Abstract:
A panel for a cathode ray tube having an inner surface with predetermined roughness, including a plurality of black matrix layers formed on the inner surface and a phosphor layer composed of red, green and blue phosphors between the black matrix layers, and the phosphor layer and the black matrix layer are formed on the inner surface of the panel after forming a transparent dielectric film, can prevent degradation of brightness and color purity of a screen caused as a result of grinding process which gives roughness to an inner surface of the panel in order to prevent mirror surface reflection on the inner surface of the panel caused by external light.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a cathode ray tube and particularly, to a panel for a cathode ray tube, capable of preventing degradation of brightness and color purity of a screen caused as a result of grinding process which gives roughness to an inner surface of the panel in order to prevent mirror surface reflection on the inner surface of the panel caused by external light. 
   2. Description of the Related Art 
   Generally, as shown in  FIG. 1 , a flat cathode ray tube includes a panel  1  having an inner surface and outer surface which are formed as a flat plane which is mounted on a front surface of the cathode ray tube, a screen film  2  which is coated on the inner side of the panel  1 , a shadow mask  3  which has a function of filtering colors of the electron beam  8  injected to the screen film  2 , a frame for supporting the shadow mask  3 , a funnel  5  which is combined on the rear surface of the panel  1  and maintains a vacuum inside the cathode ray tube, an electron gun  7  which is mounted inside a neck portion  6  which is formed at the rear of the funnel  5 , for emitting the electron beam  8 , a deflection yoke  9  for deflecting the electron beam  8  which is emitted from the electron gun  7  and an inner shield  10  which is mounted at an inner side of the panel  1 , for shielding affect of the external earth terrestrial magnetism when the electron beam progresses to the screen  2 . 
   As shown in  FIG. 2 , in the screen film  2 , a black matrix layer  11  which is made of graphite is formed as points which are positioned at a predetermined interval with a predetermined width or as a linear black matrix pattern  13  on the inner surface of the panel  1 , and a phosphor layer  12  including blue  12   a , green  12   b  and red  12   c  colors which are formed in a laminated structure while being overlapped in a predetermined region among the respective black matrix layers  11  or the upper portion, is sequentially coated at a predetermined interval. In addition, a metal layer  14  such as aluminum is deposited in the upper portion of the phosphor layer  12 . 
   In the conventional cathode ray tube, the electron beam  8  generated in the electron gun  7  is deflected by the deflection yoke  9 , and selectively reaches to the phosphor layer after passing the shadow mask  3 , thus to have the respective phosphors  12   a ,  12   b  and  12   c  emit light. Then, the emitted light passes the inside of the panel  1  and implements a final screen on the front surface of the panel  1 . 
   On the other hand, when a user looks into the screen of the image display device from the outside, external light L such as electric light and sun light is transmitted into the panel  1  from the outside, and the transmitted external light L causes external light reflection which is mirror surface reflection on the peripheral surface between the phosphor layer  12  which is coated on the inner surface of the panel  1  and the black matrix layer  11 . Such external light reflection has a disadvantage of making eyes of the user looking at the displayed screen fatigued. 
   Also, in case surface roughness of the panel  1  on which the phosphor layer  12  is coated is low, degree of the external light reflection  15  becomes deepened and to solve this, the inner surface of the panel  1  must be mechanically ground and mirror surface reflection of the inner surface of the panel  1  caused by external light must be prevented by giving rougher inner surface of the panel  1 . In addition, the external light must be scattered in order to reduce visual fatigue of the user looking at the displayed screen. 
   Here, to give a predetermined degree of surface roughness to the inner surface of the panel  1 , a method of mechanically grinding a surface of the panel  1  with abrasive composed of fine powder particles having a predetermined hardness using a flat abrasive stone or abrasive pad is used as a method of grinding processing to give a predetermined degree of surface roughness to the inner surface of the panel  1 . 
   In case the inner surface of the panel  1  is ground by the above method, the surface roughness becomes 1.5˜2.5 μm for an evaluation length of 1 mm when measuring by a mean peak to valley height method (hereinafter, as Rz: DIN 4768/1). On the other hand, in case of processing with a lower range of roughness than the surface roughness value, that is, in case of processing with a surface roughness close to that of a mirror surface, abrasive with a smaller grain size must be used, and processing time must be increased, thus to increase manufacturing cost. 
   In case of processing the surface with a larger range of roughness than the surface roughness value, that is, in case of processing with a surface roughness gets out of that of the mirror surface, the cost is decreased. However, when the phosphor layer  12  which is directly coated on the inner surface of the panel emits light while the image display device is operated and transmits light from the inner surface to the outer surface of the panel  1 , scattering of light is too extreme by high roughness of the inner surface of the panel  1 , and the light transmission rate of the panel  1  is degraded, thus to degrade brightness of the image display device. 
   On the other hand, when a phosphor layer  12  is coated on an inner surface of the panel  1  having roughness higher than 1.5 μm, generally, an average grain size of the respective fluorescent particles which comprise the phosphor layer  12  is 5 μm or higher. 
   When the phosphor having the above grain size is coated on the inner surface of the panel  1 , the phosphor layer  12  is formed by forming a periphery while being contacted on the mountain region on the inner surface of the panel  1  having a shape of the section formed as peaks and valleys which are sharp wave forms. 
   Therefore, since the size of the size of the section on the inner surface of the panel  1  is relatively smaller than the size of the average of the phosphor, the phosphor layer  12  is formed under the condition that the fluorescent particles is not infiltrated into the valley of the inner surface of the panel  1 , thus to form a pore surface  16  which is an empty space between phosphor layer  12  and the inner surface of the panel  1 . 
   The pore surface  16  promotes light scattering reflection inside the panel  1  when the phosphor emits light and the light progresses to the outer surface of the panel  1 , and brightness of the image display device is degraded by reducing efficiency of transmitting light of the phosphor layer  12  through the panel  1 . 
   On the other hand, in the process of forming the black matrix pattern  13 , materials for forming the black matrix layer  11  composed of materials such as graphite and the like are entirely coated on the inner surface of the panel  1 . Then, the surface is exposed by using a random pattern, and a final black matrix pattern  13  is developed by dividing the exposed part and the rest part. 
   The roughness of the inner surface of the panel  1  badly affects on adhesiveness between the coated graphite and the inner surface of the panel  1 , and accordingly, it is difficult to develop the peripheral line as a line having clear straightness in developing the black matrix layer  11  after exposing. Therefore, cutting that the periphery lines among the black matrix patterns  13  and between the respective phosphor layer  12  and black matrix pattern  13  which are formed having a part overlapped thereon are formed as straight lines becomes degraded, thus to degrade quality corresponding to characteristics such as brightness and color purity of the image display device conclusively. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to provide a panel for a cathode ray tube, increasing brightness of an image display device by forming a transparent dielectric film on the inner surface of a panel in order to transmit light to the outside of the panel well, and, improving color purity by improving cutting that a peripheral line between the phosphor layer and a black matrix pattern is formed as a straight line in forming the black matrix pattern, by reducing roughness of the inner surface of the panel. 
   To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a cathode ray tube having an inner surface with predetermined roughness, including a plurality of black matrix layers formed on the inner surface and a phosphor layer composed of red, green and blue phosphors between the black matrix layers, and the phosphor layer and the black matrix layer are formed on the inner surface of the panel after forming a transparent dielectric film. 
   Also, to achieve the above object, there is provided a cathode ray tube, including a plurality of black matrix layers formed on an inner surface and a phosphor layer composed of red, green and blue phosphors between the black matrix layers, and the inner and outer surfaces of the panel are substantially flat, the inner surface has predetermined surface roughness, and the phosphor layer is formed on the inner surface of the panel after forming a transparent dielectric film. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
     In the drawings: 
       FIG. 1  is a schematic view showing a general flat cathode ray tube; 
       FIG. 2  is a cross-sectional view showing structure of a image display device formed on a panel of the conventional cathode ray tube and the partially enlarged cross-sectional view thereof; 
       FIG. 3  is a cross-sectional view showing structure of an image display device formed on a panel of a cathode ray tube in accordance with the present invention and the partially enlarged cross-sectional view thereof; 
       FIG. 4A  is an enlarged view showing an inner surface of the panel before forming a transparent dielectric film; 
       FIG. 4B  is an enlarged view showing an inner surface of the panel after forming the transparent dielectric film; 
       FIG. 5A  is a plan schematic view showing cutting of a black matrix pattern before forming the transparent dielectric film; 
       FIG. 5B  is a plan schematic view showing cutting of a black matrix pattern after forming the transparent dielectric film; and 
       FIG. 6  is a graph showing brightness of an image display device in which a transparent dielectric film is formed by comparing with the brightness of the conventional device. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 3  is a cross-sectional view showing structure of an image display device of a panel of a cathode ray tube in which a transparent dielectric film  17  is formed in accordance with the present invention and the partially enlarged cross-sectional view thereof. 
   As shown in  FIG. 3 , the transparent dielectric film  17  is formed between an inner surface of a panel  100  and a phosphor layer  12 , and reduces a pore surface  16  between the phosphor layer  12  generated in the section shape of the inner surface of the panel  100  having a valley region at the center and the inner surface of the panel  100 . 
   That is, the inner surface of the panel  100  having flat inner and outer surfaces is mechanically processed by grinding, when measuring the surface roughness by a general mean peak to valley height method, a compound formed by mixing liquid sol containing an organic solvent of alcohol group and silica substance (SiO 2 ) is evenly coated on an entire surface of the inner surface of the panel  100  which is formed to having the roughness value (Rz) of 1.5˜2.5 μm corresponding to the evaluation length of 1 mm under the condition that a temperature of the inner surface of the panel  100  is maintained as 25˜35° C. The resultant material is hardened to become a mirror surface at a temperature of 150° C. or higher in a dry kiln, and as shown in  FIG. 3 , the transparent dielectric film  17  of a silica substance is formed on the whole inner surface of the panel  100 . 
   In the above process, the temperature of the inner surface of the panel  100  relates to a thickness of the transparent dielectric film  17 , if the temperature is low, the thickness becomes too thin, and accordingly, quality of the image display device can not be improved. If the temperature is high, the transparent dielectric film  17  can not be uniformly formed on the whole inner surface. 
   Also, as shown in the enlarged view of  FIG. 3 , the transparent dielectric film  17  of the silica substance is formed to be filled on the inner surface of the panel having peaks and valleys, putting the valley at the center. 
   Therefore, after forming the transparent dielectric film  17 , the surface roughness of the inner surface of the panel  100  is formed to have a small value Rz in a range of 0.5˜2.0 μm, and the roughness is formed lower than before forming the transparent dielectric film  17 . 
   That is, as shown in  FIG. 4A , the shape of the inner surface of the panel  100  was a sharp wave form before forming the transparent dielectric film  17 , and after forming the transparent dielectric film  17 , the shape is formed as a smooth wave form as shown in FIG.  4 B. 
   Also, in the conventional panel  1 , the panel is generally processed to having a surface roughness of 1.5˜2.5 μm by considering the cost and reflection by external light on a peripheral surface between the inner surface of the panel  1  and the phosphor layer  12  when processing the inner surface by grinding. On the other hand, in the present invention, since the transparent dielectric film  17  is formed even if the panel  100  is processed by grinding to have a surface roughness of 3.0˜5.0 μm which is higher than the conventional one, degradation of brightness and color purity of the image display device can be prevented. 
   The thickness of the transparent dielectric film  17  is maintained to be 0.01˜1.00 μm after forming. 
   The reason of the above is that the thickness of the transparent dielectric film  17  can be changed after forming by concentration of the compound containing silica and the rest substances, but in case of lower than a predetermined thickness, it is difficult that the characteristic of the present invention is achieved, and in case of higher than the predetermined thickness, cracks can be generated in the transparent dielectric film  17 , as heat expansion coefficients of the panel  100  and the transparent dielectric film  17  are different. 
   Therefore, if the thickness is not maintained after a predetermined degree, the effect of the present invention can not be achieved. 
   That is, the thickness of the transparent dielectric film  17  of silica substance must be formed as in following formula 1 according to the roughness of the inner surface of the panel  100  before forming the transparent dielectric film  17  of silica substance to maximize the effect of the present invention.
 
 y− 0.15&lt; Y&lt;y+ 0.15  (1)
         At this time, y=0.1 In(x+1)+0.05   x: surface roughness (μm) of the inner surface of the panel before forming the transparent dielectric film of silica substance (the surface roughness is measured corresponding to a evaluation length of 1 mm by a mean peak to valley height method)   y: optimal thickness (μm) of the transparent dielectric film according to the surface roughness of the inner surface of the panel before processing the transparent dielectric film of silica substance   Y: range of the optimal thickness (μm) of the transparent dielectric film according to the surface roughness of the inner surface of the panel before processing the transparent dielectric film of silica substance       

   On the other hand, the refraction index of the panel  100  becomes 1.45˜1.70 according to the thickness of the transparent dielectric film  17  after forming the transparent dielectric film of silica substance. 
   Hereinafter, the effect of the panel of the cathode ray tube in accordance with the present invention will be described. 
   Firstly, by coating a pore surface  16  between the inner surface of the panel  100  and the phosphor layer  12  formed thereon with a transparent dielectric film  17 , in the process of processing the inner surface of the panel  100  using abrasive, the processing can be performed by having surface roughness in a range of 3.0˜5.0 μm which is higher than the conventional roughness value (Rz) of 1.5˜2.5 μm, thus to reduce grinding cost. 
   Also, a black matrix patter having a clear peripheral line can be obtained by applying the transparent dielectric film  17  of the present invention having a low roughness, and therefore cutting which means linearity of the black matrix can be improved. 
   That is, as shown in  FIG. 6 , the panel  100  of the present invention in which the transparent dielectric film  17  is formed on the inner surface of the panel  100  having surface roughness of 3.5 μm can have brightness in the image display device about 10% higher than in the general panel  1  which was manufactured by the conventional method having a surface roughness of 2.00 μm. 
   As shown in the above, the present invention can increase brightness of the image display device by forming the transparent dielectric film on the inner surface of the panel to transmit light of the phosphor layer which emits light to the outside of the panel well, and improve cutting that the peripheral line of the phosphor layer and the black matrix patter is formed as a straight line in case of forming the black matrix pattern by lowering the roughness of the inner surface of the panel. 
   As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to be embraced by the appended claims.