Abstract
The optical conditions in quasi‐infinitely thick phosphor plaques and in translucent phosphor coatings of closed lamps are studied. Theory shows, and experiments carried out on magnesium fluorogermanate phosphors activated with Mn confirm, that the correlation between the brightness of plaques and of translucent coatings in closed lamps is rather involved. Although prolonged phosphor firing time monotonically increases the former, the latter reaches a maximum and then decreases. This and other unpredictable results render dubious the value of crude plaque brightness tests of phosphors meant for translucent coatings in lamps. Reliable measurements of phosphors for such coatings must include for the main wave lengths of excitation relative quantum efficiencies, reflectances of quasi‐infinitely thick phosphor layers, and also differential reflectivities or absorptivities in a thin phosphor layer of a known density.
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