Active Infrared sensors take a snap shot of the reflection levels from the ground. Any change in the properties of the reflected wave will indicate that a person or object is in the detection area.
Microwave sensors operate on the doppler principle have a large elliptical detection area. They register and compare the frequency of transmitted and reflected signals.
Combined technology sensors look to capitalise on the merits of both infrared and microwave technology. Microwave allows for the detection of a fast moving traffic and Infrared can detect static presence.
Beam sensor detects persons and objects when the amount of light is reduced for blocking by persons and objects.