Richard, I thought I answered your questions by saying that the security was meant for homes, to be used indoors! I have a security controller developed on 8051f340 that takes inputs from different zones. Sensors used would be PIR and pulsed infrared beams.

As bruce said, they are called IR curtains or fences. I have been informed by a manufacturer that the use of multiple beams, with each beam parallel to the other, is used to detect the breaking pattern. Some of them use patented algorithms for avoiding false triggering. I had found a parallel beam (curtain type) where you can input the alignment as a parameter for human-filtering.

A single beam, or multiple non parallel beams can false trigger easily. PIRs also false trigger at times. I have used PIR from ABB used in their BMS and had found that it can false trigger even though it did not seem to false-trigger on pets.

Two major plus points of multiple parallel beams is that, it is difficult to avoid the beams and that the false trigger issues are avoided. In most of the cases, once intrusion is detected, hooter is driven, and this becomes a nuisance in cases of false trigger.