A sensor comprising: a first light detector (302) and a second light detector (303) configured to concurrently detect light and an optical blocking filter (304). The first light detector (302) is configured to detect light that enters the sensor to produce a first detected signal the optical blocking filter (304) is configured to filter the light that enters the sensor to produce filtered light and the second light detector (303) is configured to detect the filtered light to produce a second detected signal. There is also a subtraction element configured to subtract the second detected signal from the first detected signal to produce a subtracted signal wherein the subtracted signal is a sensor signal usable for producing a physiological measurement result.
