Wolfram Czarnojan

Phosphor screens for flat cathode ray tubes

Abstract In the Zeus display panel, as in other flat-panel cathode ray tubes, the anode voltage is limited to a few kV. The effects of the anode voltage on phosphor screen performance are discussed which include energy conversion loss processes at the phosphor surface and variation of the number of luminescent centres available. Phosphor efficiency saturation has been measured in the range of 1 to 10 kV, using a pulse-width-modulation scheme. The luminance performance at different excitation conditions is predicted and confirmed experimentally. Coulomb degradation of phosphor screens is shown to increase with decreasing anode voltage. Novel ultra-thin and dense anti-degradation coatings have been developed for zinc sulphide phosphors which effectively prevent fast degradation due to chemical surface reactions. In this way a satisfactory solution has been found to obtain in the Zeus display a luminance level about equal to the best conventional tubes.

Phosphor degradation under electron excitation with varying anode voltage

A model for the variation of phosphor Coulomb degradation with anode voltage is presented, using the volume energy density as scaling parameter. With reference to the bulk phosphor degradation at 30 kV Coulomb degradation down to 2 kV has been calculated. A result of this model is that the degradation at a given charge dose increases with decreasing anode voltage. Considering the high charge doses accumulated in low-voltage displays operated at CRT brightness, only phosphors which are stable at 30 kV are expected to be applicable at low voltages. The predicted phosphor degradation at 4 kV is experimentally confirmed for oxide, silicate, and oxy-sulfide phosphors. For zinc sulfides the degradation measured at 4 kV is much faster than predicted, pointing to considerable contributions of surface-related effects.