Special methods are proposed to estimate the conductivity of a thin phosphor layer deposited onto the screen of vacuum fluorescent display and field emission display. Some phosphor compositions have been studied with the aid of these methods. The conductivity of screens has been found to be subject to the voltage applied to a display and to alter significantly under the influence of electron bombardment. Electron-induced conduction is considered as the main cause of the observed conductivity. A correlation between some features of cathodoluminescence and electron-induced conduction has been revealed. Recommendations on how to calculate the optimal mode of display operation are given.