Patent Publication Number: US-6342759-B1

Title: Color cathode ray tube having an improved phosphor screen

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. application Ser. No. 09/089,514, filed Jun. 3, 1998 now U.S. Pat. No. 6,064,147, the subject matter of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a shadow mask type color cathode ray tube, and particularly to a color cathode ray tube having a multiplicity of dot-like electron-transmissive apertures (will be also referred to as beam apertures or simply as apertures in this specification) in the useful apertured portion of the shadow mask with horizontal and vertical pitches of the apertures varied and providing a high quality display. 
     In general, in a shadow mask type color cathode ray tube, a multiplicity of beam apertures are formed in the apertured portion of the shadow mask in correspondence with the shape and position of the three-color phosphor screen formed on the inner surface of the faceplate of the panel portion. 
     The beam apertures in the shadow mask are arranged with horizontal and vertical aperture pitches determined by some formula such that triads of red, green and blue phosphor picture elements are arranged in predetermined positions. 
     As such a prior art shadow mask type color cathode ray tube, there are known those in which the aperture pitches of beam apertures in the apertured portion of the shadow mask are determined by the following formulas on the basis of the slightly curved shapes of the inner surface of the faceplate and the apertured portion of the shadow mask. 
     In one example, the horizontal aperture pitches are nearly uniform in a central portion on both sides of and in the neighborhood of the vertical center line of the apertured portion, and become progressively greater as the left or right side of the apertured portion is approached outside of the central portion. This type is hereinafter referred to as a prior art shadow mask of the first type. 
     In another example, as disclosed in Japanese Patent Laid-Open Publication No. SHO 56-41648, the vertical aperture pitches of the beam apertures become progressively smaller as one goes from the horizontal center line toward the top or bottom of the apertured portion and from the vertical center line toward the left or right side of the apertured portion at the same time. This type is hereinafter referred to as a prior art shadow mask of the second type. 
     In a color cathode ray tube employing the prior art shadow mask of the second type, the phosphor screen is configured such that the line triads of the three phosphor dots of red, green and blue colors tilt increasingly with respect to a horizontal direction as one goes from the horizontal center line toward the top or bottom of the useful phosphor screen area and from the vertical center line toward the left or right side of the useful phosphor screen area at the same time. 
     FIG. 5 is a plan view showing an arrangement of part of phosphor dots in the useful phosphor screen area of a color cathode ray tube employing the shadow mask of the second type (disclosed in Japanese Patent Laid-Open Publication No. SHO 56-41648). 
     In FIG. 5, reference numeral  31  designates a faceplate;  32  is a useful phosphor screen area;  33  is phosphor dots;  33 R is red phosphor dots;  33 G is green phosphor dots;  33 B is blue phosphor dots;  34  is the vertical center line (Y axis) of the useful phosphor screen area  32 ; and  35  is the horizontal center line (X axis) of the useful phosphor screen area  32 . 
     The faceplate  31  has a multiplicity of phosphor dots  33  in the useful phosphor screen area  32  on the inner surface thereof. The phosphor dots  33  are composed of a plurality of triads and one red phosphor dot  33 R, one green phosphor dot  33 G and one blue phosphor dot  33 B form one triad. 
     As shown in FIG. 5, the phosphor dots  33  are arranged with nearly uniform horizontal and vertical pitches such that the line triads of the three-color phosphor dots  33  tilt increasingly with decreasing distance from the corners of the useful phosphor screen area  32 , with respect to the horizontal center line  35 . 
     Specifically, the line triads of the three-color phosphors are nearly horizontal in each of the following portions in the useful phosphor screen area  32 : the approximately central portion and its neighborhood; the portion extending on the horizontal center line  35  from approximately the center to the right and left sides and its neighborhood; and the portion extending on the vertical center line  34  from approximately the center to the top and bottom and its neighborhood. At the four corner portions in the useful phosphor area  32 , the line triads of the three-color phosphors  33  tilt most largely with respect to the horizontal center line  35  and tilt decreasingly with respect to the horizontal center line  35  with increasing distance from the four corner portions. The tilt of the line triads of the three-color phosphors  33  with respect to the horizontal center line  35  at the four corner portions have upward slopes at the upper left-hand corner, downward slopes at the upper right-hand corner, downward slopes at the lower left-hand corner and upward slopes at the lower right-hand corner. In the following description, such an arrangement of a multiplicity of the phosphor dots  33  on the faceplate  31  is referred to as a tilt array arrangement. 
     In the color cathode ray tubes employing the prior art shadow mask of the first type, a multiplicity of the phosphor dots formed in the useful phosphor screen area are arranged with the horizontal aperture pitch nearly uniform in the portion on both sides and in the neighborhood of the vertical center line, and with dot pitches progressively greater toward the left or right side of the useful phosphor screen area out of the central portion. Accordingly, horizontal and diagonal pitches between two adjacent triads of phosphor dots at portions at sides and corners of the useful phosphor screen area become greater, respectively. As a result, the color cathode ray tubes employing the prior art shadow mask of the first type have a disadvantage that the resolution is deteriorated at portions at sides and corners of the phosphor screen. 
     In the color cathode ray tubes employing the prior art shadow mask of the second type, since the phosphor dots formed in the useful phosphor screen area are arranged in a tilt array arrangement, diagonal pitches between two adjacent triads of phosphor dots at portions at corners in the useful phosphor screen area become smaller. As a result, the color cathode ray tubes employing the prior art shadow mask of the second type have a disadvantage that the color purity tolerance at each corner portion on the phosphor screen is reduced, leading to the non-uniformity in displayed colors. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the above-mentioned problems and to provide a shadow mask type color cathode ray tube capable of increasing the color purity tolerance at each portion on the phosphor screen so as to prevent deterioration of the resolution, and retaining the mechanical strength of the shadow mask. 
     To achieve the above object, according to the shadow mask type color cathode ray tube of the present invention, beam apertures of the shadow mask are arranged such that the horizontal aperture pitches are nearly uniform in a central portion on both sides of and in the neighborhood of the vertical center line of the apertured portion and increase slightly with increasing distance from the central portion toward the left or right side of the apertured portion, and such that the vertical aperture pitches are arranged in such a manner as to slightly increase the tilt of the horizontal rows of the beam apertures as one goes from the vertical center line toward the left or right side of the apertured portion. 
     Here, the phosphor dots of the phosphor screen are arranged in such a manner as to satisfy the following relationship: 
     
       
         0.98 Pdo≦Pd≦1.02 Pdo  (1) 
       
     
     where Pd={Ph 2 +Pv 2 }  0.5 , and where, consider two closest phosphor dots of the same color one on each of the two adjacent horizontal rows of the phosphor dots in the phosphor screen, a horizontal dot pitch Ph is a distance between the two phosphor dots of the same color measured horizontally, a vertical dot pitch Pv is a distance between the two phosphor dots of the same color measured vertically, a diagonal dot pitch Pd is a diagonal distance between the two phosphor dots of the same color, and a central diagonal dot pitch Pdo is a diagonal dot pitch Pd measured at the central portion of the phosphor screen. 
     With this configuration, the arrangement of the beam apertures in the apertured portion of the shadow mask is such that the vertical aperture pitches are arranged to form a tilt array arrangement and the horizontal pitches of the beam apertures are nearly uniform in the central portion on both sides of and in the neighborhood of the vertical center line of the apertured portion and increase slightly with increasing distance from the central portion toward the left or right side of the apertured portion to form a varied pitch arrangement. 
     The varied pitch arrangement of the beam apertures cancels the decrease in diagonal pitches of the phosphor dots at the corners of the phosphor screen caused by the tilt array arrangement of the aperture pitches and the relationship (1) above makes the diagonal pitches of the phosphor dots nearly uniform over the entire useful phosphor screen area. Accordingly, it is possible to increase a color purity tolerance at each portion in the phosphor screen, and hence to prevent local deterioration in resolution. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings form an integral part of the specification and are to be read in conjunction therewith, in which like reference numerals designate similar components throughout the-figures, and in which: 
     FIG. 1 is a sectional view showing a schematic configuration of one embodiment of a color cathode ray tube of the present invention; 
     FIGS. 2A and 2B are configuration views showing one configuration example of a shadow mask for use in the color cathode ray tube shown in FIG. 1, wherein FIG. 2A is a perspective view of the shadow mask and FIG. 2B is a plan view thereof; 
     FIG. 3 is a view illustrating the definition of horizontal, vertical and diagonal pitches of phosphor dots of the phosphor screen formed on the inner surface of a faceplate of a panel portion for use in the color cathode ray tube shown in FIG. 1; 
     FIGS. 4A,  4 B and  4 C are characteristic diagrams showing variations in pitches of the phosphor dots of the phosphor screen formed on the inner surface of the faceplate for use in the color cathode ray tube shown in FIG. 1, wherein FIG. 4A shows one example of the horizontal dot pitches, and FIGS. 4B and 4C shows two different examples of the vertical dot pitches, respectively; and 
     FIG. 5 is a schematic configuration view showing an arrangement of phosphor dots of the phosphor screen of a cathode ray tube employing a prior art shadow mask (disclosed in Japanese Patent Laid-Open Publication No. SHO 56-41648). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A shadow mask type color cathode ray tube of the present invention includes a phosphor screen formed on the inner surface of a faceplate of a panel portion; a shadow mask closely spaced from the phosphor screen within the panel portion; and an in-line type electron gun housed in a neck portion. The shadow mask has a multiplicity of dot-like beam apertures in an apertured portion. The horizontal aperture pitches are nearly uniform in a central portion on both sides of and in the neighborhood of the vertical center line of the apertured portion and increase slightly with increasing distance from the central portion toward the left or right side of the apertured portion. The vertical aperture pitches decrease slightly as one goes from the vertical center line toward the left or right side of the apertured portion. 
     Here, the phosphor dots of the phosphor screen are arranged in such a manner as to satisfy the following relationship: 
     
       
         0.98 Pdo≦Pd≦1.02 Pdo  (1) 
       
     
     where Pd={Ph 2 +Pv 2 }  0.5 , and where, consider two closest phosphor dots of the same color one on each of the two adjacent horizontal rows of the phosphor dots in the phosphor screen, a horizontal dot pitch Ph is a distance between the two phosphor dots of the same color measured horizontally, a vertical dot pitch Pv is a distance between the two phosphor dots of the same color measured vertically, a diagonal dot pitch Pd is a diagonal distance between the two phosphor dots of the same color, and a central diagonal dot pitch Pdo is a diagonal dot pitch Pd measured at the central portion of the phosphor screen. 
     According to the shadow mask type color cathode ray tube of the present invention, the arrangement of the beam apertures in the apertured portion of the shadow mask is such that the vertical aperture pitches are arranged to form a tilt array arrangement and the horizontal pitches of the beam apertures are nearly uniform in the central portion on both sides of and in the neighborhood of the vertical center line of the apertured portion and increase slightly with increasing distance from the central portion toward the left or right side of the apertured portion to form a varied pitch arrangement. As a result, the varied pitch arrangement of the beam apertures cancels the decrease in diagonal pitches of the phosphor dots at the corners of the phosphor screen caused by the tilt array arrangement of the aperture pitches. Further, the relationship (1) between the horizontal dot pitch Ph, vertical dot pitch Pv, diagonal dot pitch Pd, and central diagonal dot pitch Pdo at the center of the phosphor screen makes the diagonal pitches of the phosphor dots substantially uniform over the entire useful phosphor screen area. As a result, a color purity tolerance increases at each portion of the phosphor screen, and local deterioration in resolution does not occur. 
     In the shadow mask type color cathode ray tube of the present invention, the varied aperture pitch arrangement in the shadow mask is effective to increase the mechanical strength of the shadow mask. 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
     FIG. 1 is a sectional view showing a schematic configuration of an embodiment of a color cathode ray tube of the present invention. 
     In FIG. 1, reference numeral  1  designates a panel portion;  1 A is a faceplate;  2  is a neck portion;  3  is a funnel portion;  4  is a phosphor screen;  5  is a shadow mask;  5 U is an apertured portion;  5 S is a skirt portion;  6  is a support frame;  7  is a deflection yoke;  8  is an in-line type electron gun;  9  is a purity adjustment magnet;  10  is a four-pole static beam convergence adjustment magnet;  11  is a six-pole static beam adjustment magnet; and  12  is an electron beam. 
     An evacuated envelope (glass bulb) of the color cathode ray tube is composed of a panel portion  1  having a large-diameter faceplate  1 A; a narrow tubular neck portion  2  housing the in-line type electron gun  8 ; and a funnel portion  3  for connecting the panel portion  1  to the neck portion  2 . The panel portion  1  has the phosphor screen  4  formed on the inner surface of the faceplate  1 A. The skirt portion  5 S of the shadow mask  5  is welded to the support frame  6  which in turn is fixed to the inner sidewall of the panel portion  1  such that the apertured portion  5 U of the shadow mask  5  faces the phosphor screen  4 . The deflection yoke  7  is mounted around the funnel portion  3  on the side thereof closer to the neck portion  2 . 
     The three electron beams  12  (only one of which is shown in FIG. 1) emitted from the in-line type electron gun  8  are deflected by the deflection yoke  7 , pass through an aperture in the shadow mask  5  and impinge upon the phosphor dots of the corresponding colors in the phosphor screen  4 . The purity adjustment magnet  9 , four-pole static beam convergence adjustment magnet  10  and six-pole static beam convergence adjustment magnet  11  are juxtaposed around the neck portion  2 . 
     The image displaying operation of the color cathode ray tube in this embodiment is substantially the same as that of a prior art color cathode ray tube of this type, and therefore, explanation thereof is omitted. 
     FIGS. 2A and 2B are configuration views showing one configuration example of the shadow mask for use in the color cathode ray tube shown in FIG. 1, wherein FIG. 2A is a perspective view showing the overall configuration of the shadow mask, and FIG. 2B is a plan view showing an arrangement of a multiplicity of beam apertures formed in an apertured portion. 
     In FIGS. 2A and 2B, reference numeral  13  designates a multiplicity of dot-like beam apertures formed in the apertured portion  5 U of the shadow mask  5 ;  14  is a vertical center line of the shadow mask  5  (apertured portion  5 U); and  15  is the horizontal center line of the shadow mask  5  (apertured portion  5 U). In addition, the same reference numerals as utilized in FIG. 1 designate corresponding portions in FIGS. 2A and 2B. 
     As shown in FIG. 2B, in the shadow mask  5  for use in the color cathode ray tube, a multiplicity of the beam apertures  13  are formed in the apertured portion like the prior art shadow mask. 
     In the shadow mask  5  in this embodiment, the arrangement of the beam apertures  13  in the apertured portion  5 U of the shadow mask is such that the horizontal pitches of the beam apertures  13  are nearly uniform in a central portion on both sides of and in the neighborhood of the vertical center line  14  of the apertured portion and increase slightly as one goes out of the central portion toward the left or right side of the apertured portion  5 U to form a varied pitch arrangement and the vertical aperture pitches of the beam apertures  13  decrease slightly as one goes from the vertical center line  14  toward the left or right side of the apertured portion  5 U, to form a varied pitch arrangement. 
     In other words, the horizontal direction of the arrangement of the beam apertures  13  tilt slightly increasingly with respect to the horizontal center line  15  as one goes from the central portion toward the corners of the apertured portion  5 U. 
     In this case, the tilt angle of the horizontal direction of the arrangement of the beam apertures  13  with respect to the horizontal center line  15  is largest at the four corners of the apertured portion  5 U and decreases with increasing distance from the four corners. 
     And the horizontal direction of the arrangement of the beam apertures  13  with respect to the horizontal center line  15  is upward-sloping at the upper left-hand corner, downward-sloping at the upper right-hand corner, downward-sloping at the lower left-hand corner and upward-sloping at the lower right-hand corner to form a so-called tilt array arrangement. 
     FIG. 3 is a view illustrating horizontal, vertical and diagonal pitches of the phosphor dots of the phosphor screen formed on the inner surface of the faceplate of the panel portion for use in the color cathode ray tube of FIG. 1 employing the shadow mask as shown in FIGS. 2A and 2B. 
     FIGS. 4A,  4 B and  4 C are views showing variations in pitches of the phosphor dots of the phosphor screen formed on the inner surface of the faceplate of the panel portion for use in the color cathode ray tube employing the shadow mask as shown in FIGS. 2A and 2B, wherein FIG. 4A shows the horizontal dot pitches, and FIGS. 4B and 4C show the vertical dot pitches. 
     In FIG. 3, character Ph designates a horizontal dot pitch; Pv is a vertical dot pitch; and Pd is a diagonal dot pitch. Although not shown in FIG. 3, character Pdo designates diagonal dot pitch in the central portion of the useful phosphor screen area. Further, reference numeral  23 R designates red phosphor dots;  23 G is green phosphor dots; and  23 B is blue phosphor dots. 
     In FIG. 4A, the abscissa designates a distance from the vertical center line toward the left or right side of the useful phosphor screen area, and the ordinate is the horizontal dot pitches Ph of the phosphor dots  23 . In FIGS. 4B and 4C, the abscissa designates a distance from the vertical center line toward the left or right side of the useful phosphor screen area, and the ordinate is the vertical dot pitches Pv of the phosphor dots  23 . Xi designates the left or right side of the useful phosphor screen area in each of FIGS. 4A,  4 B and  4 C. 
     The dot pitches of the phosphor dots  23  formed in the useful phosphor screen area in this embodiment is required to satisfy, in addition to the above-described varied pitch arrangement and tilt array arrangement, the following relationship (1) for making the diagonal dot pitches Pd substantially uniform in each portion in the useful phosphor screen area. Specifically, as shown in FIG. 3, the dot pitches of the phosphor dots  23  are arranged in such a manner as to satisfy the relationship (1): 
     
       
         0.98 Pdo≦Pd≦1.02 Pdo  (1) 
       
     
     where Pd={Ph 2 +Pv 2 }  0.5    
     In the relationship (1), the vertical dot pitch Pv is defined as a vertical distance between one horizontal row of phosphor dots  23 R,  23 G and  23 B and an other horizontal row of phosphor dots  23 R,  23 G and  23 B and adjacent to the one row of phosphor dots  23 R,  23 G and  23 B; the horizontal dot pitch Ph is defined as a horizontal distance between a first phosphor dot of a first color (a green phosphor dot  23 G, for example) in the one horizontal row and a second phosphor dot of the first color in the other horizontal row and nearest the first phosphor dot; a diagonal dot pitch Pd is defined as a diagonal distance between the first phosphor dot and the second phosphor dot; and a central diagonal dot pitch Pdo is a diagonal dot pitch Pd at the center of the useful phosphor screen area. 
     In this case, it is possible to make substantially uniform the diagonal dot pitches Pd in each portion in the useful phosphor screen area without incurring complications by selecting the diagonal dot pitches Pd everywhere in the useful phosphor screen area at a value in a range of 0.98 to 1.02 times the diagonal dot pitches Pdo in the center portion of the useful phosphor screen area. 
     The phosphor screen having such a configuration, as shown in FIG. 4A, is configured such that the horizontal dot pitches Ph are nearly uniform in a central region Hc within a distance of Xc from the vertical center line as one goes from the vertical center line toward the left or right side of the useful phosphor screen area, and increase slightly in a region Hi outside of the central region Hc as one goes toward the sides of the useful phosphor screen area. 
     Meanwhile, the vertical phosphor dot pitches Pv decrease monotonically gradually as one goes from the vertical center line toward the left or right side of the useful phosphor screen area to form a tilt array arrangement, as shown in FIG.  4 B. 
     As shown in FIG. 4C, the vertical dot pitches Pv can also be configured such that the vertical dot pitches Pv are nearly uniform in a central region Vc within a distance of Xc from the vertical center line as one goes from the vertical center line toward the left or right side of the useful phosphor screen area, and then decrease slightly in a region Vi outside of the central region Vc as one goes toward the left or right side of the useful phosphor screen area. 
     That the horizontal phosphor dot pitch Ph is “nearly uniform” in the central region Hc means that a variation of Ph from 1.0 Pho to 1.01 Pho in the central region Hc can be accepted as a manufacturing tolerance, when Pho is defined as the minimum value of Ph in the central region Hc. 
     It is preferable that the central region Hc extends not less than about two-thirds, but not more than about five-sixths of the distance from the vertical center line to the left or right side of the useful phosphor screen area. If the central region Hc has a dimension smaller than the above dimension, the phosphor dot pitches become too coarse in the critical area of the phosphor screen, resulting in pronounced deterioration in resolution and poor display quality of the color cathode ray tube. 
     On the other hand, if the central region Hc extend to more than about five-sixths of the distance from the vertical center line, diagonal phosphor dot pitches cannot be made sufficiently large at the left or right side of the useful phosphor screen area and the color purity tolerance decreases particularly at the corners of the phosphor screen, resulting in deterioration of color uniformity, the color purity tolerance being defined as a distance an electron beam can move before it reaches an adjacent phosphor dot of an unintended color. 
     The horizontal dot pitch Phi at the sides of the useful phosphor screen area is preferably in a range of about 1.03 Pho to about 1.08 Pho. 
     That the vertical phosphor dot pitch Pv is “nearly uniform” in the central region Vc means that a variation of Pv from 0.995 Pvo to 1.0 Pvo in the central region Vc can be accepted as a manufacturing tolerance, when Pvo is defined as the maximum value of Pv in the central region Vc. 
     It is preferable that the central region Vc extends not more than about two-thirds of the distance from the center to the left or right side of the useful phosphor screen area. 
     If the central region Vc extend to more than about two-thirds of the distance from the vertical center line to the sides of the useful phosphor screen area, vertical phosphor dot pitches cannot be made sufficiently small at the left or right side of the useful phosphor screen area and the sufficient tilt of the horizontal direction of the line triads of three phosphor dots of different colors (the arrangement of the tilt array of three-color phosphor dots) cannot be obtained particularly at the corners of the phosphor screen, resulting in decrease in color purity tolerance and subsequent deterioration of color uniformity. 
     The vertical dot pitch Pvi at the left or right side of the useful phosphor screen area is preferably in a range of about 0.935 PVo to about 0.985 Pvo. 
     While FIGS. 4A,  4 B and  4 C illustrate variations in pitches of the phosphor dots of the phosphor screen, the pitches of the beam apertures of the shadow mask have the same pitch arrangement and characteristics as those of the pitches of the phosphor dots of the phosphor screen. 
     As described above, according to this embodiment, since the dot pitches of the phosphor dots  23  formed in the useful phosphor screen area are arranged on the basis of the varied pitch arrangement and the tilt array arrangement, the varied pitch arrangement cancels the decrease in the diagonal dot pitches Pd of the phosphor dots  23  at the corners of the useful phosphor screen area caused by the tilt array arrangement, and further, the above-described relationship (1) makes substantially uniform the diagonal dot pitches Pd of the phosphor dots  23  in each portion of the useful phosphor screen area. 
     Since the diagonal dot pitches Pd of the phosphor dots  23  in each portion of the useful phosphor screen area  5 U are made substantially uniform, a color purity tolerance increases in the useful phosphor screen area, resulting in prevention of local deterioration in resolution. 
     Further, according to this embodiment, there can be obtained the shadow mask  5  having a high mechanical strength by adopting the varied aperture pitch arrangement for the shadow mask  5 . 
     As described above, according to the shadow mask type color cathode ray tube, a multiplicity of beam apertures are formed in the apertured portion of the shadow mask such that the vertical pitches of the beam apertures are arranged to form the tilt array arrangement, and the horizontal pitches of the beam apertures are nearly uniform in a central portion on both sides of and in the neighborhood of the vertical center line of the apertured portion and increase slightly as one goes out of the central portion toward the left or right side of the apertured portion  5 U to form a varied pitch arrangement. Accordingly, the varied pitch arrangement of the beam apertures cancels the decrease in diagonal pitches of the phosphor dots at the corners of the phosphor screen caused by the tilt array arrangement of the aperture pitches, to prevent local decrease in diagonal pitches of the phosphor dots. Also, the relationship (1) between the horizontal dot pitch Ph, vertical dot pitch Pv, diagonal dot pitch Pd, and central diagonal dot pitch Pdo makes the diagonal pitches of the phosphor dots substantially uniform over the entire useful phosphor screen area. As a result, a color purity tolerance increases at each portion in the useful phosphor screen area, and local deterioration in resolution is prevented. 
     According to the shadow mask type color cathode ray tube of the present invention, the varied pitch arrangement adopted for the shadow mask is effective to increase the mechanical strength of the shadow mask.