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 (31) which are arranged, forming a plurality of arrays (32). The aperture arrays (32) extend along the short axis (Y) of the effective part and juxtaposed along the long axis (X) of the effective part. The aperture arrays (32) are curved in different ways and spaced apart by different distances. Of the apertures (31), 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 which has heat capacity large enough to absorbs 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 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 can not be easily manufactured. Generally, a phosphor screen for used 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 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 and 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 light source, 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 shadow mask has aperture arrays juxtaposed at appropriate intervals and is curved to suppress a local doming of the effective part, and no electron-beam mislanding takes place on the phosphor screen. 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 apertures. The apertures are arranged, forming a plurality of arrays which extend along the short axis of the effective part and juxtaposed along the long axis of the effective part. The distance PH(N) between the (N-1)th and Nth arrays, counted from the array passing the center O of the effective part, is given as: PH(N) = A + BN 2 + CN 4 where A, B and C are fourth-degree functions of a Y-coordinate in a coordinate system whose origin is the center O of the effective part and whose axes are the horizontal and vertical axes of the effective part, and C is a function first decreasing and then increasing as the absolute value of the Y-coordinate, and where the values for A and B change with C such that the effective part remains substantially rectangular. The distance PH(N) between the (N-1)th and Nth arrays, which are spaced about one-third the width W of the screen from the short axis of the screen, may increase with the absolute value of the Y-coordinate and may be represented by a fourth-degree function of the Y-coordinate so as to have a transition point in the effective part with respect to the short axis of the effective part. Since the distance between any two adjacent aperture arrays is so set, the distance PHM2 between measured at a point M2 which is located in a distance of one-fourth the height H of the effective part of the mask from the long axis X of the mask as shown in FIG. 5 can be longer than the distance PHM1 between the two adjacent aperture arrays, measured at a point M1 which is located in the long axis X of the shadow mask. Moreover, the distance PHM3 between the adjacent aperture arrays, measured at a point M3 located above the point M2 as shown in FIG. 5, can be shorter than in the case where the distance between any two adjacent aperture arrays changes as a quadratic function of the distance Y from the long axis X of the effective part. The distance PH between any two adjacent aperture arrays changes as a fourth-degree function of the distance Y. Thus, the distance PHM3 can be sufficiently short even if the distance PHM2 is longer than the distance PHM1. A desired part of the effective part can therefore be curved along the short axis at a radium of curvature small enough to reduce the local doming of the shadow mask. As a result, the electron-beam mislanding on the phosphor screen can be minimized. 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 a first embodiment of the present invention; FIG. 12 is a perspective view of the shadow mask incorporated in the cathode-ray tube shown in FIG. 11; FIG. 13 is a graph representing how much the aperture arrays are spaced apart along the long axis X and long side of the effective part of the shadow mask shown in FIG. 12 and along an intermediate line extending between the long axis X and long side of the effective part; FIG. 14 is a diagram showing the arrangement of the aperture arrays made in the shadow mask shown in FIG. 12; FIGS. 15A, 15B and 15C are diagrams, each showing a relation among the distance between the shadow mask and the inner surface of the panel, the distance between any two adjacent aperture arrays, and the distance between any two adjacent phosphor stripes; FIG. 16 is a diagram showing three curves along which the shadow mask shown in FIG. 12, a first conventional shadow mask and a second conventional shadow mask are curved along the short axis; FIG. 17 is a graph representing how much the aperture arrays made in the effective part of a shadow mask used in a color cathode-ray tube according to a second embodiment of the invention are spaced apart along the long axis X and long side of the effective part and along an intermediate line extending between the long axis X and long side of the effective part; FIG. 18 is a plan view schematically showing the arrangement of the aperture arrays made in the shadow mask shown in FIG. 17; FIGS. 19A and 19B are a diagram representing how much the aperture arrays made in the effective part of a shadow mask incorporated in a color cathode-ray tube according to a third embodiment of the invention are spaced apart along the long axis X and long side of the effective part and along an intermediate line extending between the long axis X and long side of the effective part; FIGS. 20A and 20B are plan views schematically showing the arrangement of the aperture arrays made in the shadow mask shown in FIG. 19. 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 the first 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. As shown in FIG. 12, the apertures 31 are arranged in a plurality of arrays 32, which extend almost parallel to the short axis Y of the shadow mask 25 and are juxtaposed along the long axis X of the shadow mask 25. The distance PH(N) between the (N-1)th and Nth arrays 32, counted from the array 32 passing the center O of the effective part 24, is given as: PH(N) = A + BN 2 + CN 4 where A, B and C are fourth-degree functions of a Y-coordinate in a coordinate system whose origin is the center O of the effective part and whose axes are the horizontal and vertical axes of the effective part, and C is a function first decreasing and then increasing as the absolute value of the Y-coordinate. The values for A and B changes with C such that the effective part 24 remains substantially rectangular. Assume that the shadow mask 25 has 500 aperture arrays that 250 aperture arrays are juxtaposed on each side of the short axis Y, from the center O of the effective part 24 toward the left or right edge. FIG. 13 shows the relations N and PH(N) have along the long axis X. To be more specific, curve 33 shows the relation which N and PN(N) have along the long axis X of the effective part 24; curve 34 the relation which N and PH(N) have along the intermediate line extending parallel to the long side of the effective part 24 and spaced therefrom by one-fourth the height H' of the effective part 24; and curve 35 the relation which N and PH(N) have along the long side of the effective part 24. The curves 33, 34 and 35 shown in FIG. 13 indicate that as Y increases, the C of CN 4 changes differently along the long axis X of the effective part 24, the intermediate line between the long axis X and long side of the effective part 24 and the long side of the effective part 24. As curves 33 and 34 show, the C of CN 4 decreases as Y increases. The curves 33 and 34 also teach that the distance PH(190M2) by which the 189th and 190th aperture arrays are spaced apart at point M2 (FIG. 5) on the intermediate line is longer than the distance PH(190M1) by which these two adjacent aperture arrays are spaced apart at point M1 (FIG. 5) at which an electron beam passes through the mask 25 before reaching point P1 (FIG. 4) located in the long axis X of the screen 23 and at one-third the width W of the screen 23 from the short axis Y of the screen 23. As the distance Y increases, the fourth-degree function C of N increases. As can be understood from the curve 35, the distance PH(190M3) is shorter than the distance PH(190M2). It is by the distance PH(190M3) that the 189th and 190th aperture arrays are spaced apart at point M3 (FIG. 5) on the long side, at which an electron beam passes through the mask 25 before reaching point P3 (FIG. 4). Namely, the distance PH between the 189th and 190th aperture arrays of the shadow mask 25, which are spaced about one-third the width W of the screen 23 from the short axis Y of the screen 23, have values PH(190M1), PH(190M2) and PH(190M3) which have the following relationship: PH(190M2) > PH(190M3) > PH(190M1) FIG. 14 schematically illustrates the arrangement of the aperture arrays made in the upper-right section (the first quadrant) of the effective part 24 of the shadow mask 25. In this section of the effective part 24, most aperture arrays extend along the curves which are fourth-degree functions of the distance Y, and some aperture arrays close to the right edge of the effective part 24 extend almost straight. That is,the effective part 24 is substantially rectangular. The distance PH between the 189th and 190th aperture arrays provided in that portion of the mask 25, where a local doming will most likely occur to cause electron-beam mislanding on the phosphor screen 23, gradually increases from the point M1 on the long axis X of the effective portion 34 toward the point M2. Then, the distance PH gradually decreases from the point M2 toward the point M3 on the long side of the effective part 24. It will be explained a method of minimizing the electron-beam mislanding due to the local doming of the shadow mask 25. As described above, the three electron beams must correctly land on blue-emitting, green-emitting and red-emitting phosphor stripes in order to display an image having a sufficient color purity on the phosphor screen 23 which is provided on the effective art of the panel 21. To accomplish correct electron-beam landing, the distance q between the effective part 24 of the mask 24 and the effective part of the panel 21 needs to have an appropriate relation with the distance PH between any two adjacent aperture arrays 32. More specifically, the distance q and the distance PH should have such a relation that the distance d between, for example, a red-emitting phosphor stripe 37R and the adjacent blue-emitting phosphor stripe 37B is two-thirds the distance PHP between the adjacent green-emitting phosphor stripes 37G as is illustrated in FIG. 15A. If the distance q is less than the proper value, d will be less than two-thirds of the distance PHP as shown in FIG. 15B -- that is, d < 2/3 PHP. In this case, it is necessary to increase the distance q or decrease the distance PHP. On the other hand, if the distance q is greater than the proper value, d will be greater than two-thirds of the distance PHP as shown in FIG. 15C -- that is, d > 2/3 PHP, and it is necessary to decrease the distance q or increase the distance PHP. As shown in FIGS. 15A, 15B and 15C, light-absorbing stripes 38 are provided among the phosphor stripes 37B, 37G and 37R. As indicated above, the distance PH(190M2) is longer than the distance PH(190M1). It is by the distance PH(190M2) that the 189th and 190th aperture arrays are spaced apart along the intermediate line between the long axis X and long side of the effective part 24. It is by the distance PH(190M1) that the 189th and 190th aperture arrays are spaced apart along the long axis X of the effective part 24. Hence, the distance q may be increased to impart an appropriate relation to the distance q and the distance PH. A conventional shadow mask (hereinafter referred to as "first conventional shadow"), which has aperture arrays juxtaposed along the long axis such that the distance PH between any two adjacent aperture arrays does not change along the short axis Y, is curved along a curve 39 shown in FIG. 16, as viewed in a Y-Z plane containing the point M1 (FIG. 5). The conventional shadow mask 6 shown in FIG. 2 (hereinafter referred to as "second conventional mask") which has aperture arrays juxtaposed along the long axis such that the distance PH between any two adjacent aperture arrays changes as a quadratic function of the distance Y from the long axis X, is curved along a curve 41 shown in FIG. 16, as viewed in a Y-Z plane containing the point M1 (FIG. 5). The second conventional mask can have a longer distance q at the points M2 and M3 (FIG. 5) than the shadow mask which is curved along a curve 39 as viewed in the Y-Z plane. The second conventional mask therefore has a curvature along the short axis Y, large enough to reduce its local doming to some extent. The value the distance PH has at the point M3 is much greater than the value it has at the point M2. To decrease the distance PH at the point M3 appropriately, the second conventional mask must be curved in the opposite direction. To avoid this, the distance PH at the point M2 needs to be relatively short. In the shadow mask 25 shown in FIG. 12, most aperture arrays extend along the curves which are fourth-degree functions of the distance Y as described above. Thus, the mask 25 is curved along a curve 40 shown in FIG. 16, as viewed in a Y-Z plane containing the point M1 (FIG. 5). As can be understood from the curve 40, the distance PH(190M3) at the point M3 is as short as in the second conventional mask, even if the distance PH(190M2) at the point M2 is longer than in the second conventional mask. Therefore, the mask 25 need not be curved in two opposite direction along the short axis Y. Namely, the distance q can be sufficiently long at the point M2, i.e., along the intermediate line, while the distance q along the upper and lower edge is as long as in the second conventional shadow mask. As a result of this, the effective part 24 has a curvature large enough to suppress the mislanding of the electron beams passing through the effective part 24 even if the effective part 24 underwent local doming. The radii Ry of curvature at which the first and second conventional masks and the shadow mask 25 are curved along the short axis Y are as shown in the following Table 1: 1st conventional mask 2nd conventional mask Shadow mask 25 On short axis 850 mm 750 mm 650 mm On intermediate line 850 mm 750 mm 800 mm On long side 850 mm 750 mm 2200 mm As seen from Table 1, the radius Ry of curvature at which the shadow mask 25 is curved along the short axis Y, on the long axis X is 23% less than the radius Ry of curvature of the first conventional mask and 13% less than the radius Ry of the second conventional mask. On the long side, the radius Ry of curvature of the shadow mask 25 is greater than that of the first conventional mask. Nonetheless the doming, if any, of the long side part of the shadow mask 25 is small since the mask frame holding this part has heat capacity large enough to absorbs the thermal energy which the mask 25 generate when impinged with electron beams. This helps to reduce the electron-beam mislanding, despite that the radius Ry of curvature of the mask 25 is relatively large. It has been found that the mislanding occurring in a color cathode-ray tube incorporating the shadow mask 25 is 14% less than the mislanding taking place in a color cathode-ray tube comprising the second conventional mask. A color cathode-ray tube according to a second embodiment of the invention will be described, with reference to FIGS. 17 and 18. In the embodiment shown in FIG. 14, the intervals between any two adjacent aperture arrays on the long axis X of the shadow mask are different from that on the long side of the rectangular shadow mask. However, In the embodiment shown in FIG. 18, the intervals between any two adjacent aperture arrays on the long axis X of the shadow mask are substantially same as that on the long side of the rectangular shadow mask. In FIG. 18, the aperture arrays 32 made in the effective part 24 of the shadow mask extend almost parallel to the short axis Y of the shadow mask and are juxtaposed along the long axis X of the shadow mask. The distance PH(N) between any two adjacent aperture arrays 32 is given as: PH(N) = A + BN 2 + CN 4 The shadow mask of FIG. 18 is the same as the shadow mask 25 shown in FIG. 12, so far as this equation is concerned. However, the coefficients A, B and C have different values. In FIG. 17, a curve 33 shows how much the aperture arrays 32 are spaced apart along the long axis X of the effective part 24 of the mask, a curve 34 shows how much the aperture arrays 32 are spaced apart along an intermediate line extending between the long axis X and long side of the effective part 24, and a curve 35 illustrates how much the aperture arrays 32 are spaced apart along the long side of the effective part 24. The intermediate line is spaced from the long axis X by one-fourth the height H' of the effective part 24. As shown in FIG. 17, the curves 33 and 35 completely overlap. This means that any two adjacent aperture arrays are spaced apart by the same distance along the long axis X and the long side of the effective part 24. FIG. 18 schematically illustrates the arrangement of the aperture arrays made in the upper-right section (the first quadrant) of the effective part 24 of the shadow mask. As clearly shown by broken lines, the distance PH(N) between any two adjacent aperture arrays 32 is equal on the long axis X and the long side of the effective part 24. As evident from the solid curves, the distance PH(N) is longer near a point M2 than near a point M1 at which an electron beam may pass through the mask before reaching a region of the phosphor screen, where the electron-beam mislanding is most prominent. It should be noted that the point M1 is on the long axis X, whereas the point M2 is on the intermediate line spaced from the axis X by one-fourth the height H' of the effective part 24. The shadow mask having the aperture array arrangement shown in FIG. 18 achieves the same advantages as the shadow mask 25 incorporated in the first embodiment. A color cathode-ray tube according to a third embodiment of the invention will be described, with reference to FIG. 19A to 20B. A shadow mask of the third embodiment of the invention has aperture arrays having an arrangement shown in FIG. 19A. In FIG. 19A, the distance PH(N) between any two adjacent aperture arrays 32 is given as: PH(N) = A + BN 2 + CN 4 The shadow mask of FIG. 18 is the same as the shadow mask 25 shown in FIG. 12, so far as this equation is concerned. However, the coefficients A, B and C have different values. In FIG. 19A, a curve 33 shows how much the aperture arrays 32 are spaced apart along the long axis X of the effective part 24 of the mask, a curve 34 shows how much the aperture arrays 32 are spaced apart along an intermediate line extending between the long axis X and long side of the effective part 24, and a curve 35 illustrates how much the aperture arrays 32 are spaced apart along the long side of the effective part 24. The intermediate line is spaced from the long axis X by one-fourth the height H' of the effective part 24. As shown in FIG. 19A, the curves 34 and 35 completely overlap. This means that any two adjacent aperture arrays are spaced apart by the same distance along an intermediate line and the long side of the effective part 24. FIG. 20A schematically illustrates the arrangement of the aperture arrays made in the upper-right section (the first quadrant) of the effective part 24 of the shadow mask. As clearly shown by broken lines, the distance PH(N) between any two adjacent aperture arrays 32 is equal on the intermediate axis and the long side of the effective part 24. As evident from the solid curves, the distance PH(N) is longer near a point M2 than near a point M1 at which an electron beam may pass through the mask before reaching a region of the phosphor screen, where the electron-beam mislanding is most prominent. It should be noted that the point M1 is on the long axis X, whereas the point M2 is on the intermediate line spaced from the axis X by one-fourth the height H' of the effective part 24. The shadow mask having the aperture array arrangement shown in FIG. 19A achieves the same advantages as the shadow mask 25 incorporated in the first embodiment. In the modification of the third embodiment, the shadow mask has an aperture array arrangement shown in FIG. 19B. In FIG. 19B, the distance PH(N) between any two adjacent aperture arrays 32 is given as: PH(N) = A + BN 2 + CN 4 The shadow mask of FIG. 19B is the same as the shadow mask 25 shown in FIG. 12, so far as this equation is concerned. However, the coefficients A, B and C have different values. In FIG. 19B, a curve 33 shows how much the aperture arrays 32 are spaced apart along the long axis X of the effective part 24 of the mask, a curve 34 shows how much the aperture arrays 32 are spaced apart along an intermediate line extending between the long axis X and long side of the effective part 24 and spaced from the axis X by one-fourth the height H' of the effective part 24, and a curve 35 illustrates how much the aperture arrays 32 are spaced apart along the long side of the effective part 24. As the curve 34 shows, the distance PH(N) between any two adjacent aperture arrays located in an intermediate part of the effective part 24 first gradually increases from the short axis Y to the short side of the shadow mask and then gradually decreases from the intermediate part toward the short side of the effective part 24. FIG. 20B schematically illustrates the arrangement of the aperture arrays made in the upper-right section (the first quadrant) of the effective part 24 of the shadow mask. As can be understood from FIG. 20B, the distance between any two adjacent aperture arrays 32 can be obtained under the condition in which the coefficient C in the term CN 4 of the above equation corresponding to the curve 34 set to have a minus value. Since the distance PH(N) between any two adjacent aperture arrays 32 changes so along the long axis X of the effective part 24, the local doming is greatly reduced at that part of the shadow mask through which electron beams may pass through the mask before reaching the elliptical region 15 (FIG. 4) of the phosphor screen. The present invention is not limited to the embodiments described above. Rather, it may be applied to any shadow mask in which the distance PH between any two adjacent aperture arrays 32 is given as PH(N) = A + BN 2 + CN 4 . Appropriate values can be selected for the coefficients A, B and C, thereby to minimize the local doming of the shadow mask. 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 apertures. The apertures are arranged, forming a plurality of arrays which extend along the short axis of the effective part and juxtaposed along the long axis of the effective part. The distance PH(N) between the (N-1)th and Nth arrays, counted from the array passing the center O of the effective part, is given as: PH(N) = A + BN 2 + CN 4 where A, B and C are fourth-degree functions of a Y-coordinate in a coordinate system whose origin is the center O of the effective part and whose axes are the horizontal and vertical axes of the effective part, and C is a function first decreasing and then increasing as the absolute value of the Y-coordinate. The distance PH(N) between the (N-1)th and Nth arrays, which are spaced about one-third the width W of the screen from the short axis of the screen, may increase with the absolute value of the Y-coordinate and may be represented by a fourth-degree function of the Y-coordinate so as to have a transition point in the effective part with respect to the short axis of the effective part. In this case, the distance PH(N) can be optimized without altering the radium of curvature of the inner surface of the panel. The local doming of the shadow mask can therefore be reduced, suppressing the electron-beam mislanding on the phosphor screen. As a result, the color 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 (27R, 27G, 27B) 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 (31) for guiding the electron beams (27R, 27G, 27B) to said phosphor screen (23), wherein said apertures (31) are arranged, forming a plurality of arrays (32) which extend along the short axis (Y) of the effective part and juxtaposed along the long axis (X) of the effective part, characterized in that the distance PH(N) between the (N-1)th and Nth arrays, N being an ordinal number designating the position of the respective aperture array with respect to the Oth aperture array located at the center O of the effective part, is given as: PH(N) = A + BN 2 + CN 4 where A, B and C are fourth-degree functions of a Y-coordinate in a coordinate system whose origin is the center O of the effective part and whose axes (X, Y) are the horizontal and vertical axes of the effective part, and C is a function first decreasing and then increasing as the absolute value of the Y-coordinate increases from 0, and where the values for A and B change with C such that the effective part remains substantially rectangular. The color cathode-ray tube according to claim 1, characterized in that said effective part of said shadow mask (24) has two long sides substantially parallel to said long axis and two short sides substantially parallel to said short axis, and a distance by which any two adjacent aperture arrays (32) are spaced apart at the long axis is equal to a distance by which said any two adjacent aperture arrays (32) are spaced apart at either long side. The color cathode-ray tube according to claim 1, characterized in that a distance between the aperture arrays (32) which are spaced about one-third the width W of the screen (23) from either short axis (Y) of the effective part of said shadow mask increases with the absolute value of the Y-coordinate and is represented by a fourth-degree function of the Y-coordinate so as to have a transition point in the effective part with respect to the short axis (Y) of the effective part.

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, FUKUDA, KUMIO, C/O INTELLECTUAL PROP. DIV., INOUE, MASATSUGU, C/O INTELLECTUAL PROP. DIV., MURAI, TAKASHI, C/O INTELLECTUAL PROP. DIV., OHAMA, SHINJI, C/O INTELLECTUAL PROP. DIV., Saotome, Ichiro, c/o Intellectual Prop. Div., Shimizu, Norio, c/o Intellectual Prop. Div.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1995
Main classification: H01J29
IPC codes: H01J29