Film-Tech Forum ARCHIVE: Formula for Lumens vs. Screen Size?

This is topic Formula for Lumens vs. Screen Size? in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Frank Angel (Member # 248) on 06-22-2011, 11:28 AM:

Is there any general rule-of-thumb when sizing a video projector for a known screen size? For example, we have a 55ft scope screen, 0 gain -- we may need to rent a video projector. 10K lumens, 20K? Anyone have a chart to show that kind of size/light requirement?

We currently have dual Barcos R6+ Performers at 6,000 lumens each. Stacked, they were supposed to be adaquate for our screen, but the big hitch is that they are NOT stable enough to be aligned the night before a show and remain aligned (pixel for pixel) for the show the next night. Once they are turned off and cool down, you are screwed. Barco says you have to align them each time. Fine for indoors, but this is an outdoor venue and we don't have the luxury of aligning them before the show on the day-of because, well, it's out in daylight.

We have a scope picture that has to be shown and I fear using only the one Barco will not make it, thus the need to rent. I would GUESS, 12k -15k would be bright enough (remember, this is outdoors and there is more ambient light than in an indoor theatre), but was hoping there is some chart that gives real numbers.

Posted by Robert Minichino (Member # 3495) on 06-22-2011, 09:37 PM:

It depends on the format and the target brightness. Since the Barcos are 4:3, projecting scope only uses half of the imager (assuming no anamorphic), so you only get 3000 lumens out of them, and that's before color calibration which eats a little light.

I'm assuming your screen is 1 gain and not actually zero [Wink]

, and it's 23x55, that's 1,265 ft^2. To get 14 fL from a 1.0 gain screen, you'd need 1,265*14=17,710 lumens. Your two 3000 lumen projectors get you 6000 lumens total, or 4.7 fL. That's pretty dim, but it might be acceptable if you don't have much ambient light on the screen.

I'd rent something brighter, preferably with a 16:9 imager to get more light efficiency. A 20k lumen projector would be ideal, but even 10k would be way better than what you've got.

Posted by Frank Angel (Member # 248) on 06-23-2011, 12:17 AM:

Thanks Robert; yes, we are not going with what we've got. Already they are talking to a rental venue about a Christie 12K native 16:9. The DVD will be scope, but letterboxed, not anamorphic. We will just zoom it up just a bit, probably not go to full scope width and leave some frame lines on top and bottom and just mask it.

Posted by Steve Guttag (Member # 268) on 06-23-2011, 06:04 AM:

Frank, if the DVD is a 2.39 but letterboxed within the 4:3 frame, then use a scaler, FIRST to use as much of the projector's imager (while maintaining proportions) before zooming in.

Nowadays, it is odd that a widescreen movie is not anamorphic on a DVD.

Posted by Carsten Kurz (Member # 5396) on 06-23-2011, 08:26 AM:

You will not actually be throwing a letterboxed scope feature DVD on a 55ft screen?

Ouch... I do hope you mean BluRay.

- Carsten

Posted by Robert Minichino (Member # 3495) on 06-23-2011, 10:04 AM:

For what it's worth, a more recent scope DVD will be both anamorphic and letterboxed. A letterboxed-only scope DVD is YouTube quality, with only 240 lines of resolution (half the frame, vertically). At least anamorphosis gets back some of that vertical resolution.

Zooming in more to reduce the picture size is a good way to increase brightness if you don't need to fill the screen. I've done this many times before with good results.

What Steve said about scalers is well taken as well--always use as much of the imager as possible for the best brightness and, secondarily, best resolution.