Abstract: Method and apparatus for contactless locally resolved instantaneous measurement of the density and/or temperature of a gas in a measurement volume which can have a relatively large area and a relatively small thickness, wherein by laser radiation two rotational transitions of a molecular constituent of the gas excitation in which the excited state has a life time which is substantially shorter than the reciprocal collision frequency in the gas, and wherein the fluorescence intensity or absorption corresponding to the two rotational transitions is measured. From the measured intensities the density and temperature can be calculated.