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Rear Projection Mirror


cibj88

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Next year I will be working on a show that requires a 5m x 5m rear projection. We do not have a lot of space between the back wall of the theatre and the BP screen. My widest lens is a 0.8 but to get full coverage I would need 4m throw. According to the current ground plan we are a meter and a half short of this. The designer mentioned rear projection mirrors as a work around for this problem. I was hoping you guys could give me some insight into how they work, how they should be rigged, how much it can increase the image size (any useful formulas) and also how much intensity is lost from the reflection.

 

Any advice would be much appreciated!

 

Cheers.

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After struggling to quite put into words what I mean, googling came up with this link. Second page of the pdf gives a fairly good, if overly complicated idea of how they work.

 

NB: no idea who the company is, they were simply the first link on google. Other versions/systems/companies available I'm sure.

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OK, Draw yourself a triangle, showing your 5m screen as one side & you projector, 4m away as the corner of the other two.

Draw in you mirror at 2.5 m from the screen & another projector pointing at it 1.5m from it and another triangle joining these up.

 

As the angles of the triangles are the same, the ratio of the length of their sids will also be the same. So:

 

a 4m throw gives you a 5m screen, which means that 1.5m throw will need a 1.88m mirror - which is quite big!

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It might be worth seeing if you can source an ultra-wide projector. There are plenty models available around the 2000 - 3000 lumen mark, but I'm not aware of any brighter ones. I guess the other variable is how much budget there is in the job - it may be that a different projector can't be justified.

 

Every time I've used mirrors for rear projection it's been an utter faff.

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Thanks for the replies guys. It sounds like mirrors are going to be diffcult Im at drama school so the budget is tiny. And the show run including tech time fit up blah blah is going to make any hires expensive. unfortunately pannsonic don't make a wider lens than the 0.8 for the projectors I have at my dispossal. Any other ideas of how to achive the 5x5 with out mirrors wider angles but within the same limited throw distance?

 

A concurrent post has been automatically merged from this point on.

 

It might be worth seeing if you can source an ultra-wide projector. There are plenty models available around the 2000 - 3000 lumen mark, but I'm not aware of any brighter ones. I guess the other variable is how much budget there is in the job - it may be that a different projector can't be justified.

 

Every time I've used mirrors for rear projection it's been an utter faff.

 

My plan has been to piggy back my 2 5000 lumen projectors to get up to 10,000 total so it would be a shame to have to downgrade from this but I guess if I cant fill the screen I cant fill the screen.

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are the panasonic projectors you have available the kind that have built in soft-edging?

if so you can spread the image across 2 or more projectors to keep that throw distance right down.

 

oh and if you do use a mirror you need a front silvered mirror (the kind used in telescopes) or you will get a nasty double image

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Had considered screen stitching Not to sure whether the projectors do have soft edging. As far as I am aware they dont as Panasonic have discontinued the model I have and realeased a newer model that does have soft edging. Will look into it. I had heard about silvered mirrors were important.

Thanks for the reply.

are the panasonic projectors you have available the kind that have built in soft-edging?

if so you can spread the image across 2 or more projectors to keep that throw distance right down.

 

oh and if you do use a mirror you need a front silvered mirror (the kind used in telescopes) or you will get a nasty double image

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in reality, you dont want a 5 m wide image, you want a 5m high one, this is the vital as it means you are actually requiring an image 6.6m wide if its a 4/3 projector or if its a 16/9 unit 8.9m, Either way you are on a hiding to nothing as you are going to need a twin mirror Z rig and the second mirror is going to be vast if its 4:3, or impossible at 16/9. Its also not a simple as drawing a triangle as the mirror is at an angle to the screen and therefor its a trapeziod, and the top of the mirror needs to be much wider than the bottom, in reality you use a oblong mirror with the width suiting the top. If its a budget job, lets assume that a vast optical mirror isnt going to be within budget.

 

If you must rear project, within your distance you can use 2 panasonics pointed straight up and use smaller mirrors at 45 degrees. you the blend them as a pair horizontally top and bottom , which is reality vertically and all your content has to be rotated 90 degrees. however doing this gains you around 1.5m and its still a total faff, but it may just fit....

 

However id front project with a short zoom lens and rig the projector / s as high as possible you should be able to point it down at about 30 degrees and still ketsone it on the projector. The combination of wide lens and height mean that the screen shouldnt get shadowed by onstage action too much. Its going to be much easier than the rear projection options.

 

The other option would be to use something like the hitachi CPA 200 which will do the throw, but if the screen needs to go to the floor, will need flown a fair bit higher than the screen. Its also only 3k lumens and its a big screen... also they cant be stacked.

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