A system for detecting intrusion across a surface, comprising a plurality of light sources projecting an array of illuminating beams along different optical paths in the surface and a detector array system directed such that it detects along a plurality of fields of view in the surface, illumination reflected from the illuminating beams. A signal processing system detects changes along the array of fields of view, in the reflected illumination level detected by the detector system. An increase greater than a predefined level in the reflected illumination level from any field of view provides an indication of an intrusion across the surveilled surface along that field of view, at the crossing point of the direction of that field of view with the optical path whose illuminating beam generated the increase in reflected illumination from that field of view.