How and what actionscript should i use to create this kind of sensor?

my project is gonna be the one where scattered, fast-moving words (or any type of variable) will scatter away when an audience stands in the middle of a small field of scattered words. the words will be projected to the floor from above.

anyone knows how do i do this?
should i be using a webcam to do this or is there anything else i should be using?

thank you