For determining mode spectra of an optical property of a device under test (DUT) in dependence on a spectral parameter, with the mode spectra corresponding to the device's principal states of polarization (PSPs), minimum and maximum envelope values are determined for the optical property, or other measured values from which the envelope values can be determined with respect to possible state of polarization of light that is incident upon the DUT, whereby the minimum envelope values and the maximum envelope values are determined for a spectral range of interest of the spectral parameter. The mode spectra are derived for the optical property for at least one of the PSPs as a function of the spectral parameter for the spectral range of interest, whereby a partial correspondence of the mode spectra with the minimum and maximum envelope values is used for deriving the mode spectra.