Abstract:
The invention is a color cathode-ray tube having a rectangular front face ( 1 ) connected to a funnel shaped rear part ( 2 ), a neck ( 2   a ) connected to the rear part and having an electron gun ( 6 ) therein, the gun is used to generate beams intended to scan the front face under the influence of a deflection device ( 10 ) placed on the neck of the tube. The deflection device comprising correction magnets ( 20 ) placed on the periphery of the device in locations closest to the front face and a magnetic screen ( 30 ) within the tube having an opening ( 31 ) for passage of the electron beams, wherein the opening has notches ( 34 ) in its periphery. The notches are arranged so as to face at least one pair of correction magnets ( 20 ).

Description:
FIELD OF THE INVENTION  
         [0001]    The subject of the present invention is a color cathode-ray tube having a color display screen and an internal magnetic screen suitable for allowing proper correction of the geometry of the image formed on the display screen of the tube.  
         BACKGROUND OF THE INVENTION  
         [0002]    A color cathode-ray tube is composed of a glass envelope comprising an approximately rectangular front face joined to a funnel-shaped rear part terminating in a cylindrical neck. An electron gun is located in the neck and generates electron beams intended to form a color image on a luminescent screen deposited on the internal surface of the front panel. The electron beams are brought into correspondence with the phosphors on the luminescent screen by means of a perforated metal mask, called a color selection mask. The mask is attached to an approximately rectangular rigid frame with two pairs of opposed sides, one pair of short sides and one 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 paths of the electron beams so that the angles of incidence of the beams on the selection mask are not significantly modified by these components; otherwise, the points of impact of the beams on the screen will be shifted and illuminate phosphors of a color other than that desired and cause a defect known as a registration error.  
           [0003]    Cathode-ray tubes (CRTs) are all the more sensitive to these problems of registration errors with increasing angle of deflection of the electron beams. With the current trend in CRT industry to reduce the depth of the tube in relation to the size of the screen, the propensity for external magnetic fields to cause registration errors in the screen peripheral areas is greater because the required electron beam deflection angle becomes greater in such tubes.  
           [0004]    Additionally, the CRT market is seeing a greater interest in tubes having flat faceplate panel. Likewise product with such a geometry will also show an enhanced propensity for external magnetic fields to cause registration errors in the screen peripheral because the electron beam designated for the peripheral area will have to travel a greater distance to such screen areas than in comparable spherical tubes. To correct geometrical distortions of the screen images, one may find it necessary to utilize magnets which are strategically placed outside the tube.  
           [0005]    In short, because the angle of deflection of the electron beams of these trendier tubes is greater than in comparable spherical panel tubes with the normal depth, the path travelled by the electron beams emanating from the gun is comparatively longer. This makes them more sensitive to the influence of the Earth&#39;s magnetic field.  
           [0006]    Three directions of the Earth&#39;s field must be taken into consideration. The axial (north/south) field is manifested parallel to the longitudinal axis of the tube, the lateral (east/west) field along the horizontal axis and the vertical field along the vertical axis.  
           [0007]    The vertical field does not pose a major problem for registration as it is virtually constant within broad geographical areas. On the other hand, it is necessary to screen the inside of the tube against the axial and lateral fields since their influence depends on the location of the tube and, on the position in which it lies.  
           [0008]    The magnetic screens or internal magnetic shields 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. Further, the apertures, their shapes and their numbers are adapted to compensate and reduce the influences of the Earth&#39;s field on electron beam misregister. In its front part, the internal magnetic screen is closed by the color selection mask. In its rear part it has an aperture so as to allow the electron beams to pass through.  
           [0009]    It has been found that tubes of reduced depth, that is to say tubes for which the angle of horizontal deflection of the electron beams is greater than 108°, have a very high sensitivity to the Earth&#39;s magnetic field and that it is necessary for the magnetic screen to extend towards the rear of the tube as close as possible to the deflection device. However, it is no longer possible in this case to place magnets for correcting the geometry of the image in front of the deflection device because the field lines of the magnets will need to be position in the vicinity of the internal magnetic screen and with such placement the magnetic will not have an influence on the electron beams.  
           [0010]    Therefore, there is a need for a color cathode-ray tube having improved magnetic screening capabilities.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is a color cathode-ray tube comprising a rectangular front face, a funnel-shaped rear part, a neck in which an electron gun is placed in order to generate beams intended to scan the front face under the influence of a deflection device placed on the neck of the tube, the device comprising correction magnets placed on the side of its peripheral part closest to the front face, the tube furthermore comprising an internal magnetic screen, the rear part of which has an opening for passage of the electron beams. The rear part of the magnetic screen has, on its periphery, notches arranged so as to face at least one pair of correction magnets. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The invention will be more clearly understood with the aid of the following description and from the following figures.  
         [0013]    [0013]FIG. 1 shows, in section, a cathode-ray tube with its various operating members.  
         [0014]    [0014]FIGS. 2A and 2B show an example of a side and rear view of a magnetic screen according to the prior art.  
         [0015]    [0015]FIG. 3A to  3 D illustrate the influence of the Earth&#39;s magnetic field on the points of impingement of the electron beams on the screen of the tube for two known types of magnetic screen.  
         [0016]    [0016]FIGS. 4A and 4B illustrate one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    A cathode-ray tube, as illustrated in section in FIG. 1, comprises a glass envelope  3  composed of a front face  1  and a funnel-shaped rear part  2  terminating in a cylindrical neck  2   a . Deposited on the internal surface of the front face  1  is the screen  4  of luminescent materials, which is intended to reproduce an image under the impact of the electron beams  7 B,  7 G,  7 R emanating from the electron gun  6  fitted into the neck  2   a  of the tube. A shadow mask  5  perforated by holes, allows each electron beam to illuminate only the phosphors of the color corresponding to it. The mask  5  is held inside the tube by means of a metal frame  9 . A deflection device  10 , generally composed of a pair of horizontal deflection coils and a pair of vertical deflection coils, is placed on the flared rear part of the tube; it furthermore includes, close to its periphery, on the screen side, correction magnets  20 . A magnetic screen  14  is placed inside the tube and is fixed to the frame  9  supporting the mask  5 .  
         [0018]    [0018]FIGS. 2A and 2B illustrate side and rear views of a prior art internal magnetic screen  14 . The magnetic screen  14  is produced by stamping a metal sheet so as to give it a shape  21  similar to the shape of the rear part  2  of the tube. The magnetic screen  14  has a front aperture  23  intended to be attached to the frame  9 , for example, by clipping it in through apertures  26 . The rear aperture  22  is of approximately rectangular shape, the longer sides extending in the horizontal direction X. This configuration is favourable to screening against the axial component of the field but unfavourable to screening against the lateral component of the field; however, the situation is improved by virtue of nicks  25  bringing the screening to a satisfactory level. These nicks  25 , of small width, make it possible to increase the relative magnetic resistance in the horizontal direction and tend to favor screening against the lateral component of the Earth&#39;s field.  
         [0019]    [0019]FIGS. 3A to  3 D show the influence of the introduction of nicks  25  in the magnetic screen  14  on the forces due to the influence of the Earth&#39;s magnetic field which are exerted on the electron beams scanning the screen  4  of the tube. FIG. 3A illustrates an example of a magnetic screen  14  seen from the rear (the Earth&#39;s magnetic field lines being represented by arrows) and FIG. 3B illustrates the displacements of the points of impingement of the electron beams on the screen  4  of the tube which are due to the lateral component of the field.  
         [0020]    [0020]FIG. 3C shows the same magnetic screen  14  provided with nicks  25  at 6 o&#39;clock and 12 o&#39;clock; these nicks  25  increase the magnetic resistance of the magnetic screen  14  in the horizontal direction, causing the points of impingement of the electron beams to be shifted in the vertical direction, as illustrated in FIG. 3D.  
         [0021]    Reducing the depth of cathode-ray tubes requires the angle of deflection of the electron beams to be increased in order to scan the entire screen  4  of the tube. Decreases in the depth are such that the angles of deflection become greater than 108°, and can possibly reach 130°. Such tubes are very sensitive to the Earth&#39;s magnetic field and require the presence of a magnetic screen  14  extending towards the rear part  2  to be as close as possible to the deflection device  10 . Moreover, when the front face  1  of the tube is also flat, it is necessary to correct the geometry of the image formed on the screen  4  by means of magnets  20  placed in front of the deflection device  10 . It will be noted that the force of the magnets  20 , necessary to correct the geometry of the image, is very much greater for this type of tube than for tubes having a small angle of deflection (i.e., less than 108°); however, perfect correction cannot be achieved without compromising other parameters of the deflection device. It has been found that this is due to the magnetic coupling between the correction magnets  20  and the magnetic screen  14 , which must be as close as possible to the deflection device in order to prevent the Earth&#39;s field influencing the electron beams.  
         [0022]    [0022]FIGS. 4A and 4B show rear and in perspective views of a magnetic screen  30  according to the invention is equipped. FIG. 4A shows the location of two correction magnets  20  placed along the vertical axis of the tube, on each side of the horizontal plane XZ. The magnetic screen  30  according to the invention comprises, in a known manner, a surface  32  for connecting with the frame  9  supporting the mask  5 , a dished surface  33  matching the shape of the internal surface of the flared rear part  2  of the tube, and a rear aperture  31  intended for passage of the electron beams.  
         [0023]    The correction magnets  20  in this case are intended to correct the north/south geometry of the image formed on the screen  4  of the tube and are placed on the vertical axis at 6 o&#39;clock and 12 o&#39;clock. The aperture  31  lies on the main axis Z as close as possible to the deflection device  10  so as to obtain the best possible screening with respect to the Earth&#39;s magnetic field.  
         [0024]    The aperture  31  has an approximately rectangular shape, with two long sides of length R h , extending in the horizontal direction and two short sides, of length R v , extending in the vertical direction.  
         [0025]    Two notches  34  are made on the periphery of the rear aperture  22  of the screen  4  and face the correction magnets  20  when the tube is fitted with its electron-beam deflection device  10 . The depth N v  of these notches  34  is sized such that it does not appreciably modify the magnetic resistance of the horizontal parts of the screen  30  and does not reduce the screening against the Earth&#39;s field. On the other hand, the width N h  of the notch  34  must be sized to allow the correction magnets  20  to have a positive influence to maintain the geometry of the image, without being so great so as to impair the screening ability of the magnetic screen  30 .  
         [0026]    It has been found that the best compromise between the screening effect of the magnetic screen  30  and the effect of the magnets  20  on the geometry of the image is obtained by preventing magnetic coupling between the magnets  20  and the screen  30  and, to do this, the depth N v  of the notches  34  must be less than the length N h  of the notch  34 , without the length N h  exceeding 60% of the length R h  of that side of the rear aperture  31  in which the notch  34  is located.  
         [0027]    Because the notches  34  must face the correction magnets  20 , they may advantageously be placed in all directions in which the correction magnets  20  may be placed, that is to say in the 6 o&#39;clock-12 o&#39;clock vertical direction, in the 3 o&#39;clock-9 o&#39;clock horizontal direction or along the diagonals of the image.  
         [0028]    Likewise, although intended for tubes whose horizontal deflection angle is greater than 108°, the invention may advantageously be used for tubes whose deflection angle is smaller, for example when these tubes are of the high-resolution type and have a high sensitivity to external magnetic fields.