Abstract:
Shadow mask in a cathode ray tube including an effective surface, and a non-effective surface, wherein the effective surface includes elongated slots each extended in full length of the effective surface without bridges between slots in a vertical direction at left and right outermost sides of the shadow mask or outer side slots adjacent to the outermost side slots, thereby preventing wrinkles from being formed at sides of an effective surface to enhance a color purity of the picture.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a shadow mask in a cathode ray tube, and more particularly, to a shadow mask in a cathode ray tube, which has a structure that can prevent formation of wrinkles and improve deterioration of purity characteristics caused by the formation of wrinkles.  
           [0003]    2. Background of the Related Art  
           [0004]    Referring to FIG. 1, a related art color cathode ray tube is provided with a front glass called as a panel  10 , a funnel  12 , a bulb form of rear glass, a fluorescent surface  11  having a coat of luminous fluorescent material on an inside surface of the panel, an electron gun sealed in a neck of the funnel for emitting electron beams to make the fluorescent surface luminous, a shadow mask  13  for selecting a color from the electron beams passed therethrough to make the fluorescent material luminous, and a frame  15  for supporting the shadow mask  13 . A frame assembly of the frame  15  and the shadow mask  13  is fitted to the panel  10  by springs  16 . Further, there is an inner shield  17  in rear of the frame assembly in the cathode ray tube which is at a high vacuum for shielding an influence of an external geomagnetism during operation of the cathode ray tube.  
           [0005]    In the related art color cathode ray tube, upon application of an anodic voltage to the cathode ray tube, the electron beams are emitted from the electron gun, and hit onto the fluorescent surface  11  formed on the inside surface of the panel  10 , which are deflected by a deflection yoke  14  in an up, down, left, or right direction before the electron beams reach to the fluorescent surface  11 . The deflected electron beams are subjected to color selection as the electron beams pass through a plurality of slots in the shadow mask, land on the fluorescent material of the fluorescent surface, and make the fluorescent material luminous, to reproduce a picture.  
           [0006]    The shadow mask is welded to a frame structure shown in FIG. 2A, which is shown in FIG. 2B as a top view and in FIG. 2C as a perspective view. One pair of main frames  15 - 1  and subframes  15 - 2  are welded together to form a rectangular frame, wherein the pair of the main frames  15 - 1  are welded on the pair of subframes which are arrange opposite to each other opposite to each other. The shadow mask  13  having a plurality of slots formed therein as shown in FIG. 2B is welded on such a frame structure. A frame assembly as shown in FIG. 2C is formed by welding the shadow mask  13  on top of the main frames under a state the subframes  15 - 2  are bent by a compressive load T applied to the main frames  15 - 1  and releasing the compressive load applied to the main frames, to exert a tensile force to the shadow mask  13 . If an amount of elongation by the frames are not adequate, the pre-stressed shadow mask  13  assembled thus shows a serious howling caused by external vibration. Though howling in a central portion of the pre-stressed shadow mask  13  is not distinctive because an incident angle of the electron beams to the central portion is not great even if the shadow mask vibrates in an up or down direction by an external impact, the howling in a periphery and corners of the pre-stressed shadow mask  13  is distinctive because the incident angle of the electron beams to the periphery and corners of the shadow mask is great to move the electron beams heavily such that the electron beams can not land on the fluorescent material exactly, to deteriorate a color purity of the picture. Therefore, as shown in FIG. 3, the compressive load on the main frame  15 - 1  is made to vary with location of the main frame  15 - 1 , and a thickness of the shadow mask  13  is set to be below 0.1 mm for an adequate elongation of the shadow mask by means of a little compressive load. However, as shown in FIGS. 4 and 5, the effective surface  13 - 1  having the slots formed therein and the non-effective surface  13 - 2  having no slots formed therein of the pre-stressed shadow mask  13  have different modulus of elasticity owing to difference of geometry of structure, that is, the modulus of elasticity of the effective surface  13 - 1  is smaller than the modulus of elasticity of the non-effective surface  13 - 2 , to have a displacement δ of the effective surface  13 - 1  greater than a displacement of the non-effective surface  13 - 2 , to form wrinkles at left and right sides (“A” part) of the effective surface  13 - 1  as much as a difference of the displacements. The wrinkles at the sides (“A” part) of the effective surface  13  deforms the shadow mask  13 , to change locations of the slots the electron beams pass therethrough, resulting in a poor color purity of the picture reproduced on the cathode ray tube.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, the present invention is directed to a shadow mask in a cathode ray tube that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.  
           [0008]    An object of the present invention is to provide a shadow mask in a cathode ray tube, which can prevent wrinkles from being formed at sides of an effective surface for enhancing a color purity of the picture.  
           [0009]    Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
           [0010]    To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the shadow mask in a cathode ray tube includes an effective surface, and a non-effective surface, wherein the effective surface includes elongated slots each extended in full length of the effective surface without bridges between slots in a vertical direction at left and right outermost sides of the shadow mask or outer side slots adjacent to the outermost side slots.  
           [0011]    The elongated slot has a width expressed by the following inequality.  
             SWm&lt;SWg≦SWm +(+δ y )/3,  
           [0012]    where, SWg denotes width of the elongated slot, SWm denotes a width of conventional slot, and +δy denotes a vertical elongation of the shadow mask.  
           [0013]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    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:  
         [0015]    In the drawings:  
         [0016]    [0016]FIG. 1 illustrates a side view with a partial cut away view of a related art cathode ray tube;  
         [0017]    [0017]FIGS. 2A and 2C illustrate perspective views for showing assembly of a related art shadow mask;  
         [0018]    [0018]FIG. 3 illustrates a graph showing a compressive force vs. a distance in a longitudinal direction of a main frame;  
         [0019]    [0019]FIG. 4 illustrates displacements of a shadow mask in up, down, left and right directions, schematically;  
         [0020]    [0020]FIG. 5 illustrates behaviour of a related art shadow mask when subjected to a tensile strength;  
         [0021]    [0021]FIG. 6 illustrates a perspective view showing a shadow mask assembly in accordance with a preferred embodiment of the present invention;  
         [0022]    FIGS.  7 A˜ 7 D illustrate stresses in a shadow mask of the present invention, schematically; and,  
         [0023]    FIGS.  8 A˜ 8 B illustrate a slot structure of a shadow mask of the present invention, schematically.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    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. 6 illustrates a perspective view showing a shadow mask assembly in accordance with a preferred embodiment of the present invention, FIGS.  7 A˜ 7 D illustrate stresses in a shadow mask of the present invention, schematically, and FIGS.  8 A˜ 8 B illustrate a slot structure of a shadow mask of the present invention schematically, referring to which the shadow mask of the present invention will be explained.  
         [0025]    Referring to FIG. 6, the shadow mask assembly in accordance with a preferred embodiment of the present invention includes a shadow mask  13  having an effective surface  13 - 1  with slots for selecting a color from electron beams and a non-effective surface  13 - 2  to be welded to the frame  13  with no slots for selection of a color, and a frame  15  for fitting the shadow mask to a panel  10 , wherein the shadow mask is welded to a welding line  13 - 6  formed along long sides of the non-effective surface  13 - 2 . There is an elongated color selection slot  13 - 3  in each of outer most sides of the effective surface  13 - 1  in a form bridges of the slots are removed therefrom. In detail, the pre-stressed shadow mask  13  fitted to the frame is elongated by more than 0.1% (0.240 mm)+δy of a length of a vertical effective surface in a vertical direction (y-axis direction) when a tension T is applied thereto. In this instance, the shadow mask  13  is involved in shrinkage at left and right sides thereof by approx. ⅓ (0.7 mm)−δx of the vertical elongation toward a center of the shadow mask  13  inversely proportional to the vertical elongation according to a Poisson&#39;s Ratio. In a structure of the slot type shadow mask  13 , the modulus of elasticity of the effective surface  13 - 1  is lower than the modulus elasticity of the non-effective surface  13 - 2  owing to existence of slots, i.e., a difference of geometry of the effective surface  13 - 1  and the non-effective surface  13 - 2 . That is, as expressed by equation (1) below, the difference of modulus of elasticity makes the displacement δ of the effective surface  13 - 1  greater than the displacement δ of the non-effective surface  13 - 2 .  
         δ=( P×L )/( A×E )  (1),  
         [0026]    where, P denotes tensile strength, L denotes a length, A denotes a sectional area, and E denotes a modulus of elasticity.  
         [0027]    FIGS.  7 A˜ 7 D illustrate stresses in a shadow mask of the present invention schematically, wherein FIG. 7A illustrates an edge portion of the shadow mask, FIG. 7B explains a modulus of elasticity of a non-effective surface, and FIGS. 7C and 7D illustrate a modulus of elasticity of an effective surface.  
         [0028]    Referring to FIG. 7B, in the non-effective surface  13 - 2  without the slots of the shadow mask of isotropic material with uniform properties, an effective modulus of elasticity can be expressed in equation (2), below.  
         ( Ex ) eff=δx/εx , and ( Ey ) eff=δy/εy,   (2)  
         ∴(Ex)eff=(Ey)eff,  
         [0029]    where, Fx and Fy denotes tensions in respective directions applied to the non-effective surface, (δx)eff and (δy)eff denote stresses in the non-effective surface  13 - 2 , and (εx)eff and (εy)eff denote strains in respective directions.  
         [0030]    However, as shown in FIGS. 7C and 7D, since the effective surface  13 - 1  is orthotropic material in which the slot has horizontal and vertical lengths different from each other, and shows different stresses and strains when a tension is applied thereto in x-, and y-directions, the following equation (3) can be established.  
         ( Ex ) eff=δx/εx=Fx /( Pv/ 2), and ( Ey ) eff=δy/εy=Fy/Ph   (3)  
         ∴(Ex)eff≠(Ey)eff,  
         [0031]    where, Fx and Fy denotes tensions in respective directions applied to the effective surface, (δx)eff and (δy)eff denote stresses in the effective surface, and (εx)eff and (εy)eff denote strains in respective directions. That is, since the effective surface  13 - 1  of the shadow mask  13  has a greater stresses and strains in x-, and y-directions than the non-effective surface  13 - 2 , the effective surface  13 - 1  of the shadow mask  13  has a modulus of elasticity greater than the non-effective surface  13 - 2 . As strains of the effective surface  13 - 1  and the non-effective surface derived from the equation (1) are different, by removing bridges ‘B’ of the outermost slots to form one elongated slot, the wrinkles caused by a difference of displacements of the effective surface  13 - 1  and the non-effective surface  13 - 2  can be eliminated.  
         [0032]    When the shadow mask is elongated in up and down directions, since the non-effective surface  13 - 2  shrinks toward the center of the shadow mask  13  according to the Poisson&#39;s ratio, x-direction shrinkage is in general approx. 30% of y-direction elongation, i.e.,  
           x -direction shrinkage−δ x≈ ⅓×(+δ y ),  
         [0033]    where, +δy denotes a y-direction elongation, and −δx denotes an x-direction shrinkage.  
         [0034]    Therefore, since a width SWg of the outermost slot contracts when the shadow mask is elongated in up and down directions, it is preferable to set the width SWg of the outermost slot greater by an amount of the contraction in x-direction, as follows.  
           SWm&lt;SWg≦SWm+ (+δ y )/3  (4).  
         [0035]    It is apparent that the present invention may be modified or varied such that the widths of the slots are formed the smaller as it goes inward, or not only the outermost slots, but also inner slots adjacent to the outermost slots may be elongated.  
         [0036]    [0036]FIG. 8A illustrates an enlarged plan view of “B” part in FIG. 6, wherein the elongated outermost slot of the shadow mask  13  is shown to have a width greater than inner conventional slots. FIG. 8A illustrates an enlarged plan view of “B” part in FIG. 6 in accordance with another preferred embodiment of the present invention, wherein, not only the outermost slots  13 - 3  of the effective surface, but also adjacent inner slots of the outermost slots  13 - 4  are also elongated, to provide a plurality of elongated slots.  
         [0037]    As has been explained, the shadow mask in a cathode ray tube of the present invention has the following advantages.  
         [0038]    By removing bridges of outermost slots in left and right effective surfaces of the shadow mask a vertical direction tension is applied thereto, to provide an elongated slot, the poor color purity caused by deformation of the shadow mask can be improved. Moreover, by eliminating a non-uniform factor in which wrinkles at sides of the effective surface are formed the more as the vertical tension becomes the greater, and vice versa, design for elongation which gives an influence to howling becomes easy since formation of the wrinkles are prevented regardless of the elongation.  
         [0039]    It will be apparent to those skilled in the art that various modifications and variations can be made in the shadow mask in a cathode ray tube of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.