[simpits-tech] Rear projection material+mylar mirrors+projector ratios

[Rob Hommel]

Question #1
Brian, there are several materials that may be used, 
Gene and I have used a high thread count sheet manufactured by "Martha Stewart" successfully. 
On the pricey side look at diffusion glass for rear projection this is what the pros use. You can make your own if 
you are willing. Various sizes are available from most glass manufactures, they are typically quite heavy as well as
expensive. 

To make our own use a sheet of plate glass and create a diffuse surface to project your image upon. Your diffuse surface may be created using a 6X6 piece of glass
and brake fluid. Apply the brake fluid on the surface of the plate glass and using your 6X6 glass tool move the glass in a circular motion moving the brake fluid as needed 
until the entire surface of your diffuse glass appears uniformly opaque.

Question #2
I have not tried any of the methods for stretching mylar at this time, some look pretty good. I worry about my ability to do it right the first time. 
Just so as you know I think it is worthy of a try sometime in the future.

Question #3
The light projected is reduced according to what is referred to as the Chi square law. The light is reduced according to the square of the distance. Imagine if you will
that the projected image at 2 foot is a 2 foot by 2 foot square at 1700 lumins. At 8 feet the image projected is a square 8 foot by 8 foot the lumins would by reduced by 
the square of the distance in this care 8 square is 64 square feet the total lumins at any point equals 1/64th of the total lumins. Hope this helps.

Keep 'em Flying
Rob Hommel

----- Original Message -----
From: "Brian Sikkema" 
To: simpits-tech at simpits.org
Subject: [simpits-tech] Rear projection material+mylar mirrors+projector	ratios
Date: Tue, 22 Jan 2008 15:47:20 -0500


Three quick questions:

First, I vaguely remember someone posting a bit back
about rear projection materials. Someone mentioned
something that worked really well, but was on the
pricey side - could whoever it was fill me in on the
details?

Secondly, I was wondering if anyone here had played
around with making a lightweight mirror out of mylar
before. I was poking around for some info yesterday
and I found a pretty neat system for pulling all of
the corners tight, seeming to me to make a pretty
distortion-free reflection. Just wondering if anyone
here had played with it before.

And finally, I was wondering if anyone had any good
info on the ratio of projector distance vs. image
size. I'd imagine it probably varies somewhat with
projector model, but a good ballpark figure would be
nice.

Brian

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