Abstract:
The present invention relates to a flat-type cathode ray tube and more particularly, the present invention improves the brightness and color purity of picture by improving the curvature of the inner surface of the panel in order to prevent the mislanding of the electron beam, thereby preventing doming, howling, magnetic field of the earth magnetism and the like.  
     To do this, in accordance with the present invention, in a flat-type cathode ray tube comprising a panel for displaying a picture, wherein the outer surface of the panel is plane and the inner surface thereof has inner surface curvatures respectively along the vertical axis direction and the horizontal axis direction.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a flat-type cathode ray tube (CRT) and more particularly to an improvement of a curvature of a panel for the CRT.  
           [0003]    2. Description of the Related Art  
           [0004]    In general, the CRT is an image display device for displaying a picture signal received thereto as an image and is divided into a curved type having a predetermined curvature in both the inner surface and the outer surface of the panel and a flat type wherein the inner surface of the panel has a predetermined curvature, while the outer surface thereof is substantially flat.  
           [0005]    [0005]FIG. 1 shows a sectional view of a conventional curved-type CRT, and FIG. 2 shows an exploded perspective view of a shadow mask and a frame constituting the curved-type CRT of FIG. 1. FIG. 3 a  shows a top view of a panel assembly having a show mask and an inner shield combined therewith and FIG. 3 b  shows an operational view of the panel assembly.  
           [0006]    The curved-type CRT used in general will be explained with reference to the associated drawings.  
           [0007]    As shown in FIG. 1, the conventional curved-type CRT comprises a rectangular panel of which outer surface has a predetermined curvature, a nude-type funnel and an electron gun assembly.  
           [0008]    More specifically, the panel  1  comprises a phosphor screen  4  formed by depositing red, green and blue phosphors on the inner face of the panel  1  and a shadow mask  3  which is installed in the inner surface of the panel  1  at a predetermined space relationship therewith and which has a rectangular frame  7  fixed to the rear side thereof. Also, an inner shield plate  9  is attached to the rear side of the frame  7 . A funnel  2  comprises a neck portion into which an electron gun assembly for emitting electron beams  6  based on received picture signals is inserted.  
           [0009]    Further, as shown in FIG. 2, the shadow mask  3  comprises a number of fine apertures  31  for passing through electron beams emitted from the electron gun and its whole skirt portion is welded on the inner wall or outer wall of the frame  7  by means of a welding process.  
           [0010]    Also, the frame  7  includes a flat portion in parallel to its short axis direction in every edge of the outer wall thereof and has an elastic supporting member, spring  8 , fixed to one side of the flat portion.  
           [0011]    The spring  8  has a first arm portion and a second arm portion being contacted at a predetermined angle each other, wherein the first arm portion has a hole on its surface (not shown) to be held on a stud pin of the frame and the second arm portion is fixed to the flat portion by welding.  
           [0012]    As stated above, by installing the frame  7  having the shadow mask  3  fixed thereto onto the inner surface of the panel  1  and the inner shield plate  9  on the rear side of the frame  7 , a panel assembly is formed as shown in FIG. 3 a.    
           [0013]    Especially, the panel  1  has the radius of curvature defined by a level difference between the center of the inner surface of the panel  1  and the outer end of effective area thereof, and comprises stud pins  10  which are anchored on two or three edge portions of the inner wall of the panel  1 .  
           [0014]    The process for displaying picture through a conventional CRT having such construction will be described hereafter.  
           [0015]    When image signals are input to the electron gun, electron beams  6  as the image signals are emitted from the gun to the phosphor screen  4 . The electron beams  6  emitted are deflected by deflection york, pass through a number of apertures  31  of the shadow mask and radiate red, green and blue phosphors on the phosphor screen deposited the inner surface of panel  1  thereby to display the picture.  
           [0016]    At this time, about 20% of the electron beams pass through apertures  31  of the shadow mask and the rest 80% collides with the shadow mask, resulting in the rising in temperature of the shadow mask and frame.  
           [0017]    Thus, in the shadow mask comprised of AK material or invar alloy having a low thermal expansion coefficient and the frame comprised of iron having a high thermal expansion coefficient, the frame is expanded during the early rising in temperature and then the shadow mask  3  is also expanded as time passes by.  
           [0018]    Referring to the operation diagram of FIG. 3 b , the dotted line shows a shape of the shadow mask  3  and the frame  7  of the curved-type CRT before operation, while the solid line shows a shape of the shadow mask  3  and the frame  7  of the CRT, wherein the position of the shadow mask  3  and the frame  7  are shifted due to the rising in temperature caused by electron beams in operation.  
           [0019]    That is, the shadow mask  3  and the frame  7 , which were increased in volume due to their temperature rising, are shifted toward phosphor screen. Such shift of the shadow mask and the frame can be recompensed by a repelling power of the spring  8  as an elastic supporting member, however, it is difficult for the shadow mask to be returned to its position before operation.  
           [0020]    Therefore, when electron beams are scanned onto phosphors on the phosphor screen in the inner surface of the panel, an amount of variation (ΔLE) in landing of electron beam at the phosphors (referred to as “mislanding quantity (ΔLE)” hereinafter) appears because of the position change of the apertures  31  caused by the shift of the shadow mask  3 .  
           [0021]    Also, when the impact energy generated by external impact or a sound pressure of speaker arrives at the frame  7  and the shadow mask  3  through the stud pin  10  and the spring  8 , so-called domming phenomenon which causes a tremble of the shadow mask occurs. As a result, the position of the apertures  31  is changed and the mislanding quantity (ΔLE) of the electron beam occurs, thereby deteriorating transmissivity of electron beams and the brightness of a picture and lowering the color purity of picture with the incorrect beam arrangement.  
           [0022]    Further, because of a low height of the frame in the short axis direction due to the curved shape of panel, the frame does not fully wrap the outer portion of the shadow mask, thereby causing a significant deformation of the shadow mask in manufacturing it or an occurrence of black stripes on the screen due to the mislanding of the electron beam.  
           [0023]    In addition, since the inner shield plate attached on the rear side of the frame so as to prevent magnetic field of the earth magnetism has also a smooth plane shape on both the long peripheral portion and the short peripheral portion, a magnetic flux is not easily concentrated onto the long peripheral portion, the short peripheral portion as well as the corner portion. As a result, the magnetic field of the earth magnetism is not shielded effectively and thus the electron beam is deflected, resulting in occurrence of mislanding quantity (ΔLE) of the electron beam and deterioration of the color purity of picture.  
         SUMMARY OF THE INVENTION  
         [0024]    In order to solve the above mentioned problems, it is the object of the present invention to improve the brightness and color purity of picture by reducing the mislanding of the electron beam, thereby preventing doming, howling, magnetic field of the earth magnetism and the like.  
           [0025]    To do this, the present invention provides a flat-type cathode ray tube comprising a panel for displaying a picture, wherein the outer surface of the panel is plane and the inner surface thereof has inner surface curvatures respectively along the vertical axis direction and the horizontal axis direction. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a sectional view of a conventional curved -type cathode ray tube;  
         [0027]    [0027]FIG. 2 is an exploded perspective view of a shadow mask and a frame constituting the curved-type cathode ray tube of FIG. 1;  
         [0028]    [0028]FIG. 3 a  is a top view of a panel-frame assembly;  
         [0029]    [0029]FIG. 3 b  is a operational view of a panel-frame assembly;  
         [0030]    [0030]FIG. 4 is a sectional view of a flat-type cathode ray tube according to the present invention;  
         [0031]    [0031]FIG. 5 is a top view of a panel assembly of the flat-type cathode ray tube according to the present invention;  
         [0032]    [0032]FIG. 6 is a plan view of the panel of a flat-type cathode ray tube;  
         [0033]    [0033]FIGS. 7 a ,  7   b  and  7   c  are sectional views taken along the long peripheral direction, the short peripheral direction and the diagonal peripheral direction with respect to FIG. 6, respectively; and  
         [0034]    [0034]FIG. 8 a  and  8   b  are graphs showing variations in each position the inner surface curvature of the panel according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0035]    A flat-type cathode ray tube according to a preferred embodiment of the present invention will be explained with reference to the accompanying drawings.  
         [0036]    As in FIG. 4, a flat-type cathode ray tube according to the present invention comprises a rectangular flat panel  1 , a nude-type funnel  2  constituting an envelop and an electron gun.  
         [0037]    The flat panel  1  of the present invention which is a portion for displaying a picture has a safety glass for explosion which is attached to the outer surface thereof and a phosphor screen formed on the inner surface of the panel  1  by depositing regularly red, green and blue phosphors. Also, the end portion of the panel  1  is curved in its rear direction, thereby to form an inner side wall. Stud pins  10  are anchored on edge portions of the inner wall. The shadow mask  3  is installed on the rear side of the panel  1  at a predetermined space relationship therewith.  
         [0038]    In this cathode ray tube according to the present invention, only 20% of electron beams  6  composed of red, green and blue phosphors emitted from the electron gun pass through the apertures  31  of the shadow mask and radiates the phosphors to emit each color.  
         [0039]    To describe in detail, as shown in FIG. 2, the shadow mask  3  is formed of an invar material having a low thermal expansion coefficient with 0. 1 mm through 0.3 mm thickness and has a number of apertures  31  arranged thereon in regular interval. The frame  7  having a rectangular shape is formed of an iron having a high thermal expansion coefficient with 0.5 mm through 2 mm thickness. Also, a flat portion is formed on the edge of the outer wall of the frame  7  in parallel to its short axis direction.  
         [0040]    In the above described shadow mask  3 , the skirt portion of the shadow mask  3  is welded to be contacted with the inner wall of the frame or is welded to the outer wall of the frame  7  by forming the shadow mask  3  bigger than the frame  7 .  
         [0041]    And, the second arm portion of the spring  8  comprised of strong elastic material of 0.2 mm through 1.5 mm thickness is fixably welded to the flat portion of said frame  7  and the first arm portion of the spring  8  being contacted to the second arm portion at a predetermined angle includes a hole.  
         [0042]    As seen in FIGS. 4 and 5 showing a plan view of the panel for the flat-type cathode ray tube, in order to install the flame  7  having said shadow mask  3  welded thereon onto the panel  1 , said stud pin  10  is inserted into the hole of the first arm portion of the spring  8  to maintain a predetermined space relationship between the shadow mask  3  and phosphor screen  4 .  
         [0043]    The flat cathode ray tube having this configuration is not capable for preventing a conventional doming and howling phenomenon caused by the position change of a shadow mask, resulting in deterioration of brightness and color purity of a picture.  
         [0044]    Thus, in accordance with the present invention, in order to solve the defects of the conventional curved-type cathode ray tube in which an image distortion is severe and the reflection rate for reflating the external light is high, the outer surface of the panel  1  is designed to be plane and the inner surface of the panel  1  is designed in a manner that the doming, howling, explosion, etc. as described above can be prevented effectively, as described in detail hereunder.  
         [0045]    Generally, the strength of a panel is lowered as much as the panel is flatten on the curved surface and thus the panel  1  of the present invention should be firstly designed so as to be flatten with having durability to the extent of satisfying explosion according to the following equation.  
         ( Pi ) cr =(0.14×3.2/λ 2 ) Kc   equation (1) 
         [0046]    To insert the following factors into the above equation (1):  
               K                 c     =       2   /   3          (     1   -     v   2       )          E        (     t   /   R     )          2                 =       [     12        (     1   -     v   2       )       ]          1   /   4                     R   /   t                   2                   sin        (     θ   /   2     )                                     
 
         [0047]    Wherein  
         [0048]    ν: Poisson rate  
         [0049]    E: elastic coefficient  
         [0050]    R: radius of curvature  
         [0051]    Φ: angle defined by the center and the end portion  
         [0052]    Kc, λ: coefficient  
         [0053]    (Pi)cr: a critical curving weight  
         [0054]    To express the equation 1 based on the radius R of curvature of panel of the CRT, it is represented as  
         ( Pi ) cr∞ 1/ R.   
         [0055]    That is, although the strength of panel  1  is increased as the radius of curvature lowers, the curvature of the inner surface of panel is limited to the increased curvature having the durability to the extent of satisfying an explosion characteristic because an image distortion of displayed picture and the reflection of light should be lowered.  
         [0056]    Therefore, to have the radius of curvature capable of preventing the deterioration of flatness and strength of the panel  1  as well as a trembling of the mask due to the impact energy, the curvature of the inner surface of the rectangular panel is designed dividedly into the long peripheral portion, the short peripheral portion and the diagonal peripheral portion.  
         [0057]    To limit the range of the curvature of the inner surface of the panel  1  to order to achieve this effects, the long and short axises are designed dividedly  
         0.51&lt;( Zmin−Zcen )−( Zcor−Zmaj )&lt;0.6,  
         0.51&lt;( Zmaj−Zcen )−( Zcor−Zmin )&lt;0.6. 
         [0058]    wherein,  
         [0059]    Zcen is a thickness at center of the inner surface of the panel, Zmin is the difference between the thickness at the center of the inner surface of the panel and a thickness at the end portion of effective area of the long side portion, and Zcor is the difference between the thickness at the center of the inner surface of the panel and a thickness at the end portion of effective area of the diagonal portion.  
         [0060]    In the curvature structure of the shadow mask, if one direction, that is, the portion (Zmin−Zcen)−(Zcor−Zmaj) exceeds 0.6, the radius of the curvature between the long axis and the end portion of the long side portion is increased, which causes a deterioration in its supporting strength along the long axis direction and a howling phenomenon which trembles the shadow mask. As a result, since the color purity of the CRT is deteriorated in operation, it is difficult to obtain a fine image.  
         [0061]    [0061]FIG. 6 shows a plan view of inner surface of the panel according to the present invention. Sectional views of a long side portion, a short side portion and a diagonal portion which are taken along the central of the panel are shown in FIG. 7 a ,  7   b  and  7   c , respectively.  
         [0062]    [0062]FIG. 7 a  is a sectional view representing the long side portion of the panel  1 . In view of the center of the inner surface of the long side portion about the short axis direction, the height difference between the thickness at the end portion of the effective area of the long side portion on which electron beams are impinged and a thickness at the center of the inner surface of the panel is defined as Zmaj, and the difference between the thickness at a half position between the end portion of the effective area of the long side portion and its center and the thickness at the center of the inner surface is defined as Zmajp.  
         [0063]    [0063]FIG. 7 b  is a sectional view representing the short side portion of the panel  1 . In view of the center of the inner surface of the short side portion of the panel  1 , the difference between the thickness at the end portion of the effective area of the short side portion on which electron beams are impinged and a thickness at the center of the inner surface of the panel is defined as Zmin, and the difference between the thickness at a half position between the end portion of the effective area of the short side portion and its center and the thickness at the center of the inner surface is defined as Zminp.  
         [0064]    Also, FIG. 7 c  is a sectional view representing the diagonal portion of the panel  1 . The difference between the thickness at the end portion of the effective area of the diagonal portion on which electron beams are impinged and a thickness at the center of the inner surface of the panel is defined as Zcor.  
         [0065]    As an preferred embodiment of the present invention, the radius of the curvature of the inner surface of the panel for a flat-type CRT with an aspect ratio of 16:9 may have the following range when designing the panel in accordance with the above definitions.  
         0.5&lt;( Zmaj−Zcen )/( Zcor−Zcen )&lt;0.7, and  
         0.35, ( Zmin−Zcen )/( Zcor−Zcen )&lt;0.45 
         [0066]    Further, if ratio between the end portion of each axis of the effective area of the panel  1  and a half position of the its center is set to the range of 0.2&lt;(Zmajp−Zcen)/(Zmaj−Zcen)&lt;3 and 0.2&lt;(Zminp−Zcen)/(Zmin−Zcen), the doming and howling phenomenon of the shadow mask can be prevented.  
         [0067]    [0067]FIG. 8 a  depicts the height difference in the short axis from the center of the panel to the end portion of the long axis of the effective area of the panel in the long side portion. As shown, the height difference (Z point ) from the center to the end portion of the long axis of the effective area of the panel is largely increased.  
         [0068]    Also, such increase appears equal in the short side portion shown in FIG. 8 b.    
         [0069]    In graphs shown in FIG. 8 a  and  8   b , the serial  1  indicates the curvature formed in the case of 0.2&lt;Zmajp−Zcen)/(Zmaj−Zcen)&lt;0.3 and 0.2&lt;(Zminp−Zcen)/(Zmin−Zcen)&lt;0.3 and the serial  2  indicates the curvature formed in the case of 0.5&lt;(Zmaj−Zcen)−(Zcor−Zcen)&lt;0.7 and 0.35&lt;(Zmin−Zcen)−(Zcor−Zcen)&lt;0.45.  
         [0070]    Accordingly, the panel  1  can be formed such that in its thickness, its outer surface made flat and the inner surface made thin at its center, but made thick increasingly from the its upper/bottom portion towards its peripheral portion, thereby having the durability against its explosion and reducing a tremble of the shadow mask due to a impacting energy as well as the effect of magnetic field of the earth magnetism.  
         [0071]    On the one hand, since the height difference in the short axis direction from the outer end of panel curved in the rear direction to the outer surface of the panel can be increased in accordance with increase of the curvature of the panel as compared with a conventional round-type panel, the height of skirt portion of the panel is increased as well as the distance difference PA in the short axis direction from the center (Zcen) of the inner surface of the panel to the stud pin  10  made large.  
         [0072]    Further, an insertion space of the frame  7  which may be inserted into the inner portion of the panel is also wider, thereby increasing the height of the frame in the short axis direction.  
         [0073]    Such height OAH of the skirt portion  11  of the panel in the short axis direction can be given by an finite element method. As the results verified in the practical measurement, the following results are given in the comparison with an example of a panel structure of conventional 28″-screen cathode ray tube.  
                                     TABLE 1                               The Present       Item   Conventional Art   Invention                                Height of the panel   99.7   104.7       Distance between the center of   64   69       inner surface of the panel and       the stud pin (PA)                  
 
         [0074]    That is, as shown in FIG. 7 a , if the shortest distance between an extended plane passing the center of the inner surface of the panel and the center of the stud pin is referred to PA and the height of the skirt portion  11  of the panel in the short axis direction is referred to OHA, the height of the skirt portion of the panel in the short axis direction is equivalent to 0.65&lt;(PA)/(OAH)&lt;0.7.  
         [0075]    As a result, the distance between the panel and the frame is more increased than that of the conventional art, thereby improving the thermal capacity of the frame due to increase of the volume of the frame and thus reducing the thermal expansion quantity of the frame under the same condition.  
         [0076]    That is, a flat-type cathode ray tube according to preferred embodiment of the present invention is constructed such that the thermal expansion coefficient of the shadow mask is formed low, the difference in the thermal expansion coefficient of the shadow mask and the frame is set to be below 0.1, the range of the curvature of the inner surface of the panel  1  is within 0.51&lt;(Zmin−Zcen)−(Zcor−Zmaj)&lt;0.6 and 0.51&lt;(Zmaj−Zcen)−(Zcor−Zmin)&lt;0.6, and the range of the height of the skirt of the panel in the short axis direction is within 0.65&lt;(PA/OAH)&lt;0.7.  
         [0077]    Accordingly, when electron beams from the electron gun impinge in the shadow mask and cause a temperature increase of the shadow mask in operation of the CRT according to the present invention having such construction as stated above, a temperature increase in the frame is relatively low due to its increased volume even though the frame has the same thermal capacity, density and gravity as that of the conventional frame.  
         [0078]    Accordingly, the results as shown in Table 2 can be obtained.  
                       TABLE 2                       Thermal expansion quantity   Example of the   Example of the       in each position of the   present invention   conventional art       panel   maximum   minimum                   Doming portion   12 μm   22 μm       End portion of the long   10 μm   17 μm       axis       Corner portion   11 μm   16 μm                  
 
         [0079]    That is, due to a low expansion quantity of the frame, the moving quantity of the shadow mask is also low and thus the doming is decreased, whereby removing the reason of deterioration of the display quality  
         [0080]    Also, because the height of the frame in the short axis direction can be increased as compared with the conventional art, a deformation in manufacturing and operating the shadow mask is prevented.  
         [0081]    Further, due to the height increase of the frame, the inner shield plate which is installed in the rear of the frame can be sharply tapered. it causes increase of the difference of a magnetic field in the interior and exterior of the inner shield plate, thereby shielding effectively the magnetic field and preventing variation in the electron beam landing.  
         [0082]    Also, the shadow mask can be held in a fixed position along the curvature of the inner surface of the panel by means of the rectangular frame, and the inner surface of the panel is designed within the range of 0.51&lt;(Zmin−Zcen)−(Zcor−Zmaj)&lt;0.6 and 0.51&lt;(Zmaj−Zcen)−(Zcor−Zmin)&lt;0.6, thereby improving the elastic coefficient of the panel and preventing the howling (vibration) phenomenon of the shadow mask due to a impact energy.  
         [0083]    This can be seen from Table 3 showing the result of the experiment which was performed with comparison of the CRT having the curvature of the inner surface of the panel according to the present invention with a conventional CRT having the same size to the present invention.  
                                             TABLE 3                                   Inventive CRT   Conventional CRT                                        the number of inherent   135 Hz   52 Hz           vibration                      
 
         [0084]    That is, the present panel having its improved the number of the inherent vibration can absorb the impact energy due to an external vibration and the like and thus prevent the vibration of the shadow mask by reducing the impact energy transferred to the shadow mask.  
         [0085]    As described in detail above, the panel having flat-designed outer surface can solve problems such as an image distortion of display screen, light reflection and the like.  
         [0086]    Further, the inner surface of the panel is designed dividedly into the long axis direction and the short axis direction, thereby having its durability capable of satisfying the explosion characteristic. Also, the increase of the elastic power thereof prevents the howling of the shadow mask due to an impact energy, thereby reducing the landing change quantity and improving the color purity and brightness of picture.  
         [0087]    In addition, since the height in the short axis direction in accordance with flatness of the panel is increased, the thermal capacity of the frame is increased, the doming phenomenon of the shadow mask is prevented and the magnetic field of the earth magnetism is effectively shielded. Therefore, the landing change quantity can be also reduced, and the color purity of the picture can be improved. Also, the variance rate can be lowered when manufacturing the shadow mask and operating the CRT.