GB799940A - Improvements in and relating to colour television systems - Google Patents

Abstract

799,940. Colour television tubes. MORLE, C. W., (Philco Corporation). Nov. 23, 1955, No. 33587/55. Class 39(1). [Also in Group XL(b)] Colour balance is obtained in a colour television system employing stripes of luminescent materials of different colour response and efficiency by having the number or width of stripes of the materials in inverse relationship to their efficiencies. Indexing stripes of secondary emitting material are provided to control the scanning. The face plate of the colour tube is provided with a screen 40 comprising red phosphor stripes 42, 46 interposed in double frequency between the green phosphor stripes 44 and the blue phosphor stripes 48 in the sequence G, R, B, R, G, R, B, R, &c. A layer 50 of conducting material, e.g., aluminium, magnesium or beryllium covers the phosphor stripes, and indexing stripes 52 of higher secondary emission than the layer 50 are formed thereon. These stripes of caesium or magnesium oxide or a high atomic number metal such as gold, platinum or tungsten, are positioned over the green or blue stripes. The signals from the indexing stripes are used to control the relationship between the phase of the video information supplied to the control electrode of a C.R.T. and the position of the beam therein (see Group XL(b)). The widths of the strips may be modified to compensate for differences in phosphor efficiency alternatively the green or blue phosphor may be diluted to equalize efficiencies. The positioning of the indexing stripes I over the blue or green phosphor stripes, Fig. 4 (not shown) results in spurious indexing pulses which owing to the finite size of the beam spot are produced during the scanning of the red image field. These spurious pulses are symmetrically placed relative to the main pulse and they cancel out to give phase coincidence between the resultant indexing signal and the central axis of the indexing strip. The red phosphor may be zinc phosphate /manganese, the green zinc orthosilicate, and the blue zinc cadmium sulphide /silver. The indexing strips may be of a phosphor, e.g., caesium activated zinc oxide emitting ultra-violet which is detected photoelectrically, or one emitting visible light, suitable baffles being provided to exclude light from the main phosphor stripes.