I've installed a mains-switching infrared sensor unit, Cybertech LFMS-WH. It doesn't say passive sensing anywhere, but I think it is passive. The lamp switching part of the circuit works properly and the detection part is problematic.
The sensor is on the wall of a exterior corridor that opens onto a public sidewalk six meters away. The goal is for it to light up when someone enters the corridor, not when someone walks by outside. I get the impression that heat doesn't reflect off surfaces, but I do wonder if the shape of the corridor is related to the problem. It's under an overhang, so the sensor location is darker than the area of the motion.
The device has two levers of adjustment: its aim and the sensitivity ("RANGE") control. In my tests it behaved the same aimed downwards in any direction, even away from the street. With a tiny turn at around 20% of the adjustment range the device goes from seeing nothing at all to seeing me out on the sidewalk. The circuit lights up way too often. I have tried mounting an aluminum shield on its side to block the street angle, but the problem persisted.
Is this hopeless? Should I switch to a ultrasonic sensor?