COLOR CATHODE RAY TUBE

Abstract: A color cathode ray tube comprises a shadow mask structure (5) with its mask spring having 1.2-2.0 times the coefficient of thermal expansion of its support frame. This structure serves to reduce the difference in thermal expansion between the mask spring and the support frame, so that the beam landing shift is reduced to prevent the deterioration of color purity. Therefore, the color cathode ray tube can constantly maintain color purity regardless of the temperature variation in the set.

Description: The present invention relates to a color cathode ray tube incorporated in a color monitor set or in a color TV set and, particulary, to a color cathode ray tube which decreases the occurrence of beam landing error cause by the movement of a shadow mask structure resulting from a rise of the temperature in the set or a rise of the temperature of the shadow mask when the color monitor set or the color TV set is operated. A color cathode ray tube according to the preamble portion of patent claim 1 has been known from JP-A-6222354. A color cathode ray tube is generally constituted by a panel portion which is a picture screen, a neck portion for housing an electron gun, and a funnel portion for connect the panel portion to the neck portion, and is provided in the funnel portion with a deflection device for scanning an electron beam emitted from an electron gun on a fluorescent screen formed on the inner surface of the panel. Fig. 1 is a diagram schematically illustrating the structure of a cathode ray tube, wherein reference numeral 1 denotes a panel, 2 denotes a funnel, 3 denotes a neck portion, 4 denotes a fluorescent screen (screen), 5 denotes a shadow mask structure, 51 denotes panel pins for supporting the shadow mask structure, 6 denotes a magnetic shield, 7 denotes deflection yokes, 8 denotes a magnet for adjusting the purity, 9 denotes a magnet for adjusting the center beam static convergence, 10 denotes a magnet for adjusting the side beam static convergence, 11 denotes an electron gun, and B denotes the electron beams. The electron beams for R (red), G (green) and B (blue) are deflected in the horizontal direction and in the vertical direction by the deflection divice provided in the funnel portion on the way from the electron gun to the fluorescent screen, selected depending upon the colors by the shadow mask disposed in the panel portion, and impinge upon the fluorescent screen, whereby the fluorescent screen emits light in different colors so that an image is formed on the fluorescent screen. Fig. 2 is a diagram schematically illustrating the shadow mask structure which comprises a shadow mask 12 having a plurality of electron beam passing openings for selecting colors, a support frame 13 for holding the shadow mask 12, and mask springs 14 for holding the support frame 13 in the panel. The shadow mask structure 5 is held by joining the mask spring-holding holes 141 to the panel pins 51 formed on the panel. Furthermore, the mask spring holding holes 141 are positioned on a vertical axis or on a horizontal axis that passes nearly through the center of the shadow mask structure 5. In general, the shadow mask 12 is made of invar (e.g., having a coefficient of thermal expansion of 6.9 X 10 -6 /°C), the support frame 13 is made of a steel (e.g., having a coefficient of thermal expansion of 1.15 X 10 -5 /°C), and the mask springs 14 are made of a stainless steel (e.g., having a coefficient of thermal expansion of 1.04 X 10 -5 /°C). Hereinafter, the coefficient of thermal expansion means a coefficient of linear thermal expansion. In this case, even the shadow mask 12 which is nearly flat suppresses the doming of the shadow mask having low thermal expansion of the invar. In order to decrease the change with time of beam landing in the case of full-luster display, furthermore, the mask springs 14 are often made of a single material without bimetal function. When the cathode ray tube incorporated in the color monitor set or in the color TV set (hereinafter referred to as the set) is operated, the temperature in the set containing the funnel portion and the neck portion gradually rises due to heat energy generated by the circuit components in the set and reaches an equilibrium. Since the screen of the panel is exposed, it has a temperature lower than the temperature inside the set. The heat energy generated by the circuit components in the set raises the temperature in the set and, then, raises the temperature of the funnel. Moreover, the temperature of the inner shield is raised due to the radiant heat, causing the temperatures of the support frame and the mask springs to be raised, too. The temperature surrounding the cathode ray tube is lower the panel portion than the funnel portion. Furthermore, the temperature of the panel portion is lower than the temperature of the funnel portion. Therefore, the mask springs joined to the panel pins buried in the panel are less heated than the support frame, and are not thermally expanded by the same amount when the mask springs and the support frame have the same coefficient of thermal expansion. For example, a mask spring support point 141 on the short side or on the long side of the shadow mask structure and a point 131 on the support frame in the vicinity thereof are in the same straight line as a mask spring support point 141 opposed to the above-mentioned mask spring support point 141 and a point 131 on the support frame in the vicinity thereof. When their positional relationship is maintained, the shadow mask is not distorted. Actually, however, the mask springs and the support frame are not thermally expanded by the same amount, causing the shadow mask structure to be distorted. Distortion in the shadow mask structure causes the beam landing shift, deteriorating the color purity. Fig. 3 illustrates by arrows the motion of points 131 on the support frame near the mask spring support points 141 in the four-pin type shadow mask structure in which the mask springs have a coefficient of thermal expansion nearly equal to that of the support frame, i.e., in which the amount of thermal expansion of the mask springs is smaller than the amount of thermal expansion of the support frame. As described above, the motion of points 131 on the support frame near the mask spring support points 141 is caused by the difference in the thermal expansion between the mask springs 14 and the support frame 13 as a result of an increase in the temperature in the set. In the four-pin type shadow mask structure, the points 131 move in the directions of arrows; i.e., the shadow mask as a whole receives a rotational force. Fig. 4 is a diagram illustrating the motion of points 131 on the support frame near the mask spring support points 141 of a three-pin type shadow mask structure of when the thermal expansion of the mask springs is smaller than the thermal expansion of the support frame, in which the points 131 move in the directions of arrows. In the three-pin type shadow mask structure, therefore, the points 131 move in the directions of arrows, and the force is concentrated on the right upper corner portion of the shadow mask. Fig. 5 shows the directions of shift of the electron beam landing that occurs when a cathode ray tube using the three-pin type shadow mask structure shown in Fig. 4 is mounted on the color TV set. In general, the mask springs and the support frame are designed by taking into consideration the heat energy that is generated when the electron beams impinge upon the shadow mask but without taking into consideration the heat energy generated by the circuit components in the set. In a color display tube used for a color monitor set, in particular, the structure of the fluorescent screen is of the dot type and involves a stricter problem in regard to the color purity than that of the fluorescent screen structure of the stripe type. In a high definition color display tube in which the shadow mask for determining the dot pitch of the fluorescent screen has a hole pitch of smaller than 0.31 mm, furthermore, this becomes a more serious problem. Besides, in the color display tube, the number of the horizontal scanning lines must be increased. Therefore, the horizontal deflection frequency increases due to the deflection yokes, and an increased amount of heat is generated by the circuit components in the deflection yokes and in the set. The problem of heat generation becomes conspicuous, particularly in a high definition display in which the number of the horizontal scanning lines substantially exceeds 1000. The above-mentioned problem becomes serious, particularly in a high definition color display tube. JP-A-6222354 discloses a color cathode ray tube including a shadow mask structure comprising a shadow mask, a support frame for holding said shadow mask, and mask springs for holding said support frame in a panel, wherein said shadow mask is made of invar, at least one of support points where said mask springs support said panel is positioned nearly at the center of a side where said mask springs are arranged. Here, too, the thermal expansion coefficients of the frame and the springs are considerably different (i.e. 1.1 x 10 -5 /°C for the frame and 5 x 10 -6 /°C for the spring), i.e. the thermal expansion coefficient of the frame is more than twice as large than that of the spring. This leads to an undesirable distorsion of the alignment of the mask with respect to the deflection device. By using a shadow mask structure in which the mask springs have a coefficient of thermal expansion which is from 1.2 to 2.0 times as great as the coefficient of thermal expansion of the support frame, it is possible to suppress the difference between the thermal expansion of the mask spring and the thermal expansion of the support frame, preventing deterioration in the color purity caused by a beam landing shift that stems from the difference between the thermal expansion of the mask springs and the thermal expansion of the support frame, and, hence, providing a color cathode ray tube which stably maintains the color purity without being affected by a change in the temperature in the set. [Brief Description of Drawings] Fig. 1 is a sectional view of a cathode ray tube; Fig. 2 is a diagram schematically illustrating a shadow mask structure; Fig. 3 is a diagram illustrating the motion of points on the support frame near the mask spring support points in a conventional four-pin type shadow mask structure in which the coefficient of thermal expansion of the mask springs is nearly the same as the coefficient of thermal expansion of the support frame; Fig. 4 is a diagram illustrating the motion of points on the support frame near the mask spring support points in a conventional three-pin type shadow mask in which the coefficient of thermal expansion of the mask springs is nearly the same as the coefficient of thermal expansion of the support frame; Fig. 5 is a diagram showing the directions of beam landing shift in a cathode ray tube using a conventional three-pin type shadow mask structure, in which the coefficient of thermal expansion of the mask springs is nearly the same as the coefficient of thermal expansion of the support frame; Fig. 6 is a diagram for comparison of an embodiment of the present invention with a conventional example; Fig. 7 is a diagram for comparison of the amount of shift of the beam of the three-pin type shadow mask structure of the embodiment of the invention with that of the conventional example, with the lapse of time; and Fig. 8 is a diagram illustrating the relationship between the amount of shift of the beam and the ratio of the coefficient of thermal expansion of the mask springs to the coefficient of thermal expansion of the support frame. [Best Mode for Carrying Out the Invention] Fig. 6 is a table for comparison of an embodiment of the present invention with a conventional example. In the present invention, a shadow mask 12 is made of invar (coefficient of thermal expansion of 6.9 × 10 -6 /°C), a support frame 13 is made of a steel (coefficient of thermal expansion of 1.15 × 10 -5 /°C) and mask springs 14 are made of a stainless steel (coefficient of thermal expansion of 1.73 × 10 -5 /°C). Owing to the use of the shadow mask 12 of this embodiment constituted by using the above-mentioned materials, the coefficient of thermal expansion (1.73 × 10 -5 /°C) of the mask springs 14 is 1.5 times as great as the coefficient of thermal expansion (1.15 × 10 -5 /°C) of the support frame 13 when the temperature of the support frame is greatly raised because of a rise in the temperature in the set and when the temperature of the mask springs is little raised. Therefore, the difference in the amount of thermal expansion is small between the mask springs 14 and the support frame 13. The motion of the points 131 on the support frame 13 near the support points 141 of the mask springs 14 that hold the color cathode ray tube in the panel and caused by a rise in the temperature in the set, is canceled by the thermal expansion of the mask springs. The moving amount of the shadow mask welded to the support frame decreases with a decrease in the moving amount of the support frame, and the beam landing shift decreases, too. Fig. 7 shows the beam landing characteristics of when the present invention is applied to the shadow mask structure having three-pin type springs, and of when the shadow mask structure is used in a 36-cm color display tube and the display tube is operated being incorporated in the set, and shows the conventional beam landing characteristics in comparison with the beam landing characteristics of the present invention. In Fig. 7, the ordinate represents the shift of the electron beam in µm and the abscissa represents the passage of time in minutes. Line 15 represents the moving amount of a beam at the left lower corner of the panel using the conventional color display tube, and line 16 represents the moving amount of a beam at the left lower corner of the panel using the color display tube of the present invention. When the display tube is operated being mounted in the set, the temperature in the set reaches an equilibrium at 50°C. The temperature in the set is measured at an upper part of the funnel. By embodying the present invention, the amount of change in the beam landing can be greatly decreased to 5 µm from 17 µm after the operation for 100 minutes. That is, the amount of change in the beam landing can be decreased in the peripheries of the panel screen. In the above-mentioned embodiment, the mask springs 14 were made of a stainless steel. In general color cathode ray tubes, however, the mask springs 14 are often constituted by a bimetal to cope with the so-called doming. In the case of the bimetal springs, the coefficient of equivalent thermal expansion of the springs is an average value of the coefficients of thermal expansion of the two metals. Fig. 8 is a diagram illustrating the relationship among the moving amount of the beam, the temperature and the ratio of the coefficient of thermal expansion of the mask springs to the coefficient of thermal expansion of the support frame. The environmental temperature was assumed to be 40°C which is a high temperature and 0°C which is a low temperature, and the temperature difference between the inside and the outside of the set was 25°C, i.e., the temperature difference between the periphery of the panel and the periphery of the funnel was 25 °C. In Fig. 8, line 17 represents the relationship between the ratio of coefficients of thermal expansion and the amount of shift of the beam of a case where the environmental temperature is high and there is no temperature difference between the inside of the set and the outside of the set, line 18 represents the relationship of a case where the environmental temperature is low and there is no temperature difference between the inside of the set and the outside of the set, line 19 represents the relationship of a case where the environmental temperature is low and the temperature difference is 25°C between the inside of the set and the outside of the set, and line 20 represents the relationship of a case where the environmental temperature is high and the temperature difference is 25°C between the inside of the set and the outside of the set. The measurement point is in an upper part at the center of the panel, and a rightward shift beyond the measurement point is regarded to be a positive (+) movement and a leftward shift is regarded to be a negative (-) movement. When the ratio of coefficients of thermal expansion is 1.0, the environmental temperature of the whole cathode ray tube is uniform when there is no temperature difference between the outside of the set and the inside of the set irrespective of whether the environmental temperature is high or low, and the amount of shift of the beam is 0 µm. When the environmental temperature is low and there is a temperature difference between the periphery of the panel and the periphery of the funnel, or when the environmental temperature is high and there is a temperature difference between the periphery of the panel and the periphery of the funnel, the amount of shift of the beam is 25 µm. When the ratio of coefficients of thermal expansion is 2.0 and when there is no temperature difference between the outside of the set and the inside of the set, the amount of shift of the beam is -10 µm when the environmental temperature is high, and 10 µm when the environmental temperature is low. When the environmental temperature is low and there is a temperature difference between the periphery of the panel and the periphery of the funnel, the amount of shift of the beam is 0 µm. When the environmental temperature is high and there is a temperature difference between the periphery of the panel and the periphery of the funnel, the amount of shift of the beam is -20 µm. When the allowable range of the amount of shift of the beam landing is determined to be ± 20 µm from the visual point of view, then, the ratio of coefficients of thermal expansion is from 1.2 to 2.0. When the ratio of coefficients of thermal expansion is 1.71, furthermore, the amount of shift of the beam landing is ± 7 µm which is a minimum amount. [Industrial Applicability] As described above, the color cathode ray tube of the present invention is incorporated in a color monitor set or a color TV set, and is adapted to be used under the conditions where the temperature rises in the color monitor set or in the color TV set or where there takes place a temperature difference between the mask frame and the mask springs.

Claims: A color cathode ray tube including a shadow mask structure (5) comprising a shadow mask (12), a support frame (13) for holding said shadow mask (12), and mask springs (14) for holding said support frame (13) in a panel (1), wherein said shadow mask (12) is made of invar, at least one of support points (141) where said mask springs (14) support against said panel (1) is positioned nearly at the center of a side where said mask springs (14) are arranged, characterized in that said mask springs (14) have a coefficient of thermal expansion which is from 1.2 to 2.0 times as great as the coefficient of thermal expansion of said support frame (13). A color cathode ray tube according to claim 1, characterized in that said support frame (13) is made of a steel, and said mask springs (14) are made of a stainless steel. A color cathode ray tube according to claim 1, characterized in that said mask springs (14) are made of a bimetal. A color cathode ray tube according to one of the preceding claims, characterized in that the shadow mask (12) is inside a panel (1), and the fluorescent screen (4) formed on the inner surface of the panel (1) has a dotted structure, the pitch of holes formed in said shadow mask (12) is not larger than 0.31 mm. A color cathode ray tube according to claim 4, characterized said support frame (13) is made of a steel, and said mask springs (14) are made of a stainless steel. A color cathode ray tube according to claim 4, characterized in that said mask springs (14) are made of a bimetal. A color monitor set or a color TV set comprising a color cathode ray tube according to one of the preceding claims, having a panel portion (1), a funnel portion (2) and a neck portion (3), and deflection yokes (7) provided in said funnel portion (1), wherein the number of horizontal scanning lines when the set is being used is substantially not smaller than 1000. A color cathode ray tube according to claim 1, characterized in that said color cathode ray tube is not smaller than 36 cm.

Inventors: TANAKA TOSHIHIKO, TANAKA, TOSHIHIKO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1995
Main classification: H01J29
IPC codes: H01J29