I suggest when you make it, you by a cheapo mouse, and pull it
apart.  you'll have the led and detector, and the rotary
encoding wheel.  I think there's PIC software for a mouse in the
Microchip Databook, put basically all you need to do is find out
how many pulses there are to the CM (or better still a 1M for
accuracy), and divide the pulses by that to give distance.Mark