Cathode-ray tube having an electron gun with an improved main focus lens

A cathode-ray tube having an electron gun having a main focusing lens, the electron gun comprising a focusing electrode and an acceleration electrode aligned along the mean axis of emission of the electron gun, each electrode comprising an aperture of elongated form along a horizontal axis and a plate furnished with a central hole and with outer holes, disposed in proximity to the aperture of the electrode and parallel to this aperture, the three holes of each plate being aligned along an axis parallel to the horizontal axis;

• wherein the aperture of each electrode comprises a rectangular aperture whose large dimension is along the horizontal axis and terminating at its two ends in two identical semi-ellipses of order n that are symmetric with respect to the axis of the gun, and of formula:
• (
• x
• a
• )
• 2
• n
• +
• (
• y
• b
• )
• 2
• n
• =
• 1
• in which x=a cosn θ;
• y=b sinn θ,
• the parameter n has a value different from 1,
• a is half the width of the ellipses of order n along the horizontal axis and b being half the height of the ellipses of order n along the vertical axis,
• x represents the abscissa of a point lying on the ellipse of order n and y is the ordinate of a point lying on the ellipses of order n,
• θ is an angle which varies in one quadrant between 0° and 90°.