Electric lamp and discharge devices – Cathode ray tube – Beam deflecting means
Reexamination Certificate
1998-07-15
2001-04-03
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Beam deflecting means
C313S442000, C335S212000
Reexamination Certificate
active
06211610
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a color cathode ray tube including an in-line type electron gun, and in particular, a color cathode ray tube which is capable of readily compensating for misconvergence.
2. Description of the Related Art
First, a structure of a conventional color cathode ray tube is explained below with reference to FIG.
1
. The illustrated color cathode ray tube includes a bulb
1
comprised of a panel
1
b,
a neck portion
1
c,
and a funnel
1
a,
which has a truncated-conical cross-section and connects the panel
1
b
and the neck portion with each other.
A fluorescent filter film
2
is applied to an inner surface of the panel
1
b.
The flourescent film
2
includes flourescent materials for emission of three primary colors, which are separated from one another with photo-absorbing material sandwiched between them.
An in-line type electron gun
3
is installed in the neck portion
1
c
for emitting three electron beams to the flourescent film
2
to cause emission of three primary colors.
A deflecting yoke
4
is secured to the bulb
1
over the funnel portion
1
a
and the neck portion
1
c.
The deflecting yoke
4
is comprised of a bobbin
5
having a truncated-conical cross-section, a first coil
6
wound around the bobbin
5
for horizontally deflecting a magnetic field, a second coil
7
wound around the bobbin
5
for vertically deflecting a magnetic field, and a ferrite core
8
applied on an outer surface of the bobbin
5
.
Though not illustrated, the color cathode ray tube further includes an inner shield in the funnel portion
1
a,
a shadow mask facing the flourescent film in the funnel portion
1
a,
and an aid such as a purity magnet in the neck portion
1
c.
During operation, the electron gun
3
horizontally emits and accelerates three parallel electron beams, and deflects two other electron beams, between which a central electron beam is situated in such a manner that those two electron beams converge to the central electron beam.
Sawtooth current is supplied to each of the first and second coils
6
and
7
, generating horizontally and vertically deflected magnetic fields.
The three electron beams emitted from the electron gun
3
enter the deflected magnetic fields, and are deflected to a degree proportional to the intensity of the magnetic fields. These three deflected electron beams are converged onto the flourescent film
2
to emit lights. As a result, colored images appear on the panel
1
b.
In order to produce colored images having no color-misregistration, the three electron beams must be correctly directed to associated color regions in the fluorescent film
2
.
However, it is quite difficult to make the deflected magnetic fields completely symmetrical in the bulb
1
because of a dispersion in the shape in the windings of the first and second coils
6
and
7
, a dispersion in location of the first and second coils
6
and
7
when secured to the bobbin
5
, a dispersion in the axis of the electron gun in the neck portion
1
c,
and/or a gap between axes of the deflecting yoke
4
and the electron gun
3
. Accordingly, it is impossible to focus the three electron beams onto the fluorescent film
2
, and the resulting misconvergence among the electron beams in turn results in misregistration of color on the fluorescent film
2
.
This color misregistration considerably degrades the quality of images in a computer display. In order to prevent images from being degraded, the deflecting yoke
4
is set around the bulb
1
in a conventional cathode ray tube. A test pattern is displayed on the fluorescent film
2
, and deflected magnetic fields generated by the deflecting yoke
4
are compensated for, so that the test pattern is displayed in a desired shape and in a desired color, and the generated images have no color misregistration.
Many attempts have been made to compensate for deflected magnetic fields. For instance, Japanese Unexamined Patent Publication No. 55-157846 suggests the deflecting yoke illustrated in FIG.
2
. In the illustrated deflecting yoke, four magnetic pieces
9
are secured onto an outer surface of a bobbin
5
. The magnetic pieces
9
are composed of iron alloy containing nickel as a principle ingredient (commercially available in the tradename of “PERMALLOY”) and are equally spaced around the circumference of the bobbin
5
. The magnetic pieces
9
improve coma-aberration on a screen, and compensate for color misregistration horizontally and vertically in three primary colors, red (R), blue (B), and green (G), as illustrated in FIG.
3
.
Japanese Unexamined Patent Publication No. 8-115686 suggests a deflecting yoke for misconvergence. In the suggested deflecting yoke, illustrated in
FIG. 4
, magnetic pieces
10
composed of magnetic material having high magnetic permeability, such as silicon steel and “PERMALLOY,” are attached to an outer surface of the bobbin
5
in such a manner that the magnetic pieces are movable around the circumference of the bobbin
5
.
Japanese Unexamined Patent Publication No. 9-45261 suggests a deflecting yoke as illustrated in FIG.
5
. The illustrated yoke is formed with four slide rails
12
diagonally positioned reletive to the bobbin
5
at a rear end of the yoke. A magnetic piece
11
is supported along the slide rail
12
. The magnetic pieces
11
are composed of silicon steel containing 3% silicon, or magnetic materials such as ferrite and amorphous providing the same effects as those of silicon steel. A part of the magnetic flux leaking out of the deflecting yoke is cut off by appropriately adjusting the magnetic pieces
11
. As a result, a profile of magnetic flux density in the bulb
1
is adjusted, improving deformation of images.
As explained above, the conventional deflecting yokes can improve image deformation and/or color-misregistration that result from misconvergence.
If, as illustrated in
FIG. 6A
, a horizontally deflected magnetic field is asymmetrically distributed in the bulb
1
due to a dispersion in the shape in the windings of the first and second coils
6
and
7
, a dispersion in location of the first and second coils
6
and
7
when secured to the bobbin
5
, a gap between axes of the electron gun
3
and neck portion
1
c,
and/or a gap between axes of the deflecting yoke
4
and the electron gun
3
, then a magnetic flux density in a horizontal direction also becomes asymmetrical, as illustrated in
FIG. 6B
with a solid line X
1
. Forces exerting on the electron beams R, G, and B also become asymmetric as a result.
Hence, an electron beam located at a distance S from the center at the right side receives a force from a magnetic field, a force which differs in magnitude from a force received by another electron beam located at the same distance from the center at the left side, and misconvergence is generated on the fluorescent film
2
between a central electron beam G and the other two electron beams
13
B and
13
R, as illustrated in FIG.
2
C. In order to eliminate this misconvergence, it is necessary to adjust a profile of deflected magnetic flux in such a manner that a green bright line
13
G, which is a reference line, is made closer to a bright blue line
13
B or a red bright line
13
R located inside or outside the green bright line
13
G.
A horizontally deflected magnetic field is partially leaked outside the deflecting yoke
4
. Therefore, if a magnetic piece having high magnetic permeability is positioned as a compensator in a leaked magnetic field at a rear of the deflecting yoke
4
, the leaked magnetic flux is partially cut off, compensating for a profile of the magnetic flux in the bulb
1
.
By moving the magnetic piece as a compensator, a profile of the magnetic flux density is differentially varied horizontally around the center when viewed from the panel
1
b.
As a result, the electron beams located at the distance S from the center receive the same magnitude force, which ensures elimination of misconvergence.
If a horizontally deflected magnetic flux is distributed asymmetrica
Day Michael H.
McGinn & Gibb PLLC
NEC Corporation
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