as an exemplification of the principles hereof as well as the best mode of practicing same no limitation to the specific embodiment illustrated is intended in a disclosed embodiment thermally induced noise can be substantially reduced or eliminated in a photoacoustic detector by using two spaced apart radiant energy transmissive elements one element can be an optical window with high transmissivity at a pre determined wavelength the second element displaced from the window can be implemented as an optical filter as radiant energy from a local source falls on the optical window it in part passes through the window a larger portion of the incident energy results in heating the optical window the radiant energy which has passed through the window is incident on the optical filter it in part passes though the filter into a gas sensing cell or chamber the presence of the displaced optical window thermally isolates the filter which could be an infra red filter for example which is in contact with the test gas in the cell or chamber this structure substantially eliminates any error signal produced by heating of the filter fig1 illustrates an embodiment 10 of a photoacoustic detector in accordance herewith detector 10 can include a housing 12 suitable for portable or fixed use such as by attachment to a wall ceiling or other mounting.