To see a light beam from the side, crud needs to be in the air to reflect from, cough, cough.
If you just want tripwires, collimate the fricking detectors and light the room enough to cover the endpoints.
My buddy was swinging his cheap pointer around when he hit a wide-view curved mirror and got some of the dispersed reflect. He spent a bit over 2 hours wondering if that eye would ever see anything but red. Not black, red. If someone on your ground spends even 1 minute like that, guess what you lose?