Cathode-ray tube

Claim 1

A cathode-ray tube comprising an envelope containing a luminescent screen and an electron gun for producing an electron beam propagating along a longitudinal axis of the tube to the screen, said electron gun including, in order along the direction of propagation of the electron beam, a plurality of electrodes having respective electron-beam-passing apertures including: (a) a control gate and a first lens electrode defining, during operation, a first lens for forming the electron beam into a crossover; (b) a second lens electrode which together with the first lens electrode defines, during operation, a second lens for accelerating the electron beam and pre-focusing the electron beam crossover; and (c) at least a third lens electrode which together with the second lens electrode defines during operation, a third lens for focusing the electron beam crossover into a spot on the screen; characterized in that said first and second lens electrodes are spaced apart and have their respective apertures centered on the axis, the aperture in the second lens electrode having a diameter smaller than twice that of the first lens electrode, and the effective spacing S-eff between said first and second lens electrodes being smaller than 1 mm., S-eff being defined as the minimum of the function -ΔV/E(z), where ΔV is the difference between predefined operating voltages of the first and second lens electrodes, and where E(z) is the electrical field strength on the axis between the first and second lens electrodes as a function of the distance z along the axis, the effective spacing S-eff being larger than the geometrical spacing between the first and second lens electrodes.