Color cathode ray tube

Abstract: A shadow mask opposing a phosphor screen includes a substantially rectangular mask body (27) having a number of electron beam passage holes, and a substantially rectangular mask frame (30) having four side walls (28L, 28S) supporting a periphery of the mask body. Among the side walls, a pair of the side walls (28L) on long side each has a central connecting portion (32a) which is provided in a central portion thereof in lengthwise direction and fixed to the mask body, and a pair of protruding portions (34) which are provided on both sides of the central connecting portion in the lengthwise direction and protrudes in a direction apart from the mask body. Each protruding portion defines a gap (36) between the side wall and the mask body, and a width of the gap gradually increases as it goes far from the central connecting portion.

Description: The present invention relates to a shadow mask type color cathode ray tube. Generally, a color cathode ray tube comprises an envelope including a substantially rectangular panel in which a skirt portion is provided on a periphery of an effective portion thereof, and a funnel connected to the skirt portion. On the inner surface of the effective portion of the panel is formed a phosphor screen comprising three-color phosphor layers. A shadow mask is disposed inside of the phosphor screen so as to oppose it. An electron gun for emitting three electron beams is disposed in a neck of the funnel. Then, the three electron beams emitted from the electron gun are deflected by a magnetic field generated from a deflection apparatus provided outside of the funnel, and vertically and horizontally scan the phosphor screen through the shadow mask, thereby displaying color image on the phosphor screen. The shadow mask is used for selecting three electron beams and comprises a substantially rectangular mask body in which a number of electron beam passage apertures are formed on a region opposing the phosphor screen and a substantially rectangular mask frame having side walls fixed to the periphery of the mask body. As a shadow mask supporting method, according to some type thereof, substantially wedge shaped elastic supports are provided at corners of the mask frame and these elastic supports are engaged with stud pins provided at the skirt portion of the panel, so that the shadow mask is detachably supported thereby. Generally, in order to display color picture images having no deviation in color on the phosphor screen of the color cathode ray tube, three electron beams which pass the electron beam passage apertures in the mask body must be landed properly on the corresponding three color phosphor layers. For this purpose, it is necessary to keep the shadow mask at a proper position relative to the panel. To meet recent trend of multi-media, there is provided a color cathode ray tubes in which the size of three color phosphor layers and their arrangement pitch are reduced to increase its resolution. In such a color cathode ray tube, allowance to deviation of beam landing relative to the three color phosphor layers has dropped so that more accurate beam landing than ordinary color cathode ray tubes is demanded. However, in conventional color cathode ray tubes, the shadow mask is so constructed that almost entire periphery of the mask body is in contact with the side walls of the mask frame. Thus, there sometimes occurs a case in which the shadow mask may be deviated from its proper position because a undesired force is applied to the mask body due to disparity in quality of the mask frame resulting from production. Particularly in recent color cathode ray tubes having aspect ratio of 16:9, the side wall on the long side of the mask frame is much longer than the side wall on the short side as compared to ordinary color cathode ray tubes. Thus, the mask body is likely to be deformed because of insufficient strength and a disparity in distance between the opposing side walls on the long side (distance in the direction of the short axis). In manufacturing process for the color cathode ray tubes, fitting and removal of the shadow mask are carried out repeatedly. Due to a force applied to the shadow mask during this fitting and removal operation, a distortion may occur in the mask frame so that the mask body may be deformed. If such a phenomenon occurs, a deviation in beam landing to the three color phosphor layers is produced. On the other hand, in color cathode ray tubes for the multi-media, which have a small allowance in beam landing to the three color phosphor layers, it is necessary to reduce a load on the elastic supports by reducing the weight of the mask frame occupying most part of the weight of the shadow mask so as to relax the deviation of the shadow mask due to external shock. However, if the weight of the mask frame is reduced, it is likely that the mask frame is deformed due to an external shock or a force applied when the shadow mask is fitted or removed so that the shadow mask is deviated relative to the panel. Further, although such a procedure has been taken as to reduce the thickness of the mask body thereby equalizing the size and shape of the electron beam passage apertures which are a prominent reason for generation of unevenness of the phosphor screen, reducing the thickness of the mask body likely induces a reduction of the strength thereby producing a deformation of the mask body itself. There have bee provided a color cathode ray tube employing a low thermal expansion type shadow mask which is made of amber material having a high effect of suppressing a transition of electron beams due to so-called doming that the shadow mask is expanded in a direction of the phosphor screen due to thermal expansion of the shadow mask. As a measure for preventing a deformation of such a low thermal expansion type shadow mask, Jpn. Pat. Appln. KOKAI No. 5-121009 has disclosed a shadow mask in which a plurality of protruding portions which protrude outwardly are provided on side walls of the mask frame. However, even in the case in which such protruding portions are provided, since the mask body and the mask frame are in contact with each other through planes, a deformation of the mask body is generated due to disparity in production of the mask frame or deformation of the mask frame in a manufacturing process of color cathode ray tubes. As described above, design of the mask frame is important in order to display picture images having no deviation in color on the phosphor screen. However, conventional shadow masks have such a problem that the mask body or mask frame is deformed and it is difficult to display high quality images. A color cathode ray tube according to the precharacterizing part of claim 1 is disclosed in EP-A-0 004 245. US-A-4,949,009, JP-A-05121009 and EP-A-0 886 296 (having a priority of 11.03.1996 and being published on 23.12.1998) disclose further color cathode ray tubes and corresponding connections of the mask body with the mask frame. The present invention has been contrived in consideration of the above circumstances and its object is to provide a color cathode ray tube which is capable of suppressing effectively a deformation of the mask body or the mask frame and displaying high quality images. According to the present invention the above object is achieved by a color cathode ray tube according to claim 1. The dependent claims are related to further advantageous aspects of the present invention. According to one aspect of the present invention, there is provided a color cathode ray tube comprising: an envelope including a panel having a substantially rectangular effective portion in which a phosphor screen is formed on an inner surface thereof, a skirt portion provided on a periphery of the panel, and a funnel fixed to the skirt portion; a shadow mask arranged in the envelope so as to oppose the phosphor screen; and an elastic support elastically supporting the shadow mask relative to the skirt portion. The shadow mask comprises a substantially rectangular mask body in which a number of electron beam passage apertures are formed, and a substantially rectangular mask frame having four side walls supporting the periphery of the mask body. At least two side walls opposing each other include connecting portions which are provided at their central portions in a lengthwise direction and connected to the mask body, and a pair of protruding portions which are provided on both sides of the connecting portion and protrude in a direction apart from the mask body. Each of the protruding portions defines a gap between the side wall and the mask body and the gap includes that portion whose width gradually increases as it goes far from the connecting portion. Further, by defining a gap between the side wall of the mask frame and the mask body, a contact area therebetween is reduced so that influence upon the mask body of the disparity in size of the mask frame or deformation of the mask frame can be reduced. By optimizing the shape of the mask frame and fixing of the mask body relative to the mask frame as described above, it is possible to reduce deviation of beam landing which may occurs due to deviation of the mask body relative to the mask frame and deformation of the mask body resulting from a deformation of the mask frame. The invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIGS. 1 to 4 show a color cathode ray tube according to a first embodiment of the present invention, in which: FIG. 1 is a sectional view of the color cathode ray tube, FIG. 2 is a perspective view schematically showing an electron gun, a shadow mask and a phosphor screen of the color cathode ray tube, FIG. 3A is a plan view of the shadow mask, FIG. 3B is a sectional view taken along the lines IIIB-IIIB in FIG. 3A, and FIG. 4 is an enlarged plan view of a side wall of the shadow mask; FIG. 5 is a plan view showing a different modification of the side walls of the shadow mask; FIG. 6A is a plan view of the shadow mask according to a second embodiment of the present invention; FIG. 6B is a sectional view taken along the lines VIB-VIB in FIG. 6A; FIG. 7A is a plan view of the shadow mask according to a third embodiment of the present invention; FIG. 7B is a sectional view taken along the lines VIIB-VIIB in FIG. 7A; FIG. 8A is a plan view of the shadow mask according to a fourth embodiment of the present invention; and FIG. 8B is a sectional view taken along the lines VIIIB-VIIIB in FIG. 8A. Hereinafter, a color cathode ray tube according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. As shown in FIGS. 1 and 2, the color cathode ray tube comprises a vacuum envelope 10 formed of glass. The vacuum envelope 10 comprises a panel 3 including a substantially rectangular effective portion 1 and a skirt portion 2 provided on the periphery of the effective portion, and a funnel 4 attached to the skirt portion 2. On an inner surface of the effective portion 1 of the panel 3 is formed a phosphor screen 5 including stripe shaped three color phosphor layers 20B, 20G, 20R emitting blue, green and red colors, and stripe shaped light shielding layer 23 formed between the phosphor layers. A shadow mask 21 which will be described later is disposed in the envelope 10 to oppose the phosphor screen 5. An electron gun 9 for emitting three electron beams 8 is disposed within a neck 7 of the funnel 4. The three electron beams 8 emitted from the electron gun 9 are deflected by a magnetic field generated from a deflection apparatus 11 provided outside of the funnel 4, and horizontally and vertically scan the phosphor screen 5 through the shadow mask 21, thereby displaying color images on the phosphor screen. As shown in FIGS. 1 to 3B, the shadow mask 21 comprises a substantially rectangular mask body 27 and a mask frame 30 supporting the mask body 27. The mask body 27 includes a substantially rectangular effective portion 27a which is opposed to the phosphor screen 5 and has a number of electron beam passage apertures 25, and a skirt portion 26 formed on the periphery of the effective portion 27a. The mask frame 30 is formed in a substantially rectangular and has a pair of side walls 28L on its long side and a pair of side walls 28S on its short side. The mask body 27 is supported by welding the skirt portion 26 thereof to the mask frame 30. Among the side walls of the mask frame 30, the pair of the side walls 28L on the long side are fixed to the mask body 27 while only a central portion thereof in the lengthwise direction and its both ends in the lengthwise direction are in contact with the skirt portion 26 of the mask body 27, and each side wall 28L has those portions which are not in contact with the skirt portion 26 of the mask body 27. If speaking in detail, as shown in FIGS. 3A, 3B and 4, the respective side walls 28L are bent symmetrically with respect to a short axis V of the mask frame 30 such that a central connecting portion 32a and a pair of end connecting portions 32b which are in contact with the skirt portion 26 of the mask body 27 are formed in the center portion in the lengthwise direction and both ends in the lengthwise direction. On both sides of the central connecting portion 32a are wedge-shaped protruding portions 34 which protrude in a direction to be apart from the mask body 27. A distance d between the pair of the side walls 28L at the positions of the protruding portions 34 is larger than a distance dv between the side walls 28L on the short axis V and a distance ds between the side walls 28L in the vicinity of the side walls 28S on the short side. Each long side portion of the skirt portion 26 of the mask body 27 is located inside the mask frame 30 and is in contact with the central connecting portion 32a and the end connecting portions 32b of the corresponding side walls 28L and welded to the central connecting portion 32a and the end connecting portions 32b. A pair of gaps 36 are defined between each of the side walls 28L and the mask body 27 by the protruding portions 34 and located on both sides of the central connecting portions 32a. The respective gaps 36 are formed in a wedge shape whose width gradually expands from the central connecting portion 32a toward the end connecting portion 32b. The maximum width w of each gap 36 is set in about 1 to 3 mm and assuming that a distance between the short axis V and the side wall 28S is L, the length L1 of the gap 36 in the longitudinal direction of the side wall 28L is L1 = 0.7 to 0.9L. On the other hand, the skirt portion 26 on the short side of the mask body 27 is in contact with the inside of the entire length of the side wall 28S on the short side of the frame 30 and welded to the side wall 28S at several appropriate points. As shown in FIG. 1, the shadow mask 21 having the above structure is detachably supported by the panel 3 by engaging wedge shaped elastic supports 15 fixed to corner portions of the mask frame 30 with stud pins 16 protruding from the skirt portion 2 of the panel 3, respectively. In FIG. 1, reference numeral 22 denotes an internal magnetic field shielding body attached to the shadow mask 21. According to the color cathode ray tube having the above-mentioned arrangement, by forming the protruding portions 34 on the side walls 28L of the mask frame 30, the mechanical strength of the mask frame 30 is improved. Thus, it is possible to prevent the shadow mask 21 from being deformed by an external force applied thereto and effectively reduce a deviation of the shadow mask with respect to the panel 3. Particularly in a color cathode ray tube having an aspect ratio of 16:9, the side walls on the long side of the mask frame is much longer than ordinary cathode ray tubes, and therefore the mask body is likely to be deformed due to a shortage of strength and disparity of the distance between opposing side walls on the long side. However, by forming the shadow mask 21 in the structure described above, it is possible to effectively prevent the shadow mask from being deformed. Further, the gap 36 is defined between the mask body 27 and the side wall 28L by forming the protruding portions 34 on the side walls 28L of the mask frame 30. Thus, each side wall 28L is in contact with the mask body 27 only through the central connecting portion 32a and the end connecting portions 32b so that a contacting area between the mask frame 30 and the mask body 27 can be reduced largely as compared to conventional examples. As a result, it is possible to reduce undesired forces applied to the mask body 27 due to disparity of the mask frame 30 upon manufacturing. Particularly, by accurately controlling the distance between the mask frame 30 and the mask body 27, that is, only distances dv and ds, it is possible to reduce deformation of the mask body 27 thereby preventing a deviation of the shadow mask 21 with respect to the panel 3. As a result, it is possible to reduce deviation of beam landing relative to the three color phosphor layers composing the phosphor screen, thereby displaying high quality images. Further, because of formation of the protruding portions 34, the mask frame 30 is reinforced. Thus, the weight of the mask frame may be reduced so that burden on the elastic supports can be reduced. Further, even if the thickness of the mask body 27 is reduced, the deformation thereof can be prevented. In the embodiment described above, each side wall 28L of the mask frame 30 has the wedge-shaped protruding portions 34 and the gaps 36 defined by these protruding portions 34. If the side wall 28L is so structured as to be in contact with the mask body 27 through only the central portion and both ends portions in the lengthwise directions, the shapes of the protruding portions 34 and the gaps 36 may be changed in various ways as required. For example, according to a modification shown in FIG. 5, the respective protruding portions 34 are formed in substantially rectangular shape and each gap 36 is formed such that only end portion 36a on the central connecting portion 32a side gradually expand in terms of width from the central connecting portion 32a toward the end connecting portion 32b and the remaining parts are formed with a constant width. FIGS. 6A and 6B show a shadow mask 21 in the color cathode ray tube according to a second embodiment of the present invention. According to the second embodiment, in the mask frame 30, a pair of protruding portions 40 are formed on the side walls 28S on the short side as well as the side walls 28L on the long side. Each of the side walls 28S is also connected to the mask body 27 such that only its central portion 42a and both end portions 42b in the lengthwise direction are in contact with the skirt portion 26 of the mask body 27 and has some portions which are not in contact with the skirt portion 26 of the mask body 27. Namely, each side wall 28S is bent symmetrically with respect to a long axis H of the mask frame 30 such that a central connecting portion 42a and a pair of end connecting portions 42b which are in contact with the skirt portion 26 of the mask body 27 are formed in the center portion in the lengthwise direction and both ends in the lengthwise direction. Further, wedge-shaped protruding portions 40 which protrude in a direction to be apart from the mask body 27 are formed on both sides of the central connecting portions 42a. A distance at the protruding portions 40 between the pair of the side walls 28S is larger than a distance on the long axis H between the side walls 28S and further a distance in the vicinity of the long side between the side walls 28S. The skirt portion 26 of the mask body 27 is located within the mask frame 30 such that each short side portion thereof is in contact with the central connecting portion 42a and end connecting portions 42b of the corresponding side wall 28s and welded to the central connecting portions 42a and the end connecting portions 42b. A pair of gaps 44 are defined between the side wall 28s and the mask body 27 by the protruding portions 40 and located on both sides of the central connecting portion 42a. Each gap 44 is formed in wedge shape expanding gradually from the central connecting portion 42a toward the end connecting portions 42b. Other constructions of the shadow mask are the same as the first embodiment. According to the second embodiment having such a structure, the protruding portions 40 are also provided on the side walls 28s on the short side, so that the mechanical strength of the mask frame 30 is further improved and a contact area between the mask body 27 and the mask frame 30 is further reduced. Thus deformation of the mask body 27 can be further reduced thereby preventing effectively a deviation of the shadow mask 21 with respect to the panel 3. FIGS. 7A and 7B show the shadow mask 21 of the color cathode ray tube according to a third embodiment of the present invention. According to the third embodiment, four protruding portions 34 are formed on each of the side walls 28L, 28S of the mask frame 30. That is, two protruding portions 34 are formed each on one side symmetrically with respect to the short axis V on each of the side walls 28L. Each side walls 28L has middle connecting portions 32c located between the connecting portions 32a and 32b in addition to the central connecting portion 32a and the end connecting portions 32b, and the side wall 28L is welded to the skirt portion 26 of the mask body 27 through these five connecting portions. Between the side wall 28L and the skirt portion 26 are defined four gaps 36 by the protruding portions 34 such that each of the gaps 36 is formed in wedge shape gradually expanding from the central connecting portion 32a toward the end connecting portions 32b. Likewise two protruding portions 40 are formed on one side symmetrically on the side wall 28S with respect to the long axis H. Each side wall 28S has middle connecting portions 42c located between the connecting portions 42a and 42b in addition to the central and end connecting portions 42a and 42b, and each side wall 28S is welded to the skirt portion 26 of the mask body 27 through these five connecting portions. Between the side wall 28S and the skirt portion 26 are defined four gaps 36 by the protruding portions 40 such that each of the gaps is formed in wedge shape gradually expanding from the central connecting portion 42a toward the end connecting portions 42b. According to the third embodiment having such a structure, the mask frame 30 is reinforced by forming the protruding portions 34, 40, so that for example if an external shock is applied to the shadow mask 21, that shock can be dispersed depending on the number of the protruding portions. In addition, the same operation and effect as in the first and second embodiments can be obtained. FIGS. 8A and 8B show the shadow mask of the color cathode ray tube according to a fourth embodiment of the present invention. According to the fourth embodiment, one protruding portion 34 is formed on each side wall 28L on the long side of the mask frame 30, symmetrically with respect to the short axis V. Each of the protruding portions 34 is formed in a constant width along the entire length in its lengthwise direction. Thus, a distance d between the pair of the side walls 28L at the position of the protruding portions 34 is the same as a distance dv between the side walls 28L on the short axis V. An inner surface of the protrude portion 34 is of linear shape having infinite curvature. Relative to the mask frame 30, the long side portion of the mask body 27 is formed in circular shape protruding toward the side wall side of the mask frame 30. As a result, the side walls 28L are in contact with the long side of the skirt portion 26 of the mask body 27 through only the central connecting portion 32a located in the center in the lengthwise direction of the protruding portion 34 and a pair of the end connecting portions 32b located on boundaries between the protruding portion and the side wall, and welded to the skirt portion through these connecting portions. On both sides of the central connecting portions 32a are defined wedge shaped gaps 36 the width of which is expanded gradually toward the end connecting portions 32b. In the shadow mask 21, the short side of the skirt portion 26 of the mask body 27 is in contact with the almost entire length of the inner surface of the side wall 28S on the short side of the mask frame 30 and fixed at several appropriate welding points. In the shadow mask according to the fourth embodiment having such a construction, the same operation and effects as in the first embodiment described previously can be obtained.

Claims: A color cathode ray tube comprising: an envelope (10) including a panel (3) having a substantially rectangular effective portion (1) in which a phosphor screen (5) is formed on an inner surface thereof and a skirt portion (2) provided on a periphery of the panel, and a funnel (4) attached to the skirt portion; a shadow mask (21) arranged in the envelope to oppose the phosphor screen; and an elastic support (15) elastically supporting the shadow mask relative to the skirt portion; the shadow mask including a substantially rectangular mask body (27) having a number of electron beam passage apertures (25), and a substantially rectangular mask frame (30) having four side walls supporting the periphery of the mask body; wherein each of at least two side walls (28L) opposing each other of the mask frame (30) has a connecting portion (32a) which is provided at its central portion in a lengthwise direction thereof and fixed to the mask body (27), and a pair of protruding portions (34) which are provided on both sides of the connecting portion with respect to the lengthwise direction of the side wall and protrude in a direction apart from the mask body, and each of the protruding portions defining a gap (36) between the side wall and the mask body, characterized in that each gap includes a region whose width gradually increases as it goes far from the connecting portion. A color cathode ray tube according to claim 1, characterized in that each of said at least two side walls (28L) opposing each other includes a central connecting portion (32a) which is located at the central portion of the side wall in its lengthwise direction and fixed to the mask body (27), and a pair of end connecting portions (32b) which are located on both ends of the side wall in its lengthwise direction and fixed to the mask body; each of the protruding portions (34) is provided between the central connecting portion (32a) and a corresponding end connecting portion (32b) and the gap (36) defined by the protruding portion has a width continuously increasing from the central connecting portion up to the end connecting portion. A color cathode ray tube according to claim 1, characterized in that each of said at least two side walls (28L) opposing each other includes a central connecting portion (32a) which is located at a central portion of the side wall in the lengthwise direction and fixed to the mask body (27); a pair of end connecting portions (32b) which are located on both ends of the side wall in the lengthwise direction and fixed to the mask body; a middle connecting portion (32c) which is located between the central connecting portion and each end connecting portion and fixed to the mask body; a pair of first protruding portions (34) each of which is formed between the central connecting portion and the middle connecting portion and protrudes in a direction apart from the mask body; and a pair of second protruding portions (34) each of which are formed between the middle connecting portion and the end connecting portion and protrudes in a direction apart from the mask body; each of the first protruding portions definding a gap (36) whose width continuously increases from the central connecting portion up to the middle connecting portion, and each of the second protruding portions (36) defining a gap whose width continuously increases from the middle connecting portion up to the end connecting portion. A color cathode ray tube according to claim 1, characterized in that a length of the gap (36) defined between the central connecting portion (32a) and the end connecting portion (32b) in the lengthwise direction of the side wall (28L) is set to 0.7-0.9L, if a distance between the central connecting portion and the side wall (28S) on the short side is assumed to be L. A color cathode ray tube according to claim 1, characterized in that a single connecting portion (32a) is provided at each of said at least two side walls (28L) and fixed to the mask body (27) only at its central portion in the lengthwise direction. A color cathode ray tube according to claim 1, characterized in that the long side portion of the mask body (27) is formed in a circular shape protruding toward the side wall side of the mask frame (30).

Inventors: INOUE MASATSUGU, NAKAGAWA SHINICHIRO, SHIMIZU NORIO, INOUE, MASATSUGU, NAKAGAWA, SHINICHIRO, SHIMIZU, NORIO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1997
Main classification: H01J29
IPC codes: H01J29