Lead is shown to be an activator for the phosphorescence of calcium and strontium tungstates, the optimum concentration being about 0.01 mol of PbWO4 per mol of alkaline earth tungstate. In addition, it was observed that much higher concentrations of lead produce an enormous increase in both the fluorescent and phosphorescent power of strontium tungstate. The optimum concentrations of lead for this effect are 0.2 and 0.4 mol fraction for the fluorescence and phosphorescence, respectively. High concentrations of lead in calcium tungstate serve only to decrease the luminescent power.
FRANK E. SWINDELLS, "The Luminescence of Alkaline Earth Tungstates Containing Lead," J. Opt. Soc. Am. 23, 129-129 (1933)