Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
2000-11-01
2002-02-19
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S409000
Reexamination Certificate
active
06348759
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a color cathode ray tube and, particularly, to a color cathode ray tube having an electron gun that makes it possible to obtain favorable focusing characteristics over the whole fluorescent screen and that executes efficient speed modulation.
A technology for improving the picture quality of TV receivers and computer monitors can be represented by a method disclosed in Japanese Patent Laid-open No. 140428/1976, in which the scanning speed of the electron beam is modulated with a brightness-changing portion of the picture (or the image) to emphasize the contour of the picture. This method is generally called speed modulation.
Such speed modulation includes both an electromagnetic type and an electrostatic type. The speed modulation of the electromagnetic type, however, has been more generally used. The speed modulation of the electromagnetic type is produced by an electromagnetic coil attached around the neck portion of a cathode ray tube and a circuit for driving the electromagnetic coil.
FIG. 4
is a schematic sectional view illustrating a color cathode ray tube of the speed modulation type. In this color cathode ray tube, a vacuum enclosure is constituted by a panel portion
20
, a neck portion
21
and a funnel portion
22
. On the inner surface of the panel portion
20
there is formed a fluorescent screen
23
by arranging fluorescent materials of three colors in the form of a mosaic or stripes, and adjacent the back surface of the fluorescent screen
23
there is provided a shadow mask
24
which operates as a color-selection electrode. The shadow mask is held by a mask frame
25
and is supported together with a magnetic shield
26
on the inner surface of a skirt of the panel portion by a mask suspension mechanism
27
. Furthermore, an electron gun
28
of an in-line type is contained in the neck portion
21
, and a deflecting device
29
is so provided as to surround a transition region between the neck portion
21
and the funnel portion
22
.
Reference numeral
30
denotes a magnetic device for correcting the color convergence and the color purity of the electron beam. An electromagnetic coil
34
for speed modulation is further provided around the neck portion
21
.
Reference numeral
31
denotes stem pins for feeding image signals and various drive voltages to the electrodes of the electron gun,
32
denotes a getter for increasing the degree of vacuum, and
33
denotes a band for preventing implosion.
An electron beam B emitted from the electron gun is subjected to speed modulation by the magnetic field generated by the electromagnetic coil
34
surrounding the neck portion
21
to reproduce an image having a high picture quality on the fluorescent screen
23
.
FIGS. 5A and 5B
are diagrams illustrating the constitution of a conventional electron gun used for the color cathode ray tube shown in FIG.
4
.
FIG. 5A
is a side view and
FIG. 5B
is an end view as seen in the direction indicated by arrow P in FIG.
5
A.
The electron gun is constituted by a cathode
1
, a first electrode
2
, a second electrode
3
, a third electrode
4
, a fourth electrode
5
, a fifth electrode (focusing electrode)
6
, a sixth electrode (anode)
7
, and a shield cup
8
. Reference numeral
9
denotes bead glass for firmly holding the electrodes,
10
denotes a stem, and
11
denotes contact springs.
A shield cup
8
is connected to the anode
7
on the fluorescent screen side. Referring to
FIG. 5B
, openings
81
,
82
and
83
are formed in line in the bottom of the shield cup
8
for the three electron beams.
The focusing electrode
6
is divided into two parts, i.e., a first division electrode
61
and a second division electrode
62
in the axial direction of the tube. The whole electrode has a length L in the axial direction of the tube, the first division electrode
61
having a length L
1
in the axial direction of the tube, and the second division electrode
62
having a length L
2
in the axial direction of the tube, satisfying the relation L
1
≦L
2
.
FIGS. 6A and 6B
are diagrams illustrating the constitution of the second division electrode constituting the focusing electrode of FIG.
5
A.
FIG. 6A
is a front view as viewed from the anode
7
side, and
FIG. 6B
is a side view showing a portion thereof in cross section.
Openings
62
a
,
62
b
and
62
c
are formed in the second division electrode
62
on the first division electrode
61
side for the respective electron beams. A single opening
62
d is formed in the surface thereof opposed to the anode, and it has a diameter D in a direction at right angles with the in-line direction. In the electrode there are further installed an inner electrode
64
and a plate-like correction electrode
63
having openings for the respective electron beams. Reference numeral
65
denotes tabs buried in the bead glass.
An electron gun having the above-mentioned electrode constitution is disclosed in Japanese Patent Laid-open Nos. 103752/1983 and 152834/1992.
The second division electrode
62
has a length L
2
in the axial direction of the tube; the length from the surface thereof opposed to the first division electrode
61
to the electron beam passing opening in the inner electrode
64
is L
21
in the axial direction of the tube; the length of the surface thereof opposed to the anode
7
to the electron beam passing opening in the inner electrode
64
is L
22
in the axial direction of the tube; and the length from the surface thereof opposed to the anode
7
to the electron beam passing opening in the plate-like correction electrode
63
is L
23
in the axial direction of the tube. Here, the length L
21
is L
21
≧L
22
.
The main lens of the electron gun is formed in a portion shown in
FIG. 5A
, where the anode
7
and the focusing electrode
6
are opposed to each other and the focusing electrode
6
is divided into two parts, i.e., the first division electrode
61
and the second division electrode
62
in the axial direction. The electric field produced by the electromagnetic coil surrounding the neck portion enters the electrodes through gaps among the main lens-forming portion, the first division electrode
61
and the second division electrode
62
, and the electron beam passing through the main lens-forming portion and the two division electrodes is temporarily deflected by the magnetic field to control the scanning speed of the electron beam, i.e., to effect a so-called speed modulation.
The magnetic field generated by the electromagnetic coil forms eddy currents in the electrodes of the electron gun to suppress the action of speed modulation.
In order that the action of speed modulation is not suppressed by the eddy currents produced in the electrodes of the electron gun due to the magnetic field established by the electromagnetic coil, Japanese Patent Laid-open No. 146847/1980 discloses a method according to which a slit of a relatively broad width is formed in the electrodes of the electron gun at a position where the electromagnetic coil is installed.
In the above-described device, however, the electrodes for speed modulation are formed in the shape of a relatively deep box, and the gap (slit) formed between the electrodes is located relatively far away from the position of the main lens (focusing gap) toward the cathode. From the standpoint of the overall length of the electron gun, as well as the cathode ray tube, however, this structural arrangement is not suited for shortening the depth of current TVs and monitors. Besides, no consideration has been given at all concerning the efficiency of speed modulation.
SUMMARY OF THE INVENTION
According to the study conducted by the present inventors, it has been found that the effect of speed modulation can be enhanced in an electron gun having a structure as above-mentioned by contriving the position of the gap between the electrodes.
The object of the present invention is to provide a color cathode ray tube having favorable focusing characteristics over the whole fluorescent screen by efficiently execut
Misono Masayoshi
Tamura Hiroyuki
Tojo Tsutomu
Antonelli Terry Stout & Kraus LLP
Day Michael H.
Hitachi , Ltd.
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