Please do be careful when ordering banner sensors, I don't remember
laser based QS18 parts, but there they are now, with the usual warnings.
http://stevenengineering.com/pdf/04PHOTO_QS18.PDF
The led based part I listed has more than adequate range for
a beam breaking sensor for internal ball detection on a robot.
It is also quite useful to detect a given position on a rotary
shaft, such as one might use to pull back a kicker. Limit
switches tend to get mangled.
The FRC rules proscribe lasers for good reasons and the laser
based banner sensors would be proscribed at a FRC event for
these same reasons.