IR LASER LED's with focussing lenses should do the trick, though it wouldn't be able to be anywhere near the size of JMJ's laser harp due to lack of power... Yeh it is kinda like that, only instead of harp-like "strings", it would be drum-like plates, each defined and separated by the beams of light... Of course this way you could make three-dimensional zones, like invisible boxes, floating in the air which could act as triggers whenever they are entered etc... I just thought of something...A spot of coding could involve moving them around...hmmm..... imagine sitting down to a chess board with little floating boxes of light you could move around as controllers.......