Abstract: A method for determining spectral calibration data (?cal(Sd), Sd,cal(?)) of a Fabry-Perot interferometer (100) comprises: forming a plurality of filtered spectral peaks (P?1, P?2) by filtering input light (LB1) with a Fabry-Perot etalon (50) such that a first filtered peak (P?1) corresponds to a first transmittance peak (P1) of the etalon (50), and such that a second filtered peak (P?2) corresponds to a second transmittance peak (P1) of the etalon (50), using the Fabry-Perot interferometer (100) for measuring a spectral intensity distribution (M(Sd)) of the filtered spectral peaks (P?1, P?2), wherein the spectral intensity distribution (M(Sd)) is measured by varying the mirror gap (dFP) of the Fabry-Perot interferometer (100), and by providing a control signal (Sd) indicative of the mirror gap (dFP), and determining the spectral calibration data (?cal(Sd), Sd,cal(?)) by matching the measured spectral intensity distribution (M(Sd)) with the spectral transmittance (TE(?)) of the etalon (50).