Film-Tech Forum ARCHIVE: IMAX Linear Polarization

This is topic IMAX Linear Polarization in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Chase Pickett (Member # 5975) on 12-14-2010, 12:53 PM:

Does anyone know why the IMAX Digital systems use linear polarization rather than Circular polarization? It seems to me a better experience comes from circular rather than linear but there has to be a reason that IMAX uses linear, and I would like to know if anyone knows that reason.

Posted by Gordon McLeod (Member # 33) on 12-14-2010, 02:31 PM:

Cross talk is less with linear than it is with circular

Posted by Cameron Glendinning (Member # 3516) on 12-14-2010, 02:33 PM:

I am under the impression that IMAX offers linear, circlular and shutter glasses for 3D. Its been 10 years since I touched an IMAX projector but our location had both linear polorization and shutter glasses, one could use either glasses for the same screening.
After I left I heard that they stopped using the shutter glasses probably over maintainence issues. Ie the cheif projectionist no longer wanted to climb the 8 story tall screen to ajust the transmitteror perhaps the cleaning of the very expensive glasses, changing the batteries became too much ? The cheaper thow away linear glasses did provide a notably brighter image too. Ghosting was a big issue if you tilted your head.

I was told that that location now uses circlular glasses now.

Posted by Chase Pickett (Member # 5975) on 12-14-2010, 04:11 PM:

I guess I just haven't been to any IMAX screens which utilize shutter glasses or circular. The 6 or so that I have been to all utilized the linear which always annoyed me because keeping your head straight is a hard thing to do for 2 hours or so. I'm glad to hear they do use circular in some cases. But as Gordon put it cross talk is more noticeable with circular but I find it a better trade off.

Posted by John Wilson (Member # 269) on 12-14-2010, 05:20 PM:

quote: Cameron Glendinning
Ie the cheif projectionist no longer wanted to climb the 8 story tall screen to ajust the transmitteror perhaps the cleaning of the very expensive glasses, changing the batteries became too much ?

Nope.

The luvvies cleaning the glasses and re-charging the batteries decided it was not necessary to lock the battery into place (read: easier to take the battery off again to re-charge) so the batteries went missing over time as the punters could souvenir an IMAX branded battery with ease. That's when the transmitter was switched off.

Posted by Mark Gulbrandsen (Member # 72) on 12-15-2010, 09:58 PM:

That they use old polarization junk from the 50'is is simply retarted anbd assinine! Tilt your head more then 2.8 degrees either way and the cross talk is worse then Circular Polarization is. In reality they do it because the glasses are uber cheap to buy. C.P. glasses actually have linear AND C.P. filters in them and cost about three times the price per unit.

Mark

Posted by Kurt Zupin (Member # 2751) on 12-15-2010, 10:06 PM:

For your IMAX that is showing movies other then your standard IMAX Doc, Mark is correct. Price becomes a huge factor in what glasses/system the theatre runs. The shutter glasses get very expensive, as well as diffrent versions of the standard set. Most theatres don't have room in their budget to splurge on pricey glasses that are expensive to replace when some one wants a trinket with IMAX on it much like John said with the battery.

Posted by Monte L Fullmer (Member # 2797) on 12-16-2010, 04:46 AM:

Isn't that what these new 3D HDTV's use to present 3D, is with linear polarization and shutter glasses that costs almost 200 bucks for the emitter and glasses to add on to the unit?

Posted by Chris Slycord (Member # 4239) on 12-16-2010, 10:43 AM:

Depending on brand, I saw that the 3D TV glasses range from a low of ~$90 up to ~$150.

Posted by Carsten Kurz (Member # 5396) on 12-16-2010, 06:09 PM:

Lenny Lipton has something to say about using linear vs. circular glases. It is true that LP IMAX glases are more tilt-sensitive. Yet most peole say IMAX still delivers the best 3D performance. LP vs. CP has a separation efficiency faktor of 10. So even with some head tilting, the stereo separation is better than with CP. The limiting factor, though, is still the screen. But starting at 1000:1 is still better than with 100:1
Seems that, due to the large IMAX screens, IMAX also has the largest glases of them all to allow a large viewing angle.
The necessity for head movements on large IMAX screens to follow the action maybe a bad idea for LP glases - but obviously few people seem to complain. The larger size also attributes to the price, so...

Also, at the time original IMAX 3D was deployed, Ghostbusting was beyond the scope of postproduction options.

There is a digital (non IMAX) two-projector screen in germany using IMAX glases, and thus, LP Filters. Seems to be no problem.

- Carsten

Posted by Mark Gulbrandsen (Member # 72) on 12-16-2010, 08:20 PM:

I actually did tests running stacked digital projectors, an audience of over 200 people, and custom CP and linear filters at Zion Canyon on their 80 foot screen. Every one found the linear glasses extremely annoying and no one saw any difference visually between CP or Linear as far as the 3-D effect itself was concerned. The most interesting observation of that test was that about 3 or 4 people could not even see a 3-d image!! I can just imagine if the rest of the world used linear filtration for Digital just 3-D how many people would be complaining.

Mark

Posted by Carsten Kurz (Member # 5396) on 12-17-2010, 04:53 PM:

How does that comply with the general IMAX 3D performance perception?

- Carsten

Posted by Dave Macaulay (Member # 813) on 12-17-2010, 07:21 PM:

Anything Imax is poison to Mark, so take his opinions regarding them with a few pounds of salt.

Posted by Mark Gulbrandsen (Member # 72) on 12-18-2010, 09:04 AM:

Guys... Tell that to the ex-Imax people that left and designed the Master Image Unit that uses CP. Also do not forget that CP filters have a linear element AND a quarter wave element in them...

@ Carston... The audience always has the final say...

Mark

Posted by Dave Macaulay (Member # 813) on 12-18-2010, 12:27 PM:

Tell them what, Mark? I know some of the ex-Imax people at master Image and to the best of my knowledge the design was set when they joined. You may know differently but I can't see how it matters.
We've never met: I respect your immense experience and knowledge of film production and presentation. For some reason you dislike Imax and criticize everything they do. Anyone reading your posts will recognize that. Chase (I think I met him in Latvia or Estonia?) has been a member here since November 2010 and may not be aware of this bias.
CP works fine but I'd like to see a filter wheel - or LC polarization switcher - that can work with linear polarization: maybe the reason CP is used with Real-D and Master Image is because of this impossibility? For dual lens 3D, LP is an option. The crosstalk increases rapidly with head tilt... but the audience naturally "gets it" and don't tilt their heads.
Working with a real IMAX 15/70 screen the light efficiency is a major issue. Even with two 15kW lamps image brightness with two LP filters in the light path to one's eye is borderline and losing more light to CP - the glasses and filter CP polarizers have an additional light loss from the 1/4 wave plate layer after the LP layer - so a few people incapable of holding their heads up is not so important. Crosstalk is undeniably less than CP with LP (when the LP polarization axes are correctly aligned).
OK, audience experience: IMAX 15/70 3D reaction, in my experience, has been "WOW!!!" and never been "I hate how the 3D goes bad when I tilt my head". The number of "Ouch I got a headache from it" responses seems the same for all polarized systems and a bit less with XPAND or Dolby 3D.
The Imax shutter glasses were introduced for Imax Dome 3D ("SOLIDO") in 1990. LP would not work in a dome where the audience will be gazing all around and subsequently tilting their heads. A silver dome screen would also be a big disaster because of cross-illumination and contrast loss (obliteration?). To use a matte screen the only solution was LC shutter glasses. The shutter glasses were originally extended to IMAX 15/70 3D to add the near-field "3D sound" system Imax/Sonics tried out, the glasses had tiny speakers in their temples. The shutter glasses are also inherently LP glasses, and the LP axis happened to be at 45 degrees from vertical. The left and right lenses are identical with one flipped to make a pair. This makes them also "normal" 45/135 LP glasses. Some flat screen Imax 3D systems (NYC Lincoln Square for one) with the LC glasses added LP filters at the projector to reduce crostalk. I don't know if this is still done.
My opinion is that dual projector systems where both eyes are projected simultaneously are a bit less brain-annoying than alternate eye projection but that the main headache inducer is the false 3D itself, the eyes and brain get upset that closer objects are unnaturally at the same focus plane as distant objects. 3D producers seem to have realized this and don't put many objects far out of the screen and present a "window" 3D with most action occurring behind the screen plane.

Posted by Mark Gulbrandsen (Member # 72) on 12-18-2010, 06:00 PM:

Dave,

The only thing I dislike about Imax is the Liemax conversion. Seriously... it's a dam stupid thing that is going to leave Imax as just a shade of what they once were. The linear glasses are also stupid if not outright retarded. For the record, beyond that I have no grievences against Imax. So I'm not sure where you are comming up with that idea.

Mark

Posted by Gordon McLeod (Member # 33) on 12-18-2010, 06:19 PM:

actually linear is far better than circular for both light and cross talk efficiency

Posted by Steve Guttag (Member # 268) on 12-18-2010, 07:47 PM:

Linear is only superior if one holds their head straight. In actual practice...people don't sit that way (except those with perfect posture). I can understand the desire for Linear to get more light but there does need to be a reality-check on how people behave as people as to work effectively works best.

-Steve

Posted by Mark Gulbrandsen (Member # 72) on 12-19-2010, 11:25 AM:

quote: Gordon McLeod
actually linear is far better than circular for both light and cross talk efficiency

Actually this all depends on the transmissive qualities of the filter being discussed. Having had both Linear and Circular polarizers custom manufactured in the last several years one must be aware that there are a number of options as far as transmissivity is concerned. Transmissivity affects crosstalk and there is a middle of the road for both types of filters where one can easily live with the effects of them. In reality though linear is very outdated and very 50's ish! The rest of the world operates just fine with CP... and so could Imax.

@ Steve.... 100% correct.

Posted by Cameron Glendinning (Member # 3516) on 01-18-2011, 10:25 PM:

quote: Cameron Glendinning
I was told that that location now uses circlular glasses now

The person who told me this (who builds commercial 3d camera rigs) was obviously confused! I visited the theatre recently and it was definitly still linear. I doubt that IMAX does offer circular as an option. My apologies for the misinformation!