Electron-optical analog study of image tubes with different types of cathodes

Electron trajectory analog measurements for image tubes show that the serious aberrations of distortion and curvature of the image field, inherent to flat unipotential photocathodes, can be reduced by using a flat gradient cathode. Similar potential distributions as with curved cathodes are thus established. The resulting curvatures of the equipotential surfaces in the vicinity of the flat gradient cathode can be changed within a relatively wide range by varying the potentials at the gradient-cathode terminals. Pincushion distortion rapidly vanishes and even becomes barrel-shaped with increasing cathode center potential. Adequate reduction of the imagefield defect can be achieved only at the cost of introducing barrel-shaped distortion.