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.
