Glass bulb for colour picture tube, and said tube

Abstract: A colour picture tube is provided with a glass bulb (1) in which the external surface (10A) of the effective display area of the face plate (10) is formed substantially flat and a colour selection mask (20), having the curvature thereof projected toward the face plate, provided opposed to the internal surface (10B) of the face plate within the glass bulb. Therefore, the external surface is flat, the mechanical shock resistance characteristic is high, a tension can surely be applied to the colour selection mask and moreover vibration of the colour selection mask can be prevented even if external vibration is applied thereto.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to colour picture tubes, such as those used in a television receiver for household use, in a computer monitor and in a colour display, and to a glass bulb for such colour picture tubes. Description of the Related Art A colour picture tube is used in a variety of apparatuses for household use and industrial use, such as television colour picture tubes, computer monitors and colour displays or the like. In general, a glass bulb forming a colour picture tube is formed of a face plate and a funnel and the face plate and funnel are joined with a glass bonding agent. At the internal surface of the face plate, a light emitting material layer is provided to emit light of the three colours red, green and blue. Moreover, a colour selection mask is provided opposed to the internal surface of the face plate within the glass bulb. With enlargement in size of colour picture tubes, it is necessary to use a face plate having a flat surface of the glass bulb in order to satisfy the requirement for distortion-free operation and more natural display. The colour picture tube having such a flat face plate is already known, for example, from the Japanese Patent Application Laid-Open under No. HEI 7-99030. The colour picture tube disclosed in this Japanese patent application is provided with a bulb including a flat glass panel and a flat shadow mask provided opposed to the flat glass panel in this bulb. However, in the colour picture tube having a flat glass panel disclosed in the Japanese patent application laid-open No. HEI 7-99030, it is difficult to give uniform tension in both the horizontal direction (X direction) and the vertical direction (Y direction) of the colour selection mask and newly-designed facilities which are very different from the existing ones will be required. Moreover, it is also difficult to prevent vibration of the colour selection mask, caused by vibrations of external origin, and this easily results in a problem of deterioration of display quality. In addition, since the thickness of the flat glass panel is uniform, a so-called "arch effect" is not provided and strength of the glass panel against external mechanical shock is rather low and it is essential to form a thick glass panel in order to acquire sufficient strength to resist explosion. When the glass panel as a whole is formed thick as explained above, not only does image distortion increase due to refraction of light at the glass panel but also weight of the colour picture tube as a whole is increased. Here, the "arch effect" refers to the case where the face plate of the glass bulb is projected towards the outside and, if a mechanical shock is applied to the frame plate from the external side, a compressing force is applied to the internal surface of the face plate and thereby the shock resistance property of the face plate is increased. When an external mechanical shock is applied to the face plate of a flat glass bulb, the face plate may be broken easily because a tensile force is applied to the internal surface of the face plate. US patent 5,536,995 describes a cathode ray tube in which the thickness of the face plate varies along a diagonal line, with the thickness at the corners being up to 1.2 times the thickness at the centre of the face plate. In order to ensure sufficient mechanical strength for this CRT, compressive stress layers are formed in the inner and outer surfaces of the face plate. SUMMARY OF THE INVENTION It is therefore a first object of the present invention to provide a glass bulb for a colour picture tube having a flat external surface and a higher mechanical shock resistance characteristic. Moreover, it is a second object of the present invention to provide a colour picture tube which uses a flat external surface and assures higher mechanical shock resistance characteristic and moreover is easily capable of giving stable characteristic to the colour selection mask without introduction of a new technology and eliminates vibration of the colour selection mask even if external vibrations are applied. In view of achieving the first object explained above, the present invention provides a cathode ray tube comprising a glass bulb in which the external surface of the effective display area of the face plate is substantially flat and the face plate is thicker at a peripheral portion than at the center of the effective display area thereof, and a colour selection mask, having the curvature thereof projected toward the face plate, provided opposed to the internal surface of the face plate within the glass bulb, characterised in that the direction of variation of thickness of the face plate is the horizontal direction, and the face plate thickness (T) at the peripheral portion in the horizontal direction of the effective display area thereof is selected as 1.2T 0 to 1.3T 0 , where T 0 is the face plate thickness at the center of the effective display area. Here, the effective display area of the face plate means the face plate area on which an image is actually displayed when the glass bulb for colour picture tube is incorporated into the colour picture tube. Moreover, the fact that the external surface of the effective display area of the face plate is substantially flat means that the area in the manufacturing error tolerance range of the face plate is flat. For example, the manufacturing error tolerance range of the face plate in a glass bulb of 28-inch size is about 1 to 2 mm or less. In this case, the surface of the face plate can be visually assumed to be perfectly flat. Moreover, a change in thickness of the effective display area of the face plate toward the peripheral area in the horizontal direction from the center of the effective display area can be expressed using arcs or by polynomial. The curve depicted by the internal surface of the face plate when it is assumed that the glass bulb is held horizontally and the face plate is cut along the vertical surface may be a straight line or may be a curve expressed by arcs or by polynomial. As explained above, the glass bulb for colour picture tube of the present invention can realize the flat surface because the external surface of the effective display area of the face plate is substantially flat. Moreover, in regard to the internal surface of the face plate, even if the peripheral area in the horizontal direction of the effective display area is thicker by 20 to 30 % (for example, 3 to 5 mm) than the central area, the face plate may visually be assumed flat because of the phenomenon that the peripheral area can be seen floated a little by means of the refraction effect. In addition, since the peripheral area in the horizontal direction of the effective display area of the face plate is thicker than the central area of the effective display area, the so-called "arch effect" is generated. Accordingly, the glass panel has higher resistance to external mechanical shock and it is no longer necessary to form a globally thicker face plate in order to assure higher strength for explosion. In view of attaining the second object of the present invention explained above, the colour picture tube of the present invention is characterized by providing: (a) a glass bulb having a face plate of which the external surface of the effective display area is substantially flat; and (b) a colour selection mask provided opposed to the internal surface of the face plate within the glass bulb to have the curvature of projection toward the face plate. Here, curvature of the colour selection mask means the curvature of the curve depicted by a cross section of the colour selection mask when the colour selection mask is assumed to be cut at the horizontal plane. This curve should preferably be an arc and in this case, the curvature of the colour selection mask corresponds to an inverse number of the radius of the arc. A shadow mask and aperture grill are typical examples of possible types of the colour selection mask. In the case where the aperture grill type colour selection mask is used, the colour picture tube is provided with a frame member and it is preferable that the colour selection mask is attached to the frame member under the condition that the tension is applied in the vertical direction. It is not essential but is preferable for realizing higher strength of the face plate that a resin film is bonded at the external surface of the glass bulb for colour picture tube or of the face plate of the glass bulb of the colour picture tube. In the present invention, since the external surface of the face plate is substantially flat, the resin film may be bonded easily to the external surface of the face plate. The resin film may be bonded to the face plate using, for example, an acrylic pressure-sensitive bonding agent. As the resin film, polyethylene, polyethylene terephthalate, polyester and polystyrene may be listed as typical examples. The resin film may also have a multilayer structure. For instance, it is possible to give conductivity to the resin film in such a degree as is sufficient for preventing charging of the face plate by bonding a conductive material such as tin oxide (SnO 2 ) powder to the external or internal surface of the resin film using a bonding agent consisting of silicon oxide, or coating the external or internal surface of the resin film with a conductive material such as tin oxide for the purpose of evaporation or sputtering. On the other hand, it is also possible to form a hard film on the external surface of the resin film (the surface on the opposite side to the face plate) in order to improve surface hardness. As a hard film, a silicon hard coat may be listed as an example. In other words, the resin film may be coloured for controlling the transmissivity of light passing through the face plate and resin film. In addition, it is also possible to form a non-reflection layer or non-uniform area at the external surface of the resin film in order to prevent reflection of light incident from the external side. In the colour picture tube of the present invention, since a glass bulb in which the external surface of the effective display area of the face plate is substantially flat is used, a colour picture tube having a smooth display area can be realised. In addition, since the colour selection mask having the curvature projected toward the face plate is provided, when the aperture grill is used, for example, as the colour selection mask, it can effectively be prevented that the colour selection mask is vibrated due to external vibrations, because of use of the dumping wire, as is done in the related art. Moreover, since the colour selection mask can be attached to the frame member with a sufficient tension, it can also be prevented effectively that colour displacement is generated by the doming phenomenon (in which the colour selection mask is expanded because electron beams collide with the colour selection mask). In addition, even in the case where a shadow mask is used as the colour selection mask as well as the aperture grill, it is possible to introduce the structure similar to that of the related art because the shaping of the projected surface is possible. BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of the presently preferred embodiments thereof, given by way of example, which description should be considered in conjunction with the accompanying drawings in which: Fig. I is a partly cut-away schematic diagram of a colour picture tube in relation to a first embodiment of the present invention; and Fig. 2 is a partly cut-away schematic diagram of a colour picture tube in relation to a second embodiment of the present invention; and Fig. 3A and Fig. 3B are a schematic perspective view of an aperture grill type colour selection mask and a partly enlarged schematic diagram of the aperture provided to the colour selection mask, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the invention will be explained with reference to the accompanying drawings. (Embodiment 1) Fig. 1 is a schematic diagram of a partly cut-away colour picture tube of the first embodiment of the present invention. The colour picture tube of the present invention is provided with a glass bulb 1 in which the external surface 10A of the effective display area of the face plate 10 is substantially flat and a colour selection mask 20 which is provided, within the glass bulb 1, opposed to the internal surface 10B of the face plate 10 and is having the curvature projected toward the face plate 10. In regard to the face plate 10 of the glass tube 1, the external surface 10A of the effective display area is substantially flat and thickness T of the peripheral part in the horizontal direction of the effective display area of the face plate is larger than that To of the central area of the effective display area. In other words, the internal surface 10B of the face plate 10 of the glass bulb 1 has the curvature recessed toward the colour selection mask 20. The curvature of the colour selection mask 20 is larger than the curvature of the internal surface 10B of the face plate 10. When it is assumed that the glass bulb 1 is held horizontally and the face plate 10 is cut at the horizontal line, the curve depicted by the internal surface 10B of the face plate 10 may be an arc or a curve expressed by a polynomial. When such curve is expressed by an arc, an inverse number of the radius of the arc corresponds to the curvature of the internal surface 10B of the face plate 10. Moreover, when such curve is expressed by a polynomial, an inverse number of the radius of the arc connecting the three points of the peripheral area in the horizontal direction of the effective display area of the face plate 10 and the center of the effective display area is defined as the curvature of the internal surface 10B of the face plate 10. In the colour picture tube of the first preferred embodiment, since the curvature of the colour selection mask 20 is larger than the curvature of the internal surface 10B of the face plate 10, the distance in the horizontal direction from the centre of the effective display area of face plate 10 up to the peripheral area of the effective display area of the face plate 10 is longer than that up to the colour selection mask. However, in the colour picture tube of the first preferred embodiment, colour purity, particularly, in the peripheral area of the television colour picture tube can be remarkably improved by widening the pitch between the apertures 21, 21 (refer to Fig. 3B) provided in the colour selection mask 20 as it goes to the peripheral area in the horizontal direction of the face plate 10. The face plate 10 is bonded to a funnel 11 with a glass bonding agent. The face plate 10 near the funnel 11 is wound by a tension band 12 to enhance the strength of the glass bulb 1. As the schematic perspective view of Fig. 3A shows, the aperture grill type colour selection mask 20 is attached to the frame member 22 by the resistance welding method or laser welding method under the condition that the tension is applied in the vertical direction. The frame member 22 is removably attached to the face plate 10 with a fitting device 23 formed of spring means. The other structure of the colour picture tube is similar to that of existing colour picture tubes and detailed description will be omitted here. The external surface 10A of the face plate 10 is bonded to a resin film 30 formed, for example, of polyethylene terephthalate using a acrylic pressure-sensitive bonding agent. This resin film 30 is given a conductivity which is enough, for example, to prevent charging of the face plate and a silicon hard coat film is formed at the external surface of the resin film. In a 28-inch type colour picture tube, when the external surface 10A of the face plate 10 is placed on a table surface so that the surface 10A of the center of the effective display area of the face plate 10 is in contact with the table surface, a gap of about 1 to 2 mm is sometimes generated between the peripheral area in the horizontal direction of the effective display area of the face plate 10 and the table surface, a curving of the external surface 10A of the face plate 10 of such a degree is within the manufacturing error tolerance range of the face plate. The face plate within the manufacturing error tolerance range of such a degree can be seen as sufficiently smooth by the human eye. In a 28-inch type coldur picture tube, thickness T of the peripheral area in the horizontal direction of the effective display area of the face plate is set, for example, to 18 mm, while thickness T 0 at the center of the effective display area is set, for example, to 15 mm. Since the curve depicted by the internal surface 10B of the face plate 10 is set to an arc, the radius of curvature of the internal surface 10B of the face plate 10 is about 10000 mm. Since the external surface of the face plate is flat, if the internal surface of the face plate is curved in such a degree, the face plate can be seen flat also when an image is displayed. Moreover, the radius of curvature of the colour selection mask is set, for example, to about 8000 mm. In the first preferred embodiment, the pitch between the apertures 21, 21 provided in the colour selection mask 20 is set to 0.56 mm at the center of the effective display area of the face plate 10 and it is then gradually widened as it goes to the peripheral area in the horizontal direction and it is finally set to 0.8 mm at the peripheral area in the horizontal direction of the effective display area of the face plate 10. Thereby, colour purity in the peripheral area of the television colour picture tube can be improved to a large extent. (Embodiment 2) In the colour picture tube of the second embodiment, as shown in Fig. 2, the internal surface 10B of the face plate 10 of glass bulb 1 has the curvature projected toward the colour selection mask 20 and the curvature of the colour selection mask 20 is almost equal to the curvature of the internal surface 10B of the face plate 10. In more practical terms, the radius of curvature of the colour selection mask 20 is within the range from 90% to 100% of the radius of curvature of the internal surface 10B of the face plate 10. In this case, the pitch between the apertures 21, 21 provided in the colour selection mask is preferably set constant without relation to the horizontal position of the face plate 10. The present invention has been described on the basis of the preferred embodiments thereof, but the present invention is not limited thereto. The numerical data given in the above embodiments are only examples and these values may of course be varied and the structure of the colour picture tube is also exemplary rather than limitative. With the introduction of the glass bulb for colour picture tube of the present invention, not only can a colour picture tube having a flat surface be realised, but also a higher mechanical shock resistance characteristic of the glass panel against external shock can be realised and it is no longer required to make thicker the face plate in order to assure the sufficient strength against explosion. In comparison with the flat type face plate, weight of the face plate can be reduced by about 10% to 20 %. Moreover, while keeping the manufacturing system and quality of the related art, a colour picture tube having a flat display surface can be realised by introduction of the colour picture tube of the present invention. In addition, since a colour selection mask having the curvature projected toward the face plate is provided, not only vibration of the colour selection mask due to external vibrations can be prevented but also generation of colour displacement due to the doming effect can also be prevented effectively.

Claims: A cathode ray tube comprising a glass bulb (1) in which the external surface (10A) of the effective display area of the face plate (10) is substantially flat and the face plate is thicker at a peripheral portion than at the center of the effective display area thereof, and a colour selection mask (20), having the curvature thereof projected toward the face plate, provided opposed to the internal surface (10B) of the face plate within the glass bulb, characterised in that the direction of variation of thickness of the face plate is the horizontal direction and the face plate thickness (T) at the peripheral portion in the horizontal direction of the effective display area thereof is selected as 1.2T 0 to 1.3T 0 , where T 0 is the face plate thickness at the center of the effective display area. A cathode ray tube according to claim 1, wherein the internal surface (10B) of the face plate of the glass bulb has the curvature thereof recessed toward the colour selection mask (20). A cathode ray tube according to claim 1 or 2, wherein the curvature of the colour selection mask (20) is greater than the curvature of the internal surface (10B) of the face plate. A cathode ray tube according to claim 1, 2 or 3, wherein the radius of curvature of the colour selection mask (20) is 90-100% of the radius of curvature of the internal surface (10B) of the face plate. A cathode ray tube according to any previous claim, wherein a multiple layered resin film (30) is bonded to the external surface (10A) of the face plate of the glass bulb. A cathode ray tube according to any previous claim, wherein the colour selection mask (20) is formed of a frame (22) and a plurality of metal fine leads extended over the frame and the pitch of these fine metal leads is gradually widened as it goes toward the peripheral area in the widthwise direction of the face plate.

Inventors: IGUCHI YUKINOBU, SAITA KOJI, UTSUMI ICHIRO, IGUCHI, YUKINOBU, SAITA, KOJI, UTSUMI, ICHIRO
Applicants: SONY CORP, SONY CORPORATION
Application year: 1997
Main classification: H01J29
IPC codes: H01J31, H01J29, H01J9