Abstract:
A color cathode-ray tube (CRT) having an evacuated envelope with an electron gun therein for generating at least one electron beam is disclosed. The envelope further includes a faceplate panel having a luminescent screen with phosphor lines on an interior surface thereof. A tension focus mask, having a plurality of spaced-apart first electrodes, is located adjacent to an effective picture area of the screen. The plurality of spaced-apart first electrodes has a screen-facing side having a predetermined width and a relatively wider electron-gun-facing side. Each side forming sharp corner edges extending along the length of each first electrodes. A substantially continuous insulating material is deposited on the screen-facing side and on the corners of the first electrodes to shield the sharp corner edges of the first electrodes. A plurality of second electrodes are oriented substantially perpendicular to the plurality of first electrodes and are bonded thereto by the insulating material layer.

Description:
[0001]    This invention relates to a cathode-ray tube (CRT) and, more particularly to a color CRT including a tension focus mask.  
         BACKGROUND OF THE INVENTION  
         [0002]    A color cathode-ray tube (CRT) typically includes an electron gun, an aperture mask-frame assembly, and a screen. The aperture mask-frame assembly is interposed between the electron gun and the screen. The screen is located on an inner surface of a faceplate of the CRT tube. The screen has an array of three different color-emitting phosphors (e. g., green, blue and red) formed thereon. The aperture mask functions to collimate the electron beams generated in the electron gun toward appropriate color-emitting phosphors on the screen of the CRT.  
           [0003]    The aperture mask may be a focus mask. Focus masks typically comprise two sets of electrodes that are arranged orthogonal to each other, to form an array of openings. Different voltages are applied to the two sets of electrodes so as to create quadrupole focusing lenses in each opening of the mask, which are used to direct and focus the electron beams toward the appropriate color-emitting phosphors on the screen of the CRT tube.  
           [0004]    One type of focus mask is a tension focus mask, wherein at least one of the sets of electrodes is under tension. Typically, for tension focus masks, the vertical electrodes are held in tension by the mask frame. The other set of electrodes is horizontal and overlays the vertical electrodes, which are typically strands. An etching process used on a flat sheet of metal commonly forms the strands. Such an etching process forms sharp corner edges along the length of the strands.  
           [0005]    The two sets of electrodes overlap at a series of points known as junctions. At these junctions the individual elements of one set of electrodes are separated from the individual elements of the other set by an insulating material. When the different voltages are applied between the two sets of strands of the mask, to create the quadrupole focusing lenses in the openings thereof, surface flashover may occur at one or more of the junctions. Surface flashover is a breakdown process that may take place on or near the surface of the insulating material separating the two sets of strands and may lead to arcing between the strands at one or more places on the focus mask. Since the overlying wires are electrically connected to one another, all of the energy stored in the capacitance of the entire focus mask is available to arc. This stored energy may be sufficient to cause local melting of the strands and/or the insulating material and may result in an electrical short leading to the subsequent failure of the focus mask. Surface flashover has a greater risk of occurring in locations in which one of the electrodes has a sharp edge, since the local electric field can be higher at these locations.  
           [0006]    Additionally, during operation of the CRT tube, electron scattering may occur along sharp edges of the mask strands. Electron scattering along strand edges of the focus mask is undesirable because some of these electrons may strike the wrong color element, degrading the color purity of the CRT tube.  
           [0007]    Thus, a need exists for suitable tension focus masks that overcome the above-mentioned drawbacks.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention relates to a color cathode-ray tube (CRT) having an evacuated envelope with an electron gun therein for generating at least one electron beam. The envelope further includes a faceplate panel having a luminescent screen with phosphor lines on an interior surface thereof. A tension focus mask, having a plurality of spaced-apart first conductive electrodes, is located generally parallel to an effective picture area of the screen. The plurality of spaced-apart first conductive electrodes, otherwise known as strands, have a screen-facing side and electron-gun facing side. Each side of the strands have sharp corner edges extending along the length of the strands. A plurality of second conductive electrodes are oriented substantially perpendicular to the plurality of strands and separated by an insulating material deposited on the screen-facing side and corners of the strands to shield the sharp edges of the strands from the second conductive electrodes. In doing so, the present invention reduces the risk of surface flashover that would occur when sharp corners are formed using prior art etching processes.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention will now be described in greater detail, with relation to the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 is a plan view, partly in axial section, of a color cathode-ray tube (CRT) including a uniaxial tension focus mask-frame assembly embodying the present invention;  
         [0011]    [0011]FIG. 2 is a plan view of the uniaxial tension focus mask-frame assembly of FIG. 1;  
         [0012]    [0012]FIG. 3 is a side view of the mask frame-assembly taken along line  3 - 3  of FIG. 2;  
         [0013]    [0013]FIG. 4 is an enlarged section of the uniaxial tension focus mask shown within the circle  4  of FIG. 2; and  
         [0014]    [0014]FIG. 5 is an enlarged view of a portion of the uniaxial tension focus mask taken along lines  5 - 5  of FIG. 4. 
     
    
     DETAILED DESCRIPTION  
       [0015]    [0015]FIG. 1 shows a color cathode-ray tube (CRT)  10  having a glass envelope  11  comprising a faceplate panel  12  and a tubular neck  14  connected by a funnel  15 . The funnel  15  has an internal conductive coating (not shown) that is in contact with, and extends from, a first anode button  16  to the neck  14 . A second anode button  17 , located opposite the first anode button  16 , is contacted by a second conductive coating (not shown).  
         [0016]    The faceplate panel  12  comprises a viewing faceplate  18  and a peripheral flange or sidewall  20  that is sealed to the funnel  15  by a glass fort  21 . A three-color luminescent phosphor screen  22  is carried by the inner surface of the viewing faceplate  18 . The screen  22  is a line screen (not shown) that includes a multiplicity of screen elements comprised of red-emitting, green-emitting, and blue-emitting phosphor lines respectively, arranged in triads, each triad including a phosphor line of each of the three colors. Preferably, a light-absorbing matrix (not shown) separates the phosphor lines. A thin conductive layer (not shown), preferably formed of aluminum, overlies the screen  22  and provides a means for applying a uniform first anode potential to the screen  22  as well as for reflecting light, emitted from the phosphor elements, through the viewing faceplate  18 .  
         [0017]    A multi-apertured color selection electrode, or uniaxial tension focus mask  25 , is removably mounted, by conventional means, within the faceplate panel  12 , in predetermined spaced relation to the screen  22 . An electron gun  26 , shown schematically by the dashed lines in FIG. 1, is centrally mounted within the neck  14  to generate and direct three inline electron beams  28 , a center and two side or outer beams, along convergent paths through the uniaxial tension focus mask  25  to the screen  22 . The inline direction of the center of the beams  28  is approximately normal to the plane of the paper.  
         [0018]    The CRT of FIG. 1 is designed to be used with an external magnetic deflection yoke, such as the yoke  30 , shown in the neighborhood of the funnel-neck junction. When activated, the yoke  30  subjects the three electron beams  28  to magnetic fields that cause the beams to scan a horizontal and vertical rectangular raster across the screen  22 .  
         [0019]    As shown in FIG. 2, the uniaxial tension focus mask  25  (shown schematically by the dashed lines in FIG. 2) includes two horizontal sides  32 ,  34  and two vertical sides  36 ,  38 . The two horizontal sides  32 ,  34  of the uniaxial tension focus mask  25  are parallel with the central major axis, X, of the CRT while the two vertical sides  36 ,  38  are parallel with the central minor axis, Y, of the CRT. A frame  45 , for the tension focus mask  25 , includes four major members, two horizontal members  46 ,  48  to which the horizontal sides  32 ,  34  of the tension focus mask  25  are attached and two vertical members  50 ,  52  to which the second metal electrodes  60  are attached. Members  46 ,  48  are substantially parallel to the major axis, X, and each other. The curvature of members  46 ,  48  may be shaped to substantially match the specific curvature of the CRT screen (see FIG. 3). The horizontal sides  32 ,  34  of the uniaxial tension focus mask  25  are welded to the two members  46 ,  48 , which provide the necessary tension to the mask. The uniaxial tension focus mask  25  includes an apertured portion that overlies an effective picture area of the screen  22 . Referring to FIG. 4, the uniaxial tension focus mask  25  includes a plurality of first metal electrodes, or conductive strands  40 , separated by spaced slots  42  that parallel the minor axis, Y, of the CRT and the phosphor lines of the screen  22 . In the preferred embodiment slots  42  each have a width within a range of about 0.1 mm to about 0.5 mm (4-20 mils). For a color CRT having a diagonal dimension of 68 cm, the strands  40  have widths in a range of about 0.2 mm to about 0.5 mm (8-20 mils) and slot  42  widths of about 0.2 mm to about 0.5 mm (8-20 mils). In a color CRT having a diagonal dimension of 68 cm (27 V), there are about 800 strands  40 . Each of the slots  42  extends from one horizontal side  32  of the mask to the other horizontal side  34  thereof (shown in FIG. 3).  
         [0020]    Strands  40 , depicted in FIG. 5, are formed by an etching process performed on a flat metal plate. The etching process involves a sequence of operations suitable to form slots  42 . With the etching, new regions of the strands  40  are exposed. The preferred outcome is illustrated in FIG. 5 as strand  40  having a generally rectangular cross-section defined by screen-facing side  72 , electron-gun facing side  70  and side walls  75 . The etched strands  40  have associated with them a pair of relatively sharp edges at corners  43  and  44  being the top and bottom sharp edge portions shown in the embodiment of FIG. 5. As shown in FIG. 5, the edge of corners  43  at the intersection of the screen-facing side  72  and side walls  75  form corners with a relatively less sharp edge than the edges formed at corners  44 . The shaper edges formed at corners  44  are positioned as far as possible from the cross-wires  60  to reduce the probability of surface flashover or arcing between the electrodes at one or more junctions. The arcing may be sufficient to cause local melting of the electrodes, destruction of the insulator, or both and may result in electrical short, leading to the subsequent failure of the focus mask. Further, the corners  43  closest to the cross-wires  60  are typically coated with an adhesive insulating material  62 , reducing triple-point electron emission from this region and thereby also reducing the incidence of surface flashover.  
         [0021]    According to the preferred embodiment, the strands  40  each have a transverse dimension, or width, of about 0.1 mm to about 0.5 mm (4-20 mils) for both the screen-facing side  72  and the electron-gun-facing side  70 , with the screen-facing side  72  having a width about 0.025 to about 0.05 mm (1-4 mils) smaller than the width of the electron-gun-facing side  70 . Although the strands  40  may be inverted so that the wider side of the strands  40  is closest to the second conductive electrodes  60 , the above prescribed dimension of the strands  40  allows for less scatter of the electron beam  28 , thereby providing a measurable improvement in the color purity of the CRT. For example, in a conventional color CRT, the red x-coordinate is about 0.633. The red x-coordinate measured for a tension focus mask  25  incorporating the geometry described above, and shown in FIG. 5, is about 0.627, as compared with 0.613 for tension focus masks  25 , where the screen-facing side surface  72  is wider than the electron-gun-facing side  70 . A further advantage in having a narrower electron-gun-facing side  70  immediately adjacent the second conductive electrodes  60  is that the adhesive material  62  may be applied to the screen-facing side  72  and allowed to accumulate along the side walls  75  to corners  44  so as to shield the corners of the strands  40  thereby reducing the potential for surface flashover.  
         [0022]    With reference to FIGS. 4 and 5, a plurality of second conductive electrodes  60 , each having a diameter of about 0.025 mm (1 mil), are disposed substantially perpendicular to the strands  40  and are bonded to the adhesive material  62  to electrically isolate the second conductive electrodes  60  from the strands  40 . The vertical spacing, or pitch, between adjacent second conductive electrodes  60  is about 0.33 mm (13 mils) for a color CRT  10  having a diagonal dimension of 68 cm (27 V). The uniaxial tension focus mask  25 , described herein, provides a mask transmission, at the center of the screen, of about 40-45%, and requires that the second anode, or focusing voltage, δV, applied to the second metal electrodes  60 , differs from the first anode voltage applied to the strands  40  by less than about 1 kV, for a first anode voltage of about 30 kV. The combination of the strands  40  and the second conductive electrodes  60  along with the different electric potentials applied thereto function to create the quadrupole fields, which converge the electron beams  28  onto the color-emitting phosphors on the screen  22  of the CRT  10 .  
         [0023]    Although a single application of the insulative adhesive material  62  may be applied to the strands  40 , FIG. 5 illustrates the result of a multiple process for applying the adhesive material  62 . Such process includes applying a first coating of the insulative adhesive material  62 , e.g., by spraying, onto the screen-facing side  72  of the strands  40 . The strands  40 , in this example, are formed of either creep resistant steel or a low expansion alloy, such as INVAR™. The strands  40  each have a transverse dimension, or width, such that the screen-facing side  72  maintains a width about 0.025 to about 0.05 mm (1-4 mils) smaller than the width of the electron gun facing side  70 . The first coating of the insulative adhesive material  62  typically has a thickness of about 0.05 mm to about 0.1 mm (2-4 mils).  
         [0024]    After the first coating of the insulative adhesive material  62  is hardened, a second coating of the insulative adhesive material  66  is applied over the first coating of the insulative adhesive material  62 . The second coating of the insulative adhesive material  66  may optionally have a different composition from that of the first coating. The second coating of the insulative adhesive material  66  typically has a thickness of about 0.0025 mm to about 0.05 mm (0.1 to 2 mils).  
         [0025]    Thereafter, the second metal electrodes  60  are applied to the frame  45 , over the second coating of the insulative adhesive material  66 , such that the second metal electrodes  60  are substantially perpendicular to the strands  40 . The second metal electrodes  60  are applied using a winding fixture (not shown) that accurately maintains a desired spacing of, for example, about 0.33 mm (13 mils) between adjacent metal electrodes for a color CRT  10  having a diagonal dimension of about 68 cm (27 V).  
         [0026]    The assembly is heated to a temperature of about 460° C. for about 30 minutes to cure the second coating of the insulative adhesive material  66 , thereby bonding the crosswires to the second coating of the insulative adhesive material  66 . Following curing, electrical connections are made to the strands  40  and second metal electrodes  60 , and the tension focus mask  25  is inserted into a tube envelope.