The invention relates to a method to determine the wavelength dependent absorption coefficient of a turbid medium using overlapping illumination-detection areas comprising the steps of a) retrieving a calibration spectrum (CA) from a reference measurement using a reference sample； b) carrying out a measurement on an actual sample for determining the absolute reflection spectrum (Rabs) using a raw spectrum measured on the sample (Smedium) and the calibration spectrum (CΛ)； C) using the absolute reflection spectrum (Rabs) for determining the wavelength dependent absorption coefficient by minimizing the difference between the measured absolute reflection spectrum (Rabs) and a model function (Rabsmodel). wherein the model function (Rabsmodel) is modelled using a predetermined equation based on prior knowledge of the combination of a dependence of the effective photon path length (LPF) on a scattering phase function (PF)； a dependence of the absolute reflectance in the absence of absorption (Rabs0) on scattering phase function (PF). The invention further relates to a system and a computer program product for determining the wavelength dependent absorption coefficient of a turbid medium.
