The absolute efficiency of ZnCdS:Ag screens under electron beam excitation is studied. This experimental study concerns the dependence of efficiency on the excitation voltage, on the screen’s thickness, and on the phosphor’s grain size. Measurements have shown that the appropriate excitation voltage for maximum efficiency of each screen is not the maximum possible, but depends on the grain size of the phosphor. All experimental results are discussed and explained, taking into account the arrangement of grains on the screen, the internal construction of each grain, and the penetration depth of electrons in the screen.