band illumination are more robust in this case the surface is irradiated over a broad band for example with the light from a halogen lamp and the reflected light is analyzed spectrally by comparison with the illumination signal it is possible to determine the reflectivity for each wavelength and the formation of a quotient at different wavelengths provides information about the surface condition an example which may be mentioned is the alloy hastelloy x for which a quotient from two optical bandpasses around 1 6 u03bcm u03bb 1 and around 2 1 u03bcm u03bb 2 is recommended for the analysis in the case of an unoxidized surface the reflection is greater at u03bb 2 than at u03bb 1 whereas precisely the reverse is true if an oxide layer is present light of both wavelengths can be flexibly transmitted via optical waveguides to determine the bandpasses and illumination strategy the optical properties of the combustion chamber material must be known or have been determined beforehand it is advantageous if the information obtained from the monitoring of the state of the oxide layer is combined with information obtained from the measurement results of a u03bb sensor in order to set a system operating mode which is oriented to the state of the oxide layer and is optimized with regard to power and efficiency furthermore by way of example information about the local composition of the combustion gas in the turbine can be taken into consideration it being possible for this information to be obtained for example with the aid of emission analysis a further exemplary embodiment is illustrated in fig3 unlike in the exemplary embodiment illustrated in fig1 the working medium is liquefied.