If you can find someone that can write image-managing code, it can be done much more precisely, with a CCD camera, also an USB cheap one.
I mean, it's all matter of triangulation ..... looking at the spots in the image, when they displace horizontally, it's tone change, when they displace vertically, it's pitch (volume, treble, whatever other you want) change
EDIT: i just realized that in this way, you can also be the first one to build a laser-theremin, LOL (i mean, using a contituous "fan" of laser, instead beams
) ..... hey, if this happens, i want my royalties, ok ?
p
)