Light radiation absorption gas detector

A light radiation absorption gas detector for detecting the presence of a specified gas, comprising a light source, a region containing a gas to be detected through which a light beam from the source is passed, an interference filter having a substantially flat face and a light radiation absorption peak at a wavelength which facilitates detection of the specified gas, a shaft to which the filter is secured for rotation so that a perpendicular to the flat face of the filter is at an angle to the axis of the rotation, light detector means responsive to light from the source which has passed through the region and the filter for detecting predetermined light amplitude variations, and a light chopper or interrupter interposed between said source and said light detector means, wherein light from the source is directed via first lens means through the region, via second lens means from the region to the filter and via third lens means from the filter to the light detector means, and wherein light from the source is arranged to be projected towards the filter at an angle offset from the axis of shaft rotation whereby a filter pass band which includes the said wavelength is scanned consequent upon rotation of the filter, thereby to produce amplitude variations of light in the presence within the region of the specified gas, whereby detection of the specified gas by the detector means is facilitated.