Comparison of the Merits of Proxdiity and Magnetic Focusing in Image Tubes

An image tube with proximity focusing, i.e. with closely spaced photocathode and phosphor, is compared with an image tube with the simplest magnetic focusing produced by homogeneous, parallel electric and magnetic fields. Both types of focusing give a 1:1 magnification and can be applied to large image areas. As long as the homogeneity of the fields is maintained, the resolution will be uniform over the total picture area in both cases. The accuracy of focusing in the two cases is derived. While the error is a first order expression of the ratio of the initial and end velocities in the proximity focusing, the first order term disappears in the magnetic focusing and a second order expression remains for the error. This shows the superiority of the magnetic focusing. By taking account of the statistical distribution of initial velocities, the resolution obtainable with proximity focusing is calculated. Particular attention is drawn to the fact that, in an actual experiment, the errors of different origin are not always independent of each other and therefore do not add quadratically. This is demonstrated for magnetic case.