Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
2002-04-04
2004-01-06
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S412000
Reexamination Certificate
active
06674228
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to multi-beam electron guns as used in color cathode ray tubes (CRTs) and is particularly directed to a multi-layer common lens arrangement in one or more charged grids in the main focus lens of a CRT electron gun.
BACKGROUND OF THE INVENTION
A typical color CRT employs a multi-beam electron gun which directs three inline electron beams on the inner surface of the CRT's glass display screen. A magnetic deflection yoke disposed outside of the CRT's glass envelope sweeps the three electron beams in unison across the display screen in a raster-like manner. The three electron beams are aligned generally horizontally, or in the direction of each sweep across the CRT's display screen. The energetic electrons incident upon a phosphor coating disposed on the display screen's inner surface produce a video image.
Electron guns are characterized as having X-, Y-, and Z-axes respectively aligned along the width, height and length of the electron gun structure. These axes are shown in
FIG. 1
which is a longitudinal sectional view of a prior art bipotential inline electron gun
10
incorporating a common lens arrangement in its main focus lens. The Y-axis aligned with the height of the bipotential inline electron gun
10
is perpendicular to the plane of the drawing sheet. In general, the larger the electron gun is along its X- and Y-axes, or the larger its diameter, the better the resolution of the video image presented on the CRT's display screen. Over the past several years, the design of high resolution color CRT electron guns has evolved from the individual beam main lens design to the common lens design for the purpose of increasing the effective size of the electron gun. In the individual beam type of main lens design, each of the three electron beams (red, blue, green) is directed through an individually defined lens space without sharing the space with the other beams. In the common lens design, each of the three electron beams is directed through its own individual beam path as well as through a shared focusing region defined by a common beam passing aperture.
Referring to
FIG. 1
, there is shown a longitudinal sectional view of a prior art bipotential inline electron gun
10
incorporating a common lens arrangement in its main focus lens. Electron gun
10
includes an electron beam source typically comprised of three cathodes: K
R
(red), K
G
(green) and K
B
(blue). Each cathode emits electrons which are focused to a crossover along the axis of the beam by the effect of an electrode commonly referred to as the G
2
screen grid. An electrode known as the G
1
control grid is disposed between the cathodes and the G
2
screen grid and is operated at a negative potential relative to the cathodes and serves to control the intensity of the electron beams in response to the application of a video signal to the cathodes. Each of the G
1
control and G
2
screen grids includes three respective aligned apertures
12
a
,
12
b
,
12
c
and
14
a
,
14
b
,
14
c
, with corresponding apertures in each electrode in common alignment for passing a respective one of the red, green or blue color generating electron beams. The G
2
screen grid is connected to and charged by a V
G
voltage source
33
.
Electron gun
10
further includes a G
3
electrode and a G
4
electrode disposed about the three electron beams and along the path of the energetic electrons as they travel toward a display screen
40
disposed on a forward portion of the CRT's glass envelope (which is not shown in the figure for simplicity). The G
3
grid is connected to and charged by a V
F
focus voltage source
34
, while the G
4
grid is coupled to and charged by a V
A
accelerating, or anode, voltage source
35
. The lower end of the G
3
grid in facing relation to the G
2
screen grid forms, in combination with the G
1
control grid and the G
2
screen grid, a beam forming region for forming the three groups of energetic electrons emitted by the K
R
, K
G
and K
B
cathodes into three spaced electron beams. The lower end of the G
3
grid includes three inline, spaced apertures
16
a
,
16
b
and
16
c
through each of which is directed a respective electron beam.
While the G
1
control and G
2
screen grids are generally flat, the G
3
grid and a G
4
grid are cup-like in shape. Disposed within the G
3
grid is a second trio of beam passing apertures
20
a
,
20
b
and
20
c
, through each of which is directed a respective one of the electron beams. The G
3
and G
4
grids form the electron gun's main focus lens. Disposed on the upper portion of the G
3
grid in facing relation to the G
4
grid is an elongated common beam passing aperture
18
through which all three electron beams are directed. Beam passing aperture
18
extends substantially the entire width and height of the G
3
grid and typically has a chain link shape. This chain link shape includes three spaced curvilinear enlarged portions through each of which is directed a respective one of the electron beams. This chain link shaped common beam passing aperture is shown in figures discussed in the following paragraphs and is described in detail below. The common beam passing aperture may take on other common forms, e. g., race track, dog bone or elliptical, although these other shapes are not shown in the figures for simplicity.
The G
4
grid also includes an elongated common beam passing aperture
22
in facing relation to the beam passing aperture
18
of the G
3
grid. Disposed within the G
4
grid in spaced relation are three inline beam passing apertures
24
a
,
24
b
and
24
c
through each of which is directed a respective one of the electron beams. Disposed on the upper end portion of the G
4
grid is a conductive support, or convergence, cup
26
which includes plural bulb spacers
28
disposed about its circumference in a spaced manner. The support cup
26
and bulb spacer
28
combination is conventional and serves to securely maintain electron gun
10
in position in the neck portion of a CRT's glass envelope. Each of the aforementioned grids is coupled to and supported by glass beads (also not shown for simplicity) disposed in the glass envelope's neck portion.
After being subjected to the electrostatic fields produced by the accelerating and focusing voltages applied by the aforementioned grids, the focused electron beams are then directed through a magnetic deflection yoke
30
for deflecting the electron beams in a raster-like manner across a phosphor coating, or layer,
40
on the inner surface of the CRT's display screen, or glass faceplate,
42
. Disposed adjacent the inner surface of the CRT's display screen
42
is a shadow mask
36
having a larger number of apertures
36
a
therein and serving as a color selection electrode.
By directing all three electron beams through a common beam passing aperture, the effective width and height, i.e., diameter, of the electron gun is increased to provide improved video image resolution. Because the electron gun is disposed within the narrow neck portion of the CRT's glass envelope, the common lens design overcomes prior limits on the size, i.e., height and width, of the individual lens-type electron gun.
The length of the electron gun along its Z-axis may also be increased. However, increasing the length of the electron gun along its Z-axis creates a large asymmetric astigmatism which reduces video image resolution. Electron beam astigmatism is defined in terms of the difference between the horizontal focus voltage and the vertical focus voltage, or:
astigmatism=
V
FH
−V
FV
where
V
FH
=horizontal focus voltage, and
V
VF
=vertical focus voltage.
The present invention addresses the aforementioned limitations of the prior art by increasing the effective electrostatic focusing field applied to the electron beams by increasing the effective diameter of the electron gun and compensating for this increase in size by increasing the gun's length. By electrostatica
Chang Hsiang-Lin
Chen Hsing-Yao
Ma Yu-Kun
Yeh Chun-Hsien
Chunghwa Pictures Tubes, Ltd.
Emrich & -Dithmar
Patel Vip
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