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Author Topic: Lamphouse alignment John Wilson Film God Posts: 5438 From: Sydney, Australia. Registered: Dec 1999 posted 10-31-2008 01:15 AM                     What are the readings from the auditorium?  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 10-31-2008 04:21 AM                     My theater has Perlux screens and, despite many are installed in curved frames, I have the "hot spot following me" issue. I always wonder if it's something that can be fixed with a proper installation of the screens or if it's normal with high gain screens. I understand that it's normal on flat frames. Yes, you should be able to move the light up and down but if you center the light from the booth moving the lamp, and assuming that what you're seeing is caused by the silver screen, you should then have bottom lighter than top from the auditorium point of view. Marco  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-31-2008 06:48 AM                     The Perlux comes in a couple of different gains (1.4 and 1.8, I believe). Curving a screen is not something that one should do just willy-nilly. It requires a curve that takes into acount the dispersion of the screen, the projection point and the seating area. Most screens that are installed today will tend to be too shallow to properly reflect back the light and negate hot-spotting. A 1.5-1.8 gain screen will typically need a radius about 2/3rds the throw. As the theatre gets shorter/fatter in perspective, the shallower the curve will need to be to cover the entire audience...conversely, long-skinny theatres should have a deeper curve to even out the light better. The 1 in 20 rule that many seem to use (and that Schneider Optics puts in their lens program)...really only works on matte-white screens or screens with gains about 1.3 or less. It isn't so much for making the light even either. It is more for effect and I find that one can also obtain some better focus in the corners on some lenses. Steve  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 10-31-2008 08:45 AM                     Actually there are three Harkness Perlux "gain Screen surfaces available" 1.4, 1.8, and 2.2. Presently the 2.2 gain screens are only made in England but they will be produced here as well right after the first of the year... Mark  |  IP: Logged Richard May Phenomenal Film Handler Posts: 1057 From: Floral Park, NY USA Registered: Aug 2004 posted 10-31-2008 08:59 AM                     Wehave to have the silver screen because we run 3-D. I will take readings from the auditorium tomorrow and let you know.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 10-31-2008 09:21 AM                     Mine are 1.4 Thanks Steve and Mark for your replies. Marco  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 10-31-2008 09:48 AM                        If you require silver-screens-for-3D, then you are SCREWED for solving this problem if you have a high angle or a wide auditorium. Suggest you move 3D to a more appropriate venue or have a (smaller) silver screen just for 3D; rolled up or removed when "real" film is projected. Louis  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-31-2008 01:53 PM                     With a Perlux 140...you will find that a radius of 2/3 to 3/4 the throw will make it reflect reasonably evenly. Ideally, you should use some form of ray tracing to calculate the optimal radius. I suspect that your current radius is much more shallow than that. Steve  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 10-31-2008 02:48 PM                     Thanks Steve I'll measure it and I'll let you know Marco  |  IP: Logged Richard May Phenomenal Film Handler Posts: 1057 From: Floral Park, NY USA Registered: Aug 2004 posted 11-01-2008 12:24 PM                     Well I took all the measurements today from the booth and then from the orchestra. The results were stunning to say the least. The measurements from the orchestra were taken from the back row. Center of the screen was 5.0 from the booth. Center was 18.0 from the orchestra. I did get the same results as far as the light being hotter at the top than the bottom though. I just can't believe there is THAT much of a difference from the booth and the orchestra. It was blinding from down there.  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 11-01-2008 02:22 PM                     ARRRRGGGHHH!!! I can't stand this any longer!!! QUIT MEASURING FROM THE BOOTH!!! Unless you have a virtually zero projection angle, and perfectly clear OPTICAL glass in your observation ports, ANY readings from the booth (especially so with a silver screen) are TOTALLY WORTHLESS!!! The light hitting a silver screen is reflected to the audience at an angle roughly matching the angle of projection but at the opposite direction. ( I hope you get what I mean, as it's hard to say it without a picture. ) Your own measurements, when you finally went downstairs, shows that the light is more than the 16fl SMPTE standard. What you need to do is use a telephone or walkie-talkie, get one person in the booth to adjust the lamp while you take readings IN THE AUDITORIUM, MAIN FLOOR LEVEL. Adjust for 16fl and shift the vertical adjustment for the best eveness top to bottom. Be sure to only subject the lens to the light for no more than 20-30 seconds at a time, with at least as much time at rest, to avoid cooking the lens. I strongly reccommend that you go to your local library, and look for two old but excellent books: "Robert A. Mitchell's Manual of Practical Projection" or "Motion Picture Projection" Fourth edition, by James P. Cameron Both books have excellent discussion of lamphouse optics, silver screens and other projection advice and tips. They are very old, and even though they discuss carbon arcs, the optical theories and principles still apply to modern lamphouses.  |  IP: Logged Tim Reed Better Projection Pays Posts: 5246 From: Northampton, PA Registered: Sep 1999 posted 11-02-2008 12:54 AM                       quote: Richard May I just can't believe there is THAT much of a difference from the booth and the orchestra. That's the silver screen doing its thing. They're very directional. You might as well be projecting onto a mirror.  |  IP: Logged

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