The invention relates to a cathode-ray tube having an astigmatic electron lens, comprising at least two electodes spaced axially from each other, and means for supplying a first and a second potential to the electrodes, one of the electrodes at its end facing the other electrode having means to form an axial non-rotationally symmetrical electrical field between the two electrodes. The invention also relates to a device having such a cathode-ray tube.
In a cathode-ray tube it is often desired to focus an electron beam more strongly in one direction than the other. This may be necessary, for example, to compensate for astigmatism of the deflection coil or of other electron lenses in the tube. This is necessary, for example, in colour display tubes with three electron beams in a common plane and a parastigmatic self-converging deflection coil. Such a deflection coil converges the individual electron beams in a direction normal to the plane through the electron beams. The resulting vertical overfocusing cannot be compensated for by dynamically controlling the strength of the usual focusing lens as a function of the deflection, as this is done in colour display tubes having three electron beams in a delta configuration and a non-astigmatic deflection coil, because in that case horizontal underfocusing would occur.
British Patent Specification 889,005 discloses a quadrupole lens which consists of two coaxial cylindrical electrodes. The innermost electrodes are provided with apertures through which the electrical field between the two cylinders can penetrate into the space in the innermost cylinder. As a result of this, an astigmatic quadrupole field is formed in the innermost cylinder and converges the electron beam in one direction and diverges it in the direction at right angles thereto.
British Patent Specification 574,056 discloses a focusing lens in which, as a result of the design of the edge of one of the two cylindrical electrodes, an axial non-rotationally symmetrical field is obtained which converges the electron beam in one direction more strongly than in the direction at right angles thereto.