A wearable health monitoring device can include a physiological parameter measurement sensor or module configured to be in contact with a wearer's skin when the device is worn by the wearer on the wrist. The physiological parameter measurement sensor can noninvasively and optionally continuously measure one or more physiological parameters, for example, the oxygen saturation, of the wearer. The sensor can include a convex curvature to improve pressure, and therefore optical coupling, between the wearer's skin and the physiological parameter measurement sensor while balancing the pressure and the wearer's comfort. The sensor can include a light barrier between emitters and detectors and other light barriers to improve signal strength and reduce noise.