Color display tube comprising an internal magnetic shield

#The invention relates to a color display tube with an envelope comprising

#a neck portion, a funnel-shaped portion and a window portion;

#an electron gun system arranged in the neck portion;

#In the drawings:

#FIG. 1a is a cross-sectional view of a color display tube;

#FIG. 1b diagrammatically shows the origin of mislanding;

#FIG. 1a is a horizontal, cross-sectional view of a display tube comprising a glass envelope consisting of a display window 1, a funnel-shaped portion or cone 2 and a neck 3. The neck 3 accommodates an electrode system 4 comprising three electron guns for generating three electron beams 5, 6 and 7. The electron beams are generated in one plane (in this case, the plane of the drawing) and are directed towards a display screen 8 provided on the inside of the display window 1, which display screen is composed of a large number of discrete phosphor elements luminescing in red, green and blue and coated with an aluminium layer. On their way to the display screen 8, electron beams 5, 6 and 7 are deflected across the display screen 8 by means of a deflection coil system 9 which is coaxially arranged about the tube axis, and the electron beams pass through a color selection electrode 10 composed of, for example, a metal sheet having openings 11. The three electron beams 5, 6 and 7 pass through the openings 11 at a small angle with each other and, consequently, each electron beam impinges only on phosphor elements of one color. The tube further comprises a high-voltage contact (anode contact) 14 arranged in the wall of the tube. The color selection electrode 10 is suspended opposite the display screen 8. A funnel-shaped magnetic shield 16 is arranged inside the glass envelope.

#In a color display tube, electrons pass through holes of a shadow mask and are incident on a phosphor. The position of the phosphors is optimal for one orientation of the tube in one specific terrestrial field (location on earth). For another orientation or terrestrial field, the electron is incident on a different spot on the shadow mask. This leads to a distortion of the image, which is particularly disadvantageous for color monitors. In addition, the electron reaches the mask at a different angle. If the electron passes through a hole, the influence of a field at right angles to its direction of movement, causes mislanding M of the electron on the screen. See FIG. 1b. If the degree of mislanding is too large, it is even possible that a wrong phosphor is reached, resulting in color errors.

#Hereafter, a calculation is given of the degree of mislanding when the terrestrial field is not compensated for at all. In a homogeneous field of size B, the electron describes a path of radius R which is given by R=mv0/eB, where m, v0 and e are, respectively, mass, velocity and charge of the electron. At a terrestrial field of 5*10−5T (˜V½ gauss), a velocity of the electrons v0 of 108 m/sec and e/m=1.076×1011 C/kg, the value of R=11.4 m. A simple geometric consideration results in the following value for the mislanding M: M ≈ L 1 · L 2 · eB 2 ⁢ mv 0