Television Pickup Tubes Using Low-Velocity Electron-Beam Scanning

Abstract

Television pickup tubes which use a low-velocity electron beam for scanning a photosensitive target offer the possibility of avoiding some of the limitations of devices which use a high-velocity beam. Some of the advantages of the low-velocity beam are: (1) low level of spurious signals, (2) high maximum signal output, and (3) high efficiency of conversion of light into signal. The major problems encountered are: (1) ways to keep the beam in focus and (2) ways to obtain undistorted scanning of the photosensitive target. One solution to the difficulties of low-velocity-beam scanning involves the use of a uniform axial magnetic field to guide and focus the beam near the target. Because of this magneticfield, the usual type of deflection plates or deflection coils cannot be used to advantage. Two types of scanning which may be used are: (1) the release by a flying light spot of a beam of photoelectrons from successive points on a photocathode already immersed in the magnetic field and (2) the displacement of a beam from a thermionic cathode by a pair of defiection plates or coils operating in conjunction with the axial magnetic field and having an aperture at least as wide as the target to be scanned. Developmental television pickup tubes using each of these types of scanning with low-velocity electrons were tested and found to give the low spurious signal, high signal output, and high efficiency which had been expected.