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Author Topic: Periscopic projection Phil Hill I love my cootie bug Posts: 7595 From: Hollywood, CA USA Registered: Mar 2000 posted 04-02-2005 11:16 PM                  quote: Adam Martin You would never achieve focus with the lens being so far from the film plane. Adam is absolutely correct. If you want to offset the projection point, you need a periscope using mirrors. Perhaps you should use the Magic Thread Finder and look through some of the other posts in this thread like this : posted 06-02-2003 04:27 PM CT (US) (2:27 PM Local) -------------------------------------------------------------------------------- Periscopes and mirror configurations are commonly used in special venue and ride simulators to solve a myriad of projection throw limitations with both front and rear projection screens. Having designed and installed many of these, I agree with what George said in his above post. However, I would stay away from the Mylar mirrors if at all possible. In addition to the problems George mentions, I have found the large surface area of the thin membrane acts like a passive speaker/microphone diaphragm. The vibrations, especially from the subwoofers, cause the projected image to wiggle, go in-and-out of focus, and do other strange things on the screen and it is very annoying. ALWAYS use 1st surface mirrors if at all possible.  |  IP: Logged Ferdinando Innocenti Film Handler Posts: 79 From: Genova / Italy Registered: Jun 2004 posted 04-02-2005 11:33 PM                     In a periscopic projection, if you have a great distance between the first and second mirror, you must use a very big second mirror. This is the problem. I thought about what Adam said, I know he is right. But I was wandering if it possible to use a system to focus. For example, a lens which doesn't magnify the image, or a second lens just before the turret.  |  IP: Logged Tracy Bellar Film Handler Posts: 72 From: Sciotoville, Oh. Registered: Mar 2005 posted 04-03-2005 01:01 AM                        Don't forget the inverse square law. If you double the distance you cut the light in half.  |  IP: Logged Roger Frazee Film Handler Posts: 16 From: Knoxville, TN, USA Registered: Nov 2000 posted 04-03-2005 07:58 AM                     Sometimes there is no alternative to mirrors. Regal has used mirrors on three occasions, when there was no other choice. The periscope systems were built by Stewart Filmscreen in Cincinnati. The mirrors are mounted on a steel frame, encased in a heavy plastic box, and mounted in the front wall of the booth. Optical glass and front surface mirrors are used throughout. Because the mirrors are in a sealed enclosure, the system requires very little maintenance. The system is pricey, but it works well.  |  IP: Logged Tim Reed Better Projection Pays Posts: 5246 From: Northampton, PA Registered: Sep 1999 posted 04-03-2005 11:53 AM                       quote: Tracy Bellar If you double the distance you cut the light in half. Actually, the light stays the same, it is only spread out over a greater area. [ 04-16-2005, 11:41 PM: Message edited by: Tim Reed ]  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-03-2005 01:50 PM                     quote: Roger Frazee Sometimes there is no alternative to mirrors. IMHO then its time to bring in the wrecking ball and re-do the place so its right.!! Front surface mirrors, no matter how good, still degrade the image dramatically. Were I to go into a place with them and notice it I would want my money re-funded. Mark  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 04-03-2005 03:00 PM                     I have done many rear and front mirror layouts with about 50% would have been eliminated with proper preplanning of the construction space. One location the the installer created (4) periscope ports to go around the airduct and returns placed on the front projection wall with very poor results since the operators would knock the pedestals out of alinement to the mirrors and screens. Two days of relocating the air ducts to the rear of the booth and feed ducts as required solved the problem at minimal cost...no coordination on the job and a tech trying to solve a job with blinders on  |  IP: Logged Jon Morgan Film Handler Posts: 60 From: Raleigh, NC Registered: Mar 2005 posted 04-03-2005 03:20 PM                     quote: Tracy Bellar Don't forget the inverse square law. If you double the distance you cut the light in half. Actually, since it's an inverse SQUARE proportion, doubling the distance cuts the light by a factor of four.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 04-03-2005 08:39 PM                     The distance is not an issue only the size of the image If you maintain the same size of image (and don't get into silly long focal lens) the light level will stay virtually the same if the image size is maintained  |  IP: Logged Tracy Bellar Film Handler Posts: 72 From: Sciotoville, Oh. Registered: Mar 2005 posted 04-04-2005 12:42 AM                        Actually, since it's an inverse SQUARE proportion, doubling the distance cuts the light by a factor of four. If you take any distance and measure the intensity of light then either double the distance it cuts the light intensity in half. If you cut the distance in half it doubles the light. it is called the invese square because it is an exponential increase or decrease in intensity.  |  IP: Logged Peter Hall Master Film Handler Posts: 314 From: London, UK Registered: Dec 2000 posted 04-04-2005 07:34 AM                        The only time the throw comes into play re light is if there is something attenuating the beam, eg smoke (we used to do "Cinema Fumee" in a club here, serious clouds of all sorts between us and the screen) or mist (outdoor stuff). Otherwise you do lose more light on short throws and large screens however this is more to do with the lenses. We have only done one install of mirrors in 15 years, at Clapham in London. The client had an alternative of losing 33 seats per screen or installing mirrors. We put in optical front surfaced mirrors on adjustable resilient mounts, kept the bottom (larger) mirror as small as we could and it does work OK - is no fun to work with however. The Plaza in London had upwards vertical periscopes in 3 and 4 - the mirrors were in the cinema and grew when they got hot, meaning that the image distorted quite badly on a hot summer night with a full house. There is an alternative, which we are unconvinced on - as pictured. We are doing 4 of these at the moment - super low budget at the clients direction. The lens height is 400mm off the floor and you need to lie on the floor to focus - note the upside down layout of the racks. There was no alternative in this case as there is insufficient space for mirrors, however we remain concerned about the practicality of this.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 04-04-2005 09:24 AM                     Tracy that is only partly true but the image size is the only factor if one changes the focal length of the lens to maintain the same image size the amount of light will stay the same the only thing is extremely long focal length lens or atmospheric haze  |  IP: Logged Ferdinando Innocenti Film Handler Posts: 79 From: Genova / Italy Registered: Jun 2004 posted 04-05-2005 08:31 AM                     Well, I think my english is not good enough to make the situation clear. A customer of mine wants to build a second room; he’d like to install the second projector in the same booth of the first, so the projectionist could work better with platters. In fact, the second room is very small (about 80 seats), and it’s impossible to build a booth without deleting a great number of seats. Other problem: in vertical axis, the distance between the projector and the second mirror cannot be shorter than 4 meters, which is about more than a half of the distance between wall and screen. The solution I have thought (I think it is possible, but I can’t find the equipment) is: projector in the big booth, without the standard turret, turret just before the window of the second room. The focusing problem can be overcome with a kind of integrated rod (as in Barco DP100) or a special optic with focus = infinite, or a beam concentrator, or … I don’t know! I’m sure it’s possible, but I don’t know how. Can you help me?  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 04-05-2005 09:36 AM                     You can shift location of lenses if you use relay optics or condensers to create an image "in space" to allow a lens to capture the image. This concept was used in the rear screen drive-in system with 100 or more lenses picking up the image from the projector and is the concept used in telecine film / slide chain to give a common image between different formats. It is possible....but practical...no. The "collector rod" as used in digital projectors are to collimate the light source to give an even field of illumination to the image chips. Speaking of video image, why not put a video tap on the projector ( if you are running 25fps ) and use a video projector for the small room or create a custom projection head / lamphouse to fit the space of one to two seats with the platter in another location. This is being done in Sweden and Argentina. The projector is even threaded in the main booth and sent via a track to the projection room of the other cinema for projecting the show and using remote framing and focus.....Have fun  |  IP: Logged Ferdinando Innocenti Film Handler Posts: 79 From: Genova / Italy Registered: Jun 2004 posted 04-08-2005 04:31 PM                     Thanks for your advice, Richard. I tried to find something about "relay optics" but I didn't find anything. Can you help me again? and why do you say that it is possible but not practical? Thanks in advance.  |  IP: Logged

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