Patent Publication Number: US-3878427-A

Title: Apertured-mask cathode-ray tube having half-tone array of heat-absorbing areas on target surface

Description:
1451 Apr. 15, 1975 United States Patent 11 1 Godfrey APERTURED-MASK CATHODE-RAY TUBE 3,661,580 5/1972 313/92 B x HAVING HALF-TONE ARRAY OF L U s e u r B Mb. &amp; o m m 4P r NO me me E m 1 r .3 &#34;1. PA  
 HEAT-ABSORBING AREAS ON TARGET SURFACE Greenspan [75] Inventor: Richard Hugh Godfrey, Lancaster,  
 [57] ABSTRACT Cathode-ray tube comprises a target structure includ- [73] Assignee: RCA Corporation, New York, NY.  
 [22] Filed: Feb. 5, 1973 [21] Appl. No.: 329,619  
 ing a viewing screen comprised of a mosaic of phosphor areas of different emission colors, and an apertured mask having therein an array of apertures registered with phosphor areas of said screen. The target structure includes also a heat-absorbing half-tone pattern on the relatively heat-reflecting target surface facing the mask for providing tailored radiative heat transfer from the mask to the screen and, in some cm- 000 mm M 4 33 3H6 .904 m s 4&amp;0 4 30 1 3 33 m w .1 W03 W M mmh .r. &#34;3 H L h 0 r d ud Umm mum 555 [ll bodiments, tailored attenuation of the electron beam.  
 5 Claims, 6 Drawing Figures References Cited UNITED STATES PATENTS 3,392,297 Schwartz........................... 313/92 B APERTURED-MASK CATHODE-RAY TUBE HAVING HALF-TONE ARRAY OF HEAT- ABSORBING AREAS ON TARGET SURFACE BACKGROUND OF THE INVENTION This invention relates to a novel cathode-ray tube having an apertured mask and a target structure with tailored heat-transfer properties.  
  One type of cathode-ray tube that is used for television displays is referred to as a shadow-mask tube. This tube is comprised of an evacuated envelope having a viewing window, a target structure including a viewing screen comprised of a mosaic of phosphor areas (usually dots) of different emission colors supported on the inner surface of the viewing window, a shadow mask having an array of apertures therein in register with the phosphor areas and mounted in the tube in adjacent spaced relation with the viewing window, and means for projecting one or more (usually three) electron beams towards the viewing screen for selectively exciting the phosphor areas of the mosaic. The mask is of a relatively light metal sheet and has the general shape of the viewing window, which is usually spherical. The mask is mounted on a heavier mask frame, which in turn is mounted to the envelope with springs attached to the mask frame.  
  In operating a shadow-mask tube. the electron beams are made to scan a raster in a fixed pattern on the target structure. As the beams are made to scan, portions of the beams are intercepted by the mask, and other portions, referred to herein as beamlets, pass through the mask apertures and excite the desired phosphor areas. The energy in the intercepted portions of the electron beams heats the mask, causing the mask material to expand. During the initial heating-up period, the central portion of the mask heats up faster than both the frame and the peripheral portions of the mask. This initial heating causes the mask to dome, so that the central portion of the mask moves toward the viewing screen, while the edges of the mask maintain their spacing with the viewing screen. Doming of the mask may adversely affect the position of the beamlets which pass through the mask apertures and may cause misregister, that is, may cause some or all of the beamlets to miss their associated phosphor areas.  
  Some of the heat in the mask is removed by radiation back to a black coating on the funnel of the tube. Normally, the target structure includes a thin layer of a highly reflective metal, usually aluminum. Heat from the mask that is radiated forward towards the screen is reflected back by the metal layer, and insufficient heat is removed by radiation to the viewing screen. U.S. Pat. No. 3,392,297 to J. W. Schwartz suggests applying to the entire reflective metal layer an overcoating of a heat-absorptive material, such as lithium nitride, boron carbide and nickel oxide. The purpose of the heatabsorptive overcoating is to decrease the reflectivity of the back of the reflective metal layer and thereby increase the rate of the radiative heat transfer between the mask and the screen. U.S. Pat. No. 3,703,401 to S. B. Deal et a]. discloses a sprayed-on overall overcoating of carbon particles for the same purpose.  
  It has also been suggested to provide a solid pattern of heat-absorbing area on the relatively heat-reflecting target surface facing the mask, for effecting faster radiative heat transfer from the central portion of the mask than from peripheral portions of the mask. One reason for using such a pattern is to increase the rate of heating of peripheral portions of the mask and thereby re duce doming of the mask during the initial period of use. It is now desirable to provide structures which are better tailored and more smoothly graded in heatabsorbing characteristic. It is also desirable to provide tailored electron-beam attenuation across the target structure.  
 SUMMARY OF THE INVENTION The novel cathode-ray tube comprises a target struc ture including a viewing screen comprised of a mosaic of phosphor areas, and an apertured mask closely spaced from said screen and having therein an array of apertures in register with the phosphor areas of said screen. The target structure includes also a heatabsorbing half-tone pattern on the target surface facing the mask for providing tailored radiative heat transfer from the mask to the screen. By heat&#39;absorbing halftone pattern is meant any system of heat-absorbing dots or lines used in the graphic arts for producing a gradation of tones in an image.  
  In one embodiment, the half-tone pattern is comprised of discrete areas (dots) of dark-colored heatabsorbing material on the back of the heat-reflecting aluminum metal layer of the target structure. the heatabsorbing areas are in register with phosphor areas of the viewing screen, and are graded in size with the largest heat-absorbing areas at the central portion of the screen, and the smallest areas at peripheral portions of the screen.  
  By providing a heat-absorbing half-tone pattern, the ratios of heat transfer across the screen may be smoothly graded to obtain a proper balance of heat transfer from the mask to the target structure. This may be used to control doming of the mask and the misregister associated with it. Also, the attenuation of electron-beam energy in the target structure may be tailored to improve performance of the tube. By grading the sizes of the areas of the half-tone pattern, both the heat-transfer characteristic and the electron-beamattenuation characteristic may be further tailored.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal partially broken-away sectional view of a three-beam tricolor cathode-ray tube of the invention.  
  FIG. 2 is a partially broken-away view of the mask and target structure of the tube shown in FIG. 1 viewed from the electron gun.  
  FIGS. 3 to 6 are plan views of the surface of target structures showing some different half-tones patterns that can be used in tubes of the invention.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 A typical structure for the novel tube, shown in FIGS. 1 and 2, is a 25-inch (25V) 1 10 deflection rectangular color television picture tube comprised of an evacuated glass envelope 11 including a funnel 13 and a faceplate I panel 15. The interior surface of the funnel is coated with an electrically-conductive coating 17 which is entirely of a heat-absorbing material, such as graphite particles in a silicate binder. One end of the funnel 13 terminates in a neck 19 which houses an electron-gun assembly 23 so adapted to project three beams at a target structure at the opposite end of the envelope 11.  
  The target structure includes a luminescent viewing screen 25 supported on a glass viewing window 27, which window is part of the faceplate panel 15. The viewing screen 25 is comprised of a multiplicity of redemitting. green-emitting and blue-emitting phosphor dots. R. B. and G respectively (FIG. 2). adhered to the inner surface 29 of the viewing window 27. The phosphor dots generally are round and arranged in a regular repetitive order of triads of three dots. one dot of each triad being of each color-emission characteristic. The phosphor dots R. B and G are graded in size from the center to the edge of the screen. being about mils in diameter at the center of the screen and about l3 mils in diameter near the edge of the screen 25. The screen 25 is overlaid with a reflective layer 31 of aluminum metal. The reflective metal layer 31 carries a halftone pattern of heat-absorbing areas 33 consisting essentially of carbon particles. Each heat-absorbing area 33 is located above a phosphor dot and is substantially identical in size and shape as that phosphor dot.  
  Closely spaced from the viewing window 27 toward the gun assembly 23 is a metal shadow mask 35 having a multiplicity of generally round apertures 37 in regular cyclic array therein, one aperture for each triad. The apertures are graded in size, with the largest apertures being in the central portion of the mask 35 and the smallest apertures being in peripheral portions of the mask 35. In FIG. 2, segments of the heat absorbing areas 33 may be seen through the apertures 37 of the mask 35. In this example, the mask 35, which is of cold rolled steel, is blackened on both the gun side 39 and the screen side 41, as by controlled oxidation of the surfaces thereof. The mask 35 includes a peripheral skirt which is spot-welded at various points to a mask frame 43. The mask frame 43 is supported on studs 45 attached to the faceplate panel 15 by four springs attached to the mask 35.  
  The apertured mask 35 is so positioned between the gun assembly 23 and viewing window 27 that, during tube operation, an electron beamlet from each of the three beams may pass through each aperture 37 of the mask 35 at a different angle and excite a different one of the three phosphor dots of a triad of the screen 25. Thus. a first electron beam can excite all of the redemitting phosphor dots, a second electron beam can excite all of the green-emitting phosphor dots, and a third electron beam can excite all of the blue-emitting phosphor dots. The blue-emitting phosphor dots preferably consist essentially of a silver-activated zincsulfide phosphor. The green-emitting phosphor dots preferably consist of a copper-and-aluminum-activated zinc-cadmium-sulfide phosphor. The red-emitting phosphor dots preferably consist essentially of a europium-activated yttrium-oxysulfide phosphor. Other phosphor compositions may be used in place of the phosphors mentioned.  
  When the tube of FIG. 1 is turned on for operation, the electron beams heat the mask 35 as described above. Heat is removed principally by infrared radiation from the hotter mask 35 by radiative transfer to the colder target structure. However,. since lesser proportions of the peripheral areas of the target structure are covered with heat-absorbing material than is the central area of the target structure, the rate of heat transfer from the mask 35 to the target structure is faster at the central portion than at the peripheral portions thereof. The relative effect is to heat up the peripheral areas of the mask 35 and to cool down the central area of the mask 35. The mask, due to its coefficient of expansion, expands at its peripheral areas, drawing the center of the mask in a direction opposite to doming (away from the screen). A reduction in movement of the mask 35 toward the screen 25 of up to about 2 mils and reductions of beamlet movements, measured 6 to 8 inches from the screen center, of up to about 2 mils may be realized. Also, each beamlets experiences substantially the same attenuation in electron-beam energy as it passes into the phosphor areas of the target structure. The effect is to excite the phosphor areas uniformly so that the viewer sees a video image with a smoother grading in brightness.  
 EXAMPLE 2 In this example, the tube is identical in structure with the tube of Example 1, except that the heat-absorbing areas 33 of the half-tone pattern, instead of being substantially identical in size with the phosphor areas R, B and G, are made larger than the phosphor areas at the central portion of the target structure and the same size at the edge of the target structure. The heat-absorbing areas grade in size from the center to the edge of the target structure. At the central portion of the target structure, the heat-absorbing areas 33 actually overlap; somewhat further out they are tangent; and still further out they are separated from one another. The effect of having larger heat-absorbing areas at the central portion of the target structure is to increase the rate of heat removal at that location.  
 EXAMPLE 3 In this example, the tube is identical in structure with the tube of Example 1, except that the heat-absorbing areas 33 of the half-tone pattern, instead of being substantially identical in size to the phosphor areas R, B,  
 and G, are made smaller than the phosphor areas at the peripheral portions of the target structure and the same size at the center of the target structure. The heatabsorbing areas grade in size from the edge to the center of the target structure. The effect of having smaller heat-absorbing areas at peripheral portions of the target structure is to decrease the rate of heat removal at those locations, and also to reduce the attenuation of electron beam energy for the electron beamlets in the peripheral portions of the target structure. The viewer sees a video image that is brighter at the peripheral portions thereof with a smooth grading in brightness across the surface of the screen.  
 EXAMPLE 4 In this example, the tube is identical in structure with the tube of Example 1, except that the heat-absorbing areas 33 of the half-tone pattern are larger than the phosphor areas at the center of the target structure and smaller than the phosphor areas at the edge of the target structure. This example combines the characteristics set forth above for the structures of Examples 2 and 3.  
 GENERAL CONSIDERATIONS AND ALTERNATIVES The invention may be embodied in any cathode-ray tube which employs an apertured mask in combination with a target structure-The target structure includes a viewing screen comprised of a mosaic of phosphor areas. The mosaic of phosphor areas may be continuous lines of narrow widths, or round, oval or rectangular islands. Optionally. the target structure may include a light-absorbing matrix between the screen and the faceplate, for example, as described in U.S. Pat. No. 3,558,310 to Edith E. Mayaud.  
  In all of the embodiments of the invention, the gun side of the target structure carries a heat-absorbing half-tone pattern on a field that is relatively heatreflecting. By heat-absorbing half-tone pattern is meant any system of heat-absorbing dots or lines of varying sizes which may be used in the graphic arts for producing a gradation of tones (gray scale) in an image. Where a system of heat-absorbing dots is used, the dots may be of any shape and/or size, and may be separate from or overlap on one another. In some forms of the invention. heat-absorbing dots are the same shape as, and are registered over. the phosphor areas of the viewing screen, but may be of different sizes. as exemplified in Examples 1 to 4. Where a system of heat-absorbing lines is used, the heat-reflecting spaces therebetween may be of any shape and/or size. In some forms of the invention. the heat-reflecting dots are the same shape as. and are registered over, the phosphor areas. but may be of different sizes, and are surrounded by heatabsorbing material. For the purposes of the invention, a heat-absorbing area (which is the same as an infraredabsorbing area) is an area that exhibits relatively high emissivity and absorptivity of infrared radiation. The radiative heat transfer process employed in the novel tube is the transfer of heat by infrared radiation from the higher temperatured mask (from areas having relatively hight emissivity of infrared radiation) to the lower temperatured target structure and faceplate panel (to areas having relatively high absorptivity of infrared radiation).  
  Relatively high reflectivity of heat or infrared radiation is exhibited by a surface with a mirror finish which is smooth, shiny and metallic. Relatively high reflectivity is achieved to a lesser degree with any surface that is smooth or light colored, and preferably both smooth and light colored. Vapor deposited aluminum metal layers are preferred. But coatings of aluminum oxide, titanium dioxide and magnesium oxide may also be used.  
  Relatively high absorptivity of heat or infrared radiation is exhibited by surfaces with a rough or matte black finish. Such surfaces also have relatively high emissivity of infrared radiation. Relatively high absorptivity of infrared radiation is achieved to a lesser degree with any surface that is rough or dark colored and preferably both rough and dark colored. Optimum infrared absorptivity is achieved when the thickness of the infrared absorbing material is at least 1/10 of the peak wavelength of the emission from a black body. At about 80C, which is generally higher than the normal operating temperature of the mask, this wavelength is about 8440 nanometers. Coatings of black particles of materials such as graphite, carbon, manganese dioxide, nickel oxide, and black iron oxide are preferred. Dark colored materials which are brown, gray, blue, green and purple may be used. Coatings such as those disclosed in the cited Schwartz patent may be used.  
  The half-tone pattern of heat-absorbing areas may be substantially identical with the mosaic of phosphor areas as described in Examplel. Or, in part or all of the half-tone pattern may beof, areas that are larger or smaller than their associated phosphor areas. as described in Examples 2, 3 and 4. One wayof describing the half-tone pattern is to plot the locus of equally-sized heat-absorbing areas as shown in FIGS. &#39;3 to 6. Various patterns of heat-absorbing areas on a heat-reflective background may be used. FIG. 3 shows a circular pattern wherein equally-sized heat-absorbing areas are located in concentric circles 51 and 53 about the intersection 55 of the major axis 57 and minor axis 59 of the target structure. The heat-absorbing areas may be. for example. about 14.8 mils diameter at the intersection 55, 13.6 mils diameter at the inner circle 53, 12.4 mils diameter at the outer circle 51 and l 1.2 mils diameter at the target edge. The diameters of the heat-absorbing areas grade smoothly from the intersection 55 to the target edge. FIG. 4 shows a rectangular pattern wherein equally-sized heat-absorbing areas are located on rectangular contour lines 61, 63, 65 and 67 with rounded corners. FIG. 5 shows an oval pattern wherein equallysized heat-absorbing areas are located on oval contour lines 71, 73, and 77. FIG. 6 shows a pincushion pattern wherein equally-sized heat-absorbing areas are located on contour lines 81 and 83 that appear as scallops on the sides of the rectangular area of the target structure. In FIGS. 4, 5 and 6, the sizes of the heatabsorbing areas grade smoothly from the intersection of the major and minor axes to the edge&#39;of the target StlUCtLlI&#39;C.  
  The half-tone pattern of heat-absorbing areas preferably follows the pattern of phosphor areas. but this is not required. In fact, the half-tone pattern of heatabsorbing areas may be completely independent in design of the phosphor areas of the screen, which itself may be a half-tone array with any grading and distribution of sizes and shapes for the phosphor areas. The heat-absorbing areas of a particular half-tone pattern generally have substantially uniform thickness across the pattern. However. the thicknesses across the halftone pattern may be graded, and the thicknesses of heat-absorbing areas may differ for different target structures.  
  The viewing screen 25 may be fabricated by photodeposition by any method known in the prior art, for example, as disclosed in U.S. Pat. No. 3,406,068 to Harold B. Law or U.S. Pat. No. 3,558,310 to Edith E. Mayaud. The reflective metal layer may be fabricated by any method known in the prior art, for example, the process described in an article entitled, Emulsion Filming for Color Television Screens, by T. A. Saulnier, .lr., in ELECTROCHEMICAL TECHNOLOGY, 4, 31-34 (1966). The half-tone pattern of heatabsorbing areas may be produced by previously-known photo-deposition methods. In one method, the surface of the reflective metal layer 31 is coated with a mixture of particles of heat-absorbing material and a negativeacting photobinder therefor, such as carbon particles and dichromate-sensitized polyvinyl alcohol. Then, a half-tone light pattern is projected on the coating to insolubilize the areas to be retained. The coating is developed by flushing away the still-soluble portions of the coating while retaining the insolubilized portions in place. The half-tone light pattern may be produced by projecting light through a photographic master made for that purpose. Where the heat-absorbing pattern is comprised of heat-absorbing areas that are similar in shape and location to the phosphor areas. the half-tone light pattern may be produced by placing the apertured mask of the tube in position in the faceplate panel 15, and projecting light from a small area light source in one or more exposures as required through the mask apertures upon a coating containing a negative-acting photobinder. such as sensitized polyvinyl alcohol. A grading of sizes of the retained areas of heat-absorbing material may be achieved or modified. for example. by inserting a suitable neutral graded density optical filter in the light path of each exposure in the manner described in US Pat. No. 3.592.] 12 to Harry R. Frey. By controlling the light intensity and the duration of exposure for each portion of the target surface. the sizes of the heat-absorbing areas may be selectively controlled. Where the heat-absorbing pattern is comprised of heatreflecting areas that are surrounded by heat-absorbing material. the same process may be used except that a positive-acting photobinder (such as KPR available from Kodak. Rochester. NY.) is used in place of a negative-acting photobinder in the processes described above. In still another alternative method. the photobinder. instead of carrying particles of heat-absorbing material, may carry a salt which is heat-decomposable to a heat-absorbing material. such as manganous oxalate or manganous carbonate. Such a method is described in US. Pat. No. 3.365.292 to Joseph P. Fiore where such process is employed for making lightabsorbing matrices.  
 I claim:  
  1. A cathode-ray tube comprising an evacuated envelope including a viewing window. a target structure including a viewing screen comprised of a mosaic of phosphor areas of different emission colors supported on the inner surface of said viewing window. a mask having therein an array of apertures in register with phosphor areas of said viewing screen, said mask being mounted in the tube in adjacent spaced relation with said viewing screen, and means for projecting three electron beams towards said viewing screen for selectively exciting said phosphor areas of each different emission color to luminescence, said target structure having a heat-reflecting surface facing said mask and a half-tone pattern of heat-absorbing areas of darkcolored particulate material on said heat-reflecting surface facing said mask, said half-tone pattern of heatabsorbing areas providing relatively fast radiative heat transfer from said mask to said screen, said heat absorbing areas being graded and having the property of providing greater heat transfer in the central portion of said viewing screen and lesser heat transfer in the peripheral portions of said viewing screen.  
  2. The tube defined in claim 1 wherein said mosaic pattern comprises darker-colored heat-absorbing areas in a ligher-colored heat-reflecting field.  
  3. The tube defined in claim 1 wherein said heatabsorbing areas are graded in size with the largest areas at the central portion of said pattern and the smallest areas at peripheral portions of said pattern. whereby radiative heat transfer is faster at the central portion of said half-tone pattern than at peripheral portions of said half-tone pattern.  
  4. The tube defined in claim 3 wherein said heatabsorbing areas are discrete areas that are registered with phosphor areas of said viewing screen.  
  5. The tube defined in claim 4 wherein at least some of said heat-absorbing areas are smaller than the phosphor areas that they are registered with.