The efficiency of major types of cathodoluminescent phosphors has been investigated at high-brightness (up to 30 000 cd/m(2)) operational conditions using thermionic electron source. The accelerating voltage was varied from 4 to 14 kV and electron beam current density was independently varied from 2 to 50 muA/cm(2). Under those conditions both thermal quenching and current saturation of the phosphors were observed. Due to combined influence of those factors, it was found that the best way to characterize the phosphor is plotting the efficiency (in lm/W) versus the specific power density (in mW/cm(2)) graph. The resulting plots show the integrated influence of current loading and elevated temperature on the phosphor efficiency. The best efficiencies at 200 mW/cm(2) and 10 kV were measured for a green color phosphor -55 lm/W, for a blue one-10 lm/W, and for a red-13 lm/W. (C) 2003 American Vacuum Society.