A cathode ray tube of the type having a target, an electron gun for producing a beam of electrons directed toward the target, deflection means disposed along the path of the beam for deflecting the beam in two orthogonal directions, and a postaccelerating electrode surrounding the path of the beam on its way from the deflection means to the target, wherein the improvement comprises:
- (a) a scan expansion lens system disposed between the deflection means and the target and in such a position that at least the target-side end of the lens system is acted upon by the field of the postacclerating electrode, the lens system comprising:
- (1) first, second, and third tubular electrodes of substantially rectangular cross-sectional shape disposed in axial alignment to permit the passage of the beam therethrough and spaced apart from one another with a gap sufficient to provide electrical insulation therebetween, each electrode having a beam entrance end directed toward the electron gun and a beam exit end directed toward the target;
- (2) the first electrode comprising a first pair of opposite sides disposed in one of said two orthogonal directions and a second pair of opposite sides disposed at right angles with the first pair of opposite sides, the beam exit ends of the first pair of opposite sides being each curved in an arc that is convex in a first direction;
- (3) the second electrode comprising a third pair of opposite sides disposed in said one of the orthogonal directions and a fourth pair of opposite sides disposed at right angles with the third pair of opposite sides, the beam entrance ends of the third pair of opposite sides being each curved in an arc that is convex in said first direction and which is in conformity with the arcs of the beam exit ends of the first pair of opposite sides of the first electrode, the beam exit ends of the third pair of opposite sides being each curved in an arc that is convex in a second position opposite to said first direction;
- (4) the third electrode comprising a fifth pair of opposite sides disposed in said one of the orthogonal directions and a sixth pair of opposite sides disposed at right angles with the fifth pair of opposite sides, the beam entrance ends of the fifth pair of opposite sides being each curved in an arc that is convex in said second direction and which is in conformity with the arcs of the beam exit ends of the third pair of opposite sides of the second electrode;
- (5) the third electrode further comprising an end plate closing the beam exit end thereof, the end plate having formed therein an aperture which is elongated in said one of the orthogonal directions and whose geometrical center substantially coincides with the axis of the beam passing the aperture without being deflected by the deflection means;
- (b) means for applying such electrical potentials to the first, second, and third electrodes of the lens system and to the postaccelerating electrode that there are created:
- (1) a diverging electron lens within the lens system to act on the beam in said one of the orthogonal directions;
- (2) a first converging electron lens within the lens system to act on the beam in the other of the orthogonal directions for focusing the beam at a point before the aperture in the end plate of the third electrode, the first converging electron lens converging the beam to a greater extent when the beam is deflected by the deflection means in said other of the orthogonal directions than when the beam is not deflected in said other of the orthogonal directions;
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- (3) a second converging electron lens adjacent the aperture in the end plate of the third electrode to act on the beam in said other of the orthogonal directions as the beam enters the second converging electron lens in a diverging state after being converged by the first converging electron lens, the second converging electron lens being created owing to a potential difference between the third electrode of the lens system and the postaccelerating electrode and converging the beam to a greater extent when the beam is not deflected by the deflection means in said other of the orthogonal directions than when the beam is deflected in said other of the orthogonal directions.