus cameras because autofocus cameras would double polarize the light and they wouldn't let the light pass through. Circular filters add a scrambler layer to the filter that typically changes the axis of the wavelength of light roughly 90 degrees. This way the light would not be double polarized.

You can imagine that if you project an image through a circular polarizer, it may work just fine, if the extra coating is on the projector side of the filter, meaning it will polarize correctly. If you flip it the other way, it wont work at all, either way there is a loss of light. With a linear filter there is no scrambler. Now the reason you can have circular polarized filters for the eyes is because the the light will still be correctly polarized with each eye. There may be a slight loss of light from the scrambler layer, but unlike an autofocus camera, your eyes have no polarizer inside them so the scrambler layer will have no effect on your eyes other than a slight loss of light.

So linear over all is better, but circular is ok for glasses and always use linear for projecting.