1. In a security system distinguishing between changes within a scene caused by the presence of an intruder as opposed to ambient lighting fluctuations affecting scene illumination, the improvement of a method for making such a distinction when lighting changes within the scene are produced by flickering lights, comprising: obtaining a first image of the scene; comparing the first image with a reference image of the scene and obtaining therefrom a difference image composed of a set of differences between the two images; responding to said set of differences exceeding a threshold by obtaining a second image of the scene subsequent to obtaining said first image; comparing said second image with a reference image of the scene and obtaining therefrom a second difference image composed of a second set of differences between the two images; comparing said second image with said first image and obtaining therefrom a third difference image composed of a third set of differences between the two images; responding to said second and third sets of differences exceeding said threshold, identically partitioning said first and second images into a plurality of uniform segments; and comparing a portion of said plurality of uniform segments from the first image with corresponding uniform segments from the second image to determine if any differences between the respective images result from changes in the irradiation of surfaces in the scene caused by fluctuations in ambient lighting of said scene.