A variable aperture sensor has a first jaw and a second jaw that are rotatably attached. An emitter is disposed in the first jaw, and a detector is disposed in the second jaw. The jaws are adapted to attach to a tissue site so that the emitter transmits optical radiation into the tissue site and the detector receives optical radiation through a variable aperture after absorption by the tissue site. The variable aperture is disposed in the second jaw and configured to adjust the amount of the optical radiation received by the detector.