A method for imaging a quantity of gas present in the atmosphere of a selected area. The method comprises the steps of directing background infra-red radiation from the selected area into an interferometer; imaging the infra-red radiation emerging from the interferometer onto at least one infra-red detector: obtaining a plurality of Fouriertransform infra-red spectra in the 8-14 micrometer spectral region, each spectrum coitesponding to infra-red radiation collected from a different portion of the selected area; and displaying in a suitable form an infra-red image, the infra-red image comprising the plurality of infra-red spectra, or quantities derived therefrom. The temperature of the quantity of gas or ambient temperature is measured, the temperature of the background is measured, and the difference between the two measured temperatures is used to derive gas column densities from the infra-red spectra.