Film-Tech Forum ARCHIVE: Rear Projection

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Posted by Paul Salley (Member # 740) on 05-29-2003, 08:47 PM:

We have a lobby preview screen with rear projection being used with a digital projector. We are not happy with the image brightness and are looking into setting up a 35mm projection system with a Christie ELF platter (endless loop) to run trailers. My question is how can I reverse the image so it appears correctly on screen. I know other people have done this, anyone with ideas?

Posted by Steve Guttag (Member # 268) on 05-29-2003, 08:53 PM:

By far, the most popular approach is to use a 45-degree mirror to bounce the image off of on the way to the screen. The mirror needs to be a first-surface type...either glass or stretched-mylar types work well.

If brightness is the only issue...a few questions...what type of screen are you using? It is an appropriate type for the lighting conditions of the room...what are the viewing angles you wish to define as acceptable? How big is the video projector you are using?

Steve

Posted by Adam Martin (Member # 641) on 05-29-2003, 09:04 PM:

What is stopping you from mounting the video projector to the lobby ceiling and switching to a solid sheet? That seems like it would be less complicated than dealing with constantly changing out trailers on an ELF.

Good Lord, did I just condone the use of video over film?

Posted by Paul Salley (Member # 740) on 05-29-2003, 09:34 PM:

Our lobby is overlit with neon lighting and we have a very high ceiling. The neon seems to wash the image out. Plus I never liked the idea of a projector mounted up high and not being able to clean or service it easily. I,m not for sure if the screen we are using is correct, but it was installed before we took over the theater. The digital projector is a Sanyo Model PLC-XP10NA with a 150 watt lamp. Our first thought was to look for a new digital projector with a larger watt lamp. I think we would have to get other model projectors in here to test the on screen image. Below is our screen size and throw.

55 1/2 X 107

17 foot Throw

Screen Brand: SoundFold
Model: Unknown

Posted by Mike Olpin (Member # 1115) on 05-29-2003, 09:56 PM:

Let me recomend the new Christie Road Runner L6.

The brightness is increadible! We use it to project a digital preshow. It hits our 60ft screen in a decently lit auditorium while maintaining bright colors.

Posted by Adam Martin (Member # 641) on 05-29-2003, 11:11 PM:

The PLC-XP10NA is only a 1900 lumen projector. That Christie is 5200 lumens and is way over-priced. You can get a comparable Sanyo for a third less.

The film projector will be much more cost effective for your application, even though it will be a lot harder to work with. (Unless you really want to spend $20,000, in which case, drop me an email and I'll share my notes for what we're looking at purchasing.)

I get to play with a Christie Roadie X10 DLP machine (10,000 lumens!) on Monday. [Smile]

Posted by Thomas Procyk (Member # 1170) on 05-29-2003, 11:58 PM:

We had a Strong Endless Loop for a rear-projection lobby screen at the Regal I used to work at. Changing trailers on that thing was a royal pain in the ass, and one wrong move would send the whole "star-shape" of film out of whack. Then you have to break it down, set up the plungers again, re-make-up the whole 1hr 30min trailer pack.... [Roll Eyes]

We got a GREAT picture, clear, bright and sharp. They used the mirror technique Steve was talking about. Unfortunately, running the "star-shape" pattern through their crazy brain (I never saw so many of those keeper rollers around the edge!) and through a bunch of rollers to get up and around the mirror, not to mention being in an unfinished room caused a HUGE amount of static, and brainwraps were just about a weekly occurance no matter how careful we were. I tried to coencide the brain wraps around the time we had to change trailers... [Wink]

The thing hasn't been up and running since I left the place in 2000.

=TMP=

Posted by Steven Pickles (Member # 785) on 05-30-2003, 12:43 AM:

Thomas,

We had the unit running the summer of 2001, but not the endless loop. We just threaded it every 45-60 minutes and let it run through. It works very well, and patrons really like it. The projector (old Century) was not too great, though. Major oil leaks. Overall, I would rather have some other alternative.

Posted by Paul Salley (Member # 740) on 05-30-2003, 12:52 AM:

My company has spare equipment to do the 35mm setup so really no cost there. The Christie ELF platters are nothing new to us, we run 8 of them now with very little problems, although they do have a mind of their own sometimes. Does anyone have contacts that I could talk to about purchasing the mirror(s)? Is there anything special about these mirrors or do I just call my local glass/mirror company?

Thanks for all your feedback guys!

Posted by Adam Martin (Member # 641) on 05-30-2003, 02:10 AM:

The mirror needs to have the reflective surface on the front of the glass, as opposed to a regular mirror where the reflective surface is behind the glass.

Posted by John Pytlak (Member # 331) on 05-30-2003, 07:22 AM:

It is especially important to control the relative humidity when using a continuous loop platter like the ELF. Generally, 50 to 60 percent RH is best.

Posted by Thomas Procyk (Member # 1170) on 05-30-2003, 06:47 PM:

Steven -- Yep! That's what we would do when we got tired of re-making up the endless loop. I never worked with the Christie version, but anything had to be better than what we went through!

I think there was eventually a memo from Corporate that my friend told me about after I left that location saying not to use the endless loop deck anymore because of too many problems at all the locations.

I'll agree, it's freaking COOL to see FILM played in the lobby! [Cool]

=TMP=

Posted by Scott Norwood (Member # 30) on 05-30-2003, 07:46 PM:

The Brattle Theatre in Cambridge, MA. has an interesting rear projection setup. The two 35mm projectors face each other and there is a pair of 45-degree front-surface mirrors between them. There is also a 16mm projector with a separate mirror which needs to be manually put into place.

I've known several people who have worked there and they all seem to think that framing is difficult, as the masking position isn't visible from the booth.

The weak point of most rear-projection setups (film, video, and slides) seems to be the screen. Hot-spots and screen seams are commonly visible. [Frown]

Posted by John Anastasio (Member # 607) on 05-30-2003, 07:56 PM:

Another answer to the brightness problem is to simply use a second video projector with the image superimposed to double the level. It's the way the Museum of Broadcasting in NYC did it in their theater for showing old TV shows. They also had them arranged so that the scan lines doubled up, giving a really clean looking image. It gives you a backup if one fails...and you can always hook up a laptop and sell ad space on it in your lobby!

Posted by Paul Salley (Member # 740) on 05-30-2003, 08:10 PM:

Scott do you or your friends have access to any pictures of this type of setup?

Posted by Steve Guttag (Member # 268) on 06-01-2003, 06:21 PM:

The brightness of modern video projectors pretty much negates the need for doubling the projectors (Which bring about their own problems)

Screen surfaces for rear-screen are constantly getting better. We recently did a rear-screen theatre in Fayettville, NC (Cameo) and the quality of the image was stunning. 80 lines/mm in virtually all targets (ISCO Ultra-Star Plus lenses) and very little hot spotting (Harkness Translite Natural)...it was like a 1.3 gain screen in terms of appearance to the audience.

However, that surface was optimized to minimize hot spotting in low-light conditions (movie theatre lighting). There are other surfaces that are better for high-ambiant lighting (lobbies) that will hot spot much more.

Steve

Posted by System Notices (Member # 2357) on 10-23-2004, 07:32 PM:

It has been 510 days since the last post.

Posted by Tom Sauter (Member # 616) on 10-23-2004, 07:32 PM:

I'll bump up this old topic since there's little discussion of rear projection topics.

Question: All things being equal: the lamphouse wattage, lenses and screen gains, and we're in a perfect world, will you get the same luminance if you project onto a matte white screen, then turn the whole setup around and project onto a rear projection surface?

Posted by Daryl C. W. O'Shea (Member # 1303) on 10-23-2004, 07:36 PM:

No. You can't make a translucent (for rear projection) gain screen either.

Posted by Mark Gulbrandsen (Member # 72) on 10-24-2004, 12:38 PM:

Paul,
You can get custom front surface mirrors fror Precision Glass and Optics in Santa Anna,CA. I believe www.pgo.com We get our Starphire Glass from them.

But thats a timy digital projector you're running there first off. We have quite a few customers running digital projectors in lobby set ups very sucessfully. They are running 450 watt or larger Toshibas and Sanyos in most cases except the smaller locations. Jordon Commons runs a large digital unit and has about a 20 foot wide screen in its lobby area... but the place is huge lobby wise by most standards as large as many good size two story shopping malls! Practicality has alot to do with this sort of thing and digital is by far the most practical route.

One setup that a customer recently saw at a Harkins Theatre in Phoenix was a full booth set up above the ticket selling area and projecting across the lobby to a screen on the opposite wall. The "out in the open" setup of the booth would certainly fascinate alot of customers. This particuluar setup was accessable from a spiral stair case and they used a platter, not an endless loop. That makes it alot easier to do make up's and tear downs on a weekly basis or more often sometimes. Loops suck in that regard, especially if you forget to run the last show as if it were a platter.

Mark @ CLACO

Posted by Adam Martin (Member # 641) on 10-24-2004, 03:37 PM:

Photos from Harkins Arizona Mills 24 in Tempe, Arizona, that Mark is talking about. Harkins actually has this setup in a number of locations.

Posted by Brad Miller (Member # 2) on 10-24-2004, 03:52 PM:

They don't actually let cutomers walk up on that platform, do they? Most people can't seem to "not touch" stuff. I can only imagine the kind of things that could happen.

Posted by Steve Kraus (Member # 476) on 10-24-2004, 04:17 PM:

Charge them a buck a piece. It would pay for that setup.

Posted by Jason Miller (Member # 2341) on 10-24-2004, 06:23 PM:

Does that projector actually make that the Arizona Mills 25? if a brainwrap happens do you get to point at it and say "SEE... that is the crap that I get to deal with upstairs. now quit yer bitchin!"

Posted by Adam Martin (Member # 641) on 10-25-2004, 06:25 PM:

Note the black cage around the spiral staircase.

Posted by Kenneth Wuepper (Member # 1174) on 10-26-2004, 07:24 AM:

What is the size of that screen?

It looks like it is at least 2.85:1 ratio.

I don't see any speakers for the sound. Where are they and how loudly do they play?

KEN

Posted by Cory Isemann (Member # 2517) on 10-26-2004, 08:58 AM:

looks to me like there are speakers to the left and right of the screen.

Posted by Daryl C. W. O'Shea (Member # 1303) on 10-26-2004, 05:30 PM:

The screen measures 2.35:1.

Posted by Steve Scott (Member # 630) on 10-26-2004, 06:01 PM:

So, is the trailer pack 90 minutes so they can rethread it after every rush? That's not an ELF, for sure. To me, it takes the magic further out of the experience. Our lobby has a huge open space on the back center wall, and on the other side of that wall is the booth. If there wasn't only five and a half feet between that wall and my #10 projector, it'd be ideal for rear projection.

But we do have two 32" TVs below that wall, so we've already got stereo vision. Good for me, for now [Big Grin]

Posted by Frank Angel (Member # 248) on 10-27-2004, 02:03 AM:

Am I seeing things, or is the R in MIRAMAX backwards?

Posted by Daryl C. W. O'Shea (Member # 1303) on 10-27-2004, 02:07 AM:

You'd a be seein' things backwards there.

Posted by Carl Martin (Member # 1146) on 10-27-2004, 02:19 AM:

gee frank, you must be a projectionist!

Posted by Tom Sauter (Member # 616) on 11-02-2004, 07:29 AM:

Daryl C.W. O'Shea:

quote: Daryl C. W. O'Shea
You can't make a translucent (for rear projection) gain screen either.

Then why do manufacturer's specify rear projection screens with gains (either perceived or actual?)

Posted by Daryl C. W. O'Shea (Member # 1303) on 11-02-2004, 10:51 PM:

Gain is a measure of screen reflectiveness. Since you can't reflect directly off of the viewing surface when using rear projection, any gain value would be based on the screens transmissive properties. It would be quite difficult to accomplish a "gain" equal to a high gain front projection screen.

Posted by Steve Guttag (Member # 268) on 11-03-2004, 12:31 AM:

Gain in and of itself is a misnomer to begin with... A screen can never gain light...just distribute it differently. A perfect mirror (also unobtainable) has an infinate gain.

However a gain of 1.0 is such that 1 ft. candle produces 1 ft. lambert. Whether the light is reflective or transmissive is a matter of semantics rather than correctness. So if I hit a rear screen with 16 ft candles and on the otherside I read 16fL, then the "gain" is indeed 1.0 Does the meter know the light is transmitting through the screen rather than bouncing off it? Certainly there is some light loss in there somewhere since some of the light hitting from behind the screen is bouncing back but what does it matter? What is important is being able to predict the light quality such that the level is correct and the distribution is good and even.

If I put in a higher gain screen such that without increasing the light output I now read 32fL in the theatre but with a pronounced hot-spot...you would not consider that a gain of 2.0? It sure measures like it and not surprisingly, whole screen behaves like a gain screen in that the further off axis one gets the darker the image.

I suppose, one could come up with a new terminology for "Transmissive factor" or some such nonsense but then again the existing term of "gain" is already not exactly accurate since nothing is ever gained on a reflective screen either...merely distributed and absorbed.

Steve

Posted by John Hawkinson (Member # 1135) on 11-03-2004, 05:03 PM:

How about "effective gain" (or "equivalent gain")?

--jhawk

Posted by Frank Angel (Member # 248) on 11-08-2004, 11:50 AM:

OK, so you hit a piece of material with a focused image. Where the projection beam strikes a front projection screen, it is focused on that first surface and reflected back. But now we hit a rear projection screen; what do you focus on? The first surface which is the rear of the screen is not what the audience sees. There has to be enough specular material that will actually "hold" the image rather than transmit it thru the strata otherwise it simply be transparant. So for us to see the image on the audience side, somehow the focused image has to get thru to the other side and without any of the subsequent screen material defocusing it as it passes thru the remaining screen material, yet ALL of the plastic material has the same "holding" properties as the front surface meaning it will be out of focus except at one point. How can this happen? No matter how thin the strata, any point after the first surface, the material is defocusing the image. If the imaged is focused on the very last molecules of the strata, then doesn't the translucent (not transparant) surface that the bean hits first, doesn't display an out of focus image? Obviously rear screens work, so this is all wrong, but I just don't understand how they can maintain focus thru the entire screen material. For rear screens to work in theory, they would have to exist only in one plane and infinately thin, which of course they can't.