Color cathode ray tube

Abstract: A shadow mask (30) is provided in an envelope of a color cathode ray tube. The shadow mask includes a mask body (33) having a number of electron beam apertures, and a mask frame (36) having side walls (34) equipped on the peripheral portion of the mask body. The side walls of the mask frame includes a pair of long side walls (34a) extending in parallel with the long axis X and a pair of short side walls (34b) extending in parallel with the short axis Y Beads (38a) and (38b) are respectively formed on the long side walls and the short side walls. The length of each bead of the long side wall along the long axis is set in a range of 55% to 80% of the length of the long side walls along the long axis, and a width of the bead in the tube axis direction is set in a rage of 40% to 75% of the maximum width of the long side walls in the tube axis direction. The length of each bead of the short side wall along the short axis is set in a range of 55% to 80% of the length of the short side walls along the short axis, and a width of the bead in the tube axis direction is set in a rage of 40% to 75% of the maximum width of the short side walls in the tube axis direction.

Description: The present invention relates to a color cathode ray tube. In general, a color cathode ray tube comprises an envelope. The envelope has a substantially rectangular panel provided with a skirt portion at the peripheral portion of an effective surface of a curved surface, and a funnel joined with the skirt portion. On the inner surface of the panel is formed a phosphor screen which includes light absorbing layers and three-color phosphor layers formed at gaps between the light absorbing layers. An electron gun is arranged in the neck of the funnel and a deflector is mounted on the funnel. Further, a substantially rectangular shadow mask is provided in the envelope to be opposed to the phosphor screen. In this color cathode ray tube, three electron beams emitted from the electron gun are deflected by the deflector and horizontally and vertically scan the phosphor screen through electron beam apertures of the shadow mask, thereby to display a color image. In general, a shadow mask is comprised of a substantially rectangular mask body and a substantially rectangular mask frame. The mask body has a main surface portion consisting of a curved surface opposed to the phosphor screen and having a number of electron beam apertures formed therein, and a skirt portion provided at the peripheral edge of the main surface portion. The mask frame has side walls contacting with and welded to the skirt portion of the mask body. With one of methods of supporting the shadow mask, substantially wedge-like elastic support members are fixed to the side walls at the corners of the mask frame, and the support members are engaged with stud pins provided at the skirt portion of the corners of the panel, thereby detachably supporting the shadow mask on the inside of the panel. In the color cathode ray tube as described above, three electron beams which have passed the electron beam apertures of the mask body must properly land on the three color phosphor layers, respectively, in order to display an image without color deviation. Therefore, it is necessary to arrange the shadow mask at a proper alignment relationship with respect to the panel. Recently, in a color cathode ray tube such as a display tube used in a terminal apparatus of a computer in response to introduction of multi-media, the arrangement pitch of three color phosphor layers is reduced in comparison with a normal color cathode ray tube, to improve the resolution, and therefore, the margin rate for beam landing is so small that color deviation easily occurs. Accordingly, more precise beam landing is required. However, actually, in steps of manufacturing a color cathode ray tube, the shadow mask is repeatedly attached to and detached from the panel, and during the attaching and detaching operations, a stress applied on the shadow mask causes the mask frame to be distorted. The distortion of the mask frame further causes the mask body to be distorted, and as a result, beam landing is shifted from the predetermined three color phosphor layers, thereby causing color deviation. In addition, in the cathode ray tube as described above, since the margin rate for beam landing is small, it is necessary to reduce the position shifting of the shadow mask caused by any external impact as much as possible. To reduce the position shifting of the shadow mask caused by an external impact, it is also necessary to reduce loads acting on the elastic support member itself. However, if the weight of the mask frame itself is decreased to reduce loads on the elastic support member, the mechanical strength of the shadow mask against a stress applied thereto when attaching and detaching the shadow mask in steps of manufacturing a color cathode ray tube is lowered. As a result, the position shifting of the shadow mask is caused with respect to the panel. Further, with respect to the shadow mask, in order to prevent unevenness of a formed phosphor screen, the mask body is thinned so as to reduce variations of the shapes and sizes of the electron beam apertures of the mask body, thereby reducing unevenness of the phosphor screen due to variations of the electron beam apertures. However, if the mask body is thinned, the mechanical strength thereof decreases and tends to be deformed easily. Also, the shadow mask is heated by collisions of electron beams and expands, causing doming. In a shadow mask formed of a low expansion-coefficient material such as invar which efficiently restricts registration of beam landing caused by the doming, a mask frame provided with a convex portion (or bead portion) has been proposed as a counter measure against deformation. However, not only the mask frame of a conventional shadow mask but also the mask frame having the convex portion as described above is short of suitable mechanical strength and cannot respond sufficiently to variations of mask frames in manufacturing steps and deformation in assembling shadow masks. The present invention has been contrived in consideration of the above circumstances and its object is to provide a color cathode ray tube, which has an improved mechanical strength without increasing the weight of the mask frame and which can reduce mislanding of electron beams due to deformation of the mask frame. To achieve the object described above, according to the present invention there is provided a colour cathode ray tube comprising an envelope including a substantially rectangular panel having an inner surface on which a phosphor screen is formed; a substantially rectangular shadow mask provided in the envelope and opposing the phosphor screen; and an electron gun arranged in the envelope, for emitting an electron beam to the phosphor screen through the shadow mask; the shadow mask including a substantially rectangular mask body having a main surface portion opposed to the phosphor screen and provided with a number of electron beam apertures, and a substantially rectangular mask frame having side walls attached to the mask body and being in contact with a peripheral portion of the mask body; the mask frame including a long axis (X) and a short axis (Y) perpendicular to each other and perpendicularly crossing a tube axis (Z), and the side walls of the mask frame including a pair of long side walls extending in parallel with the long axis and a pair of short side walls extending in parallel with the short axis; characterized in that: each side wall of either or both pairs of side walls has a respective bead, the length of the respective beads in the respective axis direction (X, Y) perpendicular of the tube axis (Z) being set in the range of 55% to 80% of the length of the respective side wall in the respective axis direction (X, Y), and a width (W1, W2) of the respective beads in the direction of the tube axis (Z) being set in the range of 40% to 75% of the maximum width of the respective side walls in the direction of the tube axis. Since at least one pair of the side walls of the mask frame is provided with beads, and the length and width of each bead are set to a proper size matched with the side wall, the mechanical strength of the shadow mask can be improved. Accordingly, deformation of the mask body and changes of the positional relationship between the panel and the shadow mask can be effectively prevented, so that it is possible to construct a color cathode ray tube which is difficult to cause misregistration of beam landing with respect to the phosphor layers. This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features. The invention can be more fully under stood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIGS. 1 to 3C show a cathode ray tube according to an embodiment of the present invention, in which: FIG. 1 is a longitudinal cross-sectional view showing the color cathode ray tube; FIG. 2 is a perspective view schematically showing a phosphor screen, a shadow mask, and an electron gun of the color cathode ray tube; FIG. 3A is a plan view of the shadow mask; FIG. 3B is a cross-sectional view taken along a line IIIB-IIIB in FIG. 3A; FIG. 3C is a cross-sectional view taken along a line IIIC-IIIC in FIG. 3A; FIG. 4A is a plan view of a shadow mask according to a modification of the present invention; FIG. 4B is a cross-sectional view taken along a line IVB-IVB in FIG. 4A; and FIG. 4C is a cross-sectional view taken along a line IVC-IVC in FIG. 4A. In the following, a color cathode ray tube according to an embodiment of the present invention will be described in detail with reference to the drawings. As shown in FIGS. 1 and 2, a color cathode ray tube comprises a vacuum envelope 10, and the vacuum envelope comprises a substantially rectangular panel 3 having an effective surface 1 in a form of a curved surface and a skirt portion 2 standing on the peripheral edge portion of the effective surface, and a funnel 4 joined to the skirt portion of the panel. A phosphor screen 13 is formed on the inner surface of the panel 3, and the screen includes a number of strip-like light-absorbing layers 11, three-color phosphor layers 12R, 12G, and 12B embedded between the strip-like light-absorbing layers 11 for emitting light in blue, green, and red. Also, a substantially rectangular shadow mask 30 described later is arranged in the envelope 10 to oppose the phosphor screen 13 with a predetermined distance. An electron gun 6 for emitting three electron beams 7B, 7G, and 7R is arranged in a neck 5 of the funnel 4. In addition, a deflector 8 is equipped outside the funnel 4. In the color cathode ray tube having the construction mentioned above, three electron beams 7B, 7G, and 7R emitted from the electron gun 6 are deflected by the deflector 8 and horizontally and vertically scan the phosphor screen 13 through the shadow mask 30, thereby displaying a color image on the phosphor screen. As shown in FIGS. 3A to 3C, the shadow mask 30 comprises a substantially rectangular mask body 33 and a substantially rectangular mask frame 36 supporting the periphery of the mask body. The mask body 33 has a substantially rectangular main surface portion 31 formed of a curved surface opposing the phosphor screen 13, and a skirt portion 32 extending from the peripheral edge of the main surface portion 31. A number of electron beam apertures 37 are formed in the main surface portion 31. The mask frame 36 includes four side walls 34 each of which has an extending portion 35 extending from an end there and has a L-shaped cross-section. The mask body 33 is fixed to the mask frame 36 by welding the skirt portion 32 to the inner surface of the side walls 34 of the mask frame 36. According to the present embodiment, the shadow mask 30 includes beads formed in the respective side walls 34 of the mask frame 36 and projecting insides the mask frame. Specifically, the mask body 33 and the mask frame 36 have a long axis (or X-axis) and a short axis (or Y-axis) which are cross each other at right angles and perpendicular to a tube axis Z. The side walls 34 of the mask frame 36 are formed of a pair of long side walls 34a extending in parallel with the long axis X and a pair of short side walls 34b extending in parallel with the short axis Y. Further, a bead 38a is formed at the center portion of each of the long side walls 34a and extends in a direction parallel to the long axis X, and a bead 38b is formed at the center portion of each of the short side walls 34b and extends in a direction parallel to the short axis Y. The length L1 of each bead 38a in the direction of the long axis X is set in a range of 55% to 80% of the length of each long side wall 34a in the direction of the long axis X. Likewise, the length L2 of each bead 38b is set in a range of 55% to 80% of the length of each short side wall 34b in the direction of the short axis Y. The width W1 of each bead 38a in the direction of the tube axis Z is set in a range of 40% to 75% of the maximum width of each long side wall 34a in the direction of the tube axis Z, and the width W2 of each bead 38b in the direction of the tube axis Z is set in a range of 40% to 75% of the maximum width of each short side wall 34b in the direction of the tube axis Z. The shadow mask 30 constructed in the structure as described above is detachably supported on the inside of the panel 3, in a manner such that substantially wedge-shaped elastic support members 21 provided at corners of the mask frame 36 are respectively engaged with stud pins 22 erected on the inner surfaces of the corners of the skirt portion 2 of the panel 3. According to the color cathode ray tube constructed as described above, by providing the beads 38a and 38b at the side walls 34a and 34b of the mask frame 36 fixed to the mask body 33, the mechanical strength of the shadow mask 30 can be increased even when the weight of the mask frame 36 is reduced. Therefore, an unnecessary unbalanced stress is not applied to the mask body 33 when assembling the shadow mask 30, so that deformation of the mask body 33 can be prevented. In addition, deformation of the mask frame 36 can be reduced against a stress applied when the shadow mask 30 is attached to and detached from the panel 3 in the steps of manufacturing the color cathode ray tube. As a result of this, deformation of the mask body 33 and changes of the positional relationship between the panel 3 and the shadow mask 30 can be effectively prevented, and it is therefore possible to construct a color cathode ray tube which is difficult to cause mislanding of electron beams on the three-color phosphor layers 12R, 12G, and 12B. With respect to the shadow mask 30 in which the bead 38a and 38b are provided at the four side wall 34a and 34b of the mask frame 36, studies and discussions were made to the relationship between the lengths L1 and L2 and widths W1 and W2 of the beads 38a and 38b, and deformation amounts in the direction of the tube axis Z and in the direction perpendicular to the tube axis when stresses such as bending and twisting are applied to the long side walls 34a and short side walls 34b of the mask frame 36. The results shown in the following tables 1 and 2 were obtained. Among the deformation amounts shown in TABLES 1 and 2, those of the "bending of long side" show deformation amounts of the mask frame 34 where a stress in the direction of the tube axis Z is applied to the extending portion 35 of the long side wall 34a, those of the "bending of short side" show deformation amounts of the mask frame 34 where a stress in the direction of the tube axis Z is applied to the extending portion 35 of the short side wall 34b, those of the "twisting" show deformation amounts of the mask frame where stresses in the directions opposite to each other are respectively applied to the extending portion 35 of each long side wall 34a and the extending portion 35 of each short side wall 34b, and those of the "load onto corners of side walls" show deformation amounts of the mask frame where a stress in the diagonal direction is applied to the side walls. The amounts of the "load onto corners of side walls" indicates the maximum deformation amount of the portion to which the mask body 33 is to be attached. As seen from the Table 1, when the length L1 and L2 of the beads 38a and 38b are set in a range of 50% to 85% of the length of each long side wall 34a and each short side wall 34b in the direction perpendicular to the tube axis Z, unbalance between the deformation amounts of the bending of long side and the bending of short side can be reduced, and the deformation amounts of the long and short side walls can be made substantially equal to each other. With respect to twisting and load onto corners of side walls, the deformation amount of the mask frame can be reduced. According to the invention, length L1 and L2 are set in a range of 55% to 80%, wherein unbalance of the deformation amounts of the shadow mask can be further reduced and the shadow mask can be stable at a small deformation. As shown in TABLES 1 and 2, the mask frame having no beads has much higher deformation amounts of the bending of the long and short sides than the mask frame provided with beads. Hence, it can be understood that the mechanical strength of the mask frame is improved by providing the beads 38a and 38b and the deformation of the mask frame can be greatly reduced. In case of the mask frame having no beads, the deformation amount of the bending of the long side is greater than that of the bending of the short side, due to the difference in length between the long and short side walls. However, according to the present embodiment, unbalance between the bending of the long side and the bending of the short side of the mask frame is improved and the deformation amounts of the long and short side walls can be substantially equalized to each other, where the lengths L1 and L2 of the beads 38a and 38b of the long and short side walls 34a and 34b are set in a range of 55% to 80% of the lengths of the long and short side walls, respectively, and where the widths W1 and W2 of the beads 38a and 38b in the direction of the tube axis Z are set in a range of 40% to 75% of the maximum widths of the long and short side walls, respectively. Therefore, if uneven stresses are applied to the long and short side walls, the stress applied to the long and short sides of the mask body 33 can be made substantially even so that deformation of the mask body can be reduced. As described above, according to the claimed color cathode ray tube, the mechanical strength of the shadow mask 30 can be improved, so that an unbalanced unnecessary stress is not applied to the mask body 33 and deformation of the shadow mask can be prevented even if the weight of the mask frame 36 and the like is reduced. Accordingly, deformation of the mask body 33 and changes of the positional relationship between the panel 3 and the shadow mask 30 can be effectively prevented, so that it is possible to provide a color cathode ray tube of excellent quality which is difficult to cause misregistration of beam landing on the three-color phosphor layers. Note that the present invention is not limited to the above mentioned embodiment but can be variously modified within the scope of the invention. For example, in the embodiment described above, one bead 38a is provided for each of the long side walls 34a and one bead 38b is provided for each of the short side walls. However, as shown in FIGS. 4A to 4C, each of the beads 38a and 38b may be divided into a plurality of bead sectors which are disposed in parallel with relatively narrow intervals. In this case, the distances between those bead sectors positioned at both ends in their lengthwise directions are set to the lengths L1 and L2. The width of each of the bead sectors is set to the width W1 or W2. In case where each of the beads 38 is constituted by a plurality of bead sectors, the lengths of the bead sectors may differ from each other, as shown in FIGS. 4A to 4C. Also, in the above embodiment, the beads 38 are defined by projecting inwards part of each side wall of the mask frame. However, each bead may be formed by projecting outwards a part of each side wall. Further, in the above embodiment, the side walls 34 is provided with beads extending in the direction toward the mask body 33 from the side of the extending portion 35. The bead portions, however, may be provided so as to extend toward the extending portion 35 from intermediate portions of the side walls or end portions on the mask body side. In the above embodiment, beads are provided at both long and short side walls of the mask frame. However, beads may be provided at only one of the long and short side walls.

Claims: A color cathode ray tube comprising: an envelope (10) including a substantially rectangular panel (3) having an inner surface on which a phosphor screen (13) is formed; a substantially rectangular shadow mask (30) provided in the envelope and opposing the phosphor screen; and an electron gun (6) arranged in the envelope, for emitting an electron beam to the phosphor screen through the shadow mask; the shadow mask (30) including a substantially rectangular mask body (33) having a main surface portion (31) opposed to the phosphor screen and provided with a number of electron beam apertures, and a substantially rectangular mask frame (36) having side walls (34) attached to the mask body and being in contact with a peripheral portion of the mask body; the mask frame (36) including a long axis (X) and a short axis (Y) perpendicular to each other and perpendicularly crossing a tube axis (Z), and the side walls of the mask frame including a pair of long side walls (34a) extending in parallel with the long axis and a pair of short side walls (34b) extending in parallel with the short axis; characterized in that : each side wall of either or both pairs of side walls (34a, 34b) has a respective bead (38a, 38b), the length (L1, L2) of the respective beads (38a, 38b) in the respective axis direction (X, Y) perpendicular of the tube axis (Z) being set in the range of 55% to 80% of the length of the respective side wall (34a, 34b) in the respective axis direction (X, Y), and a width (W1, W2) of the respective beads (38a, 38b) in the direction of the tube axis (Z) being set in the range of 40% to 75% of the maximum width of the respective side walls (34a, 34b) in the direction of the tube axis. A color cathode ray tube according to Claim 1, characterized in that each of the beads (38a, 38b) is divided into a plurality of bead sectors which are parallel with an interval between them. A color cathode ray tube according to Claim 2, characterized in that the bead sectors of said plurality of bead sectors have lengths different from each other. A color cathode ray tube according to Claim 1, characterized in that the mask frame (36) has an extending portion (35) projecting toward an inside of the mask frame from an end edge of each of the side walls (34) in the direction of the tube axis (Z), and a L-shaped cross-section, the mask body (33) is attached to another end edges of the side walls in the direction of the tube axis, and each of the beads (38a, 38b) extends from the side on the extending portion toward the mask body. A color cathode ray tube according to Claim 1, characterized by further comprising a stud pin (22) projecting from the envelope (10), and an elastic support member (21) equipped on the mask frame (36) and elastically supporting the shadow mask on the envelope through the stud pin.

Inventors: MURAI TAKASHI, TSUCHIDA SHIGERU, MURAI, TAKASHI, TSUCHIDA, SHIGERU, Murai, Takashi, Int.Pro.Div. K. K. Toshiba, Tsuchida, Shigeru, Int.Pro.Div. K. K. Toshiba
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1998
Main classification: H01J29
IPC codes: H01J31, H01J29