"Steam/heat" ripples on the screen from the lens?

[Michael Rousselet]

I started to notice this kind of "steam/heat" effect on a section of the screen, it's very subtle but more noticeable on white/bright scenes. It looks like steam rolling down the screen. What could be causing this? Christie 4k projector.

[Philip Jones]

We have this in one of our screens and it is caused by the air conditioning when in heating mode. If you have any type of heating vents check there.

We mitigated it by altering where the flaps blow out but it is still slightly noticeable under certain conditions.

[Jim Cassedy]

I agree with what Phillip said, as we had this effect in one of the old theaters I worked at,
and it was only during the 'heating' season, and the problem was one of the heating vents
that was near the top of the screen. One other questionis if your Christie is a Xenon or
Laser projector, although I'm not sure that would make any difference.

[Randy Stankey]

It's called the "Schlieren Effect." It is caused by differences in heat/density of a medium. (Air.)

https://en.wikipedia.org/wiki/Schlieren

It can be caused by heat or, for instance, in water by differences in salinity. Let's say that you are scuba diving in the ocean while it's raining. If you look near the surface of the water, you can see the Schlieren Effect as the rain water diffuses into the salty ocean water.

In your case, it is probably caused by air conditioning ducts near the screen. Hot air comes from the ducts and diffuses into the cooler room air. The difference in air density, refracts the light, causing the effect. It could also be caused by cool air diffusing into a warm room but it's much more prevalent the other way around.

The solution, as others say, is to climb up to the ceiling near the screen, find the offending A/C ducts and move the diffuser fins so that the air stream blows away from the screen.

While you are looking for the cause, don't forget to check the booth, as well. There might be a heat/cold source near the projector lens, blowing air down, into the projector beam.

It's a 90% chance that the problem comes from the air conditioning in the auditorium but it is worthwhile to check the booth, too.

[Ryan Gallagher]

A much older 2K projector (CP-2000X) in town had this problem too, it was always present, no matter what the room/booth handlers were doing... I assumed it was in the light path internally... due to the same phenomenon of turbulent heated air progressing upward (but being inverted in the optics somewhere).

I suppose it could also be a layer of air being heated near the port glass perhaps? But room AC/Heat is just as likely to cause the problem. The diffraction patterns seemed magnified to my eye, so I suspected much closer to the projector, but that was just a guess. They almost never ran films there, just slides and live event stuff... no one every commented on it.

[Marco Giustini]

I kind of remember that effect "embedded" in a projector too as Ryan says but most of the time it was just the AC/heating from the ceiling. It's most offending when you turn it on and the room is being heavily heated/cooled. After that, the air coming to the room won't be so cold/hot and the effect will be reduced or eliminated.

[Leo Enticknap]

This was a problem at the Egyptian in winter, but only after an NC2500 was retired and replaced with a DP4K-45L, suggesting that light wavelength has something to do with making the steam visible. The fix was to start the heating up 2-3 hours before showtime. Management didn't like to do that because of the extra energy costs (nontrivial for an auditorium that size), so it would only be done before high profile shows (e.g. when there was a Q and A with a celeb after the movie and a studio tech inspected the place beforehand).

[Marcel Birgelen]

I've seen this problem with Xenon machines too, but it somehow makes sense it would be more visible with more monochromatic light sources like those of laser projectors, as this "atmospheric lensing" will probably create more unexpected interference patterns. At least that's the theory I can come up with.

[Marco Giustini]

I used to see that regularly with 35mm!

[Randy Stankey]

I don't know but I find it hard to believe that the Schlieren Effect would be visible if the air disturbance was inside the projector. If the disturbance occurred behind the image plane, it would manifest as a small loss of light. The lens would not be able to image it. If it occurred in front of the image plane, it might blur the image but the lens would not be able to resolve the Schlieren pattern.

The lens, itself, does get hot enough to cause Schlieren and the disturbed air in front of the lens could be enough to manifest the effect. I think that theory could be proved or disproved by putting a small fan in front of the projector to blow any hot air away from the lens.

[Marcel Birgelen]

I don't know but I find it hard to believe that the Schlieren Effect would be visible if the air disturbance was inside the projector. If the disturbance occurred behind the image plane, it would manifest as a small loss of light. The lens would not be able to image it. If it occurred in front of the image plane, it might blur the image but the lens would not be able to resolve the Schlieren pattern.

If it happens inside the projector, before the lens, I'd expect it to show up as a flickering effect, maybe localized to a certain part of the picture, depending on how close to one of focal point of the lens this problem occurs.

[Mark Gulbrandsen]

I have seen this at several locations from ceiling mounted HVAC registers. Never from a projector.

[Ryan Gallagher]

The one I spoke of still has that setup, maybe I’ll take a projection call there one day just to experiment and locate their local issue.

Being in Texas it is almost never heat coming out of the HVAC, and from memory it was season agnostic.

visually it was like mirage atmospheric rippling, but progressing down instead of up. That inversion is the only reason I suspected a process somewhere behind the lens. But certainly “could” have been a thermal differential to a column of cold air flowing down too.

[Marcel Birgelen]

visually it was like mirage atmospheric rippling, but progressing down instead of up. That inversion is the only reason I suspected a process somewhere behind the lens. But certainly “could” have been a thermal differential to a column of cold air flowing down too.

That's exactly how it looks.

Keep in mind that inlet temperatures for cold HVAC air are considerably below room temperature and that the hot air in the room naturally collects at the ceiling. So near the HVAC inlets, there can be a considerable temperature gradient, combined with considerable airflow and the resulting turbulence in the air. Combine this with a high-powered light-beam traveling right through this turbulence and focussed on a projection surface and you've got the perfect recipe for the "mirage phenomenon", but in this case, the inverted version.

What would be interesting to see, if light sources using monochromatic light increase the visibility of this phenomenon. Especially primary laser setups that rely on screen shakers to eliminate speckle.

My father used to work at a wind tunnel testing facility back in the 1980s, I remember them experimenting with laser-interferometry in order to visualize airflow instead of the old-school way of using smoke trails.

[Randy Stankey]

My father used to work at a wind tunnel testing facility back in the 1980s, I remember them experimenting with laser-interferometry in order to visualize airflow instead of the old-school way of using smoke trails.

There is a whole field of study in Schliren photography that has been used to study the behavior of aircraft in supersonic flight:
https://en.wikipedia.org/wiki/Schlieren_photography