The optical analysis system (20) for determining an amplitude of a principal component of an optical signal comprises a multivariate optical element (10) for reflecting the optical signal and thereby weighing the optical signal by a spectral weighing function, and a detector (9, 9P, 9N) for detecting the weighed optical signal. The optical analysis system (20) may further comprise a dispersive element (2) for spectrally dispersing the optical signal, the multivariate optical element being arranged to receive the dispersed optical signal. The blood analysis system (40) comprises the optical analysis system (20) according to the invention.