The present invention relates to a method for optically determining the concentration of a gas. The method includes using at least two luminescent dyes, the first being in-sensitive to the concentration of a gas with respect to the luminescence response (reference dye) and the second being sensitive to the concentration of a gas with respect to the luminescence response (indicator dye) the dyes show different luminescence decay times so that the resultant phase angle is indicative for the concentration of a gas, wherein the detected luminescent amplitude of the reference dye at a first moment in time is utilized to correct for sensitivity changes after the first moment.