Abstract: Light operative apparatus is located inside electro-optical instrument viewing window, such as that used by a radiation pyrometer or dimensional gage in a steel mill environment, to detect and compensate for optically attenuating objects which may accumulate on the outside surface of the viewing window. Light beam irradiates viewing window from inside and optics collects light radiation backscatter which varies as a function of light-scattering attenuating objects that have accumulated on the viewing window outside surface. A light backscatter detector feeds display, alarm and instrument compensation functions.