An apparatus is suitable for measuring a photoplethysmogram (PPG). A photoplethysmographic sensor apparatus may include a casing defining a surface, a plurality of optical emitters configured to emit radiation extending from the surface, at least one photo sensor configured to capture radiation emitted by at least a subset of the plurality of optical emitters. At least a first measurement configuration and a second configuration is defined by the plurality of optical emitters and the at least one photo sensor such that the first and the second measurement configuration provide different measurement channels by including at least partially different sets of at least one optical emitter and at least one photo sensor. The first and second measurement configurations define different spatial configurations, each of which is line symmetric with respect to an imaginary line along the surface.
