Effects of cathode bumpiness on the spatial resolution of proximity focused image tubes

In proximity focused electrostatic image tube; spatial resolution is a strong function of the cathode flatness.Topographic irregularities of the cathode surface establish transverse electric fields near the cathode.The fields impart a component of transverse momentum to the emitted electron, which, in turn,degrades the spatial resolution characteristic of the system. Calculations based on a simple model of a corrugated cathode show that, under a typical set of operating conditions, the point at which loss of resolution becomes considerable is 60 cycles/mm for a bump height of 0.2 microm and a bump period of 1 microm. For the resolution to be comparable to that predicted for a perfectly flat GaAs negative electron affinity (NEA) cathode, which is 160 cycles/mm, the bump period must be 0.1 microm, and its height must be 0.02 microm. Such small bumps represent a relatively smooth surface. Observations of surface irregularities on related NEA surfaces indicate that the bump period and height are about 1.0 micro and 0.2-0.5 microm, respectively, so that the spatial resolution in these cases is limited by the deviation from flatness as opposed to being limited by the finite random velocities of the emitted photoelectrons.