# Short Throw Daylight Rear Projection



## joeboo46 (Jul 23, 2009)

Hi, I'm an entertainment electrician currently saddled with finding a projector for an upcoming project. I know very little about projection equipment and just enough about using it to get myself in trouble. That being said I have a project coming up that will be happening outdoors starting at 7:30 PM in Northeast Pennsylvania at our town park. They want to do projections for this and it will be daylight for the first hour or so of the production so I need something i'm figuring at least 5000 ANSI Lumens or more (the screen area is partially covered). The next challenge I have is approximately a 15 foot throw and am looking to cover a 7 foot tall by 20 foot wide area (Not my idea my light plot is done and equipment ordered let me leave it at that). This will also be rear projected at a fairly steep angle so keystone adjustment is also a concern. Wondering if anyone can suggest any equipment I may be able to rent to make this happen. If more information is needed i will do my best to provide it. Just looking for some suggestions. Thank You In Advance.


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## museav (Jul 24, 2009)

I'm all for creativity but I have to say that I hate it when people commit to something prior to having any actual idea of how to do it or whether it can even be done and then dump it off on others.

I am assuming from the description that this is not a permanent installation. Based on that, have you talked to any local rental and staging companies? Since you would likely be relying on them for the equipment it might be worth getting their input. I do see a few specific challenges. For one, a 7'x20' image is 2.85:1 cinemascope format, you won't find anything other than a true digital cinema projector that directly supports that. That means that even though the image area that you use may be 7'x20', to get the 20' width the projector will likely be producing something more like a 11.25' to 15' high image and the output of the projector, and thus brightness of the image, is based on that rather than the 7' height. This means your 5,000 lumens for the 7'x20' image area becomes likely more like 8,000 to 11,000 lumens for the projector and even that might be a bit low for the application.

Second, you will almost have to somehow enclose the projection path from projector to screen. Without doing this you will have to overcome anywhere from around 1fc (twilight) to 1,000fc (daylight) or even 10,000fc (direct sunlight) on the screen. At 1,000fc, think of needing to provide millions of lumens rather than thousands.

Finally, trying to create a 20' wide image with a 15' throw is difficult enough but addressing that via a very short throw lens then in turn most likely precludes the projector being anywhere other than directly on the horizontal and vertical axis of the screen.

It almost sounds like outdoor LED screens might be a good solution, but I sort of doubt that the budget or electrical service for that is planned. Another option may be multiple projectors, maybe two widescreen format or three 4:3 format projectors, edge-blended. This would help address the throw issues and the image brightness, but might not totally address the off-axis projection and would still require light control for the projection path. Unless the only source is computer and the edge blending can be done in advance in software, you may have to look at a hardware based solution. Again, this is where talking to some local rental and staging companies that may have this equipment could help as far as the details and costs based on their inventories.


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## ruinexplorer (Jul 24, 2009)

I agree that you are in over your head. I assume that this is a project that is coming up soon, so you are looking at competing with daylight for approximately another hour prior to sunset. You will need to inform the coordinators that you won't be able to see the projection until dark. The other thing to consider is setting up the night before. You will not be able to align and focus the projection during the day.

We've discussed the problems with keystone correction on this forum. When you have an extreme short throw (as you are discussing), you need to have your projector centered on the screen. When you take a short throw lens off axis, you will be unable to keep the entire image in focus (regardless of keystone correction). The other issue that you are confronting is that you are starting out pushing the limits of visibility with the large image and ambient light, so when you take an image off axis, you will not have even lumens across your screen (again regardless of keystone correction). The ambient light can be adjusted for if, as museav suggests, you can cover the entire projection path. Think about a rear projection TV, it works because it isn't competing with other light. If you take the back off of the TV, you can't see the image unless you are in a dark room (thus eliminating the ambient light).


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## joeboo46 (Jul 24, 2009)

I don't think I am in over my head. Perhaps prefacing my question with "I know very little about projection" was the wrong thing to say. I understand how projection works more so than quite a few people I think. I get lumens, output, keystone correction, angles. I was simply asking for others input on the situation. I know this has a solution given the right equipment. I have consulted with local professional projection rental companies and they assure me this is possible. I came here first to ask so I may have been able to talk a bit more professionally to these individuals about aspect ratio, angle keystone correction, ANSI lumens some info to select the correct lens and such. 
That being said while I am projecting in daylight the area we are projecting in is mostly covered including the area where the screens will be hanging (sunlight will however make its way onto the screens.) The area we are projecting in is also shaped very much like a chimney allowing for somewhat of a completely enclosed projection area. The roof of the shell rakes up from 8' (where the projector is located) to 13' (where the screens are located) the walls also angle out from 9' to 24' in width thus creating an enclosure. I am figuring the projector will be at approximately a 23 degree angle to the screens. (Yes I know how to draw an elevation and even use a protractor). Does this info help simplify this at all?? Also today at a prodution meeting when I reminded the powers that be that they are nuts the projection size went from 7'x20' to 5'x14.5'. I actually had controlbooth open during the meeting and made some direct technical quotes from the first post I received the 1 million lumens required thing was a real crowd pleaser and also helped reinforce the fact this is harder than it appears. OK I think I am down ranting any more suggestions with that added/changed info?


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## ruinexplorer (Jul 25, 2009)

Joe, I'm sorry that you took offense to being in over your head. It's good that you are very familiar with the concepts of digital projection, my parenthetical statements are to add information. I make sure to include enough information for not only the OP, but also for other readers who may not have the same amount of experience. Some of the new information that you have provided, including the reduced size screen will make this easier to provide assistance. 

As museav noted in his post, the format that you are trying to acheive is cinemascope which is not typical to what most rental houses (if any) will have available. Since you are looking for a widescreen format, you will most likely be asking for WXGA (1366x768 pixels, 720p, 1.78:1 aspect ratio) or WUXGA (1920x1200 pixels, 1080p, 1.6:1 aspect ratio). You didn't mention what content that you were showing, so I was presuming either computer animation or video. 

Assuming the limitations that you provided, I am going to ballpark a projector that may be in the inventory of your rental house. If you were to use a Christie HD6K with a 0.67:1 HD Fixed lens, you would need 10.5 feet from the lens to fill that screen (with that projector you need to allow about 3 feet behind the lens for the physical dimension of the projector and the space required for proper air flow). You are left with 1.5 feet to spare. However, this is a short throw lens and you are not centered on the screen so you won't be able to fully focus your image. Depending on your content and how observant your audience is, it may not matter.

I hope that helps to give you an idea.


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## museav (Jul 25, 2009)

As far as being off axis, there are two things to consider, keystone and lens shift. The latter may let you vertically and possibly horizontally shift the image relative to the projector without incurring keystoning. Lens shift is related to projected image rather than to an angle, it is how up/down and left/right the image can shift relative to the 'normal' projector location. What lens shift is available, if any, is projector and lens dependent. Keystone correction can also differ from very basic vertical correction to more complex multi-point geometry correction and is again projector and lens dependent. The local staging companies should know their inventory and what capabilities it has in regards to both of these factors. However, a generalization is that very short throw lenses typically allow little, if any, lens shift and more limited keystone correction. They are also more susceptible to hot spotting especially if used with higher gain screens.

What are they going to be projecting? Realizing that it is a tradeoff, the 'standard' viewing distances for a 5' high image would be 30' for computer graphics/text and 40' for video. You can maybe stretch that to 50' or even 60' for video or very large font text, but that sounds like it may be a limited viewing distance for the application. Look at the recent Outdoor Cinema thread and you have someone looking at renting a 30'x17' air screen for civic events. So there may definitely be some balancing of viewing area and projection system involved.

Providing any additional comments or suggestions might be aided by knowing a bit more about the content and application. For example, you mentioned a lighting design and there seems to be more than just projection to the event, but we have not even touched on how the projection and lighting may interact or the potential implications of what is apparently a shell of some type with a 24' wide, 13' high opening supporting a 5' to 7' high image (with the actual screen with frame being larger). Think about the latter, because of the 2.85:1 format the projector will actually be projecting a larger image in height than the viewing image, which means you really want to mask that projected height. Even with a 5' high viewing image, the projected image from a single projector is still likely going to be 8' to 11' high and realistically that is probably the height you have to deal with in regards to image and masking, in fact you might want to mask a bit beyond that. If the shell is 13' tall where the screen is, that leaves a maximum of 5' and probably less below the screens and masking, which would seem to limit other activities and the lighting of them. The only way around that would be an LED wall or multiple edge blended projectors so that you can limit each projector to producing a 5' high image.


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## joeboo46 (Jul 25, 2009)

sorry if I was a bit bitter in my last message. I know they're are a lot of people here who don't undertstand this stuff. I learn everyday.

That being said the media that will be projected is to include (as of right now anyway) a Laptop probably powerpoint, i'm not sure about what they will be using, containing photos and some subtitle stuff. I do indeed know the source will be a computer.


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## newdaypro (Aug 3, 2009)

Everything mentioned so far is important. I do a number of outdoor events "attempting" to provide video/computer content on screens using lcd/dlp projectors. It can be difficult, so compromise and trade-offs always come into play. Find the screen size available to you...i.e. 6x8, 7.5x10, 7.9x14, etc. Next, lens focal vs throw distance become the factor to fill the screen. Several websites have lens calculators...projectorcentral.com, etc. This will tell you what lens needed to fill the screen at the distance you have available.

Any direct or ambient light at or around the rear of the screen is critical. As close to dark as possible is a must. I've had to do football stadiums and as long as the lights behind the screens were off, the front lighting didn't "kill" me. However, that being said, keep all direct lighting and as much ambient lighting as possible off the front of the screen.

I had to do a cage fight outdoors last week. With bad weather a possibilty, I took out some of my older 5500 lumen projectors. It had to be almost completely dark for those to work. I usually use 12K projectors for outdoor events to give me a "fighting chance".

Good luck on your event.


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