A sensor may be adapted to account for factors that cause irregularities in sensor measurements. A sensor may selectively absorb light from outside sources. A sensor may selectively absorb light near a region of tissue having relatively large subcutaneous anatomic structures, such as large blood vessels, and selectively reflect light near a region of tissue that is relatively free of large blood vessels or other structures. The sensor is adapted to reduce the effect of large subcutaneous anatomic structures and outside light on measurements for pulse oximetry or other spectrophotometric techniques.