Abstract: 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 (C?); 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).