Orthicon and CPS Emitron
The original iconoscope was very noisy due to the secondary electrons released from the photoelectric mosaic of the charge storage plate when the scanning beam swept it across. An obvious solution was to scan the mosaic with an electron beam, which velocity and energy were so low in a neighborhood of the plate, that no secondary electrons were emitted at all. That is, an image is projected onto the photoelectric mosaic of a charge storage plate, so that positive charges are produced and stored there due to photo-emission and capacitance, respectively. These stored charges are then "gently" discharged by a low-velocity electron scanning beam, preventing the emission of secondary electrons. Not all the electrons in the scanning beam may be absorbed in the mosaic, because the stored positive charges are proportional to the integrated intensity of the scene light. The remaining electrons are then deflected back into the anode, captured by a special grid, or deflected back into an electron multiplier.
Low-velocity scanning beam tubes have several advantages; there are low levels of spurious signals and high efficiency of conversion of light into signal, so that the signal output is maximum. However, there are serious problems as well, because the electron beam "spreads" when it scans the image's borders and corners, so that one gets an image that is well focused in the center but blurry in the borders. Another improvement is the use of a semitransparent charge storage plate. The scene image is then projected onto the back side of the plate, while the low-velocity electron beam scans the photoelectric mosaic at the front side. This configurations allows the use of a straight camera tube, because the scene to be transmitted, the charge storage plate, and the electron gun can be aligned one after the other.
In 1928, Tihanyi filed for a patent for a television apparatus where an electron beam scanned the mosaic of a charge storage plate and was deflected into the anode following a V-path. Five years latter, in 1933, Farnsworth filed for a patent where he explained for the first time that the electrons in the scanning beam must arrive with zero-velocity to the charge storage plate, so as to prevent the emission of secondary electrons. Nevertheless, neither Tihanyi nor Farnsworth considered the use of a semitransparent plate, and they never transmitted clear images with the devices described in their patents, because they were not aware that the beam alignment had to be perpendicular to the plate in order to produce a well focused image.
The first fully functional low-velocity scanning beam tube, the CPS Emitron, was invented and demonstrated by the EMI team under the supervision of Isaac Shoenberg. In 1934, the EMI engineers Blumlein and McGee filed for patents for television transmitting systems where a charge storage plate was shielded by a pair of special grids, a negative (or slightly positive) grid lay very close to the plate, and a positive one was placed further away. The velocity and energy of the electrons in the scanning beam were reduced to zero by the decelerating electric field generated by this pair of grids, and so a low-velocity scanning beam tube was obtained. The EMI team kept working on these devices, and Lubszynski discovered in 1936 that a clear image could be produced if the trajectory of the low-velocity scanning beam was nearly perpendicular (orthogonal) to the charge storage plate in a neighborhood of it. The resulting device was dubbed the cathode potential stabilized Emitron, or CPS Emitron. The industrial production and commercialization of the CPS Emitron had to wait until the end of the second world war.
On the other side of the ocean, the RCA team led by Albert Rose began working in 1937 on a low-velocity scanning beam device they dubbed the orthicon. Iams and Rose solved the problem of guiding the beam and keeping it in focus by installing specially designed deflection plates and deflection coils near the charge storage plate to provide a uniform axial magnetic field. The orthicon was the tube used in RCA's television demonstration at the 1939 New York World's Fair, its performance was similar to the image iconoscope's one, but it was also unstable under sudden flashes of bright light, producing "the appearance of a large drop of water evaporating slowly over part of the scene".
Read more about this topic: Video Camera Tube