Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
1999-12-21
2002-10-29
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S441000, C313S446000, C313S421000, C315S382000
Reexamination Certificate
active
06472808
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a cathode ray tube, and in particular to a color cathode ray tube having an electron gun employing a multistage focus lens for focusing a plurality of electron beams on a phosphor screen.
Shadow mask type color cathode ray tubes are prevailingly used as TV picture tubes and monitor tubes for information terminals. The shadow mask type color cathode ray tubes house an electron gun for emitting a plurality (usually three) of electron beams at one end of an evacuated envelope, a phosphor screen formed of phosphors coated on an inner surface of the evacuated envelope at the other end thereof for emitting light of a plurality (usually three) of colors, and a shadow mask which serves as a color selection electrode and is closely spaced from the phosphor screen. The electron beams emitted from the electron gun are deflected to scan the phosphor screen two-dimensionally by magnetic fields generated by a deflection yoke mounted externally of the evacuated envelope and to display a desired image on the phosphor screen.
FIG. 17
shows a cross-sectional view for explaining an example of the constitution of the shadow mask type color cathode ray tube, and in
FIG. 17
, reference numeral
201
denotes a panel portion forming a viewing screen,
202
denotes a neck portion for housing an electron gun,
203
denotes a funnel portion for connecting the panel portion and the neck portion,
204
denotes a phosphor screen,
205
denotes a shadow mask serving as a color selection electrode,
206
denotes a mask frame for supporting the shadow mask
205
,
207
denotes a magnetic shield for shielding extraneous magnetic fields such as the earth's magnetic field,
208
denotes a mask suspension mechanism,
9
denotes an in-line type electron gun,
10
denotes a deflection yoke,
11
denotes an internal conductive coating,
12
denote stem pins, and
13
denotes a getter.
In the case of the color cathode ray tube, the evacuated envelope is comprised of the panel portion
201
, the neck portion
202
and the funnel portion
203
, and electron beams B (one center electron beam and two side electron beams) emitted from the electron gun
9
housed in the neck portion
202
scan the phosphor screen
204
in two dimensions by the horizontal and vertical direction magnetic fields produced by the deflection yoke
10
.
The deflection yoke
10
is of a self-converging type which provides a pin cushion-like horizontal deflection magnetic field and a barrel-like vertical deflection magnetic field to converge a plurality of electron beams over the entire phosphor screen.
The electron beams B are modulated in amount by modulating signals such as video signals supplied via the stem pins
12
, are color-selected by the shadow mask
205
disposed immediately in front of the phosphor screen
204
, and impinge upon the phosphors of the corresponding colors to reproduce a desired image.
The cathode ray tubes of this kind are provided with a multistage focus lens in the electron gun and a dynamic focusing system is widely adopted where at least one of the electrodes constituting the multistage focus lens is supplied with a voltage varying dynamically, to obtain sufficiently small electron beam spots over the entire phosphor screen.
FIG. 18
is a schematic cross-sectional view of an example of an electrode structure of an electron gun employed in a color cathode ray tube, taken perpendicular to the in-line direction of three in-line electron beams.
In
FIG. 18
, reference numeral
1
denote three cathodes each having a heater incorporated therein, and electron beam generating means comprises the cathodes
1
, a first electrode
2
serving as a control electrode and a second electrode
3
serving as an accelerating electrode and the electron beam generating means forms electrons generated by the three cathodes
1
into three respective electron beams. Electron beam focusing means comprises a third electrode
4
, a fourth electrode
5
, a fifth electrode
6
and an anode
7
, and the electron beam focusing means accelerates the three electron beams and focuses them on the phosphor screen
204
. Reference numeral
8
denotes a shield cup and Eb is an anode voltage. The fifth electrode
6
is divided into a first member
61
and a second member
62
.
The third electrode
4
, the fourth electrode
5
and the first member
61
of the fifth electrode
6
form a first-stage focusing lens, and the second member
62
of the fifth electrode
6
and the anode
7
form a second-stage focusing lens.
The electrons emitted from the cathodes
1
heated by the heaters are accelerated toward the first electrode
2
serving as an electron beam control electrode by an accelerating potential of the second electrode
3
to form three electron beams. After passing through the electron beam apertures in the second electrode
3
and the third electrode
4
, the three electron beams are slightly focused by the first-stage focusing lens formed by the third electrode
4
, the fourth electrode
5
and the first member
61
of the fifth electrode
6
.
After passing through the first-stage focusing lens, the electron beams enter the second-stage focusing lens formed by the second member
62
of the fifth electrode
6
and the anode
7
and serving as a main lens.
In
FIG. 18
, reference numeral
63
denotes a correction plate electrode disposed within the second member
62
of the fifth electrode
6
and
71
is a correction plate electrode disposed within the anode
7
.
The three respective electron beams are focused while they pass through the second-stage focusing lens, then are subjected to color selection by the shadow mask
205
, and then are focused on phosphor elements of an intended color of the phosphor screen
204
to form an electron beam spot.
A first focusing voltage Vf
1
of a fixed voltage is applied to the third electrode
4
and the first member
61
of the fifth electrode
6
, and a second focusing voltage (Vf
2
+dVf) of a fixed voltage Vf
2
superposed with a dynamic voltage dVf varying in synchronism with deflection angle of the electron beams scanning the phosphor screen
204
is applied to the second member
62
of the fifth electrode
6
. With this structure, the curvature of the image field is corrected by varying the strength of the main lens according to the deflection angle of the electron beams.
In addition to the above structure, an electrostatic quadrupole lens is formed by four vertical plates
611
attached to the end of the first member
61
of the fifth electrode
6
on the second member
62
side thereof and two horizontal plates
621
attached to the end of the second member
62
of the fifth electrode
6
on the first member
61
side thereof. With the electrostatic quadrupole lens being configured so as to focus the electron beams horizontally and so as to diffuse the electron beams vertically according to increasing deflection angles of the electron beams, the electrostatic quadrupole lens corrects astigmatic deflection defocusing induced by the deflection yoke which diffuses the electron beams horizontally and focuses the electron beams vertically according to the increasing deflection angles of the electron beams. With this structure, a good focus is obtained over the entire viewing screen.
But electron guns for use in color cathode ray tubes such as TV picture tubes and display monitor tubes need to control the cross-sectional shape of the electron beams properly according to the amount of electron beam deflection so as to provide a good focus characteristic and high resolution over the entire viewing screen.
With the above electron gun, the cross-sectional shape of the electron beams entering the main lens is elongated vertically according to the increasing deflection angle of the electron beams by the astigmatism-correcting electrostatic quadrupole lens, consequently the vertical diameter of the cross section of the electron beams is influenced greatly by the deflection defocusing which compresses the vertical diameter of the cross section
Kato Shin-ichi
Miyagawa Koichi
Shirai Shoji
Uchida Go
Antonelli Terry Stout & Kraus LLP
Hitach, Ltd.
Patel Vip
Roy Sikha
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