MrMark:
I imagine there are all sorts of practical reasons this wouldn't work all that well, but that's a clever insight. I fear you may have ruined my weekend."I learn a bit every time I visit the forum.", indeed.
I do have a cert for 1800 hrs in drafting and design. It's over 40 but some things don't change.
I would suggest constricting the view of photo-transistors as light detectors to restrict light coming from off the aimed axis, distance.
How are you with tasking on Arduino? Have you ever run a status led in a project? How about a loop counter?