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Home Theater – Folding Light

When I moved to this house around 4 years ago, I found that the den is too small (15′ x 11′) to accommodate my very good Planar home theater projector, even though it is fitted with a short-throw lens.

Swapping from a long-throw lens to a short-throw lens on my expensive new projector.  (eeek!)

Swapping from a long-throw lens to a short-throw lens on my expensive new projector. (eeek!)

Well this is a problem because the image will be way too small for my 12′ diagonal screen.   There was nothing more I could do to the projector but I still needed some way to lengthen the throw.   But how, when the walls are in the way?   (click to enlarge)
SU - full shot

I searched around The Internets and found nothing.   After some thought I realized that rear-projection TVs have the same problem, needing to lengthen the throw distance to get a larger picture.   Inside they have three light-guns (R/G/B) which shine on a front-silvered mirror, which casts the light on the viewing screen.   The mirror is front-silvered to eliminate the distortion that passing through glass would otherwise cause.
mirror

Problem was, if I mount the projector to the ceiling and aim it at a mirror angled at 45°, and have that reflect to the screen…   would the image be backwards?   I had to model it.   Fortunately I’m pretty good with SketchUp, a 3D modeling app.   I constructed the model, so let’s look at this:   (click to enlarge)
SU - side-above

The projector is mounted on the ceiling and turned sideways to shine on the mirror.   The image reflects off the mirror, onto the screen.   This lengthens the throw distance, and gives me a larger image.   The further the projector from the mirror, the larger the image ends up being on the screen as throw distance is increased.   See that, the angle the light approaches the mirror, off-perpendicular, is the same angle it will reflect off, off-perpendicular.   And uh oh, the right side of the image out of the projector, hits the mirror first and ends up on the left side of the screen.   And the left side of the image out of the projector, reflects last and ends up on the right of the screen.   Backwards!   Well fortunately it was about this time that I realized the projector has a setting called “rear screen”.   This electronically swaps the image left-to-right, and did the trick.

Now;   given that all objects in the model are proportional and calibrated, I can see right there the exact size my front-silvered mirror needs to be.   I’ll make it larger than the image though, in case I ever need to mount the projector further away.   Also the projector has a zoom adjustment, so I can fine-tune with that.

So, I bought an old rear-projection front-silvered mirror on eBay for about $30, and glued it to a rigid piece of sheet-metal to which I’d already attached hinges.   It’s a lamination, for strength.   This assembly mounts to the wall with the hinges.   I have to be careful of the distance from the projector to the mirror, but I measure from the center of the lens to the center of the image on the mirror.   I also have to be careful of where to mount the mirror vertically, but this is fairly easy as the top edge should be at the same distance from the ceiling as the top of the lens.   So my protocol is mount the screen first;   then the mirror center should be centered to the screen horizontally, so mount that;   then the projector.   Minor adjustments can be made by hinging the mirror and pivoting the projector on 2 axis’, along with zoom and focus.
Pj & mirror

Works beautifully.   There are some minor losses in brightness and clarity, but not noticeable.   This whole approach seems to be novel, as there was nothing out there.

(click to enlarge)
pinnoch

shrek

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