What I'm looking for is specifically the type of sensor where somebody passing through an open door will cut a light beam (or Infra-Red) and cause something to happen.
In my case, a motion sensor will not do what I want - it could detect somebody near the door, but not going through it. It could also detect a pet moving.
I need to situate the sensor high enough that a person would trigger it, but a cat would not (unless it is riding a unicycle or walking on stilts). The door is normally open, so magnetic switches are not an option.
To make it simple to install, it should have a light source and sensor in the same unit, with a reflector at the other side of the door. Making one is, of course, an option - but buying one would be a lot easier.
I have seen commercial units online, but for the price, I believe they must be diamond encrusted platinum.