An infrared motion sensor system has an infrared (IR) sensor having a predetermined field of view, a target positioned within the field of view of the sensor which emits a spatially or temporally non-uniform pattern of IR radiation, and a processor which receives an output signal from the IR sensor, compares the received output signal to a signature temperature profile signal corresponding to the non-uniform pattern of IR radiation emitted by the target, and detects deviation of the sensor output signal from the signature temperature profile signal, indicating intervention of an object in a monitored volume between the target and sensor. The size of the target may be of the order of human size.