<p>A reflective photoplethysmograph sensor (for example mounted in an earpiece) arranged for photoplethysmograph measurements behind a subject's ear is provided. Also provided is a wearable photoplethysmograph heart rate sensor which includes a plurality of radiation detectors defining respective sensing planes which are tilted with respect to each other. Further there is provided a photoplethysmograph heart rate sensing system which compensates for motion artefacts using a dark signal which may be derived during an off phase of a duty cycle of an emitter and a photoplethysmograph system arranged to select between a plurality of detectors based on a quality measure. Combinations of the systems and sensors are also disclosed.</p>
