Improvements in design and operation of image iconoscope type camera tubes

Television camera tubes using high-velocity electrons for scanning show a number of undesirable characteristics. These include spurious signals and edge flare, which are due to non-uniform secondary electron redistribution effects and, in addition, there is no true ¿black level¿ information in the picture signal. Effects due to uneven redistribution can be considerably reduced if the average storage surface potential is shifted negatively from that reached under normal operating conditions. In an improved design of the ¿Photicon¿ image iconoscope, this potential change is achieved by flooding the storage surface with low-velocity electrons, which are emitted from a large annular photo-emissive surface surrounding the storage plate. Suitably biased electrodes direct the low-velocity electron rain in such a manner as to compensate for the non-uniformity of the residual redistribution. The use of the new tube also allows simplification of the camera control unit. As the ¿black¿ picture signal is constant in relation to the interline pulses produced by beam suppression, it is no longer necessary to evaluate the blackest part of the picture signal in order to set the ¿black level¿. A simple ¿clamp circuit¿ is sufficient and results in a very satisfactory transmission of the average brightness component of the picture.