Electric lamp and discharge devices – Cathode ray tube – Envelope
Reexamination Certificate
1998-11-10
2001-05-08
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Envelope
C313S450000
Reexamination Certificate
active
06229256
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a cathode ray tube such as a color cathode ray tube and, more particularly, it relates to the state of the high resistance conductive film applied to the inner wall surface of the neck of a cathode ray tube.
Generally, a color cathode ray tube comprises an envelope having a panel, a funnel and a neck constructed as integral parts thereof. The panel carries on the inner surface thereof a fluorescent screen (target) having three stripe-shaped or dot-shaped fluorescent layers that fluoresce respectively in blue, green and red. The panel also has therein a shadow mask provided with a large number of apertures and arranged vis-a-vis the fluorescent screen.
The neck contains therein an electron gun assembly. The electron gun assembly is adapted to emit three electron beams that proceed on a same horizontal plane and include a center beam and a pair of side beams. The three electron beams emitted from the electron gun assembly are converged toward the fluorescent screen and focused on the respective fluorescent layers of blue, green and red.
A deflection yoke arranged on the outside of the funnel produces a non-uniform magnetic field for deflecting the three electron beams emitted from the electron gun assembly in horizontal and vertical directions. Thus, the three electron beams emitted from the electron gun assembly are forced to scan the fluorescent screen both horizontally and vertically by way of the shadow mask by the non-uniform magnetic field. As a result, a color image is displayed on the screen.
Referring to
FIG. 1
of the accompanying drawings, the color cathode ray tube has an internal conductive film
7
formed on the inner surface of the envelope and extending from the funnel to the neck
3
. The internal conductive film
7
is electrically connected to the anode terminal arranged on the funnel. On the other hand, the convergence electrode
9
of the electron gun assembly
8
is electrically connected to the internal conductive film
7
by way of a bulb spacer
10
. Thus, the anode voltage supplied from the anode terminal is applied to the convergence electrode
9
by way of the internal conductive film
7
and the bulb spacer
10
.
However, in a color cathode ray tube having a configuration as described above, the converging performance of the three electron beams may change as the electric potential of the inner wall surface of the neck
3
changes with time. As a result, the three electron beams may not land on the respective fluorescent layers to give rise to a problem of color deviations in the displayed image.
More specifically, the problem occurs in the following manner.
Since the neck is made of an insulator material, or glass for instance, it is apt to become electrically charged and then discharge the accumulated electric charge. Therefore, the potential of the electric charge of the inner wall surface of the neck, i.e. the neck potential, comes to show a predetermined potential distribution pattern immediately after the application of the anode voltage under the influence of various components including the internal conductive film
7
and the convergence electrode
9
of the electron gun assembly
8
.
However, as time goes on, stray electrons generated within the neck eventually collide with the inner wall surface of the neck, thereby causing secondary electrons to be emitted from the inner wall surface, and gradually raise the neck potential. As a result, the neck potential changes with time.
The neck potential affects the electric field operating as main electron lens section of the electron gun assembly. Then, as the neck potential is not stably held to a constant level but rises with time, it gradually but remarkably permeates into the electric field of the main electron lens section. Thus, in the course of time, the neck potential changes the distribution of the electric field operating as main electron lens section. Since the neck potential permeates into the main electron lens section from the periphery thereof, it alters the tracks of the two side beams passing through a peripheral area of the main electron lens section.
Thus, color deviations occur in a color cathode ray tube adapted to emit three electron beams because of the phenomenon of the change with time of the converging performance of the electron beams, which is referred to as convergence drift.
Japanese Patent Applications KOKAI Publication Nos. 64-12449 and 5-205560 propose the use of a high resistance conductive film
17
having a coefficient of electron emission smaller than one and arranged on the inner surface of the neck as shown in FIG.
1
. The high resistance conductive film
17
is directly arranged on the inner wall surface of the neck and held in contact with the internal conductive film
7
. As a result, it can prevent the change with time of the neck potential due to the emission of secondary electrons of the neck and suppress color deviations due to convergence drift.
However, when a high resistance conductive film is arranged on the inner surface of the neck and held in contact with the internal conductive film in a manner as described in Japanese Patent Applications KOKAI Publication Nos. 64-12449 and 5-205560 and if the high resistance conductive film has a uniform film thickness as seen from
FIG. 1
, a problem arises as described below.
Referring to
FIG. 1
, if the central axis of the neck which is the axis of the tube is Z-axis, the resistance of the high resistance conductive film
17
per unit length of the Z-axis is constant. Additionally, since the neck potential is relatively high if compared with its counterpart of a cathode ray tube having no high resistance conductive film
17
, a phenomenon of field emission is apt to occur between any metal part of the electron gun assembly
8
, which may be an electrode, and the inner wall surface of the neck to give rise to a problem of reduced withstand voltage.
BRIEF SUMMARY OF THE INVENTION
In view of the above identified problem, it is therefore the object of the present invention to provide a cathode ray tube comprising a high resistance conductive film arranged on the inner wall surface of the neck to suppress any convergence drift and adapted to show an withstand voltage that is sufficiently high to effectively suppress a field emission that can occur between a metal part of the electron gun assembly, which may be an electrode, and the inner wall surface of the neck.
According to the invention, the above object is achieved by providing a cathode ray tube comprising;
an electron gun assembly for emitting a plurality of electron beams arranged in a row to proceed on a same horizontal plane and focusing on a target;
a deflection yoke for generating a deflection magnetic field to deflect the plurality of electron beams emitted from the electron gun assembly into a horizontal direction and a vertical direction, the horizontal direction and the vertical direction being rectangular relative to each other on the target;
an envelope having a neck section for containing the electron gun assembly, a panel section provided with the target and a funnel section having its inner diameter increasing from the neck section toward the panel section;
an internal conductive film arranged on the inner wall surface of the envelope and extending from the funnel section to the neck section; and
a high resistance conductive film arranged in the neck section so as to be held in contact with an end portion of the internal conductive film on the inner wall surface of the neck section and surround at least part of the electron gun assembly from the end portion, the high resistance conductive film having an electric resistance higher than that of the internal conductive film; characterized in that:
the film resistance of the high resistance conductive film per unit length of the axis of the tube running perpendicularly relative to the horizontal direction and the vertical direction is lower in a contact region located at an end of the high resistance conductive film and held in contact with
Hasegawa Takahiro
Sugawara Shigeru
Suzuki Fumihito
Guharay Karabi
Kabushiki Kaisha Toshiba
Patel Nimeshkumar D.
Pillsbury & Winthrop LLP
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