1. A system for indicating a change between a first image and a second image of a target represented by electrical signals, comprising: first means for sensing a sequence of images of the target including the first image, the second image and an additional image previous in said sequence to the first image and the second image, the first image being previous in said sequence to the second image, and for generating and transmitting said electrical signals representing said first image and said second image; second means coupled to said first means for receiving said electrical signals transmitted by said first means; third means coupled to said second means for digitizing said electrical signals; fourth means responsive to said electrical signals and coupled to said third means for determining a first processed image in response to assigning a respective weight to each image previous in said sequence to the second image, determining a second processed image in response to said first processed image and the second image and determining an optical flow field in response to said first processed image and said second processed image, said optical flow field indicating the change between the first image and the second image; fifth means coupled to said fourth means for indicating a change in the target in response to said optical flow field; and sixth means coupled to said fifth means and to said first means for controlling said first means in response to said indicated change in the target.