The present invention provides an optical analysis system for determining an amplitude of a principal component of an optical signal. The principle component is indicative of the concentration of a particular compound of various compounds of a substance that is subject to spectroscopic analysis. The optical signal is subject to a wavelength selective weighting. Spectral weighting is preferably performed by means of spatial light manipulation means in combination with a dispersive optical element. The inventive calibration mechanism and method effectively allows for an accurate positioning of the spatial light manipulation means. Calibration is based on a calibration segment on the spatial light manipulation means in combination with a reference light source and a detector.