Patent Publication Number: US-6342757-B1

Title: Cathode ray tube for multimedia

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 09/061,952, filed on Apr. 17, 1998, now abandoned, which claimed the priority of Korean patent application No. 97-23035 filed Jun. 4, 1997, the disclosures of which are incorporated herein by reference as if set forth in full. 
    
    
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a cathode ray tube (CRT) for multimedia and a method of making the same and, more particularly, to a method of making a multimedia CRT which simultaneously produces high resolution and high brightness for use in both computers requiring high-resolution and televisions or videos requiring high-brightness. 
     (b) Description of the Related Art 
     Generally, in the CRT, the electron beam emitted from the electron gun passes through the beam-guide apertures of the shadow mask having a color selection function while it is deflected by the deflection yoke. Then it excites the phosphors coated on the panel screen to produce the picture image. 
     A plurality of dot, slit or stripe-shaped apertures for guiding the electron beam are formed on the shadow mask, the shadow mask having a curvature ratio identical with that of the inside of the panel. 
     The stripe matrix type shadow mask, having a high beam-permeability, is used in the CRT for televisions or videos requiring high-brightness (collectively referred to hereinafter as “public CRT”) while the dot matrix type shadow mask, having a fine-pitch, is used in the CRT for computers or data processors requiring high-resolution (collectively referred to hereinafter as “industrial CRT”). That is to say, the public CRT requires high-brightness to display clear pictures on the screen which are capable of being viewed from a long distance. On the other hand, the industrial CRT requires high-resolution to display distinct letters or symbols on the screen. 
     The CRT using a stripe matrix type shadow mask has a beam-permeability higher than that using a dot matrix type shadow mask. Conversely, the CRT using a dot matrix type shadow mask with a fine pitch (the distance between neighboring beam-guide apertures) has a resolution higher than that using the stripe matrix type shadow mask. 
     The permeability R of the electron beam is mathematically given by the following formula: 
     
       
         R=((area of beam-guide aperture×number of beam-guide aperture)/whole area of shadow mask)×100%,  (1) 
       
     
     where the permeability of the electron beam may be calculated by the permeability per unit area or the area of the beam-guide aperture per pitch area (horizontal pitch×vertical pitch). 
     Therefore, Eq. 1 may be rewritten by 
     
       
         R=(area of beam-guide aperture/(horizontal pitch×vertical pitch))×100%.  (2) 
       
     
     Using Eqs. 1 or 2, it can be shown that the permeability R of the CRT using the dot matrix type shadow mask is about 17 to 19% while the permeability of the CRT using the stripe matrix type shadow mask is about 20 to 22%. 
     For example, the permeability of a 15-inch industrial CRT using a dot matrix type shadow mask can be given by 
     
       
         R=((2×π×0.06 2 )/(0.27×0.4677))×100=17.9%.  (3) 
       
     
     The permeability R of a 17-inch industrial CRT using a dot matrix type shadow mask is given by 
     
       
         R=((2×π×0.06 2 )/(0.26×0.4677))×100=18.6%.  (4) 
       
     
     The permeability R of a 25-inch public CRT using a stripe matrix type shadow mask is given by 
     
       
         R=((0.65×0.18)/(0.76×0.73))×100=21.08%.  (5) 
       
     
     The permeability of a 24-inch wide public CRT using a stripe matrix type shadow mask is given by 
     
       
         R=((0.6×0.175)/(0.76×0.73))×100=21.17%.  (6) 
       
     
     As described above, in the prior art, the CRT using a stripe matrix type shadow mask has a resolution unsuitable for industrial usage while the CRT using a dot matrix shadow mask has a brightness unsuitable for public usage. Therefore, one CRT cannot be used for both industrial and public purposes. 
     However, as multimedia computer systems, incorporating televisions, videos and computers to display combinations of moving and still pictures, and words, or television systems having an Internet connection function, became more developed and come into wide use, the need for a CRT having both high-brightness and high-resolution increases. 
     SUMMARY OF THE INVENTION 
     Accordingly, an embodiment of the present invention is directed to a CRT for multimedia which substantially obviates one or more of the problems due to the limitations and disadvantages of the related art. 
     An object of an embodiment of the present invention is to provide a method of making a multimedia CRT for simultaneously producing high brightness and high resolution. 
     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. 
     To accomplish these and other advantages, the multimedia CRT includes a shadow mask having a plurality of beam-guide apertures. Each of the beam-guide apertures has a stripe shape, and the ratio of the horizontal length of the effective screen to the horizontal pitch of the beam-guide apertures in the center of the shadow mask is in the range of 1:8.81×10 −4  to 1:1.23×10 −3 . 
     Furthermore, the ratio of the horizontal pitch of the beam-guide apertures in the center of the shadow mask to the thickness of the shadow mask is in the range of 1:0.25 to 1:0.56. 
     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 
     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 a particular embodiment of the invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
     FIG. 1 is a partial sectional view of a multimedia CRT according to an embodiment of the present invention; 
     FIG. 2 is a perspective view of a shadow mask according to an embodiment of the present invention; 
     FIG. 3 is a front view of a multimedia CRT showing the effective screen according to a preferred embodiment of the present invention; 
     FIG. 4 is a sectional view taken along line A—A of FIG. 2; and 
     FIG. 5 is a schematic diagram illustrating the relation of the shadow mask to the electron beam according to an embodiment of the present invention. 
     In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor(s) of carrying out the invention. As will be realized, the invention is capable of modification in various other respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     As shown in FIGS. 1 to  4 , a multimedia CRT according to a preferred embodiment of the present invention includes a shadow mask  10  having a plurality of stripe-shaped beam-guide apertures  22  where the ratio of the horizontal length L of the effective screen to the horizontal pitch P of the beam-guide apertures  22  in the center of the shadow mask  10  is in the range of 1:8.81×10 −4  to 1:1.23×10 −3 . 
     A multimedia CRT according to another preferred embodiment of the present invention includes a shadow mask  10  having a plurality of stripe-shaped beam-guide apertures  22  where the ratio of the horizontal pitch P of the beam-guide apertures  22  in the center of the shadow mask  10  to the thickness t of the shadow mask  10  is in the range of 1:0.25 to 1:0.56. 
     As shown in FIG. 1, the multimedia CRT according to an embodiment of the present invention includes a front panel  4  having a phosphor screen  2  and a funnel  6  connected to the rear of the panel  4 . The funnel  6  has a neck portion into which an electron gun  8  is fitted. In addition, a shadow mask  10  is placed directly behind the phosphor screen  2  and supported by a mask frame  12 . 
     A plurality of stud pins  14  are fixed on the side interior of the panel  4  to support the mask frame  12  through corresponding hook springs  16 . An inner shield  18  is further provided under the mask frame  12  to interrupt the influence of the earth&#39;s magnetic field. Additionally, an explosion proof band  20  is provided on the external sealing portion between the panel  4  and funnel  6 . 
     As shown in FIG. 2, the shadow mask  10  has a masking portion  24  having hundreds of thousands of fine beam-guide apertures  22 , a closed peripheral portion  26  having no apertures, and a skirt portion  28  welded to the mask frame  12 . 
     Each of the beam-guide apertures  22  has a color selection function. That is, the red (R), green (G) and blue (B) beams are focused through the beam-guide aperture  22  to hit the selected phosphors on the phosphor screen  2 . 
     The distance between the centers of the neighboring beam-guide holes is called the “pitch”. As shown in FIG. 2, the pitch can be divided into a horizontal pitch P and a vertical pitch V. In the stripe-shaped beam-guide aperture  22 , the horizontal pitch P acts as the main pitch. 
     As described above, the present invention is directed to a multimedia CRT capable of simultaneously satisfying high-brightness and high-resolution requirements. 
     For high-brightness applications, as previously noted from Eqs. 3 to 6, it is preferable that the beam-guide aperture  22  has a stripe shape capable of giving a relatively high beam-permeability R. In addition, for high-resolution applications, it is preferable that the stripe-shaped beam-guide apertures  22  (collectively referred to hereinafter as “stripes”) have a relatively small horizontal pitch P capable of ensuring an under-scanning condition. 
     The conventional 14 or 15-inch industrial CRT typically has a resolution of 640×480 or 800×600. Among them, the 800×600 resolution CRT is preferably selected because it has the advantages of producing distinct images or large amounts of letters or symbols. 
     In this respect, the inventive multimedia CRT is preferably a wide 24-inch CRT which has a 800×600 resolution and a 16:9 ratio of the width of the screen to the length of the screen. 
     A method of establishing the horizontal pitch P of the stripes  22  will be now described in connection with this wide 24-inch CRT. 
     FIG. 3 is a front view of a multimedia CRT showing a effective screen  4   a  defined by a dotted line. 
     The area of the effective screen  4   a  has a fixed value proportional to the CRT size. In industrial CRTs, an under-scanning condition, where the image producing signals are scanned on the screen portion smaller than the effective screen  4   a  by 0˜10%, produces high resolution characteristics necessary for correct information display. 
     In case of the wide 24-inch CRT shown in FIG. 3, the diagonal length of the effective screen  4   a  is 560±5 mm while the longitudinal length is 488 mm. 
     As shown in FIG. 2, a resolution of 800×600 indicates the total number of stripes  22  formed on the shadow mask  10 . Therefore, the masking portion  24  of the shadow mask  10  has stripes  22  with 800 vertical lines and 600 horizontal lines. 
     Accordingly, assuming that the area of the masking portion  24  is identical with that of the effective screen  4   a , the horizontal pitch P of the stripes  22  is given by: 
     
       
         P=the horizontal length of effective screen/number of vertical lines of stripes=488/800=0.61 (mm)  (7) 
       
     
     That is, when 800 stripes are longitudinally arranged across the effective screen  4   a , the horizontal pitch P of the stripes  22  is 0.61 mm. This value practically becomes a maximum value relating to the horizontal pitch P of the stripes  22 . 
     When an additional margin of 20 to 30% is allowed to the maximum value in view of a margin of 5 to 10% for under-scanning and a margin of 15 to 20% for manufacturing error, the horizontal pitch P of the stripes  22  is approximated as follows: 
     
       
         P=0.61×0.8=0.488≈0.49 (mm)  (8) 
       
     
     
       
         P=0.61×0.7=0.427≈0.43 (mm).  (9) 
       
     
     Eq. 8 is applicable when a margin of 20% is allowed to the horizontal pitch P of the stripes while Eq. 9 is applicable when a margin of 30% is allowed. 
     As noted from Eqs. 8 and 9, the minimum value is 0.43 to 0.49 mm when computed in view of the maximum margin. Therefore, it is preferable that the horizontal pitch P of the stripes  22  is established at 0.43 mm or more. 
     Table 1 shows the results of calculating the resolution and brightness of the wide 24-inch CRT while varying the horizontal pitch P of the stripes  22 . 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Pitch (P) 
                   
                   
               
               
                 (mm) 
                 Resolution 
                 Brightness 
               
               
                   
               
             
            
               
                 0.40 
                 ⊚ 
                 x 
               
               
                 0.43 
                 ⊚ 
                 Δ 
               
               
                 0.49 
                 ⊚ 
                 Δ 
               
               
                 0.55 
                 ∘ 
                 ∘ 
               
               
                 0.60 
                 Δ 
                 ∘ 
               
               
                   
               
            
           
         
       
     
     The resolution is determined by the focusing characteristic. That is, when the letters displayed on the periphery of the screen are unfocused, the resolution is bad. 
     As noted from Table 1, when the horizontal pitch of the stripes is over 0.60 mm, the letters at the periphery of the screen are gradually unfocused resulting in bad resolution. 
     In industrial CRTs, the brightness is good when the brightness ratio of the periphery of the screen to the center of the screen is above 90% because the pitch ratio of the beam-guide holes in the center of the shadow mask to the beam-guide holes at the periphery of the shadow mask is the same. 
     On the contrary, in public CRTs, the pitch ratio (110 to 140%) of the beam-guide holes at the center of the shadow mask to the beam-guide holes in the periphery of the shadow mask is different. Thus, the brightness is good when the brightness ratio in the center of the screen to the periphery of the screen is above 50%. 
     Therefore, in the multimedia CRT according to an embodiment of the present invention, the stripe pitch in the center of the shadow mask and the stripe pitch at periphery of the shadow mask differs with a medium range between the industrial CRT and the public CRT. Therefore, the brightness is good when the brightness ratio in the center of the screen to the periphery of the screen is above 75%. 
     Table 2 shows data measuring the brightness at the periphery of the screen when the brightness in the center of the screen is fixed with 35.5 ft-L. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Brightness ratio of 
               
               
                 Pitch (P) 
                 Brightness at periphery 
                 center to periphery 
               
               
                 (mm) 
                 of screen 
                 screen (%) 
               
               
                   
               
             
            
               
                 0.40 
                 25.9 
                 73 
               
               
                 0.43 
                 27.7 
                 78 
               
               
                 0.49 
                 31.6 
                 89 
               
               
                 0.55 
                 35.5 
                 100 
               
               
                 0.60 
                 38.7 
                 118 
               
               
                   
               
            
           
         
       
     
     It can be shown from Tables 1 and 2 that the pitch P of the stripes is in the range of 0.43 to 0.60 mm when the brightness ratio in the center of the screen to the periphery of the screen is above 75% and, hence, is good. 
     Accordingly, in the wide 24-inch multimedia CRT, the horizontal pitch P of the stripes, where the resolution and brightness are both good, is in the range of 0.43 to 0.60 mm and the optimum value of the horizontal pitch P is 0.55 mm. 
     When the horizontal pitch P is compared to the horizontal length L of the effective screen  4   a , their ratio H is given by the following Eqs. 10 and 11: 
     
       
         H=0.43/488=8.81×10 −4   (10) 
       
     
     
       
         H=0.60/488=1.23×10 −3   (11) 
       
     
     Therefore, in a multimedia CRT according to a preferred embodiment of the present invention, the ratio H of the longitudinal length L of the effective screen  4   a  to the horizontal pitch P of the stripes  22  in the center of the shadow mask  10  is established with 1:8.81×10 −4  to 1:1.23×10 −3 . 
     Accordingly, the multimedia CRT can have good brightness by making the beam-guide hole of the shadow mask into a stripe shape as well as high resolution by reducing the horizontal pitch of the stripe. 
     The relation of the horizontal pitch to the thickness of the shadow mask will be now described. 
     FIG. 4 is a cross sectional view taken along A—A line of FIG.  2 . One-side or both-side etching is performed to the shadow mask  10  to thereby form a plurality of stripes  22 . The shape of the stripe  22  is usually largely depressed in a one-sided direction. 
     In the figure, W indicates the hole size of the stripe  22  passing the electron beam  34 , TA indicates the horizontal length with respect to the left inclined side  30  of the stripe  22 , TB indicates the horizontal length with respect to the right inclined side  32  of the stripe, and B (called a “bar”) indicates the distance between the neighboring stripes  22 . 
     When the horizontal pitch P of the stripes  22  is reduced to enhance the resolution, an accompanying decrease in the size of the bar B occurs, resulting in a shadow mask  10  that becomes structurally unstable. 
     Therefore, in order to ensure structural stability, optimum size of the bar B should be considered when reducing the horizontal pitch P of the stripe  22 . 
     The size of the bar B is given by: 
     
       
         B=P−W−TA−TB.  (12) 
       
     
     At first, referring to the horizontal lengths TA and TB with respect to the inclined sides of the stripe  22 , the inclined sides of the stripe  22  forms an inclination of 15 to 60° in accordance with the incidence angle of the electron beam  34 . The stripes in the center of the shadow mask  10  may be vertically formed because the electron beam  34  is vertically incident upon the shadow mask  10 . Nevertheless, the inclined side of the stripe facing the center of the shadow mask  10  and having the horizontal length TA is formed with an inclination of 15° for easy processing. In contrast, the inclined side of the stripe facing the periphery of the shadow mask and having the horizontal length TB is formed with an inclination more than 15° in view of the deflection angle of the electron beam by a deflection yoke. 
     In order to determine the horizontal lengths TA and TB with respect to the inclined sides of the stripe, an imagined line is vertically drawn from the center of the shadow mask  10  as shown in FIG.  5 . Then, a curvature center C and a point of deflection of the electron beams D are established on the line, and an electron beam  34  is assumed to be scanned on an optional point on the shadow mask  10 . At this time, assuming that the deflection angle of the electron beam  34  is θ and the angle formed by the curvature center C and the optional point is ψ, the incidence angle φ of the electron beam  34  passing the stripe of the shadow mask  10  is represented by: 
     
       
         φ=θ−ψ.  (13) 
       
     
     Therefore, referring to FIG. 4, the horizontal lengths TA and TB (collectively indicated by “T”) with respect to the inclined sides  30  and  32  of the stripe  22  can be given by: 
     
       
         tan ψ=T/t  (14) 
       
     
     
       
         T=t×tan φ,  (15) 
       
     
     where t is the thickness of the shadow mask  10 . 
     Strictly speaking with reference to the figure, t is the thickness of the upper etching side. However, since the thickness of the lower etching side is extremely small compared to the total thickness of the shadow mask  10 , t can be assumed to be the total thickness of the shadow mask  10  in view of the small error. 
     Accordingly, the horizontal lengths TA and TB with respect to the inclined sides  30  and  32  can be measured with the thickness of the shadow mask  10  and the incidence angle φ of the electron beam  34 . 
     When the thickness t of the shadow mask  10  is 0.22 mm, the maximum deflection angle θ of the electron beam  34  is 46°, and the angle ψ is 11°, the incidence angle φ of the electron beam  34  obtained from Eq. 13 is 35°. 
     Therefore, the horizontal length TB with respect to the right inclined side  32  positioned toward the periphery of the shadow mask  10  is approximated by 
     
       
         TB=0.22×tan 35°≈0.154 (mm)  (16) 
       
     
     However, the left inclined side  30  positioned toward the center of the shadow mask  10  is not particularly affected by the incidence angle φ of the electron beam  34 . Thus, it is formed with the conventional inclination of 15° enabling easy processing of the stripe  22 . At this time, the horizontal length TA with respect to the left inclined side  30  is approximated by: 
     
       
         TA=t×tan α=0.22×tan 15°≈0.06 (mm).  (17) 
       
     
     In this way, the respective horizontal lengths TA and TB with respect to the inclined sides  30  and  32  of the stripe  22  can be measured. 
     As noted from Table 1, when the horizontal pitch P of the stripes  22  is established at an optimum value of 0.55 mm satisfying both the resolution and brightness requirements, the width W of the stripes  22  is established with 0.160 mm. When the established values are applied to Eq. 12, the bar B of the stripes  22  is given by: 
     
       
         B=0.55−0.160−0.154−0.060=0.176 (mm).  (18) 
       
     
     When the bar B of the stripes of the conventional 24-inch public CRT is measured for comparison purposes, it is given by: 
     
       
         B=0.76−0.160−0.154−0.060=0.386 (mm).  (19) 
       
     
     This value is obtained by applying the stripe pitch 0.76 mm of the shadow mask used in the conventional 24-inch public CRT to Eq. 12. As noted from Eqs. 18 and 19, when the horizontal pitch P of the stripes  22  is reduced to enhance the resolution, the bar B of the stripes  22  decreases, resulting in structural instability. 
     Therefore, the bar B of the stripes  22  should be large enough to ensure the structural stability while reducing the horizontal pitch P of the stripes  22 . 
     For this purpose, when the thickness t of the shadow mask  10  is reduced from 0.22 mm to 0.20 mm, 0.18 mm or 0.15 mm, the bar B of the stripes  22  can be computed by using Eqs. 12 and 15 as follows: 
     
       
         B=0.55−0.160−(0.20×tan 15°)−(0.20×tan 35°)=0.196 (mm),  (20) 
       
     
     
       
         B=0.55−0.160−(0.18×tan 15°)−(0.18×tan 35°)=0.216 (mm),  (21) 
       
     
     
       
         B=0.55−0.160−(0.15×tan 15°)−(0.15×tan 35°)=0.245 (mm),  (22) 
       
     
     As shown from the above equations, when the thickness t of the shadow mask  10  is reduced, the bar B of the stripes  22  becomes larger, enabling micro-processing with respect to the stripes  22 . 
     However, when the thickness t of the shadow mask  10  becomes thinner, the resulting structural weakness makes it difficult to form an appropriate curvature and may produce a doming phenomenon due to thermal expansion and a vibrating phenomenon due to the sonic pressure of the speaker. Thus, it is important that the thickness t of the shadow mask  10  should be kept within the range capable of maintaining the structural stability and mechanical strength of the shadow mask  10 . 
     In this case, it is preferable that the thickness t of the shadow mask  10  be more than 0.15 mm. When the thickness t of the shadow mask  10  is less than 0.15 mm, the stability and strength of the shadow mask  10  cannot be practically maintained. 
     Furthermore, the thickness t of the shadow mask  10  should be less than 0.24 mm. When the thickness t of the shadow mask  10  is more than 0.24 mm, the time required for the etching process increases and the smooth stripe formation suffers. 
     Accordingly, as noted from Eqs. 20 to 22, the thickness t of the shadow mask  10  is established between 0.15 and 0.24 mm, the range capable of keeping the appropriate bar B of the stripes  22 . 
     Therefore, when the thickness t of the shadow mask  10  is 0.15 mm and the horizontal pitch P of the stripes  22  is 0.60 mm, the maximum value capable of giving good brightness, the ratio S of thickness t of the shadow mask  10  to the horizontal pitch P of the stripes  22  in the center of the shadow mask  10  is given by: 
     
       
         S=0.15/0.60=0.25.  (23) 
       
     
     Furthermore, when the thickness t of the shadow mask  10  is 0.24 mm and the horizontal pitch P of the stripes  22  is 0.43 mm, the ratio S of the horizontal pitch P of the stripes  22  in the center of the shadow mask  10  to the thickness t of the shadow mask  10  is approximated as: 
     
       
         S=0.24/0.43≈0.56.  (24) 
       
     
     Accordingly, in a multimedia CRT according to another preferred embodiment of the present invention, the ratio S of the horizontal pitch P of the stripes  22  in the center of the shadow mask  10  to the thickness t of the shadow mask  10  is in the range of 1:0.25 to 1:0.56. 
     As described above, in the inventive multimedia CRT, it is possible to simultaneously realize high-resolution and high-brightness by establishing the thickness of the shadow mask and the horizontal pitch of the stripes with optimum values. Thus, it can be used for public as well as for industrial purposes. Furthermore, it is applicable in multimedia computer systems or televisions having an Internet connecting function to produce letters and pictures by incorporating various media. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the multimedia CRT and the method of making the same of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.