The invention relates to a photoplethysmography apparatus (100) comprising a source of light (110)configured to provide source light (130) of at least a first and a second spectral position directed at a tissue (140) alight detector (120) configured to detect scattered source light and to provide at least a first and a second sensor signal (127 29) indicative of the scattered source light of the first and second spectral position and a processing unit (150). The processing unit is configured to calculate a corrected sensor signal (160) indicative of a variation in blood absorbance within the tissue by removing a tissue path error signal component indicative of a variation in optical path length through the tissue over time and a light coupling error signal component indicative of a variation of source light intensity of the source light emitted at the tissue from the at least first and second sensor signals.
