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Author Topic: DLP projectors and rainbow effect
Thomas Pitt
Master Film Handler

Posts: 266
From: Leeds, West Yorkshire, UK
Registered: May 2007


 - posted 07-14-2008 03:58 PM      Profile for Thomas Pitt   Email Thomas Pitt   Send New Private Message       Edit/Delete Post 
Something that's been on my mind for a while now - when I went to see Speed Racer, it was a digital 'print'. I couldn't help noticing that if my eyes darted back and forth across the image, there was a noticeable 'rainbow' round sharp edges as if the three colors were splitting up. It was particularly noticeable on Speed Racer because of all the scenes of people's heads floating across the screen [Wink]

I also saw Journey to the Center of the Earth 3D today, in a different cinema. That too exhibited some signs of the rainbow effect if my eyes were moved rapidly around the screen, though it didn't seem as severe as on Speed Racer.

I know you often get the rainbow effect with consumer DLP projectors* (using a single chip and a color wheel), but I thought all theater-grade DLP projectors would use 3 chips.

Anyone know why the rainbow effect would crop up in digitally projected movies?

* The rainbow effect in the theatrical presentations was nowhere near as severe as I've seen on consumer projectors

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Jon P. Inghram
Expert Film Handler

Posts: 124
From: Wichita, KS USA
Registered: Jan 2007


 - posted 07-14-2008 04:14 PM      Profile for Jon P. Inghram   Email Jon P. Inghram   Send New Private Message       Edit/Delete Post 
The mirrors on a DLP chip rapidly toggle between "on" and "off" to simulate different levels of brightness, so that might be the cause.

And, not knowing how the projectors are specifically designed, I can imagine it might be that they load the new frames into the chips sequentially which might introduce enough of a delay between chips to cause a mild rainbow effect, but that's just a wild guess.

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Bruce Hansen
Jedi Master Film Handler

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From: Stone Mountain, GA, USA
Registered: Dec 1999


 - posted 07-14-2008 04:30 PM      Profile for Bruce Hansen   Email Bruce Hansen   Send New Private Message       Edit/Delete Post 
I know that Journey is 3D, was Speed 3D as well? Could this effect have something to do with the polarizer that is in front of the projector switching back and forth?

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Marco Giustini
Film God

Posts: 2713
From: Reading, UK
Registered: Nov 2007


 - posted 07-14-2008 06:15 PM      Profile for Marco Giustini   Email Marco Giustini   Send New Private Message       Edit/Delete Post 
Strange, rainbow should nor affect at all 3DLP projectors...

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Mark Gulbrandsen
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From: Music City
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 - posted 07-14-2008 07:57 PM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
You may have been seeing some moire effect. Where either the pixels of the DMD's or something in the image itself was the same size as the perforation in the screen. You then get moire effect... a form of artifact. NTSC television also suffers badly from this artifact.

Mark

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Tristan Lane
Master Film Handler

Posts: 444
From: Nampa, Idaho
Registered: Feb 2002


 - posted 07-14-2008 10:44 PM      Profile for Tristan Lane   Email Tristan Lane   Send New Private Message       Edit/Delete Post 
Normally, this type of problem is only reserved for single chip DLP projectors, and is associated with the color wheel.

The Dolby 3D wheel only has a slight tint of red and green, so I doubt it's the cause. Either way, Thomas stated that he noticed the effect during Speed Racer, which everyone knows is a 2D movie.

It could be a convergence issue. Convergence is the actual alignment of the chips to each other. Usually convergence problems are visible regardless of eye movement. It's possible that it's only slightly out of adjustment, and by moving your eyes you are able to notice the problem.

All purely speculation of course. It could be as simple as a color setting, or playback issue.

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Mark Gulbrandsen
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From: Music City
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 - posted 07-15-2008 07:42 AM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
Actually there are a few test patterns that will generate moire on the right screen surface. The first time I saw this generated from 3-DLP was in the Dolby Screening room during a training class. I also noticed it once at a local theater during Ratatiollie. So what ever type of screen surface they are/were using at that time the moire was more pronounced.

Mark

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Ian Leach
Film Handler

Posts: 7
From: Winter Park FL
Registered: Sep 2005


 - posted 09-26-2008 08:54 PM      Profile for Ian Leach   Email Ian Leach   Send New Private Message       Edit/Delete Post 
I also noticed this rainbow effect during Eagle Eye. It was most apparent in brown or orange objects that were bordered by dark objects. When my eyes darted from one side of the screen to the other, vertical red and green stripes could be seen in the dark area.
I've seen the rainbow effect on single chip DLP's before, and this was nowhere as severe.

Funny thing was that it was showing on a dual-projector Digital IMAX system.

My only thought was that since there were two DLP projectors shooting the same picture, maybe there was some latentcy between the two? The picture was sharp and clear, no problems with convergence.

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Steve Guttag
We forgot the crackers Gromit!!!

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From: Annapolis, MD
Registered: Dec 1999


 - posted 09-26-2008 09:57 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
I've noted that RED is more difficult to converge on DLPs...that is, you can get a red fringe on either side of the pixel...I've seen this show up as a color fringe, if you are looking for it.

Steve

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Mark Gulbrandsen
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From: Music City
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 - posted 09-26-2008 11:18 PM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
That is because red is ever so slightly out of focus as per green and blue... due to the way T.I. wants the prism blocks built. I woulda prefered that blue be out of focus as it only accounts for about 12% of what we see in the image... our eyes are not as sensitive to blue.

Mark

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Joe Redifer
You need a beating today

Posts: 12859
From: Denver, Colorado
Registered: May 99


 - posted 09-27-2008 12:55 AM      Profile for Joe Redifer   Author's Homepage   Email Joe Redifer   Send New Private Message       Edit/Delete Post 
Mark, the proper term is: "Our Eyes Suck At Blue".

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Mark Gulbrandsen
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From: Music City
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 - posted 09-27-2008 11:08 PM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
Joe,
My eyes don't suck at anything! The funny thing about the focal planes of the DMD's is that the lens manufacturers could easily take this into account when designing the lens. This sort of thing was done for the Technicolor Camera lenses where the focal planes of the three colors all HAD TO arrive at different focal points but arrive in absolutely perfect focus...

Mark

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Danial Simmonds
Expert Film Handler

Posts: 107
From: Kota Damansara, Selangor, Malaysia
Registered: Jan 2008


 - posted 09-30-2008 09:46 AM      Profile for Danial Simmonds   Author's Homepage   Email Danial Simmonds   Send New Private Message       Edit/Delete Post 
We played that movie Journey 3D for like 2 months..
Didnt see any rainbow or colour problem..

Mark has good eyesight :-) "Thumbs up" haha...

It most likely could be the colour setting.. i read that somewhere i cant remember. Should use a colour meter when doing this.

Regards
Dan

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System Notices
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 - posted 10-03-2009 04:28 PM      Profile for System Notices         Edit/Delete Post 

It has been 368 days since the last post.


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Jon P. Inghram
Expert Film Handler

Posts: 124
From: Wichita, KS USA
Registered: Jan 2007


 - posted 10-03-2009 04:28 PM      Profile for Jon P. Inghram   Email Jon P. Inghram   Send New Private Message       Edit/Delete Post 
We've been running CP2000-ZXs for a while now and just the other day I noticed this effect, although not directly on screen.

I was sitting to the rear and side of a projector facing away from it when noticed out of the corner of my eye that when I turned my head the light from the lens (reflected off the port glass and then off my glasses) would break into separate colors.

The easiest way to see it is to take a mirror and wobble it rapidly while viewing the reflection of the light from the lens (preferably reflected off the port glass unless you want to irritate the customers and blind yourself.)

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