An apparatus, method and computer program are provided. The apparatus, comprises: a photoplethysmography light source; a photoplethysmography light sensor configured to sense light signals emitted by the photoplethysmography light source; and an actuator configured to lift a surface of skin tissue in order to create an optical path through the skin tissue from the photoplethysmography light source to the photoplethysmography light sensor, for transmissive photoplethysmography.