[hackerspaces] Laser "touch" screen?

[B F]

It occurred to me that we have touch screens and we have laser
pointers, but can we combine the two concepts and have a laser pointer
that works on a projected image from a freehand-held pointer?  I.e.,
you could easily implement what appears to be this if working at the
computer screen, but can it be done (easily) while standing at the
lectern beside the projector screen?

I figure someone has probably invented this.  But if not, then a
simple camera specifically sensitive to the reflected red (or red+IR)
dot could be
coordinated with a few projected marks on the screen to initialize
the system.  After that the camera would simply see where the pointer
was and would respond, perhaps, to a coded strobe signal from the
pointer to actuate
whatever feature (URL link or whatever) was projected at that spot.

Any feedback?

Bruce Freeman