Gas within a ventilation circuit (12) is analyzed by a spectrometer included in gas measurement module (16) that is inserted into the respiratory circuit. The gas measurement module includes an infrared source and a movable filter member comprising at least two filter elements. The optical path length of the spectrometer is reduced. This includes removing optical components configured to collimate or focus electromagnetic radiation within the spectrometer. However, path length of the spectrometer is reduced to the point that other enhancements associated with path length reduction outweigh losses to precision and/or accuracy caused by beam expansion in the spectrometer.