Cathode-ray tube having an electric cylinder lens for the dynamic correction of electrostatic deflection defocusing

Claim 1

A cathode-ray tube having means for the dynamic correction of defocussing caused by electrostatic deflection, comprising a hermetically sealed envelope, an electron gun within said envelope for producing an electron beam along the longitudinal axis of the electron gun, a target within said envelope for receiving said electron beam, at least one pair of deflection plates on opposite sides of a plane along which said axis lies within said envelope between said electron gun and said target for electrostatically deflecting said electron beam, means for applying deflection voltage signals to said deflection plates, and an electric cylinder lens surrounding said axis within said envelope for providing corrective deflection of said electron beam to compensate for defocussing of said electron beam resulting from the electrostatic deflection of said beam by said deflection plates, said electric cylinder lens comprising two electrodes positioned substantially orthogonal to said axis, each electrode having a slot-like aperture centered relative to the axis, the longer sides of said apertures being substantially orthogonal to the direction of deflection of said electron beam, a flat grid-like electrode positioned between said electrodes substantially orthogonal to the axis, and means including two parallel rectifier elements and a grounded potentiometer, the anodes of said rectifier elements having a common connection in series with said potentiometer, the cathode of each rectifier element being connected to each deflection plate for deriving from said deflection signals a negative voltage having a value as determined by departures of the amplitude of said deflection signals from the average value thereof, and the tap of said potentiometer being connected to said grid-shaped electrode thereby to provide dynamic correction of defocussing of said beam by said deflection signals.