A wearable device is described. The wearable device includes a housing having a back cover, and an optical mask on first portions of the back cover. The back cover includes a set of windows, with a first subset of windows in the set of windows being defined by an absence of the optical mask on second portions of the back cover, and a second subset of windows in the set of windows being inset in a set of openings in the back cover. An optical barrier surrounds each window in the second subset of windows. A set of light emitters is configured to emit light through at least some of the windows in the set of windows. A set of light detectors is configured to receive light through at least some of the windows in the set of windows.