Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
1999-10-14
2003-02-25
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S414000, C313S415000, C313S442000, C313S447000, C315S368150
Reexamination Certificate
active
06525459
ABSTRACT:
BACKGROUND
1. Field of Invention
This invention relates to cathode ray tube electron guns. More particularly, the invention relates to an electron gun configuration and a method for improving the electron beam landing geometry at the extreme edges of a cathode ray tube viewing screen.
2. Related Art
Cathode ray tubes (CRTs) used in consumer electronics, e.g., television receivers, must present good picture quality. One desirable quality is uniform picture brightness and color purity over the entire viewing screen. That is, a uniformly bright white picture should result when the CRT electron gun excites all viewing screen phosphor elements to emit visible light. Another desirable quality is good focus for the displayed picture. Both qualities depend on proper landing geometry of the electron beam incident on the excited phosphor. Proper landing geometry is difficult to obtain, especially in the corners, with viewing screens that are nearly flat and that have a high width to height aspect ratio such as 16:9.
Picture uniformity requires that the beam width of the electron beam portion striking the phosphor elements be uniform over the entire phosphor area. For example,
FIG. 1
is a simplified representational plan view showing a cross section of a typical SONY® TRINITRON® CRT, such as a model 36RV, and electron beams directed to excite phosphor stripes that emit colored light. As shown, composite electron beam
20
originates from three electron sources (e.g., cathodes)
22
,
24
, and
26
. Persons skilled in the art will understand that each source
22
,
24
, and
26
is controlled by circuits that decode a television picture signal, each source emitting electrons so as to energize colored light emitting phosphors to create a color picture. Thus electron beam
20
may include component beam
28
that energizes phosphors emitting blue light, component beam
30
that energizes phosphors emitting green light, and component beam
32
that energizes phosphors emitting red light.
Beam
20
is directed against aperture grill
34
in which aperture slits
36
are defined. In this example, two slits
36
are shown. Portions
28
a
and
28
b
of beam
28
pass through the aperture slits
36
to illuminate, for example, blue phosphor stripes
38
. Similarly, portions
30
a
and
30
b
of beam
30
illuminate, for example, green phosphor stripes
40
, and portions
32
a
and
32
b
of beam
32
illuminate, for example, red phosphor stripes
42
. As shown, phosphor stripes are separated by carbon stripes
44
.
The cross-sectional area of beam
20
incident on phosphor screen
35
is the spot size. The cross-sectional shape of beam
20
incident on phosphor screen
35
is the spot shape. As discussed below, spot size and shape are important to achieving proper focus.
The width of the electron beam portions incident on the phosphor stripes is the beam width. Beam width is a critical factor in controlling the landing performance of an electron beam portion incident on a phosphor stripe.
FIG. 2
is a simplified cross-sectional view of an electron beam portion, e.g., portion
30
a
, passing though aperture slit
36
and incident on a phosphor stripe, e.g. stripe
40
. As shown, the beam width is somewhat wider than the width of aperture
36
due to scattering effects persons skilled in CRT design will understand. Persons skilled in CRT design will also understand factors that effect landing performance, such as the change in gaussian energy distribution over the beam width and the diffraction occurring as the beam passes through an aperture. For good landing performance, portion
30
a
is aligned so that the beam width uniformly overlaps carbon stripes
44
on either side of phosphor stripe
40
, shown as position
46
. Uniform phosphor stripe coverage ensures uniform energy distribution to excite the phosphor stripe for maximum brightness. It can be seen that if portion
30
a
is shifted to the left or right, for example to position
48
, landing performance may decrease. Similarly, if beam width is too wide or too narrow, landing performance decreases because the energy of the electron beam portion is not optimally distributed over the phosphor stripe. Accordingly, there is an optimum beam width and position for an electron beam portion incident on a phosphor stripe.
To ensure picture uniformity, landing performance must be the same for every beam portion incident on every phosphor stripe over the entire viewing area. Persons skilled in CRT design will understand that without any correction, landing performance in the center of the CRT viewing area differs from performance at each of the corners due to the increased deflection of the electron beam and the increased distance from gun to screen. But in addition to landing performance, good focus must be maintained over the viewing area as well. Focus performance is primarily based on spot size and shape.
Factors such as the earth's magnetic field distort spot size and shape as the beam is scanned over the aperture grill. The most severe distortions typically occur in the corners of the viewing screen. Furthermore, since the CRT viewing area is typically rectangular, horizontal and vertical spot size and shape distortions (beam cross-sectional astigmatisms) differ at the corners due to the length of the respective deflections. Persons skilled in CRT design will be familiar with various conventional correction methods such as SONY's Auto Beam Landing Correction (BLC), Multi-Astigmatism Lens System (MALS), and Extended Field Elliptical Aperture Lens (EFEAL).
To achieve good focus, the beam cross-section is shaped to ensure proper spot size and shape over the entire viewing screen. Since the spot size and shape changes as the beam is scanned across the screen, the shaping must be dynamic so as to vary with beam position. In TRINITRON® systems, the beam is shaped using an electromagnet positioned around the main focusing grid in the electron gun, as discussed below.
FIG. 3
illustrates electron gun
49
and beam shaping and deflection components used in a typical TRINITRON® CRT. As shown, three cathodes
50
a
,
50
b
, and
50
c
, produce electrons in response to signals from conventional circuits (not shown) that decode a color television picture signal. Electrons are directed as shown through a series of grids G
1
, G
2
, G
3
, G
4
, and G
5
to produce a composite electron beam that excites colored light emitting phosphors as described above. Grid G
4
is the main focusing grid, and in some electron guns component beams
54
a
,
54
b
, and
54
c
converge in grid G
4
. Conventional focusing is performed in grid G
4
using focusing elements (omitted for clarity) driven by focus voltage driver
51
that supplies focus voltage V
F
on lines
53
to terminal
53
a
on grid G
4
. Beam
54
is focused to produce good spot size as beam
54
sweeps across aperture grill
55
to illuminate phosphor coating
64
on viewing screen
66
. Persons skilled in CRT design will understand the details of beam focusing.
Persons skilled in CRT design will also understand the use of a four-pole electromagnet to alter beam spot shape. (See, e.g., U.S. Pat. No. 3,946,266, assigned to the present assignee and incorporated herein by reference.) The following brief discussion illustrates basic concepts. Electromagnet
52
with four poles is positioned around grid G
4
. As depicted in
FIG. 3
, only the top two poles
52
a
and
52
b
are shown. As described herein, the electromagnet is referred to as Dynamic Quadra-Pole (DQP) magnet.
FIG. 4
is a representational side view of DQP magnet
52
with poles
52
a
,
52
b
,
52
c
, and
52
d
positioned around grid G
4
(omitted for clarity). As shown, electron beam
54
travels out of the paper towards the viewer. DQP driver
56
is connected to DQP magnet
52
using lines
58
. DQP driver
56
controls the magnetic fields among poles
52
a
-
52
d
, represented by field lines
60
, by supplying DQP current i
DQP
along lines
58
. Thus current i
DQP
varies as a function of beam position. Persons skilled in CRT de
Patel Ashok
Roy Sikha
Skjerven Morrill LLP
Sony Corporation
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