# Rear Projection/Ultra short throw projector Question



## Tower of Terror Theater (Oct 23, 2019)

So I am trying to figure out the best way to project videos onto an approximate 8 ft span of glass doors. I would like to conceal the projector from view of spectators while giving the brightest sharpest image. Thus a rear projection approach would hide the projector. The reason I am thinking about using an ultra short throw projector is that I wonder if I can mount the projector on the outside wall above the span of glass doors and thus hide even the appearance of the projector lens with its bright light source from viewers on the inside. Is there some reason that it is not recommended to use ultra short throw projectors onto rear projection films? The next question is can I apply the film to the inside (side with the viewers) or is there a reason the film must be used on the exterior surface (same side as the projector). Lastly, assuming that I can place the projector on the exterior wall above the glass doors, the exterior wall surface only extends perhaps 5" from the parallel surface of the glass (with projection film applied). Thus in order to get an approximate 100" image on the glass door span, I would need to install a motorized projector stand inside the wall which could extend the projector the required distance (maybe a foot or 1 1/2 feet) to get the desired 100" image even with an ultra short throw projector. Lastly I guess if I try to place an ultra short throw projector, I assume that I will need digital keystone adjustment if I place it a significant distance above the glass doors. The higher I move the projector above the glass doors, the more keystone adjustment I would need correct? Thanks in advance for any input.


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## RonHebbard (Oct 23, 2019)

Tower of Terror Theater said:


> So I am trying to figure out the best way to project videos onto an approximate 8 ft span of glass doors. I would like to conceal the projector from view of spectators while giving the brightest sharpest image. Thus a rear projection approach would hide the projector. The reason I am thinking about using an ultra short throw projector is that I wonder if I can mount the projector on the outside wall above the span of glass doors and thus hide even the appearance of the projector lens with its bright light source from viewers on the inside. Is there some reason that it is not recommended to use ultra short throw projectors onto rear projection films? The next question is can I apply the film to the inside (side with the viewers) or is there a reason the film must be used on the exterior surface (same side as the projector). Lastly, assuming that I can place the projector on the exterior wall above the glass doors, the exterior wall surface only extends perhaps 5" from the parallel surface of the glass (with projection film applied). Thus in order to get an approximate 100" image on the glass door span, I would need to install a motorized projector stand inside the wall which could extend the projector the required distance (maybe a foot or 1 1/2 feet) to get the desired 100" image even with an ultra short throw projector. Lastly I guess if I try to place an ultra short throw projector, I assume that I will need digital keystone adjustment if I place it a significant distance above the glass doors. The higher I move the projector above the glass doors, the more keystone adjustment I would need correct? Thanks in advance for any input.


 *@Tower of Terror Theater* While you're waiting for a few of our up to date experts, I'll offer a few thoughts from a near-blind geezer from practically the gas age.
When 35mm slides and Kodak's Carousel projectors were King, I owned 10 PJ's and a myriad of lenses.
Light is still light, whether from a Carousel or a video projector. That said, light still travels in basically straight lines (with minor warping when passing through hot air) When you employ wide angle lenses at close range, rather than the projected beam striking the projection surface straight on, from the viewers' side, the image will appear dimmer the further off axis the viewer is. I'm likely not explaining this well. Rear projection surfaces have specified viewing angles. I'll use a few of Rosco's screens for examples since, in my era, they were among the limited options available.
Rosco's white, twin white (usable for front, rear, or simultaneous projection) light grey and slightly darker grey had reasonable viewing angles.
Rosco's black, (rear projection only) product was visually stunning, appearing dead black until you rear projected on it when it came alive with lush, intensely bright, full color, dramatic images. Rosco's black RP screen was my magical favorite BUT their black screen had an extremely narrow viewing angle.
I'll elaborate further.
Take for example:
- A 50' wide auditorium with the closest rows 30" from the stage.
- A set with a black rear screen situated equally straddling the centre-line 25' U/S from the apron / 27' 6" from the first row of patrons.
- Project from 6' behind the black RP screen with a 1" lense.
- In the centre of the screen, the PJ is shooting directly at the surface.
- Out at the edges, in all directions, the PJ is shooting anything but straight against and through the surface; closer akin to banking your pool shots off the sides of your pool table.
- When viewed by patrons seated on, or very near, the centre-line, the image was gorgeous. 
- Patrons seated House Right of centre observed the SL half of the image with zero problems while, from their perspective, the SR half of the screen remained black. - Likewise, but oposite, for patrons seated House Left of centre.
- The further back the patrons were from the front, the more of the image they saw, but still only on their side of centre.
- Similarly from the FOH booth approximately 15' above the rear of the auditorium where the upper half of the image appeared flawless while the lower half remained black.
I may not have explained this well but hopefully you've understood the concept. Projected light is still projected light, modern technology can warp images to compensate for key-stoning but how well a rear screen material bends the light passing through it towards your patrons still needs to be taken into account.
I'll employ Control Booth's 'Bat Call' to summon *@ruinexplorer* to your post. *@ruinexplorer* Would you mind replying to * @Tower of Terror Theater* 's post??
Toodleoo!
Ron Hebbard


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## macsound (Oct 23, 2019)

Fun project you've got going here.
I don't think you'll be happy with an short throw projector. Biggest reason is they're a pain to work with. Although they work beautifully when setup properly, with the models I've used, they have a "maximum projection size." In quotes because its not an actual maximum size but because the lens is so curved, you get the best looking size and shape image at the optimum distance from the screen as it's designed. Too close and you've got a skinny top edge, too far and usually the image all but disappears and you have video all over nowhere useful. 

Can you put a projector outside the glass? Yes. Your window film has to be on the outside of the glass, otherwise the glass will reflect most of the light and not the film. Same goes in reverse for projecting from inside.

I'd recommend a solution that gets the most even illumination is rear projection inside the doors. Either film directly on the glass or a rear projection screen that's floor to ceiling, wall to wall inside the doors. Get the projector centered but to avoid the hot spot, don't have it shining directly into the viewpoint of the viewer. In any case it'll look much better and be far easier than a short throw projector from the outside.


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## Tower of Terror Theater (Oct 23, 2019)

RonHebbard said:


> *@Tower of Terror Theater* While you're waiting for a few of our up to date experts, I'll offer a few thoughts from a near-blind geezer from practically the gas age.
> When 35mm slides and Kodak's Carousel projectors were King, I owned 10 PJ's and a myriad of lenses.
> Light is still light, whether from a Carousel or a video projector. That said, light still travels in basically straight lines (with minor warping when passing through hot air) When you employ wide angle lenses at close range, rather than the projected beam striking the projection surface straight on, from the viewers' side, the image will appear dimmer the further off axis the viewer is. I'm likely not explaining this well. Rear projection surfaces have specified viewing anglesl. I'll use a few of Rosco's screens for examples since, in my era, they were among the limited options available.
> Rosco's white, twin white (usable for front, rear, or simultaneous projection) light grey and slightly darker grey had reasonable viewing angles.
> ...


Thanks Ron! You are great for giving me your insight.


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## Tower of Terror Theater (Oct 23, 2019)

macsound said:


> Fun project you've got going here.
> I don't think you'll be happy with an short throw projector. Biggest reason is they're a pain to work with. Although they work beautifully when setup properly, with the models I've used, they have a "maximum projection size." In quotes because its not an actual maximum size but because the lens is so curved, you get the best looking size and shape image at the optimum distance from the screen as it's designed. Too close and you've got a skinny top edge, too far and usually the image all but disappears and you have video all over nowhere useful.
> 
> Can you put a projector outside the glass? Yes. Your window film has to be on the outside of the glass, otherwise the glass will reflect most of the light and not the film. Same goes in reverse for projecting from inside.
> ...


Thank you . I now understand why you have to place the film on the projector side. Ok I will have to digest this info and decide how to proceed. Do you know that once these films are applied to glass, can they withstand rain or will they come off. Part of the show includes simulated rain which will fall onto the exterior surface of the glass, so if the film cannot tolerate this, then this eliminates this approach or makes it additionally complicated.


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## macsound (Oct 24, 2019)

This cheap window frosting is semi-permanent which means its removable, but leaves a residue. So you're probably fine with the water effect as long as it isn't directly streaming on the edge and allows it to work off. So maybe a bead of silicone on the leading edge
https://www.homedepot.com/p/Gila-4-ft-x-6-5-ft-Frosted-Privacy-Window-Film-PFW486/100155257


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## Tower of Terror Theater (Oct 24, 2019)

macsound said:


> This cheap window frosting is semi-permanent which means its removable, but leaves a residue. So you're probably fine with the water effect as long as it isn't directly streaming on the edge and allows it to work off. So maybe a bead of silicone on the leading edge
> https://www.homedepot.com/p/Gila-4-ft-x-6-5-ft-Frosted-Privacy-Window-Film-PFW486/100155257


I want to be able to see through the glass doors during the day, so film being transparent as possible is a must.


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## macsound (Oct 24, 2019)

That makes it a bit more complicated
You could do a perforated material like the below link. It's black on the sticky side and white on the outside. So you could put the film inside the window and project from the inside.
https://www.amazon.com/dp/B01INJXEKC/?tag=controlbooth-20

Maybe someone makes frosted film thats perforated but I haven't seen it.
You might be better off having a pull down thing outside that you'd project on so you get full coverage or no coverage.


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## ruinexplorer (Oct 28, 2019)

There are some window films that are essentially transparent. The issue that you will have is that more of the direct projection will pass through the film. This results in it not being as bright and you will see the rest of the projection on the floor. The more of a "frost" film that you have, the more of the image will be seen on the film (brighter) and less will pass through to be seen on the floor (or ceiling for those who are projecting up). There are a number of digital signage films made for this purpose.

If possible, I would recommend a long throw instead of a short throw if you can keep people out of the image. Short throw lenses for the most part project out the center of the lens. This means that if you put your projector off axis, you start getting keystone which screws with your image. There are some short throw projectors that use a mirror which gives you a better even field, but they are either limited in brightness or very expensive. 

Sorry, I am a bit distracted at the moment, so if you need me to expand on any of this, I will check back later.

Cheers!


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