Abstract: A sensing system is used to sense when a stack of sheet material reaches a critical height. A light beam is shone onto a region on the surface of the stack to form a lighted area. Changes in the height of the stack of sheet material results in changes in size of the lighted area of the beam shining on the surface of the stack. A light detector monitors a zone on the stack surface in which the amount of light being monitored as shining on the zone varies in accordance with changes in size of the lighted area. The detection system detects when the lighted area has reached a predetermined size by detecting when the light shining on the zone has reached a predetermined point. The system then provides an indication that the stack surface has reached a critical height, in other words that the stack surface has reached a critical distance from the light source.