An arrangement of a thermal device and a surface reflecting and/or scattering electromagnetic radiation in the inner part of the thermal device. A source of electromagnetic radiation is arranged at a first distance (L1) from the surface, and a detector of electromagnetic radiation is arranged at a second distance (L2) from the surface. The source is configured to emit radiation to the surface, which is reflected and/or scattered from the surface as reflected radiation. The detector receives reflected radiation; and the processing unit determines data dependent on the first and/or second distance by the emitted and reflected radiation. A wall of the thermal device has a window or aperture for emitting an optical signal from the light source to the surface. An electromagnetic distance measurement device measures the thickness or the increase in the thickness of a contamination layer from a thermal device.