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Folding the Lightpath for More Distance

post #1 of 13
Thread Starter 
Now that I have my Planar PD8150 calibrated superbly with a magnificent 3D image, I'd like to set up for it to fill my Mega Screen. (Hometheatershack|Screens) It's currently mounted on the ceiling all the way back against the wall, but the image still has an unused border around it of 9" because I can only get 176" back. This is with the short-throw lens, zoomed out all the way, and I can envision an even larger screen someday.

So I want to gain some more distance by folding the lightpath. I have in mind mounting some sort of mirror some sort of way at the ceiling 45 degrees, and mounting the PJ on the back wall pointing up. I think the mirror should be able to pivot at the ceiling for best aiming, using something like a laptop hinge, or thumbscrews.

Has anyone done this sort of thing? How would you calculate mirror size, all distances (including PJ from the wall), do aiming, and what are any unforseen issues? (dust on the lens, light scatter from the mirror, etc)

About the mirrors, so far I gather that rear-projection mirrors may be suitable and are front-coated, are hyper-flat, are trapeziodal, and many are on eBay pretty cheep. The smaller the mirror the better, obviously, but how small can I get away with?

Have any academics or engineers here done this path-folding successfully, and understand it technically in-depth?
post #2 of 13
I have done many projects (cinemas, boardrooms, museums) with 1 to 2 mirrors in the image path....the largest using a 6 x 8 foot optical grade mylar mirror to a final 20 foot screen. In pre-computer days we would do a a ray-trace drawings for projector distance to final image size in both high and width axis; by doing a simple fold(s), where the mirrors should be located, the drawing you came out with an estimated size of the image at that point. The image striking the mirror would be trapazoidal from the absolute center due to the optical turn. Front surface mirrors are hard to clean and in a few situations we had to mount them within a clean box with optical glass for in / out of image with the glass as the cleaning point. Mylars we use to cover when not in use. Dust is not a big issue, only abrasived or filmy residue on the mirror.
post #3 of 13
Thread Starter 
Great overview, but got any detail?

Your software would not be an option, as this is a once-off, and also I run exclusively Linux, no M$ products.

I can see the image at the mirror trapeziodal if the mirror is not at 45 degrees, but how common is that?
post #4 of 13
The beamprint on the mirror is always trapezoidal at 45 deg because part of the mirror is farther away and the beam has expanded more.

Better check with Planar if the pj can be mounted vertically.

If it was me, I'd just turn the pj sideways.

Just make a scale drawing on paper to see how big of a mirror you need, and/or hold something up at various distances and angles from the pj and measure the beamprint.

I've got a 23" x 11 1/4" first-surface mirror from an RP project that never happened. PM me if interested.
post #5 of 13
Thread Starter 
Trying to use Sketchup for this... but kind of stuck here:

post #6 of 13
To get the beamprint on the mirror, rotate either the pj or the rear wall 90 deg about the center of the beam in the plane of the wall.
post #7 of 13
Thread Starter 
I know that I need at least 2.5' more length, so I could mount the PJ on the wall 2.5' down from... what? The center of the mirror?

I guess I have to put a mirror in there, which would be a plane of some size, rotated to 45 degrees in the beam. The bottom edge would need to be at least 4"+4" from the back wall, to allow for the projector-lens-to-feet and depth of the mount.

The wall has to be stationary, as it's a reference at the exact distance from the screen. All proportions are exact
post #8 of 13
Thread Starter 
I believe I've got it. This shows the smallest my mirror should be. I've set it a little low, for a conservative measurement.

It's important to understand that Sketchup keeps everything to exact dimensions and proportional.

It occurs to me that the pivot hinge for the mirror should be on the wall, not the ceiling, to keep it the same distance from the wall as the lens.
post #9 of 13
Is there some reason you don't want to fold the beam sideways instead of vertically?

Seems like it would be a lot less ungainly.
post #10 of 13
Thread Starter 
I didn't understand that statement when you first said it.

One thing's for sure, the beam has to be kept as high as possible all the time, so ppl walking around interfere as little as possible.

That said, mounting the projector for a side shot would make a compound bend, over and down. One corner of the screen would get there much later than the opposite corner, making it a bent trapeziod which it seems to me would be practically uncorrectable. Maybe if the mirror were adjustable in three axis', but I don't think I'm smart enough to figure out whether it's correctable.

The nature of the drawing is that I must start with the fixed screen size at a certain distance from the lens (projectorcentral), assuming it is perfectly square, and then work back to the lens at its exact size there. I can't make Sketchup behave like a light beam would, coming out of the projector and reflecting.
post #11 of 13
You can't move the pj sideways for a 90 deg fold at the mirror?

But I don't think it matters; as long as the pj is parallel to the screen there won't be keystone.

You can verify by drawing it on paper and see if the right and left side extreme ray paths are equal in length.
post #12 of 13
Thread Starter 
Maybe so. I'll look at it next weekend.
post #13 of 13
Thread Starter 
Well as my posts keep getting deleted by some secret Moderator here, I am not going to update this (or any other) thread anymore.
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