Free-D (almost)

The title is a little misleading as I already had most of the parts available, if you have to buy from scratch it may not be cheap.

This is simply a proof of principle, it’s not at all tidy and is held together by duct tape and this isn’t intended as a step by step how-to, however it does work and provides essential information to build on.

I spent some time searching round the net to see if anyone had tried to convert 2 CRT projectors to passive polarised 3D without success, I know they’re available but will no doubt be a silly price to buy as they’re intended for corporate demos, simulators etc.


Good question – it seems 3D is everywhere since that ‘blue aliens’ film was released – not the holographic 360 degree 3D we all expected of the 21st century but yet another reboot of 1950s technology. I wanted to see what it looked like without paying ££EE$$ss (yes my keyboard is too old to have a euro) to be disappointed.

How it works:

A 3D movie consists of 2 video streams, usually played side by side, each intended for one eye. Most home 3D systems play these as alternate frames and use active shutter glasses to steer each frame to the correct eye, this method needs a high refresh rate to prevent flicker and higher projector brightness as not only is twice as much information being displayed but lag in the glasses means each eye may only see ‘clearly’ around 30% of the time.

The idea was to build a ‘passive’ system using one projector for each eye each displaying one of the video streams, polarising filters on the projector lenses and passive glasses. This can be run at 50/60hz with no more flicker than a 2D display and around double the efficiency of shutter glass systems plus extra glasses can be made easily saving £50-60+ for each extra viewer and no worries if the kids sit/stand on/try to eat them.

You will need:

2 CRT Projectors each capable of displaying an image from a PC at your desired resolution. I used 1280×720@60hz. It should work just as well with 2 DLP projectors if they can be configured to overlap.

A fast PC capable of driving 2 monitors @HD resolution. Test PC was an Athlox X2 4200, 1GB ram with a geforce 7600.

3D playback software – I used Stereoscopic player.

2-3 Scrap 15-17″ LCD monitors preferably with undamaged screens.

1 Litre tin of Crown metallic emulsion.

1+ toy glasses frames

The tricky bit:

Get this out of the way first, if you can’t do it the rest is pointless and once you have the rest is easy :). You’re going to need a couple of square feet of polarising filter for those lenses and to make glasses. If you know of a cheap supplier to buy it from let me know and we can all skip this part, until then though…

You’re going to need to strip the filters from 2 LCD panels, not the front anti-glare plastic which will be scratched and isn’t capable of transmitting an undistorted image but the other side. The filter in front of the LCD backlight id usually a gloss clear finish and just what we need. Getting it off cleanly in one piece is the pain. Assuming you’ve disassembled the panel try peeling it off – (usual danger warnings and disclaimers about thin fragile glass and large amounts of force being applied to same – be careful) if you’re very lucky it might just delaminate and peel off as a nice clean sheet of plastic. If not, have fun. Sometimes heating the panel helps as does trying to tear it loose quickly like removing a plaster. The trick is to remove the pastic and leave the adhesive on the glass, if it stays on the plastic it’s nearly impossible to remove cleanly.

The screen:

Once you have the filter sorted out we can deal with the screen. For this to work we need horizontally polarised light from one projector to arrive at the eye still horizontal and vertically polarised light from the second to arrive still vertical. A traditional white screen just scatters light and will destroy the polarisation we’ve set. We need a reflective surface that doesn’t scatter – problem is commercial silver screens aren’t exactly cheap. That’s where the tin of emulsion comes in – I have to admit I didn’t expect metallic emulsion to work, I’d tested a small area using silver spray paint which worked but was messy and no fun to use indoors. I used Crown metallics ‘sophistication’ – silver to the rest of us – £19 from B&Q. For quickness I repainted my white wall screen which has the side effect of showing every crack and mark which the white paind hid to some extent. The next step will be to make a fabric screen on a timber frame.

After 2 coats, some white still shows through but it’s silver enough for a test run and not worth wasting more paint on.

Projector setup:

Once you have your polariser plastic make a template for the projector lenses then mark and cut out 2 sets of 3 filters. Make a template for your glasses frame and cut out one ‘lens’ from along the bottom of the sheet and another from the side, making sure they’re cut from the filter at 90 degrees to each other. Glue them onto the glasses frame.  The projector filters are round so can be cut from anywhere in the sheet then rotated to match later.

As a temporary test I just sat my old Barco 708 on top of the 1209 and converged the 708 as closely as possible to the image from the 1209. As neither of them have the ‘converge on green’ option it was impossible to get perfect alignment across the whole screen, mainly the line linearity was out. It has surprisingly little effect though, as long as it’s close and the vertical alignment is good – misalignment here will confuse the brain and cause headaches.

After aligning the 2D images disconnect or swich off all tubes except red on the first projector. Put on your 3D glasses and get ready to tape a filter to the front of the red CRT lens. Rotate it in front of the lens while looking at the screen with your right eye closed and your head straight until the displayed image is as dark as possible. Tape it in place and repeat for the green and blue. Do the same for the second projector but close your left eye.

If done correctly the image from the first projector will only be visible to your right eye and the image from the second projector will only be visible to your left. Now it’s time to feed it from a 3D source.


The following instructions assume a Windoze 32 box, I use Mediaportal on the cinema PC so can’t use Linux there. Make sure you have the latest nvidia drivers installed, connect both projectors to the PC and configure the dualview monitor to span the dektop across both displays, creating a 2560×720 display area (or whatever resolution your projectors can handle), you’ll probably need to use winvnc from another PC to set it up as the displays will be unreadable at times. You’ll now have 2 overlapping displays with one half of the desktop on top of the other – it can be a bit confusing trying to control the PC using them.

Run Stereoscopic player and configure it to use dual monitor output – this will send one half of the movie to one monitor and the other half to the other.

Load your 3D SBS movie and select ‘SBS right image first’ default ratio – you may need to alter these settings later but it’s a start. To play in 3D you’ll need to switch to fullscreen mode. That should be it – if the image looks ‘almost’ 3D try swapping left/right images (f7).


This system uses linear polarisation which means if you tilt your head more than a few degrees the 3D effect is lost – commercial systems use circular polarisation (if you know where to buy large sheets of circular polariser let me know). In practice this isn’t a serious problem.

For the same reason the cheap cinema/pub glasses on ebay won’t work with this – they have circular polarisers.

The silver screen tends to produce an uneven brightness compared to a white screen. It’s not really noticeable when watching a 3D film but I’d prefer to swap to a white screen for 2D.

Stereoscopic viewer doesn’t integrate with mediaportal – playing a 3D film isn’t as point-and-click as I’d like and for full brightness playing a 2D film the polarisers need to be removed from the main projector. A seperate projector/PC setup for 2D and 3D will probably be the easiest solution.

Other than those it works far better than I expected – most of the tesing was done using the Owls of ga’hoole and the end credits of Despicable me where the Minions drop the ladder out of the screen shows the effect brilliantly.

This Post Has One Comment

  1. Dear Tony
    This is great – I was thinking along these lines myself. I have two Barco 708s. Do you know if there is a ‘splitter’ that can separate the hdmi signals from a blu-ray 3d player instead of a PC?
    Polarising film should be commercially available. You could try Paradigm – as they normally provide this sort of thing commercially. No idea if its cheap or not. Anyway back to the Barcos – I don’t want to go away from CRT as I love the colour. Incidently I improve on the colour by adding light green and light red filter gels on the guns, because the red phosphor is more orange than red, and the green is yellowy green. With the filters the colour is more primary. This works so well you should try it.
    Regards Jake Purches –

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