Powder phosphors including and were excited with x‐rays, in the energy range 25–135 keV, obtained from radioisotope sources. The phosphor efficiency (light output per unit incident x‐ray energy) was measured as a function of phosphor screen weight for each of five sources. The measured efficiencies are in reasonable agreement with theoretical curves having no adjustable parameters. The curves were calculated using tabulated x‐ray energy‐absorption coefficients and measured light transmission parameters, and assuming that the efficiency of conversion of absorbed x‐ray energy to light energy is equal to the cathode‐ray efficiency.