Abstract:
A tensioned shadow mask for a cathode ray tube (CRT), including: a series of parallel strips separated by slits having a predetermined interval; a plurality of tie bars interconnecting adjacent strips to define a plurality of slits at predetermined intervals; and a plurality of dummy bridges disposed between adjacent tie bars, extending from one of the strips to the other but not interconnecting the adjacent strips, wherein a length of the dummy bridges is greater than a length of the tie bars in the longitudinal direction of the strips.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cathode ray tube (CRT), and more particularly, to a tensioned shadow mask with a color selection function. 
     2. Description of the Related Art 
     CRTs for television and computer displays employ a faceplate having on the inner side thereof a phosphor screen with a predetermined pattern, a mask and frame assembly which is an assembly of a shadow mask (hereinafter, simply referred to as a mask) and a frame, and is installed on the inner side of the faceplate, a funnel connected to the faceplate, which has a neck portion and a cone portion, and an electron gun inserted in the neck portion of the funnel, for emitting electron beams through apertures of the mask to excite the phosphor screen, and a deflection yoke installed around the cone portion of the funnel, for deflecting electron beams from the electron gun. 
     In a CRT having the above configuration, the mask for accurate landing of three electron beams emitted from the electron gun on each phosphor layer of the phosphor screen includes; a dot mask with substantially circular apertures a slot mask with parallel elongated apertures, and a tensioned mask to which tension is applied from opposite sides thereof, and having a series of parallel stripes separated by slits through which electron beams pass. 
     FIG. 1 shows an example of a tensioned mask. As shown in FIG. 1, the tensioned mask includes a plurality of strips  22  separated by slits  21  having a predetermined interval, and a plurality of tie bars  33  which interconnect the adjacent strips. The tensioned mask is supported in tension by a frame (not shown) of the tensioned mask. 
     In the mask, the tie bars  23  which interconnect the adjacent strips  22  can reduce a howling phenomenon, which occurs due to mask vibration from external impact, and unacceptable Poisson&#39;s contraction. However, if the vertical pitch of the tie bars  23  is too large, that is, if the vertical pitch (PV) of the tie bars  23  is twice or more the horizontal pitch (PH) thereof, a reflection image of the tie bars  23  is shown on the screen, which is unpleasant to viewers. 
     To avoid this problem, U.S. Pat. No. 4,926,089 discloses a tensioned mask as shown FIG.  2 . As shown FIG. 2, the tensioned mask includes a plurality of strips  31  separated by slits  32  having a predetermined interval, and tie bars  33  which interconnect the adjacent strips  31 . Also, dummy bridges  34 , which extend partially between but not interconnecting adjacent strips, are disposed between the adjacent tie bars  34  and separate each slit  32  into sub-slits having a predetermined interval. 
     In the tensioned mask, due to a technical problem in mask pattern formation, the width W 1  of the dummy bridges  34  is smaller than the width W 2  of the tie bars  33 . Thus, the reflection images by the dummy bridges  34  and the tie bars  33  have a slight difference in intensity of light. This difference raises the problem of tie bar visibility, thus deteriorating display image and making viewing unpleasant. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, an object of the present is to provide a tensioned shadow mask for a cathode ray tube (CRT), capable of eliminating the problem of tie bar visibility while enhancing display image visibility. 
     In one embodiment of the present invention, there are provided a tensioned shadow mask for a CRT, comprising: a series of parallel strips separated by slits having a predetermined interval; a plurality of tie bars interconnecting adjacent strips to define a plurality of slits at predetermined intervals; and a plurality of dummy bridges disposed between adjacent tie bars, extending from one of the strips to the other but not interconnecting the adjacent strips, wherein the dummy bridges are longer than the tie bars in the longitudinal direction of the strips. 
     In the tensioned shadow mask according to the present invention, the area of the dummy bridges is equal to that of the tie bars, or the area difference between the dummy bridges and the tie bars is in a predetermined range. The dummy bridges may extend from a strip to the next strip but not intersecting the adjacent strips, or the dummy bridges may alternately extend from the adjacent strips such that a first dummy bridge extends from one of the adjacent strips and the next dummy bridge extends from the other of the adjacent strips. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above object and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
     FIG. 1 is a plan view of a conventional tensioned mask for a color cathode ray tube (CRT); 
     FIG. 2 is a partial enlarged view of the conventional tensioned mask, illustrating an aperture configuration thereof; 
     FIG. 3 is an exploded perspective view illustrating a state where a tensioned mask for a CRT according to the present invention is secured to a frame; 
     FIG. 4 is a partial enlarged view of the tensioned mask of FIG. 3, illustrating an aperture configuration thereof; 
     FIG. 5 is a partial enlarged plan views illustrating examples of the tensioned mask according to the present invention; and 
     FIGS. 6 through 15 are photos illustrating the visibility of tie bars reflected on a phosphor screen with respect to the area difference between the tie bars and dummy bridges of tensioned masks. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 3 and 4, a tensioned mask and frame assembly includes a tensioned mask  40  and a frame  50  for supporting the tensioned mask  40  in tension. In the tension mask  40 , which is formed of a 50-100 μm-thick foil, a series of strips  41  each having a width W 1  of 190 μm are separated by slits  42  having a width of 60 μm. The slits  42  are separated by tie bars  43  which interconnect adjacent strips  41 ′ and  41 ″. The tie bars  43  are arranged in a staggered fashion in the transverse direction of the tensioned mask  40 . Also, a plurality of dummy bridges  44 , by which each slit  42  is separated into sub-slits having a predetermined interval, are disposed between the tie bars  43 , wherein the dummy bridges  44  extend from one of the adjacent strips  41 ′ and  41 ″ to the other but not interconnecting the adjacent strips  41 ′ and  41 ″. 
     FIG. 5 shows another tensioned mask having the strips with dummy bridges arranged in a different manner from that of FIG.  3 . As shown in FIG. 5, dummy bridges  45  alternately extend from one of the adjacent strips  41 ′ and  41 ″ such that a first dummy bridge extends from one of the adjacent strips  41 ′ and  41 ″ and the next dummy bridge extends from the other of the adjacent strips  41 ′ and  41 ″. 
     In the above embodiments, the length L 1  of the dummy bridges  44  and  45  is larger than the length L 2  of the tie bars  43 , and the width W 4  of the dummy bridges  44  or  45  is smaller than the width W 5  of the tie bars  43 . However, the area A 1  (=L 1 ×W 4 ) of the dummy bridges  44  is equal to the area A 2  (=L 2 ×W 5 ) of the tie bars  43 , or the area difference between the dummy bridges  44  and the tie bars  43  is in a predetermined range. The area of the dummy bridges  44  or  45  may differ from that of the tie bars  43 . However, it is preferable that the area of the dummy bridges  44  or  45  is equal to that of the tie bars  43 , so that the tie bars will not visibly stand out. Also, the area of the dummy bridges  44  or  45  may be smaller or larger than that of the tie bars  43 , as long as the area difference is in the range of 30 percent, which is expressed by |(A 1 −A 2 )|/A 2 ≦0.3. 
     Also, as shown in FIG. 3, the frame  50  of the tensioned mask and frame assembly comprises a pair of supports  51  and  52  spaced a predetermined distance, for supporting the longer side edges of the tensioned mask  40 , and a pair of elastic members  53  and  54  for applying tension to the tension mask  40 , wherein both ends of the elastic members  53  and  54  are fixed to the supports  51  and  52 . The frame configuration is not limited to the above configuration, and any configuration capable of acting tension on the tensioned mask can be adopted. 
     The tensioned mask is installed at the inner side of the faceplate, at a predetermined distance from the phosphor screen, being supported by the frame, provides a color selection function for accurate passage through the slits  42  and landing on the phosphor screen of the electron beams emitted from the electron gun. 
     The electron beams may be shielded by the tie bars  43  which define the slits  42  at predetermined intervals, or by the dummy bridges  44  or  45 , which hinders complete excitation of the phosphor screen, thus resulting in a reflection image on the screen. However, the length L 2  of the tie bars  43  is larger than the length L 1  of the dummy bridges  44  or  45 , and the area of the tie bars  43  is nearly equal to that of the dummy bridges  44  or  45 , so that the reflection image area due to the tie bars  43 , which corresponds to a nonexcited region of the phosphor screen, is nearly the same as that due to the dummy bridges  44  or  45 . As a result, a real image and a reflection image are uniformly distributed over the screen, so that viewers scarcely perceive the reflection image, thereby improving appearance uniformity. The reflection image distribution can be controlled by varying the number of tie bars  43  and dummy bridges  44  or  45 . 
     The tensioned mask of a CRT according to the present invention, having the above structure, is characterized in that the area of the dummy bridges is equal or similar to that of the tie bars, so that a decrease in resolution due to the reflection image of the tie bars can be avoided with an improved appearance uniformity. 
     The following embodiments are provided so that this disclosure will be thorough and complete. 
     EXPERIMENTAL EXAMPLE 1 
     The appearance uniformity with respect to the area difference between the tie bars and the dummy bridges was observed by varying the length of the dummy bridges relative to the length of the tie bars in a tensioned mask of a CRT for monitors. The result is shown in Table 1. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Tie bar 
                 Dummy bridge 
                   
               
             
          
           
               
                   
                 Length 
                 Width 
                 Area 
                 Length 
                 Width 
                 Area 
                 Area 
                 Appearance 
               
               
                 Sample 
                 (μm) 
                 (μm) 
                 (μm 2 ) 
                 (μm) 
                 (μm) 
                 (μm 2 ) 
                 ratio (%) 
                 Uniformity 
               
               
                   
               
             
          
           
               
                 1 
                 60 
                 60 
                 3,600 
                 60 
                 30 
                 1,800 
                 50 
                 poor 
               
               
                 2 
                 60 
                 60 
                 3,600 
                 90 
                 30 
                 1,800 
                 75 
                 moderate 
               
               
                 3 
                 60 
                 60 
                 3,600 
                 120 
                 30 
                 1,800 
                 100 
                 good 
               
               
                 4 
                 60 
                 60 
                 3,600 
                 150 
                 30 
                 1,800 
                 125 
                 moderate 
               
               
                   
               
             
          
         
       
     
     As can be noted from Table 1, the appearance uniformity is acceptable when the area of the tie bars is in a range greater than 70% and less than 130% of the area of the tie bars. 
     FIGS. 6 through 11 are photos illustrating the visibility of tie bars reflected on the phosphor screen, with respect to the area difference between the tie bars and dummy bridges of tensioned masks shown in Table 1. In particular, FIG. 7 is a macro photo in a case when the area of the dummy bridges is 50% of that of the tie bars (Sample 1 of Table 1), and FIG. 6 is a 20×-magnified photo of FIG.  7 . As shown in FIGS. 6 and 7, distinct tie bar shadows appear on the phosphor screen. 
     FIG. 9 is a macro photo showing the tie bar visibility on the phosphor screen when the area of the dummy bridges is 75% of that of the tie bar (Sample 2 of Table 1), and FIG. 8 is a 20×-magnified photo of FIG.  9 . As shown in FIG. 8, the sizes of the reflection image by the tie bars and the dummy bridges appear to be equal to each other, showing a slight difference in intensity of light therebetween. Also, as shown in FIG. 9, it is difficult to distinguish the tie bar shadows on the phosphor screen from the dummy bridges shadows thereon. 
     FIG. 11 is a macro photo showing the tie bar visibility on the phosphor screen when there is no difference in area between the tie bars and the dummy bridges (Sample 3 of Table 1), and FIG. 10 is a 20×-magnified photo of FIG.  11 . In FIG. 10, the dummy bridges that are enlarged in the longitudinal direction so as to make the area of the dummy bridges equal to that of the tie bars are visible. As shown in FIG. 11, it is difficult to distinguish the tie bar shadows from the dummy bridges shadows, and the reflection images of the tie bars and dummy bridges show uniform intensity of light. 
     Although photos of the Sample 4 in Table 1, in which the area of the dummy bridges is 125% of that of the tie bars, were not taken, the size of the reflection image of the dummy bridges on the phosphor screen was large whereas that of the tie bars was small, compared to the Sample 3. Furthermore, the reflection image of the tie bars were shown as white dots on the screen. 
     EXPERIMENTAL EXAMPLE 2 
     The appearance uniformity with respect to the area difference between the tie bars and the dummy bridges was observed by varying the length of the dummy bridges relative to the length of the tie bars in a tensioned mask of a CRT for a television. The result is shown in Table 2. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Tie bar 
                 Dummy bridge 
                   
               
             
          
           
               
                   
                 Length 
                 Width 
                 Area 
                 Length 
                 Width 
                 Area 
                 Area 
                 Appearance 
               
               
                 Sample 
                 (μm) 
                 (μm) 
                 (μm 2 ) 
                 (μm) 
                 (μm) 
                 (μm 2 ) 
                 ratio (%) 
                 Uniformity 
               
               
                   
               
             
          
           
               
                 1 
                 80 
                 195 
                 15,600 
                 80 
                 145 
                  8,700 
                 55 
                 poor 
               
               
                 2 
                 80 
                 195 
                 15,600 
                 80 
                 145 
                 11,600 
                 74 
                 moderate 
               
               
                 3 
                 80 
                 195 
                 15,600 
                 108 
                 145 
                 15,660 
                 100.3 
                 good 
               
               
                 4 
                 80 
                 195 
                 15,600 
                 140 
                 145 
                 20,300 
                 130.1 
                 moderate 
               
               
                   
               
             
          
         
       
     
     As can be understood from Table 2, the appearance uniformity is acceptable when the area difference between the tie bars and dummy bridges is in the range of 30%. 
     FIGS. 12 through 15 are photos illustrating the visibility of tie bars reflected on the phosphor screen, with respect to the area difference between the tie bars and dummy bridges of tensioned masks shown in Table 2. In particular, FIG. 13 is a macro photo in a case when the area of the dummy bridges is 55% of that of the tie bars (Sample 1 of Table 2), and FIG. 12 is a 10×-magnified photo of FIG.  13 . As shown in FIGS. 12 and 13, although the resolution is poor, due to the large horizontal pitches of the phosphor pattern and the slits of the tensioned mask for a television, compared to those for monitors (Experimental Example 1), distinct tie bar shadows appear on the screen. 
     FIG. 15 is a macro photo showing the tie bar visibility on the phosphor screen when the area of the dummy bridges is 74% of that of the tie bars (Sample 2 of Table 2), and FIG. 14 is a 10×-magnified photo of FIG.  15 . In FIG. 14, the dummy bridges that are enlarged in the longitudinal direction so as to make the area of the dummy bridges equal to that of the tie bars are distinct. As shown in FIG. 15, the reflection images of the tie bars and dummy tie bars have uniform intensity of light, so that it is difficult to distinguish the reflection image of the tie bars from that of the dummy tie bars, thus improving the appearance uniformity. 
     Although photos of the Sample 4 in Table 2, in which the area of the dummy bridges is 130% or more larger than that of the tie bars, were not taken, the size of the reflection image of the dummy bridges on the phosphor screen was large whereas that of the tie bars was small, compared to the samples described with reference to photos. Furthermore, the reflection image of the tie bars was shown as which dots the screen. 
     While the present invention has been illustrated and described with reference to specific embodiments, further modifications and alterations within the spirit and scope of this invention as defined by the appended claims will become evident to those skilled in the art.