A subject monitor includes a belt (16) that circumscribes a cyclically moving portion of a subject (14), such as a chest cavity or abdomen. A optical sensing device (18) includes a housing a tape (50) under tension to retract into the housing, at least one of the tape (50) and the housing are connected with the belt (16) a lens (56) configured to focus light on a pattern (52) that moves with the tape (50) and a photon detector (58) configured to detect light (57) reflected from the pattern (52). An optical decoder (26) determines movement of the belt (16) from the light reflected (57) from the pattern (52).
