An apparatus containing an optical emitter configured to emit optical radiation is provided. Further, the apparatus includes a first hermetically sealed measurement cell filled with a first gas. The first gas is configured to absorb the optical radiation at least partially at one or more predetermined wavelengths. Additionally, the apparatus includes a first microphone arranged in the measurement cell and configured to generate a first microphone signal on a basis of a photoacoustic excitation of the first gas by the optical radiation. The apparatus moreover includes an evaluation circuit configured to take the first microphone signal as a basis for generating a first measurement signal indicating an emission intensity of the optical emitter at the one or more predetermined wavelengths.