Abstract: Monochromatic excitation light penetrates an indicator in an indicator sp. The indicator space is closed off at the side to be brought into contact with the substance to be analyzed by means of a membrane permeable for the component of the substance whose concentration is to be ascertained. The side of the indicator space facing the light-measuring unit of the device is radiation-transmissive. The indicator is set into motion during the measurement operation. Alternatively, the substance to be analyzed, on the other side of the permeable membrane, is set into convective motion during the measurement operation. As a further alternative, both the indicator and the substance to be analyzed are set into motion in mutually perpendicular directions, which however are both parallel to the permeable membrane.