In a motion detection system, filters are applied to a series of digital images in order to determine whether changes in the properties of pixels between the current image and a reference model are due to motion of objects or to changes in lighting. The filter may be based on scaled differences between a current image and a reference model. The type of surface may be determined based on optical properties of the surface and then only the best filter for that type of surface is applied. Alternately, multiple filters may all be applied and the results combined. The processing may be reduced using a background model to determine which pixels are in the background and which are in the foreground and then only applying the filter to the foreground pixels.