A method of acquiring a circular dichroism spectrum measuring differential absorption of left and right circularly polarized light by a sample over a wavelength range of interest, the method comprising: arranging a sample in a sampling beam path; arranging a polarization modulator (46, 50) in the sampling beam path before the sample, the polarization modulator being periodically switchable between two states to output left and right circularly polarized light respectively at a polarization modulation frequency of between 20 kHz to 100 kHz; combining first and second light signals containing respective first and second frequency combs (C1, C2) into the sampling beam path, the first and second frequency combs consisting of respective finite series of discrete frequencies separated by first and second spacings defined as first and second free spectral ranges respectively; arranging a sample detector operable to output a measurement signal in the sampling beam path after the sample, the sample detector having a detection bandwidth that covers a frequency range corresponding to beat frequencies of a third frequency comb (C3') that is detected by the sample detector as the measurement signal as a result of heterodyne mixing of the first and second light signals; periodically switching the polarization modulator between its two states at a polarization modulation frequency of between 20 kHz to 100 kHz to output left and right circularly polarized light respectively; setting the frequencies of the first and second light signals such that the beat frequencies of the third frequency comb are higher than the polarization modulation frequency by between one and five orders of magnitude so that for each of the left and right circularly polarized light the sample detector's measurement signal measures transmission through the sample across the wavelength range of interest during each period of the polarization modulation. arranging a sample in a sampling beam path; arranging a polarization modulator (46, 50) in the sampling beam path before the sample, the polarization modulator being periodically switchable between two states to output left and right circularly polarized light respectively at a polarization modulation frequency of between 20 kHz to 100 kHz; combining first and second light signals containing respective first and second frequency combs (C1, C2) into the sampling beam path, the first and second frequency combs consisting of respective finite series of discrete frequencies separated by first and second spacings defined as first and second free spectral ranges respectively; arranging a sample detector operable to output a measurement signal in the sampling beam path after the sample, the sample detector having a detection bandwidth that covers a frequency range corresponding to beat frequencies of a third frequency comb (C3') that is detected by the sample detector as the measurement signal as a result of heterodyne mixing of the first and second light signals; periodically switching the polarization modulator between its two states at a polarization modulation frequency of between 20 kHz to 100 kHz to output left and right circularly polarized light respectively; setting the frequencies of the first and second light signals such that the beat frequencies of the third frequency comb are higher than the polarization modulation frequency by between one and five orders of magnitude so that for each of the left and right circularly polarized light the sample detector's measurement signal measures transmission through the sample across the wavelength range of interest during each period of the polarization modulation.