Note: Descriptions are shown in the official language in which they were submitted.
~2708B~
PHA 60060 1 1 . 5 . 1986
SLar~
Color cathode ray tube having l~Ye~ internal
magnetic shield.
This invention relates to color cathode ray
tubes having aperture masks, and more particularly relates
to improved internal magnetic shields for such tubes.
Color cathode ray tubes for color television
(color picture tubes) employing aperture masks are known
to be sensitive to external magnetic fields, especially
the earth's magnetic field, which can undesirably influence
the trajectories of the electron beams, causing shifting
and distortion of the screen raster, as well as degradation
Of colour purity and convergence. For example, shifting of
a beam trajectory to the right or left will result in a
beam landing error in a tube with vertically oriented
phosphor stripes. That is, the beam will land to the
right or left of the intended landing area on the
stripe.
For acceptable performance, therefore, such
tubes must have effective shielding from these magnetic
fields, and it has become conventional practice to incor-
porate internal magnetic shields into color picture tubes
for this purpose.
Because the effect of the ~rth's magnetic
field depends upon thelocation and orientation of the tube,
optimum shielding requires the ability to remagnetize
the shield to realign the magnetic domains after the tube
has been moved. Accordingly, these shields are custo-
marily fabricated from a soft magnetic material, such as
low carbon steel, enabling ready remagnetization each time
the television set is turned on.
The shape of the shield is in general di~-tated
by the desire to have as much of the tube volume shielded
as possible, without having the shield interfere with the
tube~s operation. For example, extending the shield too far
~ ,. j.~
~Z~ 38~3
20104-8135
back into the funnel risks interference wi~h the magnetic
deflection field for the electron beams, as well as physical
interception or "clipping" of the deflected beams. In addition,
cost considerations dictate as simple a shape as possible.
In U.S. Patent 3,867,668, the shield is eomposed of two
curved sheets of magnetic material, said to make processing,
storing, transportation and assembly convenient and easy. In
addition an external shiel~ is employed to cover the gaps created
by the opening between the two curved sheets "to provide complete
shieldlng".
It is customary to describe the effect of the earth'æ
magnetic field on beam landing in terms of two components; the
east-west effect, determined by a transverse horizontal field, and
the north-south effect, determined by an axial horizontal field.
The magnitude of these effects is the difference between the beam
landing errors in the east and west-facing dlrections, and the
north and south-facing directions, respectively. It is also
customary to describe the tube's surface in terms of the face of
an analog clock. Thus, the top is 12 o'clock, the bottom is 6
o'clock, the right side (as seen by the vlewer) is 3 o'clock and
the left side is 9 o'clock.
It ls an ob~ect of the invention to provide improved
lnternal magnetlc shields for color tubes which exhibit improved
~hlelding and whlch can be readily fabricated using present
manufacturing techniques.
In accordance with a broad aspect of the inventlon,
there is provlded a color cathode ray tube comprising: an outer
1Z708~8
20104-8135
glass envelope of integrated face panel, funnel and neck portlons;
a phosphor screen deposited on the inner surface of the face
panel, the screen comprised of an array of discrete phosphor
elements; at 1east one electron gun positioned in the neck for
directing an electron beam to the screen; and an aperture ma~k
positioned adjacent the screen for directing the beam to the
desired phosphor elements on the screen; the tube having an axis
of rotation designated the Z axis, and top, right side, bottom and
left side surfaces surrounding the Z axis, the surfaces (as seen
by the viewer) designated the 12, 3, 6 and 9 o'clock surfaces,
respectively;
an internal magnetic shield positioned between the mask and
the gun, the shield comprising a structure generally following the
contour of the funnel, said structure having a rim extending
around the periphery of the mas~ and a sidewall extending from the
rim rearward along the inner surface of the funnel toward the
neck, the structure terminating forward of the neck to define an
aperture for passing the beam to the mask and screen;
characterized in that the apertures oblong-shaped with a
ratio of length (La) to width (Wa) of from about 1.7 to 2.1 and in
that the shield has at least one slot-shaped opening in both the 6
and 12 o'clock sides of the structure, each opening aligned in the
direction of the Z axi~ of the tube.
In accordance with a preferred embodiment, there is a
pair of slots centrally located and facing each
2~
127~)88~
PHA 60060 3 1.5.1986
other above and below the Z axis in the 6 and 12 o'clock
sides of the shield.
In accordance with another aspect of the
invention, such an improved shield is fabricated by
first cutting slots into a foil sheet of soft magnetic
material, such as low carbon steel, and then forcing the
sheet over a bowl-shaped form to simultaneously stretch
and crimp the sheet into a bowl-shaped structure.
Subsequently, an aperture is cut into the shield.
Fig. 1 is a sectioned elevation view of a color
picture tube incorporating an improved internal magnetic
shield according to the invention;
Fig. 2 is a plan view of the shield of Fig. 1
as seen from the rear, showing the slots and the shield
aperture; and
Fig, 3 is a side view of the shield of Fig. 1,
showing the edge contour of one end region of the shield
aperture.
Referring now to Fig 1, there is shown a
color cathode ray tube for color television (color
picture tube) 11, including glass envelope 13 comprising
an integration of face panel 15, funnel 17 and neck 19
regions. Disposed on the interior surface of face panel
15 is cathocoluminescent screen 21, consisting of an array
of discrete phosphor elements. Positioned in the neck 19 is
mount 22 including an electron gun for generating and
directing three electron beams to screen 21, Positioned
adjacent screen 21 is a multi-apertured mask 25 for
directing the electron beams to the desired phosphor
elements on Ohe screen. Aperture mask 25 is supported by
frame member 29, which is in turn supported by studs 27
embedded in the sidewall of panel 15. Attached to mask
frame 29 is internal magnetic shield (IMS) 31 co~prised of
bowl-shaped structure 39 defining super~acent slots 41 in
the centers of facing top and bottom sidewalls thereof,
aligned with the rotational or Z axis of the tube, and also
defining aperture 47 surrounding the Z axis.
1~7088~
PHA 60060 4 1.5.1986
As can be seen from Fig. 1, the ~owl ~hapo~
structure 39 of shield 31 extends from the periphery of the
mask 25 downward along the funnel 17 toward the mount 22,
generally followingthe contour of the funnel wall, until it
terminates in a mid-region between the mask and mount to
define aperture 47. While a further extension o~ the shield
would result in improved shielding, it would also present
the risk of interference with the magnetic beam deflection
field produced by external deflection means (not shown)
10 positioned i~ the transition region between the funnel and
the neck, as well as actual physical interception
(clippin~) of the electron beams.
Referring now to ~ig. 2, the shield 31 is
seen in plan view from the rear or mount end of the tube,
15 as viewed along the Z axis. In accordance with the
conventional practice of labeling regions of the tube as
though the screened panel were the face of an analog clock,
the top, left side, bottom, and right side of the shieJd
are labeled the 12, 3, 6 and 9 o'clock sides, respectively.
20 As can be seen, the slots 41a and 41b are located in the
centres of the 6 and 12 o'clock sides.
The slots may also be located off-center, for
example within a central region of the 6 and 12 o'clock
sides, this region defined as extending up to 40 percent of
25 the length La ofthe aperture. There may also be more than
one slot in a sidewall, but preferably there will be the
same number o~ slots in each sidewall. In addition, the
facing slots need not be superjacent as shown, but may be
displaced relative to one another, within the central
30 region defined.
The slots have a height h which is from about
50 to 95 percent of the height Hs of the shield from the
rim 48 to the straight edge 49 of the aperture 47, and a
width w which is from about 1 to 10 percent of the length Lr
35 of the shield at the rim 48.
The aperture 47 is oblong-shaped, having a
length-to-width ratio of from about 1.7 to 2.1, and having
1Z7~8~
PHA 60060 5 1.5.1986
a central region defined by straight edges 49 and two
end regions defined by curved edges 50. As may be seen
more clearly in Figs, 1 and 3, the central region of the
aperture lies in a plane normal to the Z axis, while the
end regions lie in upwardly curving surfaces,
resulting in the curved edge profile seen in Fig. 3.
The aperture has a width Wa which is from about
49 to 51 percent o~ the width Wr of the shield at the rim
48, and a length La which is from about 65 to 69 percent of
10 the length Lr. In addition, the amount of upward
curvature of the end regions of the aperture, resulting from
the intersection of the bowl-shaped sidewalls with the
oblong-shaped aperture, is indicated by the height Hc of
the shield from the rim 48 to the outer point 52 of
15 curved edge 50, and this height Hc may range from about 85
to 90 percent of the height Hs.
Within the above ranges, the aperture has a
larger length-to-width ratio than the apertures of certain
prior shield designs (about 2 versus 1.3). This change has
20 the effect ofimproving north-south shielding, but at the
expense of slightly reduced east-west shielding. East-west
shielding is at least partially recovered, however, by
introducing the slots into the 6 and 12 o'clock sidewalls.
By way of example, 19V mini neck color tubes
25 having a shield of the invention and a prior art shield were
fabricated and compared for registration differences. The
shield of the invention was as shown in the sidewalls, and
an oblong-shaped aperture with upwardly curving end
regions, with the following dimensi~ns in inches:
30 Hs = 4.0, Lr = 15.0, Wr = 10.9, Hc = 3.75, Wa = 5.37,
La = 10.75, w = 0.3, and h = 3.9. The prior art shield had
no slots and its aperture was entirely located in a plane
normal to the Z axis, and had the following dimensions:
Hs = 3.0, Lr = 15.0, Wr = 10.9, Wa = 7.4 and La = 9.6.
35 Registrations (difference between stripe location and
actual beam landing location; also known as beam landing
error) were measured on several sample tubes for each type
PHA 60060 6 1.5.1986
nf shield and averaged. Measurements were taken at the
four corners ofthe screen for each sample tube oriented
successively in the west, south, east, north and west
directions. Registration differences between north and
south facing orientations averaged about o 6 mils or is
less for the inventive shield versus about 1.2 mils for
the prior art hield. Registration differences between east
and west facing orientations averaged about o.6 mils for the
inventive shield versus about 0.5 mils for the prior art
shield. The significant improvement in north-south shiel-
ding at the expense of a slight decrease in east-west
shielding resulted in a beneficial correction of the effects
of external magnetic fields, and a consequent overall
improvement in tube performance.
In accordance with another aspect of the
invention, the shield is fabricated from a foil sheet of
soft magnetic material, such as low carbon steel. The
sheet is shaped by forcing it over a mold, resulting in
simultaneously stretching and crimping it into the desired
shape. The crimps are represented in the drawings by the
randomly distributed loopedand wavy lines.
The slots are preferably formed before shaping
(such as by cutting or punching) because after shaping the
slots lie in a curved surface and forming in such a surface
is more difficult than in a flat surface. Because the slots
are located in an area of the foil which undergoes consi-
derable stretching during shaping, it is important that
the slots adhere to the dimensional and other limitations
set forth herein, in order to avoid tearing of the foil or
30 deformation of the slots or both, during shaping. For the
~ame reasons, the aperture is preferably formed after
shaping.
While there have been shown and described what
are at present considered to be the preferred embodiments
of the invention, it will be obvious to those skilled in the
art that various changes and modifications may be made
therein without departing from the scope ofthe invention as
defined by the appended Claims.
