Apparatus for determining presence of a gas in a gas detection volume , the apparatus comprising one or more retarders configured to spectrally modulate polarisation of received radiation in accordance with a plurality of polarised spectral modulation profiles which are offset in phase from each other, the received radiation having passed through a gas detection volume which may comprise the gas to be detected, the radiation output from the one or more retarders comprising radiation having polarisation spectrally modulated in accordance with the said plurality of polarised spectral modulation profiles in a common beam of radiation; one or more polarisers ; and radiation detectors to detect radiation output from the one or more retarders filtered for respective polarisation states by the one or more polarisers , the radiation detectors selectively and separately detecting on different radiation detectors at the same time polarised radiation conforming to each of at least first and second of the said polarised spectral modulation profiles to thereby provide at least respective first and second polarisation-dependent radiation intensity measurements from which the presence of the gas in the detection volume can be determined wherein the said polarised spectral modulation profiles have alternating maxima and minima which are substantially periodic with frequency of radiation within the selected frequency range, the frequency spacing between successive maxima being substantially equal to the frequency spacing between peaks of a pair of spectral features of the gas in the detection volume within the selected frequency range. one or more retarders configured to spectrally modulate polarisation of received radiation in accordance with a plurality of polarised spectral modulation profiles which are offset in phase from each other, the received radiation having passed through a gas detection volume which may comprise the gas to be detected, the radiation output from the one or more retarders comprising radiation having polarisation spectrally modulated in accordance with the said plurality of polarised spectral modulation profiles in a common beam of radiation; one or more polarisers ; and radiation detectors to detect radiation output from the one or more retarders filtered for respective polarisation states by the one or more polarisers , the radiation detectors selectively and separately detecting on different radiation detectors at the same time polarised radiation conforming to each of at least first and second of the said polarised spectral modulation profiles to thereby provide at least respective first and second polarisation-dependent radiation intensity measurements from which the presence of the gas in the detection volume can be determined wherein the said polarised spectral modulation profiles have alternating maxima and minima which are substantially periodic with frequency of radiation within the selected frequency range, the frequency spacing between successive maxima being substantially equal to the frequency spacing between peaks of a pair of spectral features of the gas in the detection volume within the selected frequency range. <EOS>