my profile | my password | search | faq & rules | forum home

» Film-Tech Forum ARCHIVE   » Operations   » Digital Cinema Forum   » NEC 800 Mirage like pattern on screen

Author Topic: NEC 800 Mirage like pattern on screen Richard May Phenomenal Film Handler Posts: 1057 From: Floral Park, NY USA Registered: Aug 2004 posted 04-14-2013 06:35 PM                     You know that wavy heat like pattern we all saw with film? You see it especially on bright sky scenes. Like looking in the distance in the blazing heat at an asphalt road. What is that again?????  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 04-14-2013 06:59 PM                     Hello Richard, This effect (defect) is called MOIRE. It is caused by the interference between the patterns of dots from the projector and from the screen. Like two sound frequencies BEATING together, these two mathematical frequencies visibly combine to make this effect. I hope this is helpful. KEN  |  IP: Logged Richard May Phenomenal Film Handler Posts: 1057 From: Floral Park, NY USA Registered: Aug 2004 posted 04-14-2013 07:12 PM                     Hey Ken, I do know what you're talking about. We had that in 2 other screens with video projectors. We went to microperf screens and it went away. This is different. It actually moves around the screen like flowing waves of water.  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 04-14-2013 07:22 PM                     OK! If you were to touch the screen can you feel any slight movement like might be caused by air handling drafts? Is there any vibration of the machine from a blower motor or similar mechanical device? Something is changing the frequency of the dot patterns and the beats are therefore varying across the screen.  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 04-14-2013 07:37 PM                     Most likely hot air from your HVAC system. Try shutting down the HVAC and see if it is still there, probably not. Then turn HVAC back on... usually the "mirage" waves come back immediately.  |  IP: Logged Dennis Benjamin Phenomenal Film Handler Posts: 1445 From: Denton, MD Registered: Feb 2002 posted 04-14-2013 07:41 PM                        Yes, heat/air and dust particles between the lens and the screen can actually show up in the picture. Crazy stuff, this digital equipment..  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 04-14-2013 07:51 PM                     It's called Schlieren. http://en.wikipedia.org/wiki/Schlieren Caused by heat making warm air rise and flow through a mass of cooler air. The difference in air density refracts the light and causes a moving shadow effect on your screen. As others have said, it might be caused by your HVAC system. It could also be caused by any heat source near the projector or in line between the projector and screen. Theoretically, it could be caused by heat rising off the projector motor. (In which case, it would be a warning sign that your projector motor is overheating!) It could even be caused by a door leading outside letting cold air into a warm room. Mostly nothing to worry about unless it becomes a distraction to your customers.  |  IP: Logged Gavin Lewarne Master Film Handler Posts: 278 From: Plymouth, UK Registered: Aug 2009 posted 04-14-2013 08:22 PM                     Project a totally white test attern. Go stand in front of screen and watch the heat waves rising off the shadow of your head. Pretty weird at first  |  IP: Logged Barry Floyd Phenomenal Film Handler Posts: 1079 From: Lebanon, Tennessee, USA Registered: Mar 2000 posted 04-14-2013 10:37 PM                        We haven't seen any weird "heat" related apparitions, but did notice a really cool effect of shooting a Barco DP2K-23B through falling snow. First weekend we opened it snowed on Sunday night and you could see that the snow flakes falling through the projector beam were individually colored. Same effect in the rain, but with snow it's much more pronounced. I've run 35mm in the snow and rain and it didn't do the same thing.  |  IP: Logged Richard May Phenomenal Film Handler Posts: 1057 From: Floral Park, NY USA Registered: Aug 2004 posted 04-15-2013 12:06 PM                     Were actually running the A/C at this point, but I think Gavin nailed it. It's the body heat from people. I ran white light with nobody in the theater and it wasn't there. As soon as the theater filled up, it was there. The big problem is the ceiling is only 11 feet high. This sucks!!!  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 04-15-2013 12:45 PM                     What about increasing air circulation? The idea would be to stir up the air more, hopefully, to homogenize the air mass between the projector and screen. It might take a lot of fan power to do this. You might have to create a virtual windstorm. In that case, you would be better off leaving well enough alone. We, as operators and technicians, are much more tuned in to minor problems than the average public. 90% of your customers will probably never notice. 90% of those who do notice probably won't care. Yes, if you can solve the problem, you should fix it but don't kill yourself for such minor things. If adjusting the HVAC can solve this, do it but, unless you are getting complaints, don't worry about it. Schlieren has been visible on the screens at Tinseltown, here in Erie, ever since the building opened. Only a handful of people have ever noticed. I'm one of them. The only other people who I know have said anything also worked in the theater. The majority of those were projector operators. Never has a customer complained in almost twenty years.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 04-15-2013 01:08 PM                     When I'm standing right in front of our screen, I can see these 'schlieren' from our heating system. However, from the audiences seats, it is hardly visible at all. And it only happens during 'heavy' heating phases, not all day. I could imagine that in some scenarios, it could actually be slightly annoying for the audience. - Carsten  |  IP: Logged Richard May Phenomenal Film Handler Posts: 1057 From: Floral Park, NY USA Registered: Aug 2004 posted 04-15-2013 01:47 PM                     Or film festivals like we have now where the film makers are here watching everything.  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 04-15-2013 03:56 PM                     I agree that it should be fixed if you can. But, at what cost? Would simply adjusting the diffusers on your HVAC work? Simple solution. If it would require revamping the system or remodeling the building, that would not be worth it. In my experience, it is likely to take a lot of work. In which case, there will be little you can do. Film makers can be notoriously picky. To an extent, they should be but this is something they'll just have to live with unless you can find a simple solution. Yes. Try but don't kill yourself over it.  |  IP: Logged

All times are Central (GMT -6:00)

Printer-friendly view of this topic

Hop To: Select a Forum: Category: Operations -------------------- Digital Cinema Forum Film Handlers' Forum Large Format Forum Ground Level Feature Info, Trailer Attachments & REAL Credit Offsets Category: Community -------------------- Film-Yak Film Handlers' Movie Reviews The Afterlife Bob Maar's Joke-A-Thon Category: Classified -------------------- Equipment and Supplies Wanted/For Sale Help Wanted/Seeking Employment Category: System -------------------- Software Beta Testing Forum

The Film-Tech Forums are designed for various members related to the cinema industry to express their opinions, viewpoints and testimonials on various products, services and events based upon speculation, personal knowledge and factual information through use, therefore all views represented here allow no liability upon the publishers of this web site and the owners of said views assume no liability for any ill will resulting from these postings. The posts made here are for educational as well as entertainment purposes and as such anyone viewing this portion of the website must accept these views as statements of the author of that opinion and agrees to release the authors from any and all liability.