Emission reserve mapping of CRT-type cathodes

The "dip test" was recommended in 1956 to evaluate the emission activity of cathodes, and this method is now in common use for evaluation of cathodes in microwave tubes and in other electron devices wherein unvarying geometry and field configuration permit simple application of the test and interpretation of results. Application of the "dip test" to CRT cathodes, however, involves complexities associated with a) variation of emission current density from center to edge of the cathode, b) variation of the emitting area and current density with intensity modulation (grid voltage), and c) variation of the grid-cathode geometry which takes place during the cooling period of the dip test. A method has now been developed for accurately evaluating the cathode activity for varying radii of concentric circular cathode areas. A figure of merit is introduced which characterizes the cathode emission.