the humble LED as a light sensor....

[idiot savant]

some background for using LED's for bi-directional communication:

http://www.merl.com/publications/TR2003-035/

LED's as sensors
http://projects.dimension-x.net/technology-and-projects/ledsensors/

Looks like there may be some use for this kind of thing in the FX community as a replacement(or alternative) for LDR's.
Bill Bowden has a circuit here:
http://ourworld.compuserve.com/homepages/Bill_Bowden/page8.htm#light.gif

that can output a control voltage based on the light sensing capabilities of the LED. Looks like you could easily interface it to an envelope follower or LFO driving a LED.

imagine being able to control something like the resonant peaks of a filter in 3D using a matrix of such sensors like here:

http://www.hackaday.com/entry/1234000873073550/

watch the video link, it's way cool!

how practical would this approach be?

[Meanderthal]

imagine being able to control something like the resonant peaks of a filter in 3D using a matrix of such sensors like here:

 I have that gizmo. It's the Alesis air synth. 2 versions, I have the cheaper one. Infrared leds for sensing. It was so much fun that after a week I actually had to take it to my drummer's house so I could protect my sanity and get some sleep! I use it for wind sounds(noise patch with resonant filter) and spacey weird stuff, when they let me. The "theremin" patch is VERY hard to control, so it's not very useful as a musical instrument, but as a noise machine it's OUTSTANDING, except you can't edit the patches.

 But controlling the resonant peaks of a filter is where this thing really shines! So, yes, the concept works very well for this.

[Meanderthal]

Here'a a link to a review of the one I have.  Would be nice to see other stuff with this type controller.

http://www.soundonsound.com/sos/Mar03/articles/alesisairsynth.asp