Abstract: A sensor apparatus for detecting constituents of a material, the apparatus comprising: a first sensor, which interacts with the material, configured to measure an optical spectrum of the material and is coupled to an evaluation device that is configured to output an output value relating to the level of the one or more constituents in the material with the aid of the measured spectrum and calibration data; a second sensor configured to analyse the material examined by the first sensor and to output a signal relating to the level of the one or more constituents in the material; and a calibration data generating device configured to generate the calibration data for the evaluation device with the aid of the signal of the second sensor.