A respiration gas monitor device comprising: a respired air flow path for carrying respired air; an infrared light source arranged to launch infrared light through the respired air flow path; an optical detector arranged to detect the infrared light after passing through the respired air flow path; an absorption line bandpass filter having a passband that encompasses an absorption line of a target gas; a reference line bandpass filter having a passband over which the respired air is transparent; a control device (62, 64, 70, 80, 90) operative to switch the RGM device between: a monitoring state in which the absorption line bandpass filter is in the path of the infrared light and the reference line bandpass filter is not in the path of the infrared light, and a calibration state in which the reference line bandpass filter is in the path of the infrared light and the absorption line bandpass filter is not in the path of the infrared light; and characterised by an electro-optical beam steering device operable at: a first electric bias implementing the monitoring state by steering the infrared light to an optical path that passes through the absorption line bandpass filter and does not pass through the reference line bandpass filter, and a second electric bias implementing the calibration state by steering the infrared light to an optical path that passes through the reference line bandpass filter and does not pass through the absorption line bandpass filter. a respired air flow path for carrying respired air; an infrared light source arranged to launch infrared light through the respired air flow path; an optical detector arranged to detect the infrared light after passing through the respired air flow path; an absorption line bandpass filter having a passband that encompasses an absorption line of a target gas; a reference line bandpass filter having a passband over which the respired air is transparent; a control device (62, 64, 70, 80, 90) operative to switch the RGM device between: a monitoring state in which the absorption line bandpass filter is in the path of the infrared light and the reference line bandpass filter is not in the path of the infrared light, and a calibration state in which the reference line bandpass filter is in the path of the infrared light and the absorption line bandpass filter is not in the path of the infrared light; and characterised by an electro-optical beam steering device operable at: a first electric bias implementing the monitoring state by steering the infrared light to an optical path that passes through the absorption line bandpass filter and does not pass through the reference line bandpass filter, and a second electric bias implementing the calibration state by steering the infrared light to an optical path that passes through the reference line bandpass filter and does not pass through the absorption line bandpass filter. a monitoring state in which the absorption line bandpass filter is in the path of the infrared light and the reference line bandpass filter is not in the path of the infrared light, and a calibration state in which the reference line bandpass filter is in the path of the infrared light and the absorption line bandpass filter is not in the path of the infrared light; and characterised by an electro-optical beam steering device operable at: a first electric bias implementing the monitoring state by steering the infrared light to an optical path that passes through the absorption line bandpass filter and does not pass through the reference line bandpass filter, and a second electric bias implementing the calibration state by steering the infrared light to an optical path that passes through the reference line bandpass filter and does not pass through the absorption line bandpass filter. a first electric bias implementing the monitoring state by steering the infrared light to an optical path that passes through the absorption line bandpass filter and does not pass through the reference line bandpass filter, and a second electric bias implementing the calibration state by steering the infrared light to an optical path that passes through the reference line bandpass filter and does not pass through the absorption line bandpass filter.