Patent Publication Number: US-6911769-B2

Title: Lateral magnetic shielding for color CRT

Description:
BACKGROUND OF THE INVENTION 
   The subject of the present invention is a color cathode-ray tube having an internal magnetic screen, and more particularly a cathode-ray tube whose color selection mask is held under tension by a support frame. 
   A color cathode-ray tube is composed of a glass envelope comprising an approximately rectangular front face joined to a funnel-shaped part terminating in a cylindrical neck. An electron gun is placed in the neck and generates electron beams intended to form a color image on a luminescent screen placed on the internal surface of the front panel. The electron beams are kept in correspondence with the phosphors of the luminescent screen by means of a perforated metal mask called the color selection mask. The mask is attached to an approximately rectangular rigid frame with two pairs of opposed sides—a pair of short sides and a pair of long sides. An internal magnetic screen is generally placed in the funnel-shaped part of the envelope and is joined to the rear of the frame. The primary objective of this magnetic screen is to reduce the influence of the components of the Earth&#39;s magnetic field on the trajectories of the electron beams so that the angles of incidence of the said beams on the selection mask are not significantly modified by these components; if this were not so, the points of impact of the beams on the screen would be moved and illuminate phosphors of a color other than that desired. 
   SUMMARY OF THE INVENTION 
   The magnetic screens of the prior art are designed to have a shape matching as closely as possible the internal surface of the funnel-shaped part of the envelope, with openings. The shape and number of openings are specifically designed, for particular types of tubes, to counteract the Earth&#39;s field prevent the beams from illuminating the incorrect phosphor. 
   However, it was apparent that tubes incorporating a mask held under tension between two opposed sides of a frame had a greater sensitivity to the Earth&#39;s magnetic field and that the magnetic screens of the prior art could not provide a solution to this sensitivity problem. 
   This sensitivity seems to be problematic in tubes whose mask is held under tension between two opposed sides of the frame and whose other two sides have a free edge extending towards the screen, the end of which is placed beneath the surface of the mask. This frame structure is generally used because this structure makes it possible, among other things, to lighten the frame/mask assembly and to reduce its material cost, since it is not necessary for the heights of the long and short sides to be identical in order for the frame to fulfil its mechanical functions. 
   The tube according to the invention does not have this sensitivity to the Earth&#39;s magnetic field and to achieve this it comprises a glass envelope having a front face inside which is a luminescent screen, the said front face being sealed to a funnel-shaped part, a color selection mask placed so as to face the luminescent screen and held under tension by an approximately rectangular frame having a pair of long sides and a pair of short sides, the mask being fixed under tension to a first pair of opposed sides, the edges of the sides of the frame which constitute the second pair extending towards the screen and lying beneath the surface of the mask, and a main magnetic screen fixed to the frame and extending to the rear of the funnel-shaped part of the tube&#39;s envelope,wherein the tube includes complementary means for making a screen against the Earth&#39;s magnetic field, these means being placed so as to cover, at least partially, the open space between the edges of the second pair of sides and the surface of the mask. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and its advantages will be more clearly understood from the description below and from the drawings among which: 
       FIG. 1  shows, in section, a cathode-ray tube according to the prior art; 
       FIG. 2  shows a magnetic screen according to the prior art; 
       FIG. 3  shows, in perspective, a frame/tensioned-mask assembly; 
       FIG. 4  shows, in perspective, a first embodiment of the invention; and 
       FIG. 5  illustrates a second embodiment of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a cathode-ray tube  10  comprising a funnel-shaped part  11  and an approximately rectangular front face  12 , the two parts being joined together by means of a glass frit seal  13 . A luminescent screen  15  is deposited on the internal surface of the front face  12 . The luminescent screen  15  is composed of three arrays of luminescent materials emitting in the three primary colors—red, green and blue—when they are excited by the three electron beams output by the electron gun  17  placed in the cylindrical neck  18 . A color selection electrode  16  is held at a precise distance from the screen  15  by a frame  21  whose cross section is generally in the form of an L. The electrode  16  keeps the three electron beams output by the gun  17  in correspondence with the three arrays of luminescent materials. The electrode  16  includes a peripheral skirt  14  to be inserted inside the edges of the frame  21  extending towards the screen  15 . 
   Because electrons are charged particles, the electron beams output by the gun  17  are liable to be deflected by the Earth&#39;s magnetic field. As a result, depending on the orientation of the tube  10  with respect to the Earth&#39;s magnetic field, the electron beams may illuminate phosphors of an array not corresponding to them, causing discoloration of the image formed on the screen of the tube. These tubes therefore have problems called register problems. The effects of the Earth&#39;s magnetic field are conventionally minimized by a magnetic screen  22  placed inside the tube along the path of the electron beams, that is to say in the funnel-shaped part  11 . This magnetic screen  22 , as illustrated for example in  FIG. 2 , has a rear opening  23  and a front opening  24  to be attached to the perimeter of the frame  21  by soldering or welding or by clips. 
   The color selection electrode  16 , the frame  21  and the magnetic screen  22 , all manufactured from ferromagnetic alloys, have hitherto provided protection against the Earth&#39;s magnetic field. However, it seems that the latest generations of tubes  10  with a flat front face  12 , in which the color selection electrode  16  is tensioned along a direction, for example along the vertical direction Y, are very sensitive to the Earth&#39;s magnetic field. 
   The frame/mask structure of these tubes is illustrated in FIG.  3 . 
   The frame  21 , of approximately rectangular shape, comprises a pair of vertical short sides  40  and a pair of horizontal long sides  30 . The sides of the frame  21  have an L-shaped cross section and the color selection mask  16  is held under tension by welding or soldering its horizontal edges to first edges  31  of the frame  21  on the long sides  30 . In order to minimize the weight of the frame  21 , the short sides  40  have a smaller height than that of the long sides  30 . As such, second edges  41  of the frame  21  on said short sides  40  which lie below the mask  16  leave a gap  45  in the Z direction perpendicular to the surface of the color selection electrode  16  between the second edges  41 . 
   Within the context of the invention, it has been demonstrated that the register problems of the tubes incorporating this type of frame/mask structure were due to high sensitivity to the horizontal component of the magnetic field. It was discovered that this sensitivity arose from the unprotected gap  45  between the color selection electrode  16  and the second edges  41  of the frame  21 . 
   In order to screen against this horizontal component, two ferromagnetic metal plates  50  are placed for example inside the tube  10 , each along each vertical short side  40 , in the gap  45  so as to cover, preferably completely, the said gap  45 . This embodiment is illustrated in  FIG. 4  in which the metal plates  50  are held in place by being soldered or welded to the vertical short sides  40  of the frame  21 . This arrangement is not limiting as to the metal plates  50 : metal plates  50  may also, advantageously, be placed outside the tube, along the vertical short sides, so as to cover the gap  45 . The plates  50  may be made either of the same material as the main magnetic screen  22  or in any ferromagnetic material. 
   In the alternative embodiment in  FIG. 5 , the magnetic screen  22  comprises at least two lateral flanges  51  extending towards the screen  15  and intended to cover the short sides  40  of the frame  21  and, in particular, the gap  45 . This embodiment is advantageous since it does not require additional pieces to protect against the horizontal component of the Earth&#39;s magnetic field. 
   The table below illustrates the improvements with regard to the register (deviation measured at specific points on the luminescent screen  15  between the phosphor and the point of impact of the electron beam corresponding to it) which are obtained by the lateral magnetic flanges  51  according to the invention, compared with the same frame/mask structure not having these lateral magnetic flanges  51 . 
   
     
       
         
             
             
             
             
             
           
             
                 
             
             
                 
                 
               Register 
                 
                 
             
             
                 
                 
               without 
               Register 
             
             
               Variation 
               Position of 
               the 
               with the 
               Improvement 
             
             
               of the 
               the 
               lateral 
               lateral 
               due to the 
             
             
               magnetic 
               measurement 
               flanges 
               flanges 
               lateral 
             
             
               field 
               point 
               (in mils) 
               (in mils) 
               flanges 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
               Vertical: 
               Corner 
               0.4 
               0.3 
               Slight 
             
             
               380 mG 
             
             
               Vertical: 
               Between 
             
             
               380 mG 
               corner and 
               −0.5 
               −0.4 
               Slight 
             
             
                 
               3 o&#39; clock 
             
             
               Vertical: 
               3 o&#39; clock 
               0 
               0 
               Unchanged 
             
             
               380 mG 
             
             
               Horizontal 
             
             
               along Z 
             
             
               500 mG 
               Corner 
               0.8 
               0.5 
               40% 
             
             
               500 mG 
               Between 
             
             
                 
               corner and 
               1.6 
               0.9 
               43% 
             
             
                 
               o&#39; clock 
             
             
               Horizontal 
             
             
               along X 
             
             
               500 mG 
               Corner 
               1.6 
               0.9 
               43% 
             
             
               500 mG 
               Between 
             
             
                 
               corner and 
               2.3 
               1.2 
               47% 
             
             
                 
               3 o&#39; clock 
             
             
                 
             
          
         
       
     
   
   The 3 o&#39;clock measurement point conventionally corresponds to the middle of the vertical peripheral edge of the luminescent screen. 
   These results are obtained under the following test conditions: 
   firstly, the tube  10  is subjected to a magnetic field variation in the vertical direction of about 380 milligauss; 
   secondly, the tube  10  is subjected to horizontal field variations of 500 mG along the main axis Z of the tube  10  and then along the horizontal axis X so as to check the variations in register when the receiver incorporating the tube  10  has its orientation changed. 
   After each measurement, the tube  10  is degaussed in a manner equivalent to the degaussing undergone by the receiver after each tensioning. 
   The invention provides an appreciable improvement in the performance of the tube  10  against the Earth&#39;s magnetic field, and most of all against the horizontal component, which makes it possible to obtain a suitable register whatever the orientation of the image screen. 
   The invention may also apply in the same way if the color selection electrode  16  is held under tension between the short sides  40  of the frame  21  and there is another gap between the edges  31  of the long sides  30  and the surface of the color selection electrode  16 . In this case, the additional means will have to be placed so as to cover this gap in order to screen against the vertical component of the Earth&#39;s magnetic field.