Described herein are systems and methods for optically isolating components of an optical sensor in physiological monitoring devices worn by a user to sense, measure, and/or display physiological information. An optical sensor may be mounted in the rear face of the device, emit light proximate a targeted area of a user's body, and detect light reflected from the targeted area. Optically isolating structure may be located at least partially between one or more components of the optical sensor to ensure that light detected by the sensor is light reflected from the targeted area rather than light emitted directly from a light source and/or ambient light. The optically isolating structure may, in some cases, extend between a contact surface of the monitoring device to a base portion of the sensor components or a surface of a circuit board to which the sensor components are mounted.