An intrusion detection system comprises a pair of optical lenses arranged a predetermined distance apart and having overlapping fields of view within an area to be monitored to form a common field of view; at least one light-sensitive device responsive to light from each of the optical lenses; a range detector responsive to signals from the light-sensitive device and operable to determine a range to an object within the common field of view; and a range discriminator for setting at least one range gate to sense objects within the common field of view at predetermined ranges and for ignoring objects outside of the predetermined ranges.