Method for sensor calibration

The present invention provides a method of determining the amount of an optical probe species binding to or releasing from an optical sensor surface. The method is characterized in that: the determination comprises the steps of: a) determining, at one single wavelength or at more than one wavelength, a physical measurand (xi) that is related to the absorptivity of the probe, and b) correlating the value of the measurand to the amount of the optical probe species binding to or releasing from the surface, respectively, wherein the physical measurand (xi) of the step a) is a physical measurand in which the contribution from the refractive index is substantially zero. The present invention further provides different uses of a peak width as well as a computer program product and reagent kits for the method.