Motion sensor using linear array of infrared detectors

A motion sensor includes an infrared detector having a single linear array of at least four detector elements disposed on a substrate. The motion sensor also includes an optical system to simultaneously direct electromagnetic energy from two or more curtains of monitored volumes onto the linear array of detector elements. The two or more curtains of the monitored volumes having a tilt angle from a vertical orientation of the motion sensor to create a quadrature horizontal spatial relationship between the monitored volumes. Signals from the linear array of detector elements are then processed to detect a quadrature phase relationship between the signals, and an indication of human presence is provided if a quadrature phase relationship is found.