The performance of high gradient superconducting RF cavities for electron accelerators is mainly limited by field emission. Major improvements have been recently obtained using different surface conditioning techniques confirming the involvement of metallic particles in field emission enhancement. In this paper we present the results obtained with an optical apparatus attached to an RF copper cavity equipped with a removable sample which is subjected to high RF fields (E pk > 40 MV/m). Stable

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... ght spots are observed on the sample surface and their intensities and optical spectra are measured as a function of the surface electric field. The total emitted current is simultaneously measured by an isolated hollow electrode facing the sample. Particles of different types were deliberately sprinkled over the sample surface and the luminous features are studied. Light intensity, spectral power density and evolution of the luminous sites provide useful information for understanding the field emission phenomena and the conditioning effects.