Abstract: Automatic image-based volumetric detection of an object in a space is achieved based on a combinational characteristic produced by combining a first characteristic of a reference image associated with the space when empty, and second characteristic of a detection image associated with the space at a time when object detection is active. The combinational characteristic is independent of relative image intensity between the reference image and the detection image. The object detection operation is thus provided with a degree of independence relative to the respective illumination conditions during capture of the reference and detection images.