1. A security system monitoring a premise to detect intrusions onto the premise comprising: means acquiring sequential multiple video images of a scene of the premise, a processor processing said images to compare said video images to determine differences in said video images of said scene, to classify the cause of said differences based on physical characteristics of an object causing the differences, and to provide an indication only if said source is classified as movement of a predetermined class; and, an alarm unit responsive to said indication to provide an alarm.