GB753452A - Improvements in or relating to cathode ray tubes - Google Patents

Abstract

753,452. Colour television tubes. PHILCO CORPORATION. Aug. 1, 1952 [Sept. 14, 1951], No. 19512/52. Class 39 (1). The screen described in Specification 753,449, [Group XX], comprising a combination of fluorescent strips and indexing strips, is modified by mixing the indexing material with the material of some of the fluorescent strips, preferably those responsive to beam impingement in the same primary colour. The screen comprises glass support 32, Fig. 2, carrying a thin transparent conductive film 33 of stannic oxide and, over the film 33, fluorescent strips 34, 35, 36. A material or materials of different secondary emissive ratio from the red and blue strips may be mixed with the green strips. The added material or materials are such as not to affect the phosphor chemically to impair the brightness or colouration of its light emission, and are preferably mixed with the phosphor before deposition. They may be mixed with the phosphor or phosphors whose light emission efficiency is greatest so as to achieve colour balance. The added material may be uniformly or non-uniformly distributed, and in the latter case may be of higher concentration near the surface exposed to the beam. Since the secondary emissivities of the phosphors are different, green usually having the lowest value, more additional material will be required if it is desired to mix it with the green than with the other phosphors to produce the same differences in the collected output. The green, blue and red phosphors may be zinc orthosilicate, calcium magnesium silicate and zinc phosphate respectively, and the indexing material may be magnesium oxide, gold or tungsten. In an example the green phosphor contains 25-75 per cent by weight of magnesium oxide. The phosphors may be deposited by silk screening, settling or spraying, preferably the latter. The screen base may be sprayed over its whole surface successively with the different phosphor materials, one of which is mixed with magnesium oxide, and the vehicle being polyvinyl acetate and ammonium dichromate. When a single layer has been deposited it is exposed to light through narrow slits in a mask, the irradiated portions thus being fixed and the remaining portions then washed off. The mask is moved or a different mask used for each set of strips. Specifications 753,448, [Group XXXV], and 753,551, [Group XL (b)], also are referred to. Reference has been directed by the Comptroller to Specification 669,848.