Color tube having shield correcting for terrestrial magnetism

The color picture tube is provided with a terrestrial magnetism correcting member contained in the tube and combined with a shadow mask assembly. The correcting member is shaped as a rectangular box made up of a pair of vertical magnetic side plates extending from the opposed sides of the supporting frame of the shadow mask and a second pair of horizontal magnetic plates interconnecting the upper and lower ends of the vertical side plates.

BACKGROUND OF THE INVENTION 
This invention relates to a color picture tube with stripe shaped 
phosphors, and more particularly to a color picture tube wherein a 
terrestrial magnetism compensating or correcting member is contained in 
the envelope of the tube. 
In a conventional color picture tube, due to the effect of terrestrial 
magnetism the electron beams are deviated from their normal paths so that 
they do not land on the correct positions on the fluorescent screen with 
the result that the color purity of the reproduced picture is impaired 
thus degrading the quality of the picture. 
For this reason, in a large size color picture tube, for the purpose of 
preventing terrestrial magnetism from affecting the electron beams, a 
magnetic shielding member is disposed on the outer wall or inside of the 
tube but in most of the small size color picture tubes it has been 
impractical to provide such magnetic shielding member from the standpoint 
of cost and construction. 
The relationship between terrestrial magnetism and the landing of the 
electron beam on the fluorescent screen will first be considered. 
The horizontal intensity of the terrestrial magnetism is the largest at or 
near the equator but it decreases gradually toward the north and south 
magnetic poles of the earth, while the relation becomes contrary as to the 
vertical intensity of the terrestrial magnetism. For example, the vertical 
component of the terrestrial magnetism which greatly affects the color 
picture tube with stripe shaped phosphors varies from 0 to 0.7 gauss 
between the equator and the poles. The relationship between the vertical 
component of the terrestrial magnetism and the beam landing is shown in 
FIG. 1 in which the outer frame represents the size of the picture. In 
this example, the tube has a size of 12 inches. The data represents the 
error at portions where the data are written, and the arrows show the 
direction in which error occurs, the horizontal direction in this example. 
Taking 0.35 gauss as an example, which represents the average value of the 
vertical component in Japan, and taking as a reference a case wherein the 
effect of the horizontal terrestrial magnetism is zero (0 gauss) the 
landing error of various portions of the picture is one half of that of 
FIG. 1 as shown by FIG. 2. Where the landing error at the center of the 
picture is adjusted to be zero by a purity adjusting correction magnet 
mounted on the outside of the neck portion of the envelope in a manner 
well known in the art, the landing error can be reduced by 28.5 microns 
throughout the picture, as shown in FIG. 3. 
However, as can be noted from FIG. 3, in a color picture tube not provided 
with terrestrial magnetism compensating means, although it is possible to 
reduce to zero the landing error at the center of the picture by the 
purity adjusting correction magnet, it is impossible to reduce to zero the 
landing error on both sides of the center. This is due to the fact that 
the length of travel of the electron beams increases toward the periphery 
of the picture so that the electrons are more liable to be affected by the 
terrestrial magnetism thus increasing the deviation of the electron beams. 
This landing error increases at higher latitude areas. 
In addition to the vertical component, the horizontal component of the 
terrestrial magnetism also causes landing error in the axial direction. 
The landing error in Japan caused by the horizontal component is the 
largest when the tube axis is directed in the south to north direction, as 
shown in FIG. 4. Accordingly, in a conventional color picture tube landing 
errors are caused by both the vertical and horizontal components of the 
terrestrial magnetism. 
For the purpose of decreasing the beam landing error caused by the 
terrestrial magnetism throughout the picture, it has been proposed to 
include a terrestrial magnetism correction member in the tube. 
In one example shown in FIG. 5, a quadilateral frustum shaped shielding 
member 15 is mounted on the back of a supporting frame 11 of a shadow mask 
assembly 10, the shielding member having a base aligned with the periphery 
of the funnel 12a of an envelope 12. Reference numeral 13 represents a 
shadow mask. Such construction is disclosed, for example, in U.S. Pat. No. 
3,549,932. 
Although this construction can substantially decrease the adverse effect of 
the terrestrial magnetism in comparison to a conventional color picture 
tube not provided with such shielding member, there arises the following 
problems. 
Since the size of the magnetic shielding member is the same or larger than 
the size of the shadow mask a large space is necessary for the surface 
treatment of the magnetic shielding member, thus decreasing the treating 
capacity. Furthermore, during the operation of the tube, the temperature 
of the shadow mask increases due to the scanning of the electron beams, 
but as the inner surface or the impinged surface of the shadow mask is 
bounded by the shielding member, heat radiation is prevented thus causing 
deformation of the shadow mask. Moreover, with this construction it is 
necessary to increase the size of the demagnetizing coil wound about the 
envelope near the fluorescent screen for the purpose of demagnetizing the 
shadow mask, since a substantial portion of the demagnetizing flux flows 
through the shielding member instead of the shadow mask. This results in a 
more incomplete demagnetization of the shadow mask than with a tube not 
using the shielding member. Accordingly, to demagnetize the shadow mask as 
desired, it is necessary to increase the demagnetizing current. For the 
reason described above, use of an internal shield for correcting the 
terrestrial magnetism as shown in FIG. 5 results in such defects as 
increasing the manufacturing difficulty and degradation of the tube 
characteristics. These disadvantages are more serious in a small size 
color picture tube. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of this invention to provide an 
improved color picture tube with stripe shaped phosphors that can minimize 
the beam landing error on a fluorescent screen caused by terrestrial 
magnetism. 
Another object of this invention is to provide a color picture tube with 
stripe shaped phosphors capable of decreasing the demagnetizing current 
required for the shadow mask in a tube employing a terrestrial magnetism 
correcting member. 
A further object of this invention is to provide an improved color picture 
tube provided with stripe shaped phosphors capable of increasing heat 
radiation of the shadow mask. 
According to this invention, there is provided a color picture tube with 
stripe shaped phosphors of the type comprising an envelope including a 
panel with a fluorescent screen made of the stripe shaped phosphors, a 
funnel and a neck, a shadow mask assembly disposed at a predetermined 
distance from the fluorescent screen, the shadow mask assembly including a 
rectangular supporting frame and a shadow mask supported thereby on the 
side thereof facing the fluorescent screen, an electron gun assembly 
contained in the neck, and a terrestrial magnetism correcting member 
mounted on the side of the supporting frame facing the electron gun 
assembly, wherein the terrestrial magnetism correcting member comprises a 
pair of vertical magnetic side plates extending from the opposed vertical 
sides of the supporting frame toward the neck, and a second pair of 
horizontal magnetic side plates interconnecting the upper and lower ends 
of the first pair of vertical magnetic side plates on the edges thereof 
remote from the shadow mask.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 6 shows one embodiment of the color picture tube according to the 
invention and provided with stripe shaped phosphors. The color picture 
tube 20 shown therein comprises an envelope made up of a panel 21a, a 
funnel 21b and a neck 21c. On the inside of the panel 21a is formed a 
fluorescent screen 22 coated with well known stripes of phosphors of red, 
blue and green colors. The fluorescent screen 22 may be of the black 
stripe type wherein graphite is coated between the stripes of the 
phosphors, or may be of any other type. A shadow mask assembly 23 is 
disposed in front of the fluorescent screen 22 at a predetermined 
distance. The shadow mask assembly 23 comprises a rectangular shadow mask 
23a, a rectangular supporting frame 23b for supporting the same, and leaf 
springs 23c secured to the supporting frame 23b for supporting the 
assembly on the inner wall of the panel 21a. 
A terrestrial magnetism correcting member or magnetic shielding member 25 
according to this invention is mounted on the back of the supporting frame 
23b. The terrestrial magnetism correcting member 25 is made of magnetic 
material and shaped as a rectangular box and includes a pair of 
rectangular vertical magnetic side plates 26 and 27 extending in parallel 
from the rear sides of the supporting frame 23b toward the neck 21c. The 
correction member 25 further comprises a pair of horizontal magnetic side 
plates 28 and 29 extending in parallel between the ends of the side plates 
26 and 27 or between the upper and lower ends 26a, 27a 26b and 27b. The 
side plates 26 and 27 are secured to the supporting frame 23b of the 
shadow mask assembly 23 by suitable means as welding. The first pair of 
side plates 26, 27 and the second pair of side plates 28 and 29 are 
assembled by welding, for example or stamped from a single metal plate. 
Thus, a space 30 with its upper end opened is defined by the upper 
horizontal side of the supporting frame 23b, the upper ends of the 
vertical side plates 26 and 27 and the upper horizontal side plate 28. In 
the same manner, a space (not shown) with its lower end opened is defined 
by the lower horizontal side of the vertical side plates 26 and 27 and the 
lower horizontal side plate 29. The vertical edges of the vertical side 
plates 26 and 27 remote from the shadow mask 23a, and the horizontal side 
plates 28 and 29 define an opening 32 in a plane perpendicular to the tube 
axis. The size of this opening is selected so that the electron beams 38 
emitted by an electron gun assembly 34 contained in the neck 21c can land 
on the effective area of the fluorescent screen 22 formed on the inner 
surface of the panel 21a. A demagnetizing coil 35 for demagnetizing the 
shadow mask is wound about the tube contiguous to the funnel 21b of the 
panel 21a. Further, near the joint between the funnel 21b and the neck 21c 
is disposed an assembly 36 of a vertical deflection coil and a horizontal 
deflection coil to deflect the electron beams 38 in the vertical and 
horizontal directions for causing them to impinge upon the predetermined 
portions of the fluorescent screen 22. 
With this construction, the vertical component of the terrestrial magnetism 
which passes near the vertical sides of the picture and which otherwise 
would tend to deflect the electron beams in the horizontal direction, are 
attracted by the vertical magnetic side plates 26 and 27 thus changing the 
terrestrial magnetic flux distribution in an area through which the 
electron beam passes. More particularly, the intensity of the terrestrial 
magnetic field near the vertical side plates 26 and 27 decreases so that 
the lines of magnetic force in this area distort outwardly in the 
horizontal direction with respect to the vertical plane containing the 
tube axis passing through the center of the picture. Consequently, as 
shown in FIG. 7, on both sides of the horizontal axis of the picture, the 
landing errors of the beam passing through the magnetic field are equal to 
that at the center of the picture. The electron beams toward the corners 
of the picture pass through the field distorted outwardly in the 
horizontal direction so that the beams would be subjected to a force in 
the perpendicular direction. As a consequence, the electron beams passing 
through the terrestrial magnetism correcting member 25 toward the shadow 
mask assembly 23 and the fluorescent screen 22 are deflected in the 
horizontal and vertical directions by the correcting member 25. It should 
be noted, however, that since the fluorescent screen comprises stripe 
shaped phosphors, the beam landing error in the vertical direction caused 
by the horizontal component of the terrestrial magnetism is not necessary 
to be considered. In other words, it is necessary to consider only the 
horizontal beam deflection error caused by the vertical component so that 
it is possible to further decrease the beam landing error in comparison to 
a fluorescent screen that comprises dot matrix type phosphors. 
The field distortion described above is also afforded by the horizontal 
magnetic side plates 28 and 29 thus further decreasing the beam landing 
error. 
The horizontal side plates 28 and 29 of this invention are provided between 
the upper and lower ends of the vertical magnetic side plates 26 and 27 to 
define upper and lower openings 30 so that the substantial portion of the 
magnetic field produced by demagnetizing coil 35 which is most effective 
to demagnetize the shadow mask passes substantially only through the 
shadow mask 23a and the frame 23b having a small reluctance. Accordingly, 
different from the prior art construction, the terrestrial magnetism 
correcting member 25 does not decrease the demagnetizing effect. 
In addition to not adversely affecting the demagnetizing effect described 
above, since the terrestrial magnetism correcting member 25 of this 
invention is provided with upper and lower openings 30 the heat generated 
or stored in the shadow mask 23a due to the impingement of the electron 
beams is dissipated more rapidly through these openings so that it is 
possible to more efficiently dissipate the heat than with the prior art 
correcting member 15 shown in FIG. 5. In one embodiment of the invention 
the width of the vertical side plates 26 and 27 is made equal to that of 
the frame 23b of the shadow mask assembly 23 and the width of the 
horizontal side plates 28 and 29 is made to be equal to approximately 
twice of the width of the metal surface 23b, on the incident side of the 
electron beams of the frame 23b of the shadow mask assembly 23. In this 
structure the beam landing error was measured when the vertical component 
of the terrestrial magnetism was varied from 0 to 0.7 gauss which 
correspond to the equator and the north or south pole, respectively, and 
the result of measurement is shown in FIG. 7. FIG. 8 shows the 
distribution of the beam landing error after purity adjustment to provide 
0.35 gauss which corresponds to the vertical component of the terrestrial 
magnetism in Japan. Thus, after the purity adjustment, the beam landing 
error caused by the vertical component is decreased to such an extent that 
the viewer can scarcely perceive it on the fluorescent screen. As shown in 
FIG. 9, the beam landing error caused by the horizontal component becomes 
maximum toward the north and south. Thus, as shown in FIG. 9, according to 
this invention the overall beam landing error caused by the terrestrial 
magnetism can be reduced to a value much smaller than the prior art 
correcting member. More particularly, the horizontal component is also 
attracted toward the vertical side plates to slightly decrease the 
intensity of the field at these positions. As a result, the magnetic lines 
of flux due to terrestrial magnetism are deformed towwards the vertical 
side plates with the result that the vertical component is strengthened 
whereas the horizontal component is weakened. Consequently, it is 
necessary to consider only the horizontal direction error due to the 
vertical component with regard to the landing error on the stripe shaped 
phosphors, which means that a decrease in the overall beam landing error 
is achieved. 
FIG. 10 shows a modification of the shadow mask assembly and the 
terrestrial magnetism correcting member according to this invention which 
differ from the embodiment shown in FIG. 6. In FIG. 10 inclined magnetic 
plates 40a--40d which are substantially parallel with the portions of the 
inner wall of the funnel at the corners of the envelope, are provided at 
the joints of the vertical and horizontal magnetic side plates 26, 27, 28 
and 29. These plates may be welded to other magnetic plates directly or 
indirectly. 
With this construction, it is possible to project the terrestrial magnetism 
correcting member more toward the electron gun assembly than with the 
construction shown in FIG. 6. As this extent of projection increases, the 
openings 30 and 31 become larger, thus increasing the demagnetizing flux 
acting upon the shadow mask and the frame thereby increasing the 
demagnetizing effect. 
When the magnetic plates are welded together, the surface treatment, that 
is blocking thereof can be made simply, thus decreasing the cost of 
manufacturing. 
It should be understood that the invention is not limited to the specific 
embodiments described above and that many changes and modifications will 
readily occur to one skilled in the art in the light of the above 
teachings. For example, instead of providing inclined plates 40a--40d at 
the corner joints between the vertical and horizontal side plates the 
portions of the horizontal side plates near the funnel may be inclined to 
match the shape of the inner wall of the funnel.