The spectral distribution for the excitation of phosphors is so erratic that it seems at present impossible to predict the location of strong excitation bands. Out of many phosphors examined in rare gas discharges, none was found to exceed in brightness the calcium tungstate and zinc silicate already in commercial use. The light output of the latter, however, was considerably improved by the elimination of the free silica usually present, which evidently has an appreciable absorption to radiation in the Schumann region. On the reasonable assumption that the resonance radiation is the source of excitation, the efficiency of fluorescence emitted by the zinc orthosilicate phosphor in the neon discharge denotes a quantum conversion near unity, just as in the low pressure mercury discharge.
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