| 摘要 |
1308113 Cathode ray tubes PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 19 April 1971 [11 April 1970] 26608/71 Heading H1D An electron gun in a display tube comprising a cathode 6, a first control grid 7, a second accelerating grid 8 and a third grid 9, is characterized by the provision of an astigmatic lens element. In Fig. 2 the lens element comprises two spaced-apart semi-circular plates 16 and 17 on the plate part 13 of the second grid on either side of a circular aperture 14. At low beam currents the beam spot on the screen is thereby elongated vertically so that the line structure is less apparent; at high beam currents the spot is circular, the larger spot being sufficient to mask the line structure and any elongation would reduce definition. In a modification, Fig. 6 (not shown), the plate 13 is covered with a circular plate provided with a central rectangular aperture. In another arrangement, Fig. 7 (not shown), the plate 13 is provided with a rectangular bulge extending towards the first grid. Other arrangements mentioned are the provision of non-circular beam apertures in either the second or third grids, or the provision of slots in cylindrical parts of the second or third grids surrounded by electrodes at different potentials. In a shadow mask-type colour display tube Moire patterns may be prevented in this way. |
