In a cloud height measuring device, the attenuation caused by deposits on the output and/or receiver windows of the device (e.g. by rain and/or dirt) are compensated for by measuring the degree of reflection of light from the window(s) when illuminated by light pulses. By means of a special measuring sequence in a processor unit included in the cloud height measuring device, light pulses separate from those used for the cloud height measurement itself, can be employed to assess the extent of the fouling. The measurement energy can then be controlled during the cloud height measurement proper, so that the total attenuation, caused by rain and dirt on the windows, is properly compensated for.
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