I think that is a common misunderstanding of the inverse square law, I've heard it before. People ask if they get a short throw lens if it will make the image brighter because it's closer.
The answer is NO, it's not the distance it's the size of the image which the brightness is proportional to.

I see where the confusion happens, if you have two matching projectors (with the same lens) and one is 5ft away from the screen and the other is 10ft away from the screen: then the one that is 10ft way will be twice and high and twice as wide, therefore 4x the sq feet of image surface.
The same amount of light is spread over 4x the area making it 1/4 as bright.

It's the increased area that makes it dimmer, but it's the distance that makes the area larger, thus the mistaken connection.

So lets clear that up, by taking one of those projectors and putting in a wide angle lens, one that makes the image 5ft wide from 5ft away. The other has a long throw lens that makes the image 5ft wide from 10ft away.
Now both images are 5ft wide, from the same lumen projector and ... both images are the same brightness.

We move the short throw projector back to 10ft away, and it's now got a 10ft wide image, and is 4x the area and 1/4 as bright.

so we can change distance, or the beam spread of the lens, and both things change the area of the projection. Now distance is decoupled from area, but we see that it's area that is proportional to brightness not distance.

Now just for fun in case anyone needs it lets look at the inverse sq law and how it is about Area.So far that was nice and easy with 5ft and 10ft, lets put the long throw lens back in both projectors and put one at 5ft and one at 15ft. Away from the screen.
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I've mentioned that I love Isadora, and I have a lot of patches that might be useful so I'm going to try and share a few here.   If you have any problems or questions about Isadora feel free to ask me, and I'll see if I know or can figure out how to help.

You will have to have "the newest" or close to newest version of Isadora to open my example files (1.2.9 on the Mac, or 9b51 on the PC)

One of the things I love the most about Isadora is the Quad Distort Actor.   I had been using my own cobbled together software in which this was the primary feature, and when I started using Isadora I though that it was so much better than everything I had tried, if only it had this one more feature.  Well Mark added the feature and I was hooked.

The simple explanation is that a Quad Distortion actor is just Like a Projector but you can control it's corner points to correct for even radical keystone distortions.

I think this is obviously useful, but I want to point out some of the great things that it makes possible.
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DPI:

Would you be wiling to share any patches?

Hal Eagar:

Yes I would love to, It's always hard to find time to clean up and document something. And you are never sure if anyone will care, but if they do and it could save time for someone then I'd be really please to do so. I learn so much from looking at other peoples patches and code.
OK let me find some.

However, I was much more impressed by a product from the German company Woehburk, especially their Cristalline, glass projection screen. I thought that it showed a brighter image than the other products, with better contrast and color. Perhaps I was charmed by the small scale, as I wanted to design some kind of suspended 'chandelier' type array of small glass projection screens. I really like the way that the image is only visible from one side, which makes it appear quite magically, if you're just passing by...

In this video I compare three different Woehburk screens and then compare my favorite with an opaque rear projection sample from 3M's Vicuity range.

Participants:

PERFORMANCE: SOUNDING
February 17-March 13 2010  -  $18 8:30PM

HERE Arts Center at 145 6th Ave (entrance on Dominick Street).
http://www.here.org

I know this show has a lot more than the video design to offer, but so far the video aspect is all I've gotten to see.  It's a lush abstract immersive video that plays to the theatrical non cinematic.  In other words worth seeing.

It's nothing short of amazing the amount and variety of video surfaces and environments that can be packed into a small blackbox like HERE. 

Directed by Kristin Marting 
Written by Jennifer Gibbs
Video by Tal Yarden

Featuring Bessie Award-winning Okwui Okpokwasili* as Ledaand an ensemble cast featuringTodd d'Amour, Ana Kayne, Irene Longshore, Rudy Mungaray, Michael Pemberton*, Stephen Reyes*  
Music Kamala Sankaram Set Nick Vaughan Light Rie Ono Seo Costumes Elizabeth Bourgeois Sound Jane Shaw Technical Direction Nate Lemoine Stage Management Emily Rea Casting Judy BowmanAssistants Jane Jones, Jennifer Kraus, Elenna Mosoff, Zarrin Whyte, Taili Wu