Apparatus (10) and a method for visually monitoring a scene and detecting motion of an intruder within the scene. A camera (C) continually views the scene and produces a representative signal. A processor (12) processes the signal and produces an image (f2) represented by the signal. This image is compared with a similar image (f1) of the scene from a previous point in time. Segments of the later image which differ from segments of the earlier image are identified. A discriminator (14) evaluates these segments to determine if the differences result simply from lighting changes, or the movement of an intruder within the scene. If caused by an intruder, an appropriate indication is provided. An algorithm is employed by which differences from one image to another caused by lighting changes, the effects of motion of objects established within the scene, noise, and aliasing effects are identified so as not to produce false alarms. Those remaining segments are further evaluated to determine if there is an intruder present within the scene.
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