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The quantum efficiency of a photoemitting material is determined by a combination of physical properties. It has been found that the requirements are met best by semiconductors of low work function surfaces. In the light of these general considerations, a short survey is given of the photocathodes available heretofore. A more detailed description will then be given of the new multi-alkali cathodes, particularly those of the composition of Sb-K-Na and Sb-K-Na-Cs. Their advantages for use in scintillation counters are discussed.