Image transfer properties of proximity focused image tubes

Experimental measurements of the lateral (i.e., radial) spreading of photoelectrons from a multialkali photocathode in an unfocused image dissector tube have shown that (a) the distribution of the radial emission energies is approximately Maxwellian and (b) the radial emission energies are smaller than expected based on the difference between the input photon energies and the long wavelength threshold energy of the multialkali photocathode ( congruent with1.4 eV). Both results are favorable with respect to the predicted electron optical image transfer properties of proximity focused image tubes. The observed data indicate that at least a 40-lp/mm or a 1600-lp/40-mm image field diameter should be resolvable for white light input to commercially available tubes-and that this resolving power should increase to at least 60 lp/mm for input wavelengths longer than approximately 600 nm and decrease to 30 lp/mm or less for input wavelengths shorter than approximately 400 nm.