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

[Ryan Gallagher]

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.

The venue I refer to is a large concert hall, the pattern is uniform across the whole screen with no obvious density variation indicating a source above or below. They almost never deploy their full size cinema screen, and I see it on a much smaller flown screen that has quite some distance to warm air layer that might be trapped near the ceiling (in fact it's within the flyhouse, so it has like 100ft of headroom). When the handlers are all on there tends to be an pressure gradient from the house to the stage... so air flows towards the screen (and perhaps down when reaching it).

Part of me wonders, if not stemming from a thermal effect somewhere in the optics, if maybe the long throw is what is accentuating the effect? At those distances the projector image more and more resembles a collimated light source. If some people also remember seeing it with 35mm, then maybe the long throw and a region of turbulent air is the common element?

[Randy Stankey]

Does your venue have a fly tower? You could be seeing a "chimney effect". Warm air rises up, into the tower, carrying cooler air through the proscenium, past the screen.

Another cause could be the people, themselves. If you put 100 people all in the same room, the heat from their bodies might cause a mass of rising air that could cause schlieren.
I've actually seen schliren rising from the top of a person's head on a very cold day, outdoors.

[Ryan Gallagher]

Does your venue have a fly tower? You could be seeing a "chimney effect". Warm air rises up, into the tower, carrying cooler air through the proscenium, past the screen.

Another cause could be the people, themselves. If you put 100 people all in the same room, the heat from their bodies might cause a mass of rising air that could cause schlieren.
I've actually seen schliren rising from the top of a person's head on a very cold day, outdoors.

Yes that one does. That all seems logical except that the pattern has an overall downward movement, not up as one would expect with a chimney effect or a warm air boundary propitiating its natural direction.

[Randy Stankey]

Have you ever seen smoke billow from a fireplace, into a room? Since a stage, fly tower and proscenium have the same general form as a fireplace, only gigantic, hot and cold air could be billowing and swirling in front of the proscenium and screen, much as the smoke from a fireplace billows smoke from the firebox.

Is it possible for you to close the main rag behind the screen? Maybe some black stage goods? That would block or, at least, slow down the air flow.