Film-Tech Forum ARCHIVE: NEC 800 Mirage like pattern on screen

This is topic NEC 800 Mirage like pattern on screen in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Richard May (Member # 2627) on 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?????

Posted by Kenneth Wuepper (Member # 1174) on 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

Posted by Richard May (Member # 2627) on 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.

Posted by Kenneth Wuepper (Member # 1174) on 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.

Posted by Dave Macaulay (Member # 813) on 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.

Posted by Dennis Benjamin (Member # 1137) on 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..

Posted by Randy Stankey (Member # 64) on 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.

Posted by Gavin Lewarne (Member # 5397) on 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

Posted by Barry Floyd (Member # 385) on 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.

Posted by Richard May (Member # 2627) on 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!!!

Posted by Randy Stankey (Member # 64) on 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.

Posted by Carsten Kurz (Member # 5396) on 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

Posted by Richard May (Member # 2627) on 04-15-2013, 01:47 PM:

Or film festivals like we have now where the film makers are here watching everything.

Posted by Randy Stankey (Member # 64) on 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.