Color display tube having an external magnetic shield

Color display tube having a display screen with a pattern of phosphor elements. To reduce mislanding errors of the electron beams, particularly in the y-direction, the color display tube is arranged within an external magnetic shield which is constituted by an open arrangement (frame) of bars of a soft-magnetic material. The bars jointly constitute, for example the ribs of a block.

The invention relates to a colour display tube having an envelope and 
comprising means for generating electrons, a transparent display window, a 
display screen having a pattern of phosphor elements on the inner surface 
of the window and means for directing electrons onto the display screen. 
Such a colour display tube may be, for example of the box-shaped type or of 
the type in which the display window is connected by means of a 
funnel-shaped envelope portion having a neck portion accommodating the 
means for generating electrons, a colour selection means arranged opposite 
the display screen and an internal magnetic shield arranged within the 
funnel-shaped portion and having two long side walls parallel to the long 
axis of the display screen, (the x-axis), two short side walls parallel to 
the short axis of the display screen (the y-axis), and a gun-sided open 
end which extends transversely to the longitudinal axis of the display 
tube. 
A colour selection means is herein understood to mean, for example, an 
apertured shadow mask sheet or a wire mask. 
In a (colour) display tube the earth's magnetic field deflects the electron 
paths, which without any measures may be so large that the electrons 
impinge upon the wrong phosphor element (mislanding) and produce a 
discolouration of the picture. Particularly the component of the earth's 
magnetic field in the axial direction of the display tube (referred to as 
the axial field) plays an important role in this respect, which may become 
manifest as a lack of colour or even as a colour impurity in the comers of 
the display screen. 
A known measure of reducing mislandings due to the earth's magnetic field 
is the use of an internal magnetic shield which, together with the 
similarly ferromagnetic shadow mask, partially shields the earth's 
magnetic field. The shape of such a shield roughly follows the contours of 
the envelope of the display tube. This means that the (funnel-shaped) 
shield has two long, trapezoidal sides which are parallel to the long axis 
(the x-axis) of the display screen and two short, trapezoidal sides which 
are parallel to the short axis (the y-axis) of the display screen. 
The short sides of the shield are often provided from the gun-sided 
aperture (17, FIG. 3) with triangular recesses (18, 19, FIG. 3) so as to 
reduce mislanding in the comers due to the axial field. If relatively 
small tubes and a relatively large pitch of the elements of the phosphor 
pattern on the display screen are used, an acceptable result may be 
achieved in this way. However, if larger display tubes and/or a smaller 
pitch of the phosphor elements are used, this type of solution does not 
always ensure a sufficient colour purity. To (further) reduce the 
influence of the earth's magnetic field, an external shield is sometimes 
provided, either or not in combination with an internal shield. For this 
purpose, a metal cover is arranged in the tube with a more or less tight 
fit. See, for example U.S. Pat. No. 4,845,402. 
It is an object of the present invention to provide a shielded colour 
display tube which effectively reduces the detrimental effect of the 
earth's magnetic field on colour purity without an external cover being 
necessary. 
According to the invention, a colour display tube of the type described in 
the opening paragraph is therefore characterized in that it is positioned 
within a volume which is defined by an open arrangement or frame of bars 
of a soft- magnetic material. 
The cross-sections of the bars and the magnetic permeability of the bar 
material may be chosen to be such that a given shielding effect is 
achieved. 
In a simple embodiment the bars, whose ends are interconnected, are 
arranged, for example in such a way that they constitute the ribs of a 
block. Then they may be arranged, for example against the walls of the 
cabinet of a TV set or a monitor. 
In a complicated embodiment they may define a block whose sides are 
constituted by networks or grating of bars. 
When the colour display tube is switched on, it is important to demagnetize 
the arrangement of soft-magnetic bars (preferably by means of an 
alternating field decreasing in strength with respect to time) so as to 
make use of the high anhysteresis permeability. For this purpose, a 
demagnetization coil which can be connected to an energizing circuit may 
be used. 
The larger the volume which is defined by the arrangement of bars, the more 
magnetic flux of the colour display tube can be captured. 
These and other aspects of the invention will be apparent from and 
elucidated with reference to the embodiments described hereinafter.

The display tube shown in a horizontal cross-section in FIG. 1a comprises a 
glass envelope consisting of a display window 1, a cone 2 and a neck 3. 
The neck 3 accommodates an electrode system 4 with three electron guns for 
generating three electron beams 5, 6 and 7. The electron beams are 
generated in one plane (here the plane of the drawing) and are directed 
onto a display screen 8 arranged internally on the display window 1 and 
consisting of a large number of red, green and blue luminescing phosphor 
elements coated with an aluminium backing. On their path to the display 
screen 8, the electron beams 5, 6 and 7 are deflected across the display 
screen 8 by means of a deflection coil system 9 arranged coaxially around 
the tube axis and pass a colour selection electrode 10, here consisting of 
a metal plate with apertures 11. The three electron beams 5, 6 and 7 pass 
the apertures 11 at a small angle and consequently impinge on phosphor 
elements of one colour only. A funnel-shaped magnetic shield 16 is 
arranged within the glass envelope. 
In a colour display tube electrons pass through apertures of a shadow mask 
and impinge upon a phosphor. The position of the phosphors is optimal for 
one tube orientation in one given earth's magnetic field (earth location). 
For a different orientation or earth's magnetic field, the electron 
impinges upon another spot on the shadow mask. This causes a distortion of 
the picture which is particularly detrimental in colour monitors. 
Moreover, the electron reaches the mask at a different angle. If it passes 
through an aperture, it is incident with a given mislanding M on the 
screen under the influence of a field transverse to its direction of 
movement. See FIG. 1b. If this mislanding is too large, the wrong phosphor 
element may even be reached, thus causing colour errors. 
A computation of the extent of mislanding for the case where the earth's 
magnetic field is not compensated for at all will be given hereinafter. In 
a homogeneous field having a size B the electron describes a path having a 
radius R which is given by R=mv.sub.o /eB, with m, v.sub.o and e denoting 
mass, velocity and charge, respectively, of the electron. At an earth's 
magnetic field of 5*10.sup.-5 T(.about.1/2 gauss), an electron velocity 
v.sub.o of 10.sup.8 m/sec and e/m=1.76.times.10.sup.11 C/kg, this results 
in R=11.4 m. A simple geometrical consideration then yields for the 
mislanding M: 
##EQU1## 
in which 1.sub.1 is the distance between the electron source and the 
shadow mask and 1.sub.2 is the distance between the shadow mask and the 
screen. It is important to reduce the mislanding as much as possible 
because this may immediately lead to, for example a greater brightness of 
the tube. If the tube is larger, 1.sub.1 and 1.sub.2 both increase, so 
that the mislanding will become quadratically larger. 
The direction of the disturbing magnetic field in the tube depends on the 
location and orientation of the apparatus. To adapt the magnetization of 
the shield to the field which is present in a given situation, this shield 
is demagnetized by means of a decreasing alternating field whenever the 
tube is switched on. 
The shields necessarily have a gun-sided open end. This means that there 
can be no question of a total shielding. 
The invention is based on the surprising recognition that a considerable 
decrease of mislandings due to the earth's magnetic field can be achieved 
by placing the display tube in an open arrangement (frame) 14 (FIG. 1); 15 
(FIG. 2) of bars of a soft-magnetic material. 
To simplify the explanation, FIG. 2 shows a definition of a system of axes 
in a display tube and of locations on the screen. Here, only the component 
of the earth's magnetic field in the z-direction is considered, which is 
the field known as the axial field. 
The Table states computed values of the mislandings of the electrons in the 
north-east corner (NE) of the display screen for a 66 inch FS tube having 
an acceleration voltage of 27.5 kV. A frame comprising (12) bars having a 
cross-section of 5.times.5 mm and a magnetic permeability of 
1.times.10.sup.6 has been taken as an example. 
Dimensions of the frame: in the x-direction 570 mm, y-direction 430 mm, 
z-direction 240 mm. Dimensions of the mask: 522.times.401 mm, distance 
between the mask and the edge of the internal shield: 180 mm. The Table 
below states the mislandings in the north-east corner of the screen at a 
mask-screen distance of 10 mm, a deflection point-mask distance of 278 mm 
with and without an external frame. 
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field without with 
direction 
field (A/m) 
M.sub.x (.mu.m) 
M.sub.y (.mu.m) 
M.sub.x (.mu.m) 
M.sub.y (.mu.m) 
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lateral 
15 -4.8 -21.1 -2.5 -11.0 
axial 15 -4.5 17.6 -2.3 12.1 
vertical 
35 46.6 6.8 23.6 3.7 
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It will be evident that the mislandings in all directions are approximately 
halved in this example by providing the external frame. 
A demagnetization coil (not shown) is preferably arranged around the 
magnetic frame so as to demagnetize the frame when the tube is switched 
on. 
The bars of the magnetic frame may be arranged efficiently against the 
inner walls of a cabinet accommodating the display tube, like the cabinet 
20 shown in FIG. 4. 
Since the display screen is larger in width than in height, mislandings 
(due to the axial field) in the y-direction are larger than in the 
x-direction. In display tubes in which the phosphors are arranged in 
vertical rows, the y-mislandings are not important. However, the phosphors 
are arranged as dots in high-resolution tubes. Here, mislandings in the 
y-direction are as troublesome as in the x-direction. Since the 
y-mislandings are naturally larger because of the aspect ratio of the 
screen, this should require extra attention in these tubes. This also 
applies to tubes having a display screen with a 9:16 aspect ratio, which 
tubes are more elongate than the conventional display screens having a 3:4 
aspect ratio. 
In these cases the use of an external magnetic frame is thus of particular 
importance. 
A shielding effect will generally be noticeable if the following condition: 
##EQU2## 
is satisfied for a frame having the shape of a block as shown in FIG. 2. 
Here, c is the dimension of the rib of the block parallel to the direction 
of the magnetic field considered (in accordance with the Table) and a and 
b are the dimensions of the two ribs perpendicular thereto. S denotes the 
cross-section of the bars. At a value of the magnetic permeability .mu. 
which is ten times larger than the value indicated above, there will 
generally be saturation of the effect.