Electron optical ray tracing in cathode lenses

In cathode lenses with high anode potential, as used in image tubes, each object point is imaged by a narrow ray bundle, Requirements for a ray-tracing method are 1) computation of a principal ray with allowance for total aberration, and 2) computation of other rays in the imaging bundle in a manner which is "paraxial" with respect to the principal ray; i.e., small departures from the principal ray, due to initial energies, are computed directly on a linear basis, using potential derivatives at the principal ray only. The method described fulfills these requirements and allows accurate determination of focal surfaces, displaying the effects of field curvature, astigmatism, and distortion. Accuracy is demonstrated by comparison with theoretical results and further supported by practical results.