PhotoEye sensor, allowed?

[DjMaddius]

Photoeye sensor referring to the sensor which most vending machines use to see if the item has dropped. We are looking at using 3 of these to determine if we have balls in our storing area or not. Instead of normal kill switches.

I'm assuming that it would be allowed as it is only a Digital signal but before we use any I'd like to make sure. I can't seem to find anything in the manual prohibiting them.