Patent Publication Number: US-6707241-B2

Title: Color cathode-ray tube

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-155445, filed May 24, 2001, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a color cathode-ray tube having a shadow mask. 
     2. Description of the Related Art 
     In general, a color cathode-ray tube includes a glass vacuum envelope which comprises a substantially rectangular panel having an effective portion, a funnel connected to the panel, and a cylindrical neck connected to a small-diameter portion of the funnel. On the inner face of the effective portion of the panel is formed a phosphor screen which includes dot-shaped or striped three-color phosphor layers emitting blue, green, and red lights, and black color light shielding layers. Further, in the vacuum envelope, a shadow mask having many electron beam passage apertures is arranged to oppose the phosphor screen. A an inline type electron gun for emitting three electron beams arranged in a line is located in the neck. A deflecting yoke is mounted from the outer periphery of the neck to the outer peripheral surface of the funnel. 
     In the color cathode-ray tube having the above-described structure, the three electron beams emitted from the electron gun are deflected horizontally and vertically by means of horizontal and vertical deflecting magnetic fields that are generated by the deflection yoke. The phosphor screen is horizontally scanned at high frequency and vertically scanned at low frequency with the electron beams that are passed through the shadow mask, whereupon a color image is displayed. At this time, the electron beams emitted from the electron gun are projected onto the phosphor screen at a given angle of incidence, and are sorted by the shadow mask in accordance with the angle of incidence on the shadow mask. Therefore, the three electron beams correspond, in a one-to-one correspondence, to the respective colors of red, blue and green of the phosphor screen. 
     In the color cathode-ray tube described above, the shadow mask is formed in a gradually-curving dome shape, and has a substantially rectangular mask surface in which many electron beam passage apertures having a diameter of about 100 μm are formed, and a skirt portion extending substantial perpendicularly from the peripheral edge of the mask surface toward the electron gun. Further, the shadow mask is fixed to a mask frame via the skirt portion, and the mask frame is removably supported via elastic supports at stud pins provided on the panel. 
     Usually, the shadow mask is formed in a desired shape by press molding a flat base mask, in which electron beam passage apertures are formed in advance. Further, the shadow mask is fixed to the mask frame by welding tongue portions, which are respectively formed at the corner portions and the substantially central portions of the long sides and short sides of the skirt portion, to the mask frame, thereby constituting a shadow mask assembly. 
     In the above-described color cathode-ray tube, in order to reduce doming of the shadow mask at the time of displaying high-intensity images, the shadow mask is formed of Invar material (an iron-nickel alloy) as a low thermal expansion material. 
     However, in a color cathode-ray tube to be used as a monitor of a computer or the like, reverse display such as window display or the like is general. In a 17 to 19 inch tube, electric current of 500 to 700 μA is irradiated as an electron beam onto the shadow mask. Therefore, there is much heat generation of the shadow mask, and mask doming cannot be completely prevented. 
     When the color cathode-ray tube displays a high-intensity image, the directions of the electron beams, which have passed through the electron beam passage apertures of the shadow mask, are changed in the direction of the center of the screen due to the mask doming at the intermediate portion of the screen. Therefore, the beam spots of the electron beams formed on the phosphor screen cannot correctly land on the phosphor layer of the target color, and as a result, the color purity of the formed image deteriorates. 
     Further, the mask frame is mounted is formed of an inexpensive iron material, from the standpoint of costs. Therefore, the coefficients of thermal expansion of the mask frame and the shadow mask are about 10 times different from each other, and a difference arises in the amounts of thermal expansion of the shadow mask and the mask frame, at the time of high-luminance display. 
     When the shadow mask assembly is heated at 400 to 500° C. in the manufacturing process, an extremely thin shadow mask is pulled by thermal expansion of the mask frame due to the difference in thermal expansions and the effective portion of the mask locally deforms particular at the vicinities of the welded points between the mask skirt portions and the mask frame. In consideration of such thermal expansion of the mask frame in the heating process, the diameter of the whole skirt portion of the shadow mask is formed larger than the inner diameter of the mask frame so that the skirt portion is pushed into the mask frame when the shadow mask is assembled to the frame, thereby preventing the shadow mask from being deformed in the heating process. In this case, however, as the color cathode ray tube displays high-luminance images, the effective portion of the mask locally deforms toward the electron gun due to the thermal expansion of the frame, in particular at the vicinities of the welded points of the long side portions and the short side portions of the shadow mask. In this case, at the peripheral portion of the screen, the directions of the electron beams, which have passed through the shadow mask, are changed further toward the outer peripheral direction of the screen than the target positions. 
     As described above, at the intermediate portion of the screen and at the peripheral portion of the screen, the landing positions of the electron beams which have passed through the shadow mask are changed in the opposite directions, and so-called landing reversal occurs. It is extremely difficult to completely suppress the deterioration in color purity caused by the landing reversal. Usually, attempts to balance the intermediate portion and the peripheral portion of the screen by correction by a holder or the like, have been carried out. 
     Further, in the case of the shadow mask to be used in the color cathode-ray tube with a flat screen, because the radius of curvature of the mask surface is large, the change in the landing position due to mask doming at the time of thermal expansion markedly increases. Therefore, with such a shadow mask, attempts to suppress mask doming and to suppress the deterioration in color purity by using an extremely low thermal expansion Invar material having a smaller coefficient of thermal expansion than that of Invar material, or the like, have been carried out. 
     However, the extremely low thermal expansion Invar is a material which is extremely hard to press-mold, and it is extremely difficult to mold a shadow mask having a large radius of curvature at a stable quality. Further, if an expensive material such as the extremely low thermal expansion Invar or the like is used as the shadow mask material which accounts for a large portion of the manufacturing costs of the color cathode-ray tube, the manufacturing costs greatly increase. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been contrived in consideration of above-described points, and an object of the present invention is to provide a color cathode-ray tube which can suppress deterioration in color purity without causing an increase in costs. 
     In order to achieve the above-described object, a color cathode-ray tube according to an aspect of the present invention comprises: a panel having a phosphor screen on an inner surface of the panel; an electron gun located opposite the phosphor screen and configured to emit electron beams toward the phosphor screen; a shadow mask located opposite the phosphor screen and having a large number of electron beam passage apertures through which the electron beams pass; and a substantially rectangular mask frame supporting a peripheral portion of the shadow mask. 
     The shadow mask includes a substantially rectangular mask main surface having the electron beam passage apertures, and a skirt portion extending from a peripheral edge of the mask main surface in a direction substantially perpendicular to the mask main surface, 
     the mask main surface has a major axis extending perpendicular to a tube axis, a minor axis extending perpendicular to the tube axis and the major axis, and a diagonal axis extending through the tube axis, 
     the skirt portion has a pair of long side skirt portions positioned on both sides of the major axis, a pair of short side skirt portions positioned on both sides of the minor axis, and a plurality of notches extending from an extended edge of the skirt portion toward the peripheral edge of the mask main surface, 
     The notches are formed in the long side skirt portions and the short side skirt portions, and include a plurality of first notches defining first tongue portions respectively at substantially central portions of the respective long side skirt portions and substantially central portions of the respective short side skirt portions, and second notches provided as pairs so as to sandwich respective corner portions of the skirt portion and defining second tongue portions respectively, and the mask frame is disposed at the outside of the skirt portion and welded to the first and second tongue portions. 
     At the respective corner portions of the skirt portion, when a length from a peripheral edge of the mask surface to welding points of the second tongue portions is Lwd, and a length from the peripheral edge of the mask main surface to a distal end of at least one of the second notches is Lcd, Lcd and Lwd have the relationship of: Lcd≦Lwd≦2Lcd. 
     Further, in accordance with a color cathode-ray tube according to another aspect of the present invention, the mask frame has a pair of long side walls facing the long side skirt portions and a pair of long side walls facing the short side skirt portions, and assuming that an angle between the major axis and a diagonal axis connecting a center of the mask frame and a diagonal point at which extensions of the long side walls and the short side walls intersect is θad, and an angle between the major axis and a diagonal welding axis connecting the welding point of the second tongue portion and the center of the mask frame is θwd, θad and θwd have the relationship of θad&lt;θwd. 
     According to the color cathode-ray tube structured as described above, by setting the relationship between Lcd and Lwd to Lcd≦Lwd≦2Lcd, due to thermal expansion of the mask frame, the tensile force applied to the diagonal welding points of the shadow mask is applied to the entire mask main surface, and mask doming is suppressed. 
     Further, in a laterally-long color cathode-ray tube whose screen size is 4:3 or 16:9, because mask doming is largest on the major axis X, by making the relationship be θad&lt;θwd, the rigidity in the major axis direction at the corner portions of the skirt portion increases at the diagonal welding points. Therefore, the force of pulling the shadow mask applied in the major axis direction increases, and mask doming in the major axis direction can be effectively suppressed and landing reversal of electron beams at the intermediate and peripheral portions of the screen can be reduced. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently embodiment of the invention, and together with the general description given above and the detailed description of the embodiment given below, serve to explain the principles of the invention. 
     FIG. 1 is a sectional view showing a color cathode-ray tube according to an embodiment of the present invention; 
     FIG. 2 is a perspective view showing a shadow mask in the color cathode-ray tub;. 
     FIG. 3 is a perspective view showing a corner portion of the shadow mask; 
     FIG. 4 is an exploded perspective view showing respective one portions of the shadow mask and a mask frame; 
     FIG. 5 is a plan view showing positions of diagonal welding points of the corner portion of the shadow mask and the mask frame; 
     FIG. 6 is a perspective view showing welding points of a skirt portion at respective long sides of the shadow mask; 
     FIG. 7 is a graph showing the relationship between Lwd and the PD amount of the shadow mask; 
     FIG. 8 is a graph showing the relationship between Lcd and the PD amount of the shadow mask; 
     FIG. 9 is a graph showing the relationship between θwd and the PD amount of the shadow mask; and 
     FIG. 10 is a graph showing, in comparison, PDh-R of the color cathode-ray tube according to the embodiment and PDh-R of a conventional color cathode-ray tube. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a color cathode-ray tube according to an embodiment of the present invention, for example, a 19-inch color cathode-ray tube whose ratio of the screen size is 4:3, will be described in detail with reference to the accompanying drawings. 
     As shown in FIG. 1, the color cathode-ray tube comprises a vacuum envelope  10  formed from glass. The vacuum envelope  10  has a substantially rectangular panel  1  whose outer surface is flat and which has a skirt portion  2  at the peripheral portion, a funnel  4  joined to the skirt portion of the panel, and a cylindrical neck  3  joined to a small-diameter portion of the funnel. 
     Formed on the inner surface of the panel  1  is a phosphor screen  6  that includes a plurality of dotted or striped phosphor layers, which emit red, green, and blue lights, individually, and black light-shielding layers. A deflection yoke  7  having horizontal and vertical deflection coils is mounted on the outer periphery from the neck  3  to the funnel  4 . Further, an electron gun  9  is disposed in the neck  3 . The electron gun  9  emits three electron beams arranged in one line, which are a center beam  8 G and a pair of side beams  8 R,  8 B passing on the same horizontal plane, toward the phosphor layers of the phosphor screen  6 . Because the loci of the electron beams are changed by effects of magnetic fields, an inner shield  11  shielding external magnetic fields is disposed at the inner side of the joined portion of the panel  1  and the funnel  4 . 
     A shadow mask  12  is provided in the vacuum envelope  10 . The shadow mask  12  faces the phosphor screen  6  functioning as a phosphor screen, and is attached to a rectangular mask frame  14 . The shadow mask  12 , as will be described later, has a mask main surface  20  in which a large number of electron beam passage apertures are formed, and a skirt portion  18  which extends from the peripheral edge of the mask main surface and is fixed to the mask frame  14 . The shadow mask  12  is formed by press molding. The shadow mask  12  is detachably supported on the panel  1  in a manner such that elastic support members  15  fixed to the mask frame  14  individually engage stud pins  17  that protrude from the inner surface of the skirt portion  2  of the panel  1 . 
     The vacuum envelope  10  including the panel  1  has a tube axis Z extending so as to pass through the center of the panel and the electron gun  9 , a major axis (horizontal axis) X extending perpendicular to the tube axis, and a minor axis (vertical axis) Y extending perpendicular to the tube axis and the major axis. 
     In the color cathode-ray tube having the above-described structure, the three electron beams  8 B,  8 G and  8 R emitted from the electron gun  9  are deflected by the deflection yoke  7  mounted on the outer side of the funnel  4 . The phosphor screen  6  is scanned horizontally and vertically with the electron beams that are passed through the electron beam passage apertures of the shadow mask  12 , whereupon a color image is displayed. 
     As shown in FIG. 2, the shadow mask  12  is formed by press molding, and has the substantially rectangular mask main surface  20  formed in a gently curving dome shape and the skirt portion  18  extending along the entire periphery of the mask main surface, which are formed integrally. The skirt portion  18  extends substantially at right angles to the mask effective portion from its peripheral edge. The mask main surface  20  has a substantially rectangular holed portion  20   a  and a rectangular frame shape non-holed portion  20   b  surrounding the periphery of the holed portion. A large number of electron beam passage apertures  19  are formed at a predetermined arrangement pitch at the holed portion  20   a.    
     The skirt portion  18  has a pair of long side skirt portions  18   a  respectively extending from the long sides of the mask main surface  20  and positioned on both sides of the major axis X, and a pair of short side skirt portions  18   b  respectively extending from the short sides of the mask main surface and positioned on both sides of the minor axis Y. A plurality of first notches  25  are formed in the long side skirt portions  18   a  and the short side skirt portions  18   b , respectively, and extend from the extended edges of the skirt portions toward the peripheral edge  21  of the mask main surface  20 . These first notches  25  are disposed so as to be substantially symmetrical with respect to the major axis X, the minor axis Y, and a diagonal axis D of the shadow mask  12 . Further, first tongue portions  19  which are welding portions, are formed at the central portions of the respective long side skirt portions  18   a  and the central portions of the respective short side skirt portions  18   b.    
     As shown in FIG.  2  and FIG. 3, second notches  26  are formed at the both sides of the respective corner portions of the skirt portion  18  and respectively extend from the extended edges of the skirt portion toward the peripheral edge  21  of the mask main surface  20 . Further, second tongue portions  30  are defined at the respective corner portions of the skirt portion  18  by these pairs of second notches  26 . 
     As shown in FIG. 1, FIG. 2, and FIG. 4, the mask frame  14  is formed in a rectangular form, and has a pair of long side walls  14   a  and a pair of short side walls  14   b  respectively corresponding to the long side skirt portions  18   a  and the short side skirt portions  18   b  of the shadow mask  12 . The above-described shadow mask  12  is disposed at the inside of the mask frame  14 , and for example, is welded to the mask frame  14  at eight points which are the first tongue portions  19  positioned at the central portions in the longitudinal direction of the respective long side skirt portions  18   a , the first tongue portions  19  positioned at the central portions in the longitudinal direction of the respective long side skirt portions  18   b , and the second tongue portions  30  positioned at the respective corner portions. Note that respective welding points  32 ,  34  are shown by the X marks. 
     Next, the respective welding points  32  of the shadow mask  12  will be described in detail. As shown in FIG. 3, at the second tongue portion  30  provided at each corner of the skirt portion  18 , assuming that a length from the peripheral edge  21  of the mask main surface  20  to the welding point  32  is Lwd and a length from the peripheral edge  21  to the upper end of the second notch  26  is Lcd, the welding point  32  and the second notch  26  are set so as to satisfy the relationship of: 
     
       
         Lcd≦Lwd≦2Lcd. 
       
     
     Note that, in the present embodiment, at each corner portion of the skirt portion  18 , each of the pairs of notches  26  positioned on both sides of the second tongue portion  30  and the welding point  32  are respectively set to satisfy the above-described relationship. However, it suffices that at least one of the second notches  26  satisfies the above-described relationship. 
     As shown in FIG. 5, assuming that an angle between the major axis X and a diagonal axis DF connecting a center C of the mask frame  14  and a diagonal corner point  40  at which the long side wall  14   a  and the short side wall  14   b  of the mask frame  14  intersect is θad, and an angle between the major axis X and a diagonal welding axis DW extending through the center C of the mask frame and the welding point  32  is θwd, the welding point  32  at each corner portion of the skirt portion  18  is set so as to satisfy the relationship of: 
     
       
         θad&lt;θwd, and preferably θad&lt;θwd&lt;1.2θad. 
       
     
     On the other hand, as shown in FIG.  2  and FIG. 6, assuming that a length from the extended edge of the skirt portion to the welding point  34  is Lw1, and a length from the peripheral edge  21  of the mask main surface  20  to the extended edge of the skirt portion  18  is Ls1, the welding points  34  of the respective long side skirt portions  18   a  and short side skirt portions  18   b  of the skirt portion  18  are set to satisfy the relationship of: 
     
       
         Lw1≦Ls1/3. 
       
     
     Note that, although FIG. 6 shows the welding point  34  of the long side skirt portion  18   a , the welding point  34  of the short side skirt portion  18   b  as well is set to satisfy the above-described relationship. 
     In the color cathode-ray tube having the shadow mask  12  of the above-described structure, the length Lcd from the peripheral edge  21  of the mask main surface  20  to the upper end of the second notch  26  was fixed to, for example, 3 mm, and the color offset characteristic (PD amount) at the time of changing Lwd was investigated. At this time, the PD amount of the intermediate portion of the screen spaced by 140 mm from the center of the screen toward the major axis X end (hereinafter called PDh-M), and the PD amount of the major axis end portion of the screen spaced by 180 mm from the center of the screen toward the major axis X end (hereinafter called PDh-E) were investigated. 
     As a result, as shown in FIG. 7, with respect to the PD reversal amount (hereinafter called PDh-R) which is the difference between the PDh-M amount and the PDh-E amount at the time of displaying a high-luminance image, assuming that the inner direction of the screen is minus and the outer direction of the screen is plus, and the shorter Lwd is, the smaller PDh-R is, and the longer Lwd is, the larger PDh-R is. However, if Lwd is made to be too short, although PDh-R is small, PDh-E is too large. 
     Accordingly, the relationship between Lcd and Lwd is preferably set to: 
     
       
         Lcd≦Lwd≦2Lcd. 
       
     
     Note that, in the present embodiment, Lwd=5 mm. 
     When the Lcd of the skirt portion  18  is lengthened, the strength of the diagonal portion of the mask skirt portion increases. The strength of welding at the corner portions of the skirt portion  18  and the mask frame  14  can be adjusted by adjusting the Lcd. 
     As shown in FIG. 8, as a result of investigating the PD amount while changing Lcd in a state in which Lwd was fixed at Lwd=5 mm, the longer Lcd is, the more PDh-R decreases. However, PDh-E at the time of displaying a high-luminance image increases. Further, if Lcd is 1 mm or less, the rigidity of the shadow mask itself deteriorates, and handling and workability of the shadow mask in a production line deteriorate. Further, if the length of Lcd is 5 mm or more, the moldability of the shadow mask deteriorates. From this, Lcd is preferably 2 mm or more, and in the present embodiment, Lcd=3 mm. 
     Moreover, as a result of investigating the PD characteristic while changing θwd in a state of fixing Lwd=5 mm and Lcd=3 mm, as shown in FIG. 9, if θwd is made large, although PDh-M, PDh-E, and PDh-R decrease gradually, the PD amount at the vicinity of the end of the minor axis Y of the screen (hereinafter called Pdv-E) deteriorates markedly. Therefore, the relationship is set to: 
     
       
         θad&lt;θwd, and preferably, to θad&lt;θwd&lt;1.2θad, and in the present embodiment, θwd=37.5°, and θad=36.9°. 
       
     
     Further, at the shadow mask  12 , the welding points  34  between the long side skirt portions  18   a  of the skirt portion  18  and the mask frame  14 , and the welding points  34  between the short side skirt portions  18   b  and the mask frame are preferably provided at positions as close as possible to the extended edge of the skirt portion  18 . 
     This is because, if the skirt portion  18  is pulled toward the outer side by thermal expansion of the mask frame  14  at the respective welding points  34  of the skirt portion  18 , local deformation in the curved surface generate at the peripheral edge portion of the mask main surface  20  due to the bending moment of the skirt portion  18 , and the color purity deteriorates. 
     Here, the welding points  34  of the respective long side skirt portions  18   a  and the respective short side skirt portions  18   b  are provided so as to satisfy the relationship of Lw1≦Ls1/3. In the present embodiment, Lw1=17 mm, and Ls1=3 mm. 
     According to the color cathode-ray tube structured as described above, when the tensile force is applied to the shadow mask  12  due to thermal expansion of the mask frame  14 , in particular, when the force of pulling the shadow mask is applied to the welding points of the respective corner portions of the mask skirt portion, the force is applied to the entire curved surface of the shadow mask, and doming of the shadow mask can be effectively suppressed. 
     Further, in a laterally-long color cathode-ray tube whose screen size is 4:3 or 16:9, mask doming is largest on the horizontal axis, namely, on the major axis X. Therefore, by making the welding points of the corner portions be θD&lt;θDW, the rigidity in the major axis direction of the mask skirt at the corner portions of the mask skirt portion increases. Accordingly, the force of pulling the shadow mask applied in the major axis direction increases, and mask doming in the major axis direction can be suppressed. 
     However, if the welding points at the long side skirt portions and the short side skirt portions of the shadow mask are set close to the peripheral edge of the mask surface, thermal expansion of the mask frame is applied to the shadow mask only at vicinities of the welding points of the skirt portion, and local deterioration in color purity arises. 
     Therefore, by making Lw1≦Ls1/3 and setting the distances between the welding points of the long side and short side skirt portions and the mask main surface to be long, local deformation of the mask main surface at the time of expansion of the mask frame can be prevented. As the result, landing reversal of electron beams at the intermediate and peripheral portions of the screen can be reduced, and deterioration of color purity can be suppressed. 
     In accordance with the above description, it is possible to prevent fluctuations in the space (q value) between the phosphor screen and the shadow mask due to doming of the shadow mask, and to suppress deterioration of color purity of a displayed image. Further, as the shadow mask material, a conventional Invar material can be used, and an increase in the manufacturing costs is not caused. 
     Comparing the PDh-R of the color cathode-ray tube according to the present embodiment and the PDh-R of a conventional color cathode-ray tube, as shown in FIG. 10, in the color cathode-ray tube according to the present embodiment, the PD reversal amount is reduced by about 40% by suppressing the mask doming. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 
     For example, the number, depth, and the like of the notches provided at the skirt portion  18  are not limited to those in the embodiments described above, and can be changed as needed.