Film-Tech Forum ARCHIVE: Desiging a curved screen for a 2-projector setup

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Posted by Michael Schaffer (Member # 1204) on 04-12-2007, 10:04 PM:

Not for changeover, but for one digital and one film, each offset 2'6" from the centerline. What's the best way to establish an optimum curvature for a gain screen (1.4) which doesn't result in excessive side distortion?

Posted by Steve Guttag (Member # 268) on 04-12-2007, 10:19 PM:

It will be the same ray tracing but you will want it based on the projectors on center line. If you only had one projector then the screen would be effectively angled towards that one projector (not the same degree as the offset but so the light is reflected best back onto the audience).

Steve

Posted by Kyle Anderson (Member # 3504) on 04-12-2007, 11:47 PM:

Though it's annoying up in the booth, you can install tracks on the floor and have both projectors going through the same porthole (just move one over out of the way); it can be done well if you do it right, and it can save a LOT of money trying to adjust a screen for 2 projectors. long tracks are a must ( > 10 feet) so threading isn't a royal pain in the ass.

Posted by Phillip Grace (Member # 2262) on 04-13-2007, 02:34 AM:

How about trying to find some way of stacking the projectors so that both beams are along the vertical centre-line of the screen? What is the shape and size of the digital projector? You could stand a small on on end in front of the 35mm machine and use an angled mirror to divert the beam. There may also be a partial solution to sidewise distortions using lens shift for both film and digital projectors.

Posted by Michael Schaffer (Member # 1204) on 04-13-2007, 04:01 AM:

quote: Steve Guttag
It will be the same ray tracing but you will want it based on the projectors on center line.

You mean traced as if the light came from a "phantom projector" dead center? Does that still give you a fairly even spread (I would think that the left projector would illuminate the audience more right of center, and the other way around, although that may not even be noticeable to the naked eye)?
Do you prefer to curve on a radius around the lens or so that the light is concentrated into a spot in the back half of the auditorium?

Posted by Steve Guttag (Member # 268) on 04-13-2007, 08:36 AM:

Yes, based on a phantom projector and yes, the left projector will light to the right more and the left projector will light to the left a bit more...not horribly so...you are only talking about a 2.5' offset over how long a throw?

I curve the screen so the maximum light is reflected back onto the bulk of the audience. Curving based strictly on the throw really can't use a gain screen or hot spotting will result.

Steve