In-line plural beams cathode ray tube having color phosphor element strips spaced from each other by intervening light absorbing areas and slit-shaped aperture mask

#This invention relates to a color cathode ray tube, and more particularly to a cathode ray tube provided with a screen in which color phosphor element strips are spaced from each other by intervening light absorbing strips.

#With the prior art black matrix type color cathode ray tube, the inner wall of the faceplate is provided with a phosphor screen in which there are scattered a large number of groups of three triangularly arranged phosphor dots producing red, blue and green lights, and the other portions of the screen than those where said phosphor dots are formed are coated with a black light absorbing layer. A shadow mask disposed adjacent to the phosphor screen is perforated with a plurality of holes shaped like the dots of the phosphor screen with one hole so positioned as to face one group of said three triangularly arranged phosphor dots. In the neck portion of the cathode ray tube envelope is received an electron gun unit consisting of three triangularly arranged electron guns for emitting three electron beams through said mask holes. According to the prior art cathode ray tube, the mask holes are enlarged by re-etching so as to cause electron beams having a larger diameter than the phosphor dots to impinge thereon. Namely, an electron beam passing through one mask hole has a larger area of incidence than the surface area of each of the three phosphor dots.

#With the conventional color cathode ray tube, the light absorbing area occupies as much as about 50 percent of the entire screen area, offering the advantage of reducing the reflection of external light, reproducing an image bearing distinct color contrast and permitting the application of a faceplate consisting of higher light permeability material than possible before due to said decreased reflection of external light. With such prior art color cathode ray tube, however, the phosphor dots are surrounded by a light absorbing layer, so that electron beam dots deflected perchance to said light absorbing layer can not be converted to visible light. Accordingly, proper adjustment of color purity is extremely difficult, particularly where electron beams passing through the mask holes do not fall exactly on the phosphor dots, failing to meet the demand for reproduction of a brighter, more distinct image. To obtain a brighter image, it may be contemplated to form phosphor elements in the shape of a strip in order to provide a larger illuminated area and to use an aperture grill as a color distinguishing mask. However, a mask made of such aperture grill can not be formed into a spherical shape because of its small mechanical strength. Further, mere change of the phosphor dots of the conventional delta gun color cathode ray tube to phosphor strips leads to the appreciable distortion of the triple arrangement of electron beams on the peripheral edges of an image. Accordingly, a light absorbing layer concurrently used as a guard band which is disposed on the peripheral edges of the image can not admit of a broader limit to the displacement of electron beam landing on the phosphor elements, resulting in the failure to adjust the color purity and whiteness of an image.

#The present invention can be more fully understood from the following detailed description when taken in connection with reference to the accompanying drawings, in whcih:

#Fig. 1 is a perspective view of a cathode ray tube according to an embodiment of this invention;

#FIG. 2 illustrates, partly in enlargement, the phosphor screen of FIG. 1;

#As seen from FIG. 1, the cathode ray tube of this invention comprises a funnel portion 11; a face portion 12 formed on the front side of said funnel portion 11; an aperture mask 13 perforated with slits 21 and disposed inside of the face portion 12 so as to closely face a phosphor screen mounted on the inner wall of said face portion 12; and an in-line electron gun unit 15 received in the neck portion of the cathode ray tube envelope and consisting of three linearly arranged electron guns to emit three electron beams in the horizontal direction. The deflection section of the cathode ray tube enevelope has its periphery surrounded with a deflection coil 16 to deflect emitted election beams.

#The inner wall of the face portion 12 is coated, as shown in FIG. 2, with a phosphor screen 19 which comprises a large number of parallel vertical light absorbing strips 17 made of black light absorbing material, for example, graphite and arranged at a prescribed space; and a plurality of parallel vertical phosphor element strips 18 spaced from each other by said light absorbing strips 17 which intervene between said phosphor element strips 18. These phosphor element strips 18 consist of a plurality of groups each consisting of three strips 18R, 18G and 18B producing red, green and blue lights respectively and arranged in a prescribed order. The aperture mask 13 is perforated with a plurality of rows of separate vertically elongate slits 21 arranged parallel with the phosphor element strips 18, said slits 21 being spaced from each other at a prescribed vertical interval K and a prescribed horizontal pitch P. Each row of said slits 21 corresponds to each phosphor element strip 18, the horizontal width D.sub.s of each slit 21 being made larger than the horizontal width Dp of each phosphor element strip 18. The electron gun unit 15 received in the neck portion 14 of the cathode ray tube envelope is of in-line type consisting of three electron guns 15a, 15b and 15c linearly arranged in the horizontal direction. The forward end of said electron gun unit 15 is fitted with a magnet 20 for converging electron beams emitted from the three elelctron guns 15a, 15b and 15c to a single point on the aperture mask 13.

#There will now be described the method of preparing the phosphor screen 19 of a color cathode ray tube according to an embodiment of this invention. Referring to the first step of FIG. 5, the inner wall of the face plate 12 is coated with a film of polyvinyl alcohol (PVA) 30. Said PVA film 30 is exposed to a light supplied from a linear light source 31 through the slits 21 of the aperture mask 13. The reason why said linear light source 31 is used is that light beams brought to the PVA film 30 through the slits 21 overlap each other in the vertical direction, thereby substantially eliminating any nonilluminated portion on the surface of the PVA film 30 which might result from those sections of the mask 13 which intervene between the adjacent ones of the vertically arranged slits, with the result that the surface of the PVA film 30 is exposed to a vertically continuous beam of light. During said exposure step, the linear light source 31 has its position varied so as to form in a prescribed order strips of phosphor elements producing red, green and blue lights. In the second step, the nonexposed portions of the PVA film 30 are etched, while the exposed portions of said PVA film 30 are developed to form PVA strips 32. In the third step, the inner wall of the faceplate 12 on which there were previously formed said PVA strips 32 is coated all over with a layer 33 of light absorbing material, for example, graphite. In the fourth step, the coated inner wall of the face plate 12 is dipped in a solution of hydrogen peroxide to fall off those portions of the graphite layer 33 which are deposited thereon together with said PVA strips 32. The other portions of the graphite layer 33 remain intact to form graphite strips 34. The inner wall of the faceplate 12 thus treated is washed with sprayed warm water fully to eliminate any remnant of the dissolved PVA strips 32 and the unnecessary portions of the graphite layer 33. In the fifth step, the inner wall of the faceplate 12 now bearing said graphite strips 34 is coated all over with a phosphor slurry 35 prepared by mixing a green light-producing phosphor element with a solution of polyvinyl alcohol. Those portions of the coated layer of said phosphor slurry 35 which are to be positioned in the interspaces between the respective adjacent graphite strips 34 are exposed to a continuous strip form of light from the same linear light source 31 as used in step 1 through the aperture mask 13. In the sixth step, the non-exposed portions of the coated phosphor slurry layer 35 are etched, while the exposed portions thereof are developed to form a green light producing phosphor strip 18G. Later there are formed in succession the other phosphor element strips 18B and 18R producing blue and red lights respectively, thereby finishing the phosphor screen 19 illustrated in FIG. 2.