| 摘要 |
An electron gun for use in a cathode ray tube (CRT) includes a source of energetic electrons, i.e., a cathode, a beam forming region for forming the energetic electrons into a narrow beam, an arrangement for deflecting the beam over a display screen, and a focus arrangement for focusing the beam on the display screen in the form of a small spot in forming a video image. The beam forming region includes a G1 control grid, a G2 screen grid and a portion of a G3 grid in facing relation to the G2 grid, where each grid includes a respective aperture through which the electron beam is directed, with the apertures aligned along a common axis. The size of the aperture on the side of the G2 grid in facing relation to the G1 grid forms a first focusing lens and is smaller than the size of the aperture on the opposed side of the G2 grid in facing relation to the G3 grid which forms a second focusing lens. The larger size of the G2 aperture in facing relation to the G3 grid reduces electron beam convergence and spherical aberration of a video image formed by the beam on the display screen. The aperture of the G2 grid may increase in diameter in proceeding from the low side, i.e., the G1 control grid facing side, to the high side, i.e., the G3 grid facing side, in a step-wise manner or in the general shape of a cone. The invention is adapted for use in a single beam projection CRT as well as in a multi-beam color CRT.
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