"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.

[Ryan Gallagher]

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.

Such options depend entirely on how it plays in the live events, screen is not the only focus. It hangs upstage of the main by a few line-sets though. Ballet uses it for walkin and season bumper videos, in their case it has a mid stage black hung behind it. But other shows it will just be floating above talking heads etc. I did run one or two film events there before they deaded their movie screen to the grid to free up that lineset, don’t recall if it was visible on the big screen or not, but that screen and masking effectively fills the proscenium opening, very different air currents at play when it is in.

it’s fairly academic as I’m not really in that space much anymore after taking the FT paramount job. It’s just where I saw the effect regularly, and their equipment has not been upgraded yet.

Their inclusion of a perforated film screen and DCI projector is amusing though, they never added a media server or proper film sound processing or speaker placements. I’m guessing it was maybe intended as a phase two install that never came to pass. Or they had some hope of it being suitable for film special events with less gear to rent in. But other than screen and projector, surround cabs are the next biggest headache in a venue that size, and should have been included, the rest is relatively easy to load in.

[Randy Stankey]

I sounds like you are in the same situation that I was when I worked at Mercyhurst.

I had bosses and admins who wanted to "show movies" in a multi-use venue but didn't want to spend the time and money to do things the way they really need to be done. They would always whine and complain when things didn't look or sound right but when they were told how to fix the problem, would always try to weasel their way around the problem.

We were in a live-performance concert hall and you know as well as we all do that putting a movie sound system in a concert hall is like making a jackalope...like trying to put deer horns on a rabbit. It just doesn't work right unless you are willing to put in a lot of time and effort.

I'd try and try to tell them things like having to buy certain types of speakers or a certain type of sound processor. They'd always whine and complain because they didn't want to spend the money. When their hair-brained solutions didn't work, they'd always come back to me, asking why. I'd tell them the same things I told them a hundred times and, then, the cycle would start all over again.