GB1231985A - - Google Patents

Abstract

1,231,985. Cathode - ray display tubes. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 6 Feb., 1970 [8 Feb., 1969], No. 5807/70. Heading H1D. In a cathode-ray display tube using at least two beams which pass through two cylindrical electrodes, forming a common focusing lens, at different distances from the axis of the electrodes, the effective boundary between the electrodes lies at axial positions which vary round the circumference of the lens to compensate for the otherwise unequal effect the lens would have on the beams. The tube may be a colour television tube of the shadow mask type using three co-planar beams which are focused and converged by the lens; the electron gun may comprise three cathodes aligned with respective apertures in the first three grids (Fig. 2, not shown) which are followed by the fourth and fifth grids 80 and 81 shown in Fig. 8. The opposed edges of these grids have the same diameter and follow a sinusoidal pattern which prevents astigmatism in the outside beams 84 and 86. In this case the four axial indentations are equal, but the two indentations lying along the vertical axis may be greater than the two lying along the horizontal axis 87, and this will distort all three beams into a line focus which may be desirable to avoid MoirÚ effects; alternatively they may be smaller in order to distort initially elliptical beams into spot foci. It is not necessary for the edges of both electrodes to be curved, and when the electrodes are of unequal diameters the effective boundary is the radial projection of the inner one on to the outer one, or an imaginary extension of the outer one. The outer electrode may be a wall coating.