Ion spot

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An ion spot is a nearly circular shadow in the center of a historic cathode ray tube screen . It was created by heavy metal ions that emanate from the cathode in addition to the desired electrons . Over time, these ions destroy the sensitive luminous layer on the screen.

Ion spot on a television screen

Such ions can emerge from the cathode surface on the one hand (in the form of metal ions), but also arise from the ionization of remaining gas atoms in the cathode ray tube , as this can never have an absolute vacuum . Because of their mass, ions are more difficult to deflect than electrons, which are guided over the entire fluorescent screen by the image deflection . This creates a clearly defined spot in the middle of the screen.

Ion spots spoiled the visual impression in the early days of television , especially in the pre-war but also in the early post-war period, so that the picture tubes - which were expensive at the time - soon became unusable despite good cathode emission values.

View of the bent beam system of an ion trap; the permanent magnet has been moved to the right to take pictures

As a remedy, ion traps were used from around 1950 to suppress the ion spot: The beam generation system installed at an angle in the tube neck ensures that the ions reach the metal wall of the tube anode in the axial direction . The ion trap magnet , a weak permanent magnet , is attached externally to the tube neck and adjusted so that the electron beam is deflected again in the direction of the tube axis and thus towards the luminous layer. The massive ions are hardly affected by the weak magnetic field and still hit the anode of the beam generation system, where they do no relevant damage.

At the end of the 1950s it was recognized that an aluminum layer with a thickness of approx. 0.2 µm applied in front of the luminous layer (from the point of view of the beam generation system) could replace the ion trap (so-called aluminum-backed screen). The ions get stuck in the layer, the much smaller electrons fly almost unhindered to the luminescent layer. At the same time, the light is prevented from being radiated backwards into the tube bulb, which increases the brightness of the light spot (reflection on the aluminum layer) and the contrast (no brightening of dark areas of the image by scattered light from the tube bulb).

literature

  • Heinz Richter: TV for everyone . In: Radio technology for everyone . Three. Franckh'sche Verlagshandlung, Stuttgart 1955, p. 102 ff .
  • Rudolf Goldammer: The television receiver . 3. Edition. Franzis-Verlag, Munich 1958, p. 22nd ff .
  • Otto Limann: TV technology without ballast . 8th edition. Franzis-Verlag, Munich 1969, p. 38 ff .
  • Werner Espe: Materials science of high vacuum technology . tape 1 : Metals and metallically conductive materials . VEB Deutscher Verlag der Wissenschaften, Berlin 1957.
  • Werner Espe: Materials science of high vacuum technology . tape 3 : auxiliary materials . VEB Deutscher Verlag der Wissenschaften, Berlin 1961.