Apparatus and method for determining presence of a gas

A method for determining presence of a gas, the method comprising:

• one or more retarders spectrally modulating polarisation of received linearly polarised radiation in accordance with a plurality of polarised spectral modulation profiles which are offset in phase from each other, 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;
• and
• radiation detectors detecting radiation output from the one or more retarders filtered for respective polarisation states by one or more polarisers, the radiation detectors selectively and separately detecting on different 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, and
• determining the presence of the gas based on the at least first and second polarisation dependent radiation intensity measurements;
• wherein the said polarised spectral modulation profiles have alternating maxima and minima which are substantially periodic with frequency of radiation within a 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 within the selected frequency range.