Color cathode-ray tube

Abstract: A color cathode-ray tube comprising a phosphor screen (23) and a shadow mask (24) having a curved, substantially rectangular effective part facing the phosphor screen (23). The shadow mask (24) has a number of elongated apertures (41) which are arranged, forming a plurality of arrays (42). The aperture arrays (42) extend along the short axis (Y) of the effective part and juxtaposed along the long axis (X) of the effective part. The aperture arrays (42) are curved in different ways and spaced apart by different distances. Of the apertures (41), those made in a section extending for one-fourth the width of the effective part from either short side thereof are inclined in one direction to the short axis (Y), and those made in any other section are inclined in the opposite direction to the short axis (Y). The phosphor stripes constituting the phosphor screen (24) are not bent zigzag, whereby the cathode-ray tube can display images having high color purity.

Description: The present invention relates to a color cathode-ray tube of shadow mask type, and more particularly to a color cathode-ray tube comprising a phosphor screen and a shadow mask which has an effective part having arrays of apertures extending parallel to the short axis of the effective part and juxtaposed along the long axis thereof. The aperture arrays are spaced apart, and the apertures of each array are inclined such that electron beams passing through the apertures of the shadow mask land at desired positions on the phosphor screen, enhancing the quality of the phosphor screen. Generally, a color cathode-ray tube comprises a panel 2, a funnel 3, a shadow mask 6, an electron gun 9, and a beam-deflecting unit 10, as illustrated in FIG. 1. The panel 2 and the funnel 3 are connected together, forming an envelope. The panel 2 has an effective part 1. Provided on the inner surface of the effective part 1 is a phosphor screen 4. The screen 4 consists of blue-emitting phosphor layers, green-emitting phosphor layers and red-emitting phosphor layers. The shadow mask 6 is provided in the envelope and faces the phosphor screen 4. The mask 6 has an effective part 5 which is substantially rectangular. The effective part 5 is curved and has arrays of apertures. The electron gun 9 is provided in the neck 7 of the funnel 3, for emitting three electron beams 8B, 8G and 8R. The beam-deflecting unit 10 is located outside the envelope, more precisely mounted on the funnel 3. In operation, the beams 8B, 8G and 8R emitted from the gun 9 are deflected in horizontal and vertical planes, pass through the apertures of the shadow mask 6, and are applied onto the phosphor screen 4, whereby the cathode-ray tube displays a color image. Various color cathode-ray tubes which have the structure described above are known. One of them is an in-line color cathode-ray tube, in which three electron beams 8B, 8G and 8R travel in the same horizontal plane. The blue-emitting phosphor layers, green-emitting phosphor layers and red-emitting phosphor layers which constitute the phosphor screen 4 of the in-line cathode-ray tube are elongated stripes which extend vertically. The shadow mask 6 of the cathode-ray tube has arrays of apertures in its effective part. The aperture arrays extend along the short axis Y of the effective part 5 and are juxtaposed along the long axis X of the effective part 5. The shadow mask 6 is a color-selecting electrode. The electron beams 8B, 8G and 8R are guided through each aperture of the mask 6, traveling at different angles with respect to the mask 6. The beams 8B, 8G and 8R must land correctly on the adjacent blue-emitting phosphor stripe, green-emitting phosphor stripe and red-emitting phosphor stripe of the screen 4, respectively. Otherwise, the in-line color cathode-ray tube cannot display an image having high color purity. To achieve correct landing of the beams, the apertures of the shadow mask 6 need to be aligned with the phosphor stripes all the time the cathode-ray tube operates. More precisely, throughout the operation of the cathode-ray tube, the mask 6 must be held at such a position that the distance q between its effective part 5 and the effective part 1 of the panel 2 remains within a limited range. Due to the operating principle of a shadow-mask type color cathode-ray tube, only one third or less of each electron beam emitted from the gun passes through an aperture of the shadow mask 6 and reaches the phosphor screen 4. The other part of the electron beam impinges on the mask 6 and is converted into thermal energy, heating the shadow mask 6. Thus heated, the shadow mask 6 warps toward the phosphor screen 4 as indicated by the one-dot, one-dash line shown in FIG. 2, because it is made of low-carbon steel which has a large coefficient of thermal expansion. Due to this warping, known as "doming," the apertures change their positions. Consequently, the distance q between its effective part 5 and the effective part 1 of the panel 2 decreases. If the distance q excessively decreases to a value outside the limited range, each electron beam will fail to land on the target phosphor stripe 11, and the cathode-ray tube will display an image having insufficient color purity. The erroneous electron-beam landing caused by the doming of the shadow mask 6 is known as "mislanding." The degree of mislanding greatly depends on the luminance of the image to display, the period of displaying that image, and the like. When the image displayed has a high-luminance part, a so-called local doming develops as illustrated in FIG. 2 within a short period of time. The local doming causes great electron-beam mislanding. To analyze the mislanding caused by local doming, experiments were conducted. In the experiments, a window-like pattern 14 was displayed on the phosphor screen of a color cathode-ray tube as shown in FIG. 3, by using a pattern signal generator. Formed by applying large-current electron beams to the screen, the pattern 14 had high luminance. It extended along the short axis Y of the phosphor screen. The window-like pattern 14 changed in shape and position, due to the electron-beam mislanding. The mislanding was the greatest when the pattern 14 was displayed at a distance of about W/3 from the short axis Y of the screen, where W is the width of the screen. To be more precise, the mislanding was most prominent in the elliptical region 15 of the screen, which is shown in FIG. 4. Why the electron-beam mislanding was most prominent in the region 15 will be discussed with reference to FIG.5. If the pattern 14 is displayed in the central region of the screen shown in FIG. 3, the central part of the shadow mask will undergo thermal expansion. In this case, the mislanding of beams will be trivial since the beams passing through the apertures made in that central part are deflected by small angles. The farther the pattern 14 is located from the short axis Y of the screen, the greater the incident angles of the electron beams applied to form the pattern. The greater the incident angles, the more prominent the electron-beam mislanding of the beams. Nonetheless, if the pattern 14 is displayed in the left or right edge region of the screen, the mislanding will be small. This is because the deforming of the shadow mask is suppressed by the rigid frame which holds the shadow mask. Hence, the mislanding resulting from the thermal expansion of the shadow mask is the greatest when the pattern 14 is at a distance of about one-third the width W of the screen, from the short axis Y of the screen. The upper and lower edge parts of the shadow mask will be deformed but a little if the shadow mask expands when heated, because they are fastened to the frame which is rigid and strong. Furthermore, the frame has a heat capacity large enough to absorb the thermal energy the left, right, upper and lower edge parts of the shadow mask generate when impinged with electron beams. This helps to reduce the deforming of the edge parts of the shadow mask. Thus, the electron-beam mislanding was most prominent in the elliptical region 15 (FIG. 4) of the phosphor screen. This region 15 faces an elliptical region of the shadow mask, whose center is on the long axis X of the mask and spaced from the short axis Y of the mask by about one-third the width of the mask and whose upper and lower edges are at a distance of about one-fourth the height of the mask, from the long axis X of the mask. Various methods have been devised to minimize the doming of a shadow mask. One of them is to impart a large curvature to the effective part of the shadow mask, that is, to increase the radius of curvature of the effective part. As experiments show, the doming can be reduced more effectively by decreasing the curvature along the short axis of the mask than by decreasing the curvature along the long axis. The curvature of the effective part of the shadow mask is determined by the curvature of the inner surface of the effective panel part and the deflection characteristic of the beam-deflecting unit, such that the effective parts of the mask and panel are spaced apart by an appropriate distance q. Therefore, when the curvature of the effective part of the mask is altered, the curvature of the inner surface of the effective panel part must be changed in the same fashion. To increase the curvature of the effective part of the mask, thereby to minimize the doming of the mask, it is necessary to increase the curvature of the inner surface of the effective panel part to the same value. The curvature of the inner surface of the effective panel part may not be increased in the case of a large-screen color cathode-ray tube and a recently developed color cathode-ray tube with a wide screen having an aspect ratio of 16:9. With these cathode-ray tubes there is the trend that the outer surface of the effective panel part has small curvature and is almost flat. If the curvature of the inner surface of the effective panel part is increased, the central part of the panel will be far more thinner than the edge parts, impairing the operating characteristic of the cathode-ray tube. If the curvature of the effective mask part is increased, while the curvature of the inner surface of the effective panel part remains relatively small, the distance q between the effective parts of the mask and panel will be different from the desired value. As is known in the art, the difference between the actual and desired values of the distance q can be compensated for by adjusting the intervals between the aperture arrays made in the effective part of the shadow mask. A shadow mask is known in which the intervals between the aperture arrays gradually increase from the short axis toward the left and right edge of the mask, and whose effective part is curved along the long axis at a large curvature. The effective part of this shadow mask cannot, however, be curved along the short axis, much enough to prevent the doming of the mask. To increase the curvature along the short axis, the aperture arrays must be arranged such that the distance between any two adjacent aperture arrays gradually increases from the long axis of the mask toward the upper and lower edges of the mask. If all aperture arrays are so arranged, the effective part of the shadow mask cannot remain rectangular. Consequently, the cathode-ray tube cannot have a rectangular screen. Shadow masks free of this problem are disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 5-1574 (corresponding to U.S. Patent No. 4,691,138) and Jpn. Pat. Appln. KOKOKU Publication No. 5-42772 (corresponding to U.S. Patent No. 4,631,441). The shadow mask disclosed in either publication is characterized in that the aperture arrays are less spaced apart near either short axis than in each corner section. The corner sections can therefore be curved along the short axis at a small radius of curvature, while enabling a cathode-ray tube to have a rectangular screen. The distance PH between any two adjacent aperture arrays is given as: PH = a + bX 2 + cX 4 where X and Y are coordinates in a coordinate system whose origin is the center of the effective part and whose axes are the horizontal and vertical axes of the effective part, and a, b and c are quadratic functions of Y. As the distance Y from the long axis X of the effective part changes, the distance PH changes as quadratic function of Y. The curvature at which the effective part of the mask is curved along the short axis Y can only be large uniformly. The local doming of the shadow mask can be suppressed, but not sufficiently to minimize the electron-beam mislanding in the elliptical region 15 (FIG. 4) of the phosphor screen. To minimize the local doming, that part of the shadow mask through which the electron beams are applied onto the elliptical region 15 of the screen must be curved along the short axis Y at a great curvature. This part of the mask cannot be curved so unless PHM2 > PHM1. As shown in FIG. 5, PHM1 is the distance between the two adjacent aperture arrays, measured at a point M1 which is located in the long axis X of the shadow mask 6 and which corresponds to the center P1 of the elliptical region 15 (FIG. 4) of the screen. As shown in FIG. 5, too, PHM2 is the distance between the two adjacent aperture arrays, measured at a point M2 which is located in a distance of one-fourth the height H' of the effective part of the mask 6 from the long axis X of the mask 6 and which corresponds to the upper end P2 of the elliptical region 15 (FIG. 4) of the screen. If the distance PHM2 is longer than the distance PHM1, however, the distance PHM3 between the adjacent aperture arrays, measured at a point M3 located on a long side of the rectangular shadow mask 6, will be longer than the distance PHM2 as is indicated by broken lines in FIG. 5. This is inevitably because the distance PH between any two adjacent aperture arrays changes as a quadratic function of the distance Y from the long axis X of the effective part. For the shadow mask 6 to have a rectangular effective part, it is required that the distance between other adjacent aperture arrays be extremely short at another points on the long side of the rectangular shadow mask. If the shadow mask 6 is curved in accordance with the distance on the point M3, the distance q between the effective part of the mask and the panel will be excessive long. As a consequence, the effective surface of the shadow mask is so curved as to be turned. Thus, the shadow mask can not be easily manufactured. Generally, a phosphor screen for use in color cathode-ray tubes is manufactured by photolithography. To be more specific, first, a phosphor slurry made of mainly blue-emitting phosphor and photosensitive resin is coated on the inner surface of the panel and subsequently dried, forming a phosphor layer. Then, the phosphor layer is exposed to the light beams applied through the shadow mask. The layer, thus light-exposed, is developed, forming blue-emitting phosphor stripes on the inner surface of the panel. The sequence of these steps are repeated for two phosphor slurries containing green-emitting phosphor and red-emitting phosphor, respectively, thereby forming green-emitting phosphor stripes and red-emitting phosphor stripes on the inner surface of the panel. In the step of exposing each phosphor layer to light beams, these are applied from a light source to the shadow mask through an optical lens system in the same paths as electron beams will be applied from the electron gun to the shadow mask. The light beams passing through the apertures of the shadow mask are applied onto each phosphor layer formed on the inner surface of the panel. The phosphor stripes formed by developing the phosphor layer therefore assume specific positional relation with the apertures of the mask. An in-line color cathode-ray tube has a phosphor screen consisting of blue-, green- and red-emitting phosphor stripes formed on the inner surface of the panel and black stripes arranged between the phosphor stripes, and a shadow mask having vertical arrays of elongated apertures. Even if the spot an electron beam passing through one of the apertures forms on the target phosphor stripe moves in the lengthwise direction of the stripe (namely, along the short axis Y of the phosphor screen), the color purity will not affected. Therefore it is unnecessary to apply light beams to the shadow mask in the substantially same paths as the electron gun will emits electron beams to the shadow mask. To form a phosphor screen in the in-line color cathode-ray tube, an elongated light source is used which extends along the aperture arrays made in the shadow mask. The elongated light source serves to shorten the exposure time very much and to form a phosphor-stripe pattern with high precision. A problem will arise if an elongated light source is used. The inner surface of the panel is curved along not only the long axis X, but also the short axis Y. Thus, as shown in FIGS. 6 and 7, the light beams Ep emitted from the ends AL and BL of the light source Ls pass through the apertures of the shadow mask 6, reaching points AP and BP on the inner surface of the panel 2. The points AP and BP are spaced apart in horizontal direction by a distance Δ1, because the axis of the light source Ls and the axes of aperture arrays do not exist in the same plane. Consequently, although the phosphor stripes 16B, 16G and 16R provided on the central part of the panel 2 are straight as desired, as is illustrated in FIG. 8B, the phosphor stripes 16B, 16G and 16R are bent zigzag on the four edge parts of the panel 2, as is shown in FIG. 8C. The zigzagging of the stripes, known as "light-source bending," lowers the quality of the edge parts of the phosphor screen. In order to prevent a decrease in the quality of the phosphor screen, a shutter is used in the step of exposing each inner phosphor layer to light beams. That is, a movable shutter having a window is located between the panel and the shadow mask, preventing the entire phosphor layer from being exposed to light at a time. When the shutter is moved, the elongated light source is inclined, so that the axis of the aperture pattern formed on the phosphor layer may be in the same plane as the axis of the elongated light source. This exposure method requires a complex exposure device and a long exposure time. Recently, a new method is widely employed, in which an optical lens system adjusts the path of the light beams applied from the elongated light source, applying the beams onto the entire phosphor layer at a time, without inclining the elongated light source. The phosphor stripes formed by the new exposure method are bent zigzag, though slightly, on the four edge parts of the panel, because an optical lens system is used. U.S. Patent No. 4,691,138 (KOKOKU Publication No. 5-1574) discloses two shadow masks which serve to form phosphor stripes which extend straight even on the four edge parts of the panel. As shown in FIG. 9A, the first mask has aperture arrays 18 made in its effective part 5. Of the apertures made in the section extending for one-fourth the width W of the effective part 5 from either short side thereof, those located near either long side of the effective part are not inclined at angles PI of positive values as indicted by the curve I shown in FIG. 9B. Further, of these apertures, those located near an intermediate line 19 spaced from either long side of the effective part 5 by one-third the height H thereof are inclined at angles KII of negative values, as is indicated by the curve II shown in FIG. 9C. As shown in FIG. 10A, the second mask has aperture arrays 18 made in its effective part 5. Of the apertures made in the section defined above, those located near either long side of the effective part are inclined at various angles PI as indicated by the curve I shown in FIG. 10B. Of these apertures, those located near an intermediate line 19 defined above are inclined at various angles PII as indicated by the curve II shown in FIG. 10C. In either shadow mask disclosed in U.S. Patent No. 4,631,441, the apertures made in each corner section of the effective part 5 are not inclined sufficiently to prevent the forming of zigzag phosphor stripes. An object of the present invention is to provide a color cathode-ray tube in which the apertures of each array made in the shadow mask are inclined such that the phosphor stripes formed on the panel extend straight even on the four edge parts of the panel, and which can therefore display images having high color purity. A color cathode-ray tube according to the present invention is defined in claim 1. According to the invention, there is provided a color cathode-ray tube which comprises a panel having a substantially rectangular effective part, a phosphor screen provided on the inner surface of the effective part of the panel, and a shadow mask having a curved, substantially rectangular effective part facing the phosphor screen and having a number of elongated apertures. The elongated apertures are arranged, forming arrays which extend along the short axis of the effective part and which are juxtaposed along the long axis of the effective part. The aperture arrays are curved in different ways. The elongated apertures are inclined at different angles to the short axis of the effective part. More precisely, of the apertures made in the section extending for one-fourth the width of the effective part from either short side thereof, those located near either long side of the effective part are more inclined than those located near the long axis of the effective part. For the apertures made in the section extending for one-third the height of the effective part from either long side thereof, the angle changes from the short axis of the effective part toward either short side thereof, first increasing gradually to a maximum positive value and then decreasing to 0° or to a negative value. The position each elongated aperture assumes in the effective part is represented by coordinates (x, y) in a coordinate system whose origin is the center of the effective part and whose axes are the long axis X and short axis Y of the effective part, where x is a fourth-degree function or a higher-degree function of y. Thus, the apertures made in any corner of the effective part are more inclined than those made in any other portion of the effective part. An elongated light source used to from the phosphor screen can therefore be located, with its axis existing in the same plane as the axis of the aperture pattern formed on the inner surface of the panel. Hence, the phosphor stripes formed are not bent zigzag, even on'the four edge parts of the panel. Furthermore, since the inclination angle of the apertures made in the section extending for one-third the height of the effective part from either long side thereof changes from the short axis of the effective part toward either short side thereof, first increasing gradually to a maximum positive value and then decreasing to 0° or to a negative value, the aperture arrays provided in this section are spaced apart by a long distance. On the other hand, the aperture arrays are spaced apart by a short distance along the long axis of the effective part, whereby the local doming of this section is suppressed sufficiently, whereby the cathode-ray tube can display images having high color purity. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a sectional view of a conventional color cathode-ray tube; FIG. 2 is a diagram explaining the electron-beam mislanding which occurs in the cathode-ray tube shown in FIG. 1, due to the doming of the shadow mask; FIG. 3 is a diagram explaining how a local doming of the shadow mask takes place in the cathode-ray tube shown in FIG. 1; FIG. 4 is a diagram showing the region of the phosphor screen, where the electron-beam mislanding occurs due to the local doming of the shadow mask shown in FIG. 3; FIG. 5 is a diagram explaining the problem with a conventional shadow mask in which the distance between any two adjacent aperture arrays increases as a quadratic function of the distance Y from the long axis X of the effective part; FIG. 6 is a diagram explaining why the phosphor stripes are bent zigzag on the four edge parts of the panel in a conventional color cathode-ray tube; FIG. 7 is another diagram explaining why the phosphor stripes are bent zigzag on the four edge parts of the panel in the conventional color cathode-ray tube; FIG. 8A is a plan view of the phosphor screen of the conventional color cathode-ray tube; FIG. 8B is a diagram showing the shape of the phosphor stripes formed on the central part of the panel; FIG. 8C is a diagram illustrating the shape of the phosphor stripes formed on the four edge parts of the panel; FIG. 9A is a diagram showing the aperture arrays made in a conventional shadow mask; FIG. 9B is a graph representing how much the apertures arranged along the long side of the conventional shadow mask are inclined to the short axis Y of the mask; FIG. 9C is a graph representing how much the apertures arranged along an intermediate line spaced from the long side of the mask by one-third the height of the effective part of the mask are inclined to the short axis Y of the conventional shadow mask; FIG. 10A is a diagram showing the aperture arrays made in another conventional shadow mask; FIG. 10B is a graph representing how much the apertures arranged along the long side of the mask shown in FIG. 10A are inclined to the short axis of the mask; FIG. 10C is a graph representing how much the apertures arranged along an intermediate line spaced from the long side of the mask are inclined to the short axis Y of the shadow mask; FIG. 11 is a sectional view of a color cathode-ray tube according to an embodiment of the present invention; FIG. 12A is a diagram showing the aperture arrays made in the shadow mask incorporated in a color cathode-ray tube according to a first embodiment of the present invention; FIG. 12B is a graph representing how much the apertures arranged along the long side of the mask shown in FIG. 12A are inclined to the short axis of the mask; FIG. 12C is a graph representing how much the apertures arranged along an intermediate line spaced from the long side of the mask are inclined to the short axis Y of the shadow mask; FIG. 13 is a perspective view illustrating the positional relation between the elongated light source for applying light on phosphor layers and the aperture arrays made in the shadow mask shown in FIG. 12A; FIG. 14 is a diagram showing how much the aperture arrays made in the effective part of the shadow mask shown in FIG. 12A are spaced apart along the long axis X of the effective part; FIG. 15 is a diagram explaining how the doming of the shadow mask shown in FIG. 14 is suppressed; FIG. 16A is a diagram showing the apertures made in the shadow mask incorporated in a color cathode-ray tube according to a second embodiment of the invention; FIG. 16B is a graph representing how much the apertures arranged along the long side of the mask are inclined to the short axis Y of the mask shown in FIG. 16A; FIG. 16C is a graph representing how much the apertures arranged along an intermediate line spaced from the long side of the mask are inclined to the short axis Y of the mask shown in FIG. 16A; FIG. 17A is a diagram showing the apertures made in the shadow mask incorporated in a color cathode-ray tube according to a third embodiment of the invention; FIG. 17B is a graph representing how much the apertures arranged along the long side of the mask are inclined to the short axis Y of the mask shown in FIG. 17A; and FIG. 17C is a graph representing how much the apertures arranged along an intermediate line spaced from the long side of the mask are inclined to the short axis Y of the mask shown in FIG. 17A. Embodiments of the present invention, which are color cathode-ray tubes, will be described in detail with reference to the accompanying drawings. FIG. 11 shows a color cathode-ray tube according to an embodiment of the invention. As shown in FIG. 11, the cathode-ray tube comprises a panel 21, a funnel 22, a phosphor screen 23, a shadow mask 25, an electron gun 28, and a beam-deflecting unit 29. The panel 21 and the funnel 22 are connected together, forming an envelope. The phosphor screen 23 is provided on the inner surface of the effective part 1 of the panel 21. The screen 23 consists of blue-emitting phosphor layers, green-emitting phosphor layers and red-emitting phosphor layers. The shadow mask 25 is provided in the envelope and faces the phosphor screen 23. The mask 25 has an effective part 24 which is substantially rectangular. The effective part 24 is curved and has apertures. The electron gun 28 is provided in the neck 26 of the funnel 22, for emitting three electron beams 27B, 27G and 27R. The beam-deflecting unit 29 is located outside the envelope, more precisely mounted on the funnel 22. In operation, the beams 27B, 27G and 27R emitted from the gun 28 are deflected in horizontal and vertical planes, pass through the apertures of the shadow mask 25, and are applied onto the phosphor screen 23, whereby the cathode-ray tube displays a color image. A color cathode-ray tube according to the first embodiment of the present invention will be described, with reference to FIGS. 12A to 12C and FIGS. 13 to 15. FIG. 12A shows the aperture arrays made in the effective part 24 of the shadow mask which is incorporated in the color cathode-ray tube. As shown in FIG. 12A, each aperture 41 is an elongated one. The apertures are arranged, forming arrays 42 which extend along the short axis Y of the effective part and juxtaposed along the long axis X of the effective part. More precisely, the arrays 42 curve differently. The apertures of each array 42 are inclined to the short axis Y of the effective part 24. Here, an aperture 41 will be considered to be inclined by a positive angle  if it is inclined toward the short axis Y of the effective part 24. As indicated by the curve 43 shown in FIG. 12B, all apertures 41 on the long side of the effective part 24 are inclined at positive angles . Of these apertures 41, the one located in a region to the short side from a line along the short axis, which passes through a point at a distance of one-fourth the width W of the effective part 24 from the short side thereof are inclined at the greatest positive angle . As indicated by the curve 45 shown in FIG. 12C, some of the apertures 41 on an intermediate line 44 spaced from the long side of the effective part 24 by one-third the height H of the effective part 24 are inclined by positive angles . The other apertures 41 on the line 44 are inclined at negative angles . More specifically, for the apertures 41 on the intermediate line 44, the angle  gradually changes from the short axis Y toward the short side of the effective part 24, first increasing to a maximum positive value, then decreasing to a maximum negative value, and finally increasing to 0. Since the apertures 41 are inclined so; an elongated light source 48 used to from the phosphor screen 23 by photolithography can be located, with its axis existing in the same plane as the axis of the aperture pattern formed on the inner source of the panel 21 as is illustrated in FIG. 13. Therefore, the phosphor stripes 47 formed by the photolithography are not bent zigzag, even on the four edge parts of the panel 21. As shown in FIG. 14, for the apertures on an intermediate line 49 extending parallel to the long axis X of the effective part 24 and spaced from the long axis X by one-fourth the height H of the effective part 24, the angle  gradually changes from the short axis Y toward the short side of the effective part 24, first increasing to a maximum positive value, then decreasing to a maximum negative value, and finally increasing to 0. Hence, two adjacent aperture arrays 42 are spaced apart more at a point P2 on the intermediate line 49 than at a point P1 on the long axis X or at a point P3 on the long side. As shown in FIG. 15, the distance q between the effective part 24 of the shadow mask 25 and the inner surface of the effective panel part 20 is therefore long at the point P2 and short at the point P1. In other words, the effective part 24 of the mask 25 has a short radius Ry of curvature at the point P1. The local doming of the effective part 24 is suppressed effectively. A color cathode-ray tube according to the second embodiment of the invention will be described, with reference to FIGS. 16A, 16B and 16C. FIG. 16A shows the apertures 41 made in the shadow mask incorporated in this color cathode-ray tube. As evident from FIGS. 16A and 16B, for the apertures 41 on the long side of the effective part 24 of the mask, the angle  gradually changes from the short axis Y of the effective part 24 toward the short side of thereof, first decreasing to a maximum negative value, then increasing to a maximum positive value, and finally decreasing to 0. As shown in FIG. 16C, for the apertures 41 on an intermediate line extending parallel to the long axis X of the effective part 24 and spaced from the long axis X by one-third of the height H of the effective part 24, the angle  gradually changes from the short axis Y toward the short side of the effective part 24, first increasing to a maximum positive value, then decreasing to a maximum negative value, and finally increasing to 0. The apertures 41 are more inclined than the apertures of the shadow mask (FIG. 12A) incorporated in the first embodiment, so as to form phosphor stripes by photolithography, which are not bent zigzag, even on the four edge parts of the panel 21. Particularly, the apertures 41 located near the point P2 (FIG. 4) on an intermediate line parallel to the long axis X are inclined very much, whereby the effective part 24 has a shorter radius Ry of curvature at the point P1. The local doming of the effective part 24 is suppressed more effectively than in the shadow mask provided in the first embodiment. The position which the center of each aperture 41 assumes in the effective part 24 can be represented by coordinates (x, y) in a coordinate system whose origin is the center of the effective part 24 and whose axes are the long axis X and short axis Y of the effective part 24. If the upper and lower halves of the effective part 24 are symmetrical with respect to the long axis X, the position of the aperture 41 is represented as an even function, provided that x is a fourth-degree function or a higher-degree function of y. A color cathode-ray tube according to the third embodiment of the invention will be described, with reference to FIGS. 17A, 17B and 17C. The shadow mask incorporated in this cathode-ray tube is characterized in that its upper and lower halves are symmetrical with respect to the long axis Y. FIG. 17A shows the apertures 41 made in the effective part 24 of the shadow mask. The position of each aperture 41 is represented by coordinates (x, y) in a coordinate system whose origin is the center of the effective part 24 and whose axes are the long axis X and short axis Y of the effective part 24. The value for x is a sixth-degree function of y. As shown in FIG. 17A, the arrays 42 of apertures meander, and the apertures 41 are inclined to the short axis Y of the effective part 24. More precisely, for the apertures 41 on the long side of the effective part 24, the angle  gradually changes from the short axis Y of the effective part 24 toward the short side of thereof, first decreasing to a maximum negative value, then increasing to a maximum positive value, and finally decreasing to 0 as indicated by the curve 43 shown in FIG. 17B. For the apertures 41 on an intermediate line extending parallel to the long axis X and spaced from the long axis X by one-third of the height H of the effective part 24, the angle  gradually changes from the short axis Y toward the short side of the effective part 24, first increasing to a maximum positive value, then decreasing to a maximum negative value, and finally increasing to 0 as indicated by the curve 45 shown in FIG. 17C. The shadow mask shown in FIG. 17A achieves the same advantages as the shadow mask (FIG. 16A) incorporated in the second embodiment, though it differs in that x is a higher-degree function of y. As has been described, the present invention can provide a color cathode-ray tube which comprises a panel having a substantially rectangular effective part, a phosphor screen provided on the inner surface of the effective part of the panel, and a shadow mask having a curved, substantially rectangular effective part facing the phosphor screen and having a number of elongated apertures. The elongated apertures are arranged, forming arrays which extend along the short axis of the effective part and which are juxtaposed along the long axis of the effective part. The aperture arrays are curved in different ways. The elongated apertures are inclined at different angles to the short axis of the effective part. More precisely, of the apertures made in the section extending for one-fourth the width of the effective part from either short side thereof, those located near either long side of the effective part are more inclined than those located near the long axis of the effective part. For the apertures made in the section extending for one-third the height of the effective part from either long side thereof, the angle changes from the short axis of the effective part toward either short side thereof, first increasing gradually to a maximum positive value and then decreasing to 0° or to a negative value. Hence, an elongated light source used to form the phosphor screen by photolithography can be located, with its axis existing in the same plane as the axis of the aperture pattern formed on the inner surface of the panel. Therefore, the phosphor stripes formed by the photolithography are not bent zigzag, even on the four edge parts of the panel. Further, since the angles of the elongated apertures made in the section extending for one-third the height of the effective part from either long side thereof change as described above, the distance between any two adjacent aperture arrays in this section is relatively long. This section of the effective part therefore has a shorter radius of curvature. As a result, the local doming of the section is suppressed sufficiently, whereby the cathode-ray tube can display images having high color purity.

Claims: A color cathode-ray tube comprising: electron-beam generating means (28) for generating electron beams (27R, 27G, 27B) in in-line fashion; a panel (21) having a substantially rectangular effective part which has a curved inner surface; a phosphor screen (23) provided on the inner surface of the effective part of said panel (21), for emitting red, green and blue light rays when excited by the electron beams generated by said electron-beam generating means (28); and a shadow mask (24) having a curved, substantially rectangular effective part facing the phosphor screen (23) and having a number of apertures (41) for guiding the electron beams (27R, 27G, 27B) to said phosphor screen (23), the effective part of the shadow mask having two short sides parallel to a short axis (Y) and two long sides parallel to a long axis (X), the two axis passing through the center of the effective part, a width extending between the two short sides, and a height extending between the two long sides, wherein said apertures (41) are elongated and arranged, forming a plurality of arrays (42) which extend along the short axis (Y) of the effective part and juxtaposed along the long axis (X) of the effective part and each of which is curved in a specific way depending on its position on the shadow mask, wherein the elongeted apertures of each array are inclined extending along the axis of the array; characterized in that the elongated apertures (41) are inclined at different angles to the short axis (Y) of said effective part such that in a section extending for one-fourth the width of the effective part from either short side thereof, the elongated apertures (41) and respective portions of aperture arrays (42) located near either long side of the effective part are more inclined than those located near the long axis (X) of the effective part, wherein the angle of inclination of each elongated aperture is such that its one end near the long side is located closer to the short axis (Y) than its other end near the long axis, respresenting an angle of positive value; and for the apertures (41) and respective portions of aperture arrays (42) made on a line extending for one-third the height of the effective part from either long side thereof, the angle changes from the short axis (Y) of the effective part toward either short side thereof, first increasing gradually to a maximum positive value and then decreasing to 0° or to a negative value. The color cathode-ray tube according to claim 1, characterized in that a position each elongated aperture (41) assumes in said effective part is represented by coordinates (x, y) in a coordinate system whose origin is the center of the effective part and whose axes are the long axis X and short axis Y of the effective part, where x is a fourth-degree even function or a higher-degree even function of y.

Inventors: FUKUDA KUMIO, INOUE MASATSUGU, MURAI TAKASHI, OHAMA SHINJI, SAOTOME ICHIRO, SHIMIZU NORIO, FUKUDA, KUMIO, INOUE, MASATSUGU, MURAI, TAKASHI, OHAMA, SHINJI, SAOTOME, ICHIRO, SHIMIZU, NORIO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1995
Main classification: H01J29
IPC codes: H01J29