A system for determining the area of a sash opening (112; 126; 210) in a fume hood (100; 120; 200) formed by at least one movable sash panel (108; 128a, 128b, 128c, 128d; 208a, 208b) mounted over a hood opening (106; 202) to an enclosure structure (102; 122) of the fume hood (100; 120; 200) to reduce the area of the sash opening (112; 126; 210) when the at least one sash panel (108; 128a, 128b, 128c, 128d; 208a, 208b) is moved in one direction or to expand the area of the sash opening (112; 126; 210) when moved in the other direction to provide access to a work surface (104; 124; 204) in the fume hood enclosure space, the system comprising: an emitter and sensor panel (220; 280; 281; 520) mounted in a fume hood enclosure space, the emitter and sensor panel (220; 280; 281; 520) comprising a first light emitter mounted on one side of an image sensor (230; 283), and a second light emitter mounted on an opposite side of the image sensor, the first and second light emitters configured to illuminate the fume hood enclosure space; a first reflective marker mounted on one edge of the sash opening to reflect light from at least the first light emitter; a second reflective marker mounted on an opposite edge of the sash opening (112; 126; 210) to re- fleet light from at least the second light emitter, where at least one of the edges of the sash opening (112; 126; 210) with either the first or second reflective marker (240a; 240b) is formed by an edge of one of the movable sash panels (108; 128a, 128b, 128c, 128d; 208a, 208b); and a sash position tracking unit configured to determine a distance between the first and second reflective markers (240a; 240b) based on a distance between a first marker image of the first reflective marker and a second marker image of the second reflective marker in an image detected by the image sensor (230; 283), where the distance between the first and second reflective markers (240a; 240b) is used to determine the area of the sash opening (112; 126; 210). an emitter and sensor panel (220; 280; 281; 520) mounted in a fume hood enclosure space, the emitter and sensor panel (220; 280; 281; 520) comprising a first light emitter mounted on one side of an image sensor (230; 283), and a second light emitter mounted on an opposite side of the image sensor, the first and second light emitters configured to illuminate the fume hood enclosure space; a first reflective marker mounted on one edge of the sash opening to reflect light from at least the first light emitter; a second reflective marker mounted on an opposite edge of the sash opening (112; 126; 210) to re- fleet light from at least the second light emitter, where at least one of the edges of the sash opening (112; 126; 210) with either the first or second reflective marker (240a; 240b) is formed by an edge of one of the movable sash panels (108; 128a, 128b, 128c, 128d; 208a, 208b); and a sash position tracking unit configured to determine a distance between the first and second reflective markers (240a; 240b) based on a distance between a first marker image of the first reflective marker and a second marker image of the second reflective marker in an image detected by the image sensor (230; 283), where the distance between the first and second reflective markers (240a; 240b) is used to determine the area of the sash opening (112; 126; 210).