Color picture tube having an internal magnetic shield

A color picture tube includes a faceplate and a funnel attached to the faceplate. The faceplate has a horizontal dimension M.sub.h and a vertical dimension M.sub.v, with the dimensions having a ratio M.sub.h /M.sub.v greater than one. A magnetic shield is located within the tube. The magnetic shield has a front aperture in the proximity of the faceplate and a rear aperture remote from the faceplate. The front aperture has a horizontal dimension F.sub.h and a vertical dimension F.sub.v and the rear aperture has a horizontal dimension R.sub.h and a vertical dimension R.sub.v. The top and bottom of the rear aperature are close to the sides of the funnel and the sides of the rear aperture are spaced from the funnel by a spacing consistent with the entrance of the electron beams at maximum deflection into the shield.

This invention relates to a color picture tube, and particularly to such a 
tube having an internal magnetic shield providing improved shielding in 
three magnetic fields. 
BACKGROUND 
A color picture tube includes a faceplate and a funnel which are integrally 
joined together. The inside surface of the faceplate is covered with a 
phosphor screen composed of triads of phosphor elements which emit the 
three primary colors of light, red, green and blue when impacted by 
electrons. An electron gun is mounted in a neck portion which is attached 
to the funnel in a position remote from the faceplate. The electron gun 
provides three electron beams which are used to scan the phosphor pixels 
to cause the desired image to be displayed. A shadow mask is arranged in 
the proximity of the phosphor screen and is used as a color selection 
electrode to assure that each of the three electron beams impacts the 
phosphor of the proper light emitting color. Thus, for example, the 
electron beam which is modulated with the red data impacts the phosphor 
pixels which emit red light. Because the electrons are charged particles, 
the Earth's magnetic field has an influence on their trajectories which 
can cause the electrons to impact a phosphor of the improper color, a 
phenomena known as misregistry. For this reason, a magnetic shield is 
commonly used, either in the interior or on the exterior, of the picture 
tube, to shield a substantial portion of the electron beam trajectories 
from the influence of the Earth's magnetic field. Most recent tubes 
utilize an interior magnetic shield (IMS) which is attached to the shadow 
mask and extends toward the electron gun. 
The magnetic effect on electron beams, which causes misregistry, occurs in 
the directions which are perpendicular to the longitudinal axis of the 
tube. For this reason, various changes in the configuration, or structure, 
of the internal magnetic shield can beneficially influence the 
misregistration in one direction and adversely influence it in an 
orthogonal direction. Misregistry must be corrected in all three field 
directions: axial, horizontal, and vertical. The axial (north-south) field 
acts parallel to the longitudinal axis of the tube. The horizontal 
(east-west) and vertical fields act along the horizontal and vertical axes 
of the faceplate, respectively. In the early prior art, the vertical field 
was shielded from the interior of the tube by enclosing the interior of 
the tube as completely as possible. This entailed attaching an internal 
shield to the mask and minimizing the size of the opening facing the 
electron gun. In later prior art, the axial field was reshaped to have a 
vertical component by the formation of V-notches on the sides of the 
shield which enlarged the rear opening facing the electron gun but 
degraded the vertical-field shielding. Since the horizontal field is 
reshaped by the shadow mask, the shield generally interferes with this 
function by the shadow mask, the shield generally interferes with this 
function of the shadow mask. This interference is reduced in the prior art 
by placing vertical cuts in the shield to section it horizontally, e.g., 
by placing vertical slots along the minor axis of the shield. These cuts, 
or slots, further reduce the enclosure of the tube interior by the shield, 
thus further degrading the vertical-field shielding ability of the shield. 
Thus, the prior art is generally deficient in providing adequate shielding 
in all three fields. The present invention is directed to a tube having an 
internal magnetic shield which has a favorable influence on electron beam 
misregistry in all directions. 
SUMMARY 
A color picture tube includes a faceplate and a funnel attached to the 
faceplate. The faceplate has a horizontal dimension M.sub.h and a vertical 
dimension M.sub.v, with the dimensions having a ratio M.sub.h /M.sub.v 
greater than one. A magnetic shield is located within the tube. The 
magnetic shield has a front aperture in the proximity of the faceplate and 
a rear aperture remote from the faceplate. The front aperture has a 
horizontal dimension F.sub.h and a vertical dimension F.sub.v and the rear 
aperture has a horizontal dimension R.sub.h and a vertical dimension 
R.sub.v. The top and bottom of the rear aperture are close to the sides of 
the funnel and the sides of the rear aperture are spaced from the funnel 
by a spacing consistent with the entrance of the electron beams at maximum 
deflection into the shield.

DETAILED DESCRIPTION 
In FIG. 1, a color picture tube 10 includes a funnel 11 and a faceplate 12 
which are integrally joined at a frit seal line 13. A phosphor screen 14 
is arranged on the inside surface of the faceplate 12. The phosphor screen 
14 is composed of triads of phosphors each of which emits one of the three 
primary colors of light when impacted by three electron beams. A shadow 
mask 16 is spaced from the phosphor screen 14 and is used to direct the 
three electron beams to the phosphors which emit the appropriate colors of 
light. An electron gun 17 is arranged in a neck portion 18 of the 
kinescope 10 and provides the three electron beams which are used to scan 
the phosphors of the screen 14. 
The electrons are charged particles, and accordingly the electron beams are 
subject to deflection because of the influence of the Earth's magnetic 
field. The effects of the Earth's magnetic field are minimized by 
utilizing an interior magnetic shield 19. The shield 19 is composed of a 
ferromagnetic material, such as cold rolled steel, which bends or 
redirects the magnetic field lines of the Earth around the electron beams 
to minimize the effects on the beams as they pass through the shield. This 
is an important feature because the electron beam deflection caused by the 
Earth's magnetic field can cause a particular electron beam to hit a 
phosphor of the wrong light emitting color, thus resulting in misregistry 
and thereby degrading the quality of the image display. Additionally, when 
a television receiver including the tube is moved from one position to 
another, the relative position of the axis of the tube with respect to the 
Earth's magnetic field changes, thereby possibly causing substantial 
degradation of the image display because of additional misregistration of 
the electron beams. 
In FIGS. 1 and 2, the magnetic shield 19 is supported on a shadow mask 
frame 21, which also is ferromagnetic, so that the two are magnetically 
coupled. The magnetic shield 19 includes a front aperture 22, arranged in 
the proximity of the faceplate 12, and a rear aperture 23, which is 
arranged remote from the faceplate 12 and faces the electron gun 17. The 
magnetic shield 19 lies within the interior surface of the funnel 11. The 
rear aperture 23 permits entry of the electron beams into the shield and 
must be large enough to accept all the beams. Between the front aperture 
22 and the rear aperture 23, the shield must lie within the interior 
surface of the funnel 11 but not so far from the sides of the funnel that 
the shield intercepts the electron beams. Thus, the interior funnel 
surface and the surface of the space filled by the electron beams define a 
shell or gap in which no electrons flow. 
In the prior art, a shield is placed in a tube so that the separation of 
the shield from the surfaces of the funnel are approximately the same at 
the sides of the shield as it is at the top and bottom of the shield. Such 
spacing minimizes problems associated with putting the shield into the 
funnel and also alleviates problems associated with the prior art shield 
intercepting the electron beams. In the novel shield 19, the top and 
bottom of the shield are made to lie close to the top and bottom of the 
funnel and the sides of the shield 19 are made to lie as far as possible 
from the sides of the funnel without intercepting the beams. Thus, because 
the funnel transforms from a horizontally rectangular to a vertically 
rectangular cross-section at a location in the proximity of the rear of 
the shield, the height of the shield will be greater than the width of the 
shield at that location. At the very rear of the shield 19, the width of 
the rear aperture 23 is only large enough to admit the electron beams and 
the height of aperture 23 exceeds the width of the aperture. 
In FIG. 2, the faceplate 12 has a major horizontal dimension M.sub.h and a 
minor vertical dimension M.sub.v. These dimensions are selected so that 
the aspect ratio M.sub.h /M.sub.v is greater than one. In the proximity of 
the front aperture 22 the magnetic shield 19 is arranged within the 
faceplate 12, as shown in FIG. 2, and the front aperture 22 has a 
horizontal dimension F.sub.h and a vertical dimension F.sub.v. In the 
vicinity of the front aperture 22, the magnetic shield 19 is configured 
very similarly to the configuration of the faceplate 12. Accordingly, the 
dimensions F.sub.h and F.sub.v of the front aperture 22 are selected so 
that the aspect ratio F.sub.h /F.sub.v is greater than one. The rear 
aperture 23 is centered in the shield 19 and has a horizontal dimension 
R.sub.h and a vertical dimension R.sub.v. When the shield 19 is properly 
arranged in the funnel 11 of the tube 10, the top and bottom of the funnel 
are substantially closer to the top and bottom of the rear aperture 23 
than the sides of the funnel are to the sides of the aperture 23. The 
dimensions R.sub.h and R.sub.v of the aperture 23 are selected so that the 
aperture is no larger than is necessary for the entry of the deflected 
electron beams into the rear aperture 23. The dimensions R.sub.h and 
R.sub.v therefore preferably are selected so that the aspect ratio R.sub.h 
/R.sub.v is less than one. A rear aperture 23 which is dimensioned in 
accordance with these criteria, or with an aspect ratio R.sub.h /R.sub.v 
less than one, substantially decreases misregistry in a north magnetic 
field by shaping the field to have a large vertical component. Misregistry 
is decreased in the east-west field by presenting a flat magnetice field 
to the electron beams. 
Utilizing the novel shield 19, the enclosure of the interior of the tube 10 
is kept as complete as is compatible with the maximum deflection of the 
electron beams through the shield 19. The narrow rear aperture 23 reshapes 
the axial field to have a vertical component, thereby improving registry 
in much the same manner as the prior art shield with V-notches does. The 
shield 19 is also as high vertically as is compatible with the funnel 
interior, thereby further enhancing the redirection of the axial field to 
have a vertical component. 
For horizontal fields, the shield would ideally be absent, so that the mask 
can reshape this field to have axial and vertical components. Given the 
presence of the shield, ideally such shield should be a flat plate in the 
minor axis plane, i.e., orthogonal to the horizontal field, since shield 
plates orthogonal to a field do not alter the field. Of course, a flat 
plate is not compatible with the transmission of electron beams. The best 
possible approximation is a box-like structure that has minimal width and 
maximal height. The shield 19, disclosed herein, provides such a structure 
.