Film-Tech Forum ARCHIVE: 'Ghosting' on DLP showings

This is topic 'Ghosting' on DLP showings in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Thomas Pitt (Member # 4322) on 07-23-2008, 01:18 AM:

At all the DLP showings I've been to so far, there has been a 'ghost' image below the main image when the digital projector comes on. This image is on-screen throughout the movie, but is most noticeable when you have a dark screen with bright bits - such as opening credits. It's the same orientation (not flipped or mirrored), just a lot dimmer than the main image.

The problem seems worse on installations where Real-D has been added. Anyone know what might be causing this, and is there anything that can be done to eliminate the ghost image?

Posted by Steve Guttag (Member # 268) on 07-23-2008, 06:05 AM:

It isn't a function of the DLP...most likely it is a re-reflection from the port glass. If this is in your theatre, try removing the port glass and see if it goes away.

You are more likely to have this sort of thing if the glass is in any way perpindicular to the plane of the lens orientation. Ideall, the glass should be angled to reflect the image back down to the booth floor...this also prevents reflecting into any theatres that may be across the booth (back-to-back theatres).

Additionally, the port glass should be of an anti-reflective type to minimize this and to maximize the light on the screen.

Steve

Posted by Mark Gulbrandsen (Member # 72) on 07-23-2008, 08:29 AM:

quote: Steve Guttag
You are more likely to have this sort of thing if the glass is in any way perpindicular to the plane of the lens orientation. Ideall, the glass should be angled to reflect the image back down to the booth floor...this also prevents reflecting into any theatres that may be across the booth (back-to-back theatres).

Yes, but NO MORE than about 6 to 7 degrees angled as per Dr. Richard Vetter....

Mark

Posted by Bill Enos (Member # 440) on 07-23-2008, 02:26 PM:

Who is Dr. Richard Vetter?

Posted by Dick Vaughan (Member # 540) on 07-23-2008, 02:36 PM:

from in70mm.com

About Richard Vetter
1. I was appointed Director of the Todd-AO Camera Department in 1963. The main object was to update the 65mm system, to continue to provide equipment and services to the film industry and to integrate the cameras with Dimension 150 optical and projection systems. Between 1963 and 1971 several features were produced and released (Sound of Music in Todd-AO, The Bible in D-150, Hello Dolly in Todd-AO, Patton in D-150, The Last Valley in Todd-AO and Baraka in Todd-AO. Between 1969 and 1984 several World's Fair pictures were produced in D-150, Todd-AO and StereoSpace 3-D.

2. In the late 1960's we perceived that 70mm releases were fading and decided to develop a 35mm anamorphic system. In conjunction with NAC of Japan, I invented a superior optical system for which we received an Academy Award for its superiority: Todd-AO 35. Roman Polanski was the first to produce Macbeth in our new 35mm version in 1972. Sam Peckinpah, Dino de Laurentiis and other prominent producers used Todd-AO 35. The 35mm equipment and lenses were sold to Cinema Products in the late 1980.s. Todd-AO 35mm lenses continue to be used by Dino de Laurentiis and other producers. Over 40 feature films have been produced using Todd-AO 35 lenses.

3. I was concurrently Vice President and Technical Director for United Artists Theatres (parent owner of Todd-AO) from 1970 to 1997. We received numerous patents for improvements to production and exhibition equipment and systems.

4. Many skilled individuals contributed to Todd-AO: Mike Todd (the genius behind Todd-AO), Marshall and Robert Naify, Paul Neilsen, Douglas Fries, Dan Lemeiter, Darryl Gray, and the list goes on and on.

Posted by Galen Murphy-Fahlgren (Member # 4535) on 07-23-2008, 03:38 PM:

quote: Mark Gulbrandsen
Yes, but NO MORE than about 6 to 7 degrees angled as per Dr. Richard Vetter....

Mark

If you had a downward projector angle due to stadium seating, wouldn't you end up with vertical portholes? This is a concern of mine, since my ports are angled, I think, too steeply downwards, and my projectors are angled down as well, meaning that the port is angled probably 20 degrees off parallel with the film plane. With my understanding of optics, this causes unnecessary light loss on screen.

Posted by Aaron Sisemore (Member # 145) on 07-23-2008, 05:14 PM:

quote: Thomas Pitt
The problem seems worse on installations where Real-D has been added. Anyone know what might be causing this, and is there anything that can be done to eliminate the ghost image?

The cause in the case of RealD is the image is reflecting off the portglass and then off of the RealD polarizing filter unit. I have the problem too, but instead of showing up on the screen, it manifests itself as being projected onto the backs of heads and seats. Placing a small curtain just below the bottom of the projected image cured the problem.

-Aaron

Posted by Bob Koch (Member # 1062) on 07-28-2008, 01:23 PM:

I think that this is the first time ever, that I have seen Marshall and Robert Naify refered to as "skilled individuals".

Posted by Frank Angel (Member # 248) on 07-31-2008, 03:06 AM:

My projectors are at fairly steep downward angle because the booth is five stories up. The port glass is simply in a frame that is mounted directly on the wall and parallel to it -- very easy and very cheap. We don't have double glass, only the single piece of glass on the inside booth wall. The wall is about 15 inches thick and it has soundproof tile on all four sides to dampen any noise excape. A second pane is unnecessary. The NYC old fire code required double wall thickness; actually the booth is a poured concrete box inside a cinder block box.

The angle of the projector insures that the port glass will not reflect back to the lens.