Film-Tech Forum ARCHIVE: Ghost on Large Dual Projection 3D

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Posted by Alejandro Mir (Member # 7129) on 10-29-2014, 04:00 PM:

Hi all,
I recently installed a large dual 3D projection system with two Barco DP4K-32B and Dual IMB Showvault, using Schneider circular polarizers. The screen is an MDI Stereoview 2.8 gain, size 75`x40` and the throwing distance is 90`.
Everything seemed perfect until we played subtitled trailers... Hobbit and Exodus... there was a terrible ghost, very noticeable. I performed all the usual checks: confirmed each projector delivers the correct image, removed the porthole glasses to avoid any depolarization, checked another piece of silver screen surface, checked that the Master Image glasses match the polarization system, but the problem persists.
There's no doubt to me that the image from one eye is crosstalking to the other one. I measured a crosstalk rate of 22 with the Shcneider polarizers, also tested Master Image polarizers and the rate was 43. The ghost was sligtly lower, but still unusable.
I also tested linear polarizers and glasses, getting a rate of 75 and a barely acceptable ghost, but still not too good.
I've done few duals before and never seen this... I already tried everything posible without success...
Does anyone knows what the reason may be? Am I reaching the limit of the polarizing system? I doubt so...
Any suggestion? [Eek!]

regards
Alex Mir
Argentina

Posted by Carsten Kurz (Member # 5396) on 10-29-2014, 05:18 PM:

Where did you measure the crosstalk - 'in flight', or on the screen?

Have you used these polarizers before?

- Carsten

Posted by Jim Cassedy (Member # 4115) on 10-29-2014, 05:59 PM:

Halloween is this Friday.
I've been seeing a lot of ghosts this week. . . [Big Grin]

Posted by Marcel Birgelen (Member # 6801) on 10-29-2014, 06:06 PM:

Subtitles on a large screen using polarized 3D solutions and NO ghosting? Dual projector or not, I've never really seen it happen. It varies somewhere between awful and acceptable. Subtitles and 3D are no friends.

You seemingly already checked everything obvious, including other patches of silver screen. You've got a high-gain screen, two of the most powerful DCI projectors, so... maybe your picture is just too bright to make it work?

There will always be some crosstalk, even the best shutter-glasses do still leak some light.

Posted by Joe Elliott (Member # 4006) on 10-29-2014, 09:50 PM:

Where are the subtitles from? Are they part of the preview, or are they rendered by the projector? Most servers now have built in ghostbusting as part of the software, but if the subtitles are being rendered by the projector, it never goes through that ghostbusting.

Posted by Mark Gulbrandsen (Member # 72) on 10-30-2014, 09:58 AM:

You have to have the correct transmissivity of polarizers to be able to minimize crosstalk (Ghosting) And indeed some large screen films are intended to be used with linear polarizers ONLY and you will still get a teeny bit of ghosting in some scenes. In the LF systems I installed I supplied both CP and linear polarizers with the systems and I believe they are 48% transmissive. The amount of transmissivity also determines the amount of crosstalk as well. We use the same MDI screen.

Mark

Posted by Alejandro Mir (Member # 7129) on 10-30-2014, 10:29 AM:

Thank you all for the fast response...
I agree there is always a certain amount of ghost on 3D, being worst on high contrasted images... like subtitles...
To evaluate the crosstalk on dual projectors, I throw White Test pattern on left projector only, thru the left polarizer, and measure the light on the screen thru left and right glass.
Ie, 5Fl on left and 0.23Fl on right gives me a ratio of 22...
I used those polarizers before and they worked ok.
I also tested the MI ones on this theatre, and their ratio was 43, but still too bad on the subtitles...
Another strange detail is that the same MI polarizer set gave me a ratio of 60 in another theatre, instead of the 43 I was having... their efficiency should not vary from site to site.
I also tested linear polarizers there an got a ratio of 75, when the theory says I should get around 150.
I made a crazy test: took my led maglite close to the screen and measure the ratio with the linear, it was about 150! the only difference was the light source and the higher light intensity.
I also played the 3D sync test pattern, the black sides look blueish, not too dark nor a dark grey. It becomes real black when closing the dowser of one projector, confirming the crosstalk.

I'm starting to consider I may have a combination of issues:
- the seating distance is too close, too inmersive, and ghost is more visible.
- the screen gain is too high and projectors too bright, I will do some math to compare light intensity with previous projects.
- I use a PCA meter to measure the light, it may not be precise enough when measuring dark images.

Something I also noticed is that the Light Engines of this two projectors are different tan the ones before... is there any chance that the light those projectors generate may depolarize the image?

Thanks again!

Posted by Marcel Birgelen (Member # 6801) on 10-30-2014, 02:06 PM:

I've seen this blueish hue in dark parts of the picture in several 3D setups with silver screens, especially those with blue walls [Wink]

. It's probably a result of the light, bouncing off of the walls, back onto the screen.

Do you get different readings in the center and in the edges? If you're sitting really close to a large screen, the viewing angles towards the edges become a significant factor.

Posted by Marco Giustini (Member # 4544) on 10-30-2014, 02:08 PM:

Try and use one of those RealD test charts, where you can see the ghosting based on the colour of the background compared to the vertical patches.

Also, have you enabled ghostbusting on the server? On Doremi you need a licence, but on a Dolby you can enable it yourself.

Posted by George Germisashvili (Member # 6597) on 11-02-2014, 01:22 AM:

Hi! If it possible to test without projection window glasses? Before try to adjust the 3D system angle.