Abstract:
A slot type shadow mask for use in a color picture tube. The mask includes a mask plate member. The plate member is formed by a front plate and a rear plate which are joined to each other. Bridges of the front and rear plates divide slot holes within the rear and front plates, respectively.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a slot type shadow mask for use in a shadow mask type color picture tube. 
     2. Description of the Background Art 
     As shown in FIG. 7, a shadow mask type color picture tube typically comprises three electron guns 1, a fluorescent screen 2 and a shadow mask 3. Electron beams B from the guns 1 cause the screen 2 to glow red, green and blue. The shadow mask 3 permits only certain of the electron beams B to pass through beam apertures 3A such that stripes of fluorescent material on the fluorescent screen 2 are caused to glow in response to image signals. 
     The mask 3 includes about a hundred thousand to hundreds of thousands of the strip-shaped beam apertures 3A. The apertures 3A have a horizontal dimension of 0.08 to 0.25 mm and a vertical dimension of 0.3 to 1.0 mm. The apertures 3A are defined within a steel plate which is 0.1 to 0.3 mm thick. The apertures 3A are arranged in a regular bricklaying manner. The apertures 3A are opened/formed by etching. 
     To form bright color images, it is necessary to increase the proportion of electron beams which are transmitted through the shadow mask 3. The width of each aperture is a function of (1) the arrangement of the different fluorescent materials on the screen 2 and (2) the transverse pitch between the beam apertures 3A. The bridges cannot be too thin and the length of each beam aperture cannot be too long or else the shadow mask will be broken during the molding, working and mounting of the shadow mask on the color picture tube. 
     The slot type shadow mask 13 shown in FIG. 8 has been proposed in U.S. Pat. No. 4,293,792. The mask 13 provides increased electron beam transmission. 
     The mask 13 has a large number of longitudinal beam apertures 13A which are vertically and transversely arrayed on a single mask plate member 14. Bridges 16a and 16b are alternately interposed between the beam apertures 13A. The bridges 16a are displaced toward a front surface S 1  and the bridges 16b are displaced toward a rear surface S 2 . The mask plate member 14 has a thickness T. The thickness of the bridges 16a, 16b is less than T. 
     The shadow mask 13 is fabricated by applying photoresist films to front and rear surfaces of a thin metal plate, drying the same, contact-printing shadow mask patterns on the front and rear surfaces, spray etching and then separating the photoresist films. 
     To displace the bridges 16a toward the 
     surfaces S 1  and to displace the bridges 16b toward the rear surfaces S 2 , it is necessary to etch the single mask plate member 14 from the front and rear surfaces. To do this, the etching speed, the pressure of the etching solution and the like must be strictly controlled. This means that the dimensions of the beam apertures 13A as well as the dimensions of the bridges 16a and 16b are simultaneously controlled during etching on the premise that the thickness of the thin metal plate, the thickness of the photoresist films, the sensitivity of the photoresist material and the like are not variable. However, it is necessary to perform etching while most emphasizing the opening dimensions of the beam apertures 13A in practice. Hence, if the thickness of the materials is variable, the thicknesses of the bridges 16a and 16b are correspondingly variable If the bridges are too thin, strength is reduced and the bridges may be broken during molding/working. In sum, it is difficult to fabricate the mask 13 by etching. 
     Further, the positions of the beam apertures and the stripes of fluorescent materials must correctly coincide with each other. 
     However, since the shadow mask forms an anode of the color picture tube with the fluorescent materials, the percentage of electron beam transmission through the shadow mask is only about 20%. Power loss at the anode reaches 80%. When anode power is about 25 watts (W) in a 20-inch color picture tube, 20 W of power is expended by the shadow mask. This causes the temperature of the shadow mask to increase by about 40° C. The mask expands by about 100 μm as a result. Such thermal expansion prevents coincidence of the electron beams and the fluorescent materials. The picture tube&#39;s color purity is therefore reduced. 
     In this regard, generally known is a shadow mask which is formed by two plate members so that corresponding slot holes of the second plate member are brought into close contact with slot holes of the first plate member or opposed to the same with small clearances thereby to increase the strength of the shadow mask, as disclosed in Japanese Patent Laid-Open No. 79170/1974. Japanese Patent Laid-Open No. 131676/1974 discloses preventing local thermal expansion by stacking two sliced shadow masks (plate members) so that a large number of openings entirely overlap each other. However, the percentage of electron beam transmission in these masks is changed by displacement of the positions of the slot holes (openings) of the overlapped plate members due to thermal expansion of master patterns (which are employed for printing shadow mask patterns), distortion caused during etching of the shadow mask, inaccurate alignment of the plate members and the like. 
     Further, transmission irregularity caused by inaccurate hole configurations within a plate member is compounded when such plate members are overlapped. 
     U.S. Pat. No. 4,374,452 discloses a post-focusing type color picture tube with means for improving the brightness of displayed images by increasing the percentage of electron beam transmission in a color selector. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-described problems. An object of the invention is to provide a slot type shadow mask which can be easily manufactured, which has excellent strength and which transmits a high percentage of electron beams with a small amount of transmission irregularity. 
     The present invention relates to a shadow mask with beam apertures for controlling electron beam transmission within a color picture tube. The shadow mask includes: a front plate having a large number of slots and bridges; and a rear plate having a large number of slots and bridges. The rear plate is joined to the front plate. The front plate and the rear plate are arranged with respect to each other such that the bridges of the front plate and the rear plate divide the slots of the rear plate and the front plate, respectively, into the beam apertures. 
     According to the present invention, the front plate and the rear plate are separately etched to define the longitudinal slot holes. During such etching, bridges of prescribed dimensions are formed by controlling only the dimensions of the slot holes. The bridges are as thick as the respective plate. Thus, strength is improved such that the shadow mask is not broken during molding/working. 
     By reducing the thickness of the bridges, the percentage of electron beam transmission is improved, particularly in peripheral edge portions of the shadow mask. 
     Further, the present invention eliminates deviation in the percentage of electron beam transmission through the mask caused by overlapping the front and rear plates. In the present invention, transmission irregularity is substantially determined by the plate member with the smaller openings. The beam apertures of the slot type shadow mask are in the form of strips. Hence the areas of light transmitting portions are largely influenced by the opening width of the beam apertures, i.e., the smaller opening width. Influence caused by deviation in bridge width is small. 
     Preferably, the thickness of the plate members is a function of the opening width of the slot holes. Preferably, the plate member with the smaller openings is thinner than the other plate member. The holes through the thinner plate member can be etched more accurately. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged perspective view showing an essential part of a slot type shadow mask according to the present invention, 
     FIG. 2 is an enlarged front view showing an essential part of a slot type shadow mask according to another embodiment of the present invention, 
     FIG. 3 is a sectional view along the line III--III in FIG. 2, 
     FIG. 4 is an enlarged sectional view of still another embodiment, 
     FIGS. 5a to 5c and 6a to 6d are sectional views of slot holes of further embodiments, 
     FIG. 7 is a schematic diagram of a color picture tube, and 
     FIG. 8 is an enlarged perspective view showing an essential part of a conventional slot type shadow mask. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a slot type shadow mask 3 includes a front plate 4a and a rear plate 4b which are joined to each other to form a mask plate member 4. Longitudinal slot holes 5a and 5b are vertically and transversely arrayed/formed in the front plate 4a and the rear plate 4b by etching from both sides. 
     The slot holes 5a and 5b are vertically displaced by half a pitch from each other so that bridges 6a and 6b mutually uniformly divide the slot holes 5b and 5a when the plates 4a and 4b are joined to each other. Beam apertures 3A are defined by the slot holes 5a, 5b and the bridges 6a, 6b. 
     The plates 4a, 4b can be formed of thin plates of aluminum killed steel or Invar alloy having a nickel content of 36%. The plates 4a, 4b are joined to each other by spot welding the peripheral edge portions (the skirt portions) of the shadow mask. Alternately, the plates 4a, 4b can be adhered together by polyimide resin. 
     Strip-shaped slot holes 5a and 5b illustrated in FIGS. 2 and 3 are formed by etching both of the plates 4a, 4b from a front surface side S 1 . The holes 5a, 5b have a transverse dimension of about 0.2 mm and a vertical dimension of about 1.2 mm. The holes 5a, 5b are regularly arranged vertically and transversely in the plates 4a and 4b. The mask is formed by etching each of the plates 4a, 4b from a front surface side S 1 , each one of the slot holes being conically opened/formed. When the plates 4a, 4b are joined to each other, the bridges 6a, 6b uniformly divide the slot holes 5b, 5a to define the beam apertures 3A, as in the embodiment shown in FIG. 1. Thus, the bridges 6a are displaced toward the front surface S 1  side of the shadow mask and the bridges 6b are displaced toward a rear surface S 2  side. 
     The bridges 6a, 6b are reduced in thickness as compared with the overall mask plate member such that the percentage of transmission of electron beams B is improved. 
     In the mask illustrated in FIG. 4, bridges 6a and 6b are not alternately displaced toward a front surface side S 1  and a rear surface side S 2 . Instead, there are two of the bridges 6a for every one of the bridges 6b. The bridges 6a and 6b are periodically displaced on the front surface side S 1  and the rear surface side S 2  of the shadow mask. 
     The ratio of displacement of the bridges 6a and 6b is not restricted to 2:1. An integer-to-integer ratio such as 3:2 or 4:5 may be employed. Further, periodic displacement is not necessarily required. 
     FIG. 5a shows an example in which the slot hole opening width l 1  of a front plate 4a is smaller than the slot hole opening width l 2  of a rear plate 4b. Thus, for example, assuming that l 1  is 200 m and l 2  is 240 μm, the slot holes have a difference of 20 μm on either side. In this case, the percentage of electron beam transmission is influenced only by the slot holes of the front plate 4a. 
     FIG. 5b shows an example in which a rear plate 4b is inverted from the state illustrated in FIG. 5a. Again, the percentage of electron beam transmission is influenced only by the slot holes of the front plate 4a. 
     FIG. 5c shows an example in which the slot hole opening width of a rear plate 4b is smaller than the slot hole opening width of a front plate 4a. In this case, the percentage of electron beam transmission is influenced by the slot holes of the rear plate 4b. 
     The difference in width on each side of the slot holes is generally 5 to 50 μm. The difference is variable because the array pitch of the slot holes and the slot hole opening width vary with the resolution required for the shadow mask. 
     A superior shadow mask (from a practical standpoint) can be obtained by making the opening width of slot holes on one side of the mask smaller than the opening width of slot holes provided on the other side of the mask. Unlike the prior art, the percentage of electron beam transmission is not adversely influenced by distortion of the larger slot holes. 
     In FIGS. 6a to 6d, the plate members have different slot hole opening widths. The thicknesses of the plate members are also taken into consideration to further reduce influence on the percentage of electron beam transmission. In particular, adverse influence on electron beam transmission can be reduced by the thickness of the plate member having the smaller slot opening width. 
     In general, slot holes can be more accurately etched through the thin plate member. Since transmission irregularities are caused by incorrect hole configuration, such irregularities are reduced when a thin plate member is provided with thin slot hole openings. Transmission irregularities caused by the thick plate member do not exert an adverse influence. 
     A shadow mask formed by joining plate members with thicknesses of 0.2 mm and 0.3 mm was found to cause less transmission irregularity than a shadow mask formed by joining two plate members each of which was 0.25 mm thick. 
     The embodiments shown in FIGS. 5a to 5c and 6a to 6d are illustrated on the assumption that electron beams are transmitted upwardly. 
     It is understood that the bridges of one plate need not equally divide the slot holes of the other plate. Further, alternative joining methods for joining the plates may be used. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.