Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
2001-04-03
2003-09-23
O'Shea, Sandra (Department: 2875)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C315S003000, C315S015000
Reexamination Certificate
active
06624559
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cathode ray tube whose velocity modulating effect is enhanced and more specifically to a cathode ray tube which enables to display high quality images having remarkable contrast by preventing the decrease of velocity modulating effect caused by eddy current caused by velocity modulating magnetic field generated in electrodes composing an electron gun.
2. Related Art
Various contrivances for displaying high definition and high contrast images have been made to improve the imaging quality of a cathode ray tube for displaying TV images and of a cathode ray tube of an information terminal such as a personal computer.
For instance, there has been known an aperture compensating method of stressing white components by a signal obtained by differentiating an image signal to clearly display outlines. However, this method has had drawbacks that there is a case when it deteriorates image quality in contrary by generating unnecessary white peaks and deteriorating the contrast and that only the right side (downstream side of the horizontal scan direction) of the contrast boundary of an image can be always corrected.
There has been also a velocity modulation of changing electron beam scanning velocity corresponding to the brightness level of an image. This method is to control the scan of an electron beam. The scan of electron beam stop momently after quickening the scanning velocity momently when the electron beam scans horizontally from the black level to the white level by the differential output of the image signal. The scan of electoron beam quicken momently after stopping the scan momently when the electron beam scans horizontally from the white level to the black level.
The density of electron beam is low and the image is dark at the spot where the scan rate is fast. The density of electron beam is high and the image is bright at the spot where the scan stops. Thereby, a high contrast and good quality image is displayed by increasing the black level areas, by narrowing the white level area and by increasing the brightness by increasing the current density.
While there are electrostatic and electromagnetic type velocity modulations, a cathode ray tube using the electromagnetic type velocity modulation which has been currently widely adopted will be explained below.
FIG. 13
is a diagrammatic sectional view for explaining a structural example of the main part of the cathode ray tube which adopts the conventional electromagnetic type velocity modulation and which comprises a cathode K, a first electrode
1
(control electrode), a second electrode
2
(first accelerating electrode), a third electrode
3
(second accelerating electrode), a fourth electrode
4
(focusing electrode) and a fifth electrode
5
(anode electrode).
The cathode ray tube has a panel portion (not shown) having a phosphor screen and a vacuum envelope comprising a funnel portion
22
and a neck portion
23
. An electron gun is housed within the neck portion
23
and a deflecting yoke
30
is externally mounted around the transition area between the neck portion
23
and the funnel portion
22
.
The cathode ray tube also has a convergence regulating and color purity regulating correcting magnetic device
31
externally mounted at the neck portion
23
where the electron gun is housed at the position leaning toward the cathode side from the position where the deflecting yoke
30
is externally mounted and a velocity modulating coil
32
externally mounted at the neck portion
23
at the position leaning toward the cathode side from the position where the correcting magnetic device
31
is externally mounted.
The fourth electrode
4
, i.e., the focusing electrode, is a relatively deep (long in the tube axial direction) cylindrical electrode as a whole and its inside is an almost equipotential space. Positive (scan direction) or negative (reverse direction from the scan direction) deflection in the horizontal scan direction acts momentarily on the electron beam passing through the fourth electrode
4
by a magnetic field caused by current flowing through the velocity modulating coil
32
.
The direction of the positive deflection is the same with the horizontal deflecting direction caused by the deflecting yoke
30
, so that the horizontal scan velocity of the electron beam on the screen becomes fast. The direction of the negative deflection is opposite from the horizontal deflecting direction caused by the deflecting yoke
30
, so that the velocity of the electron beam on the screen becomes almost zero, thus enhancing the contrast and improving the image quality as described above.
While the velocity modulating coil
32
is mounted at any place on the way where the electron beam passes in principle, it must be mounted at the place distant from the deflecting yoke
30
by a predetermined distance so that no interference occurs with it.
Accordingly, the velocity modulating coil
32
cannot but be mounted at the place toward the cathode K rather than the fourth electrode
4
, i.e., the focusing electrode. Ideally, it is disposed at the outer periphery of the fourth electrode
4
composing the focusing electrode as shown in FIG.
13
.
However, because the relatively large convergence regulating and purity regulating correcting magnetic device
31
is attached to the outside of the neck portion where the fourth electrode
4
is located from the relationship of disposition of the parts at the neck portion, the velocity modulating coil
32
is attached to the position leaned toward the third electrode
3
rather than the fourth electrode
4
.
Because frequency of current flowing the velocity modulating coil
32
is high and the fourth electrode
4
is made of non-magnetic metallic material such as stainless steel similarly to the other electrodes, eddy current is generated within the electrode when magnetic field acts on it from the velocity modulating coil
32
.
The eddy current suppresses the magnetic flux acting on the space of the fourth electrode
4
and diminishes the velocity modulating effect.
FIG. 14
is a sectional view for explaining one structural example of a conventional electron gun. The same reference numerals therein denote the same or corresponding parts in FIG.
13
. The fourth electrode (focusing electrode)
4
is divided into a first cylindrical focusing electrode
4
B (fourth bottom electrode) and a second cylindrical focusing electrode
4
T (fourth top electrode) in the tube axial direction.
The first cylindrical focusing electrode
4
B(fourth bottom electrode) is electrically connected with the second cylindrical focusing electrode
4
T(fourth top electrode) by a connecting line
7
disposed at the outside of the respective electrodes so as to have equal potential. It is noted that the third electrode
3
and the fifth electrode
5
have the equal potential, focusing voltage Vf is applied to the first cylindrical focusing electrode
4
B and they are electrically connected by connectors
8
, respectively.
The reference numerals
1
t
,
2
t
,
3
t
,
4
t
−
1
,
4
t
−
2
and
5
t
are electrode supports (bead supports) for embedding and fixing the first electrode
1
, the second electrode
2
, the third electrode
3
, the first cylindrical focusing electrode
4
B, the second cylindrical focusing electrode
4
T and the fifth electrode
5
to an insulating support (bead glass)
6
, respectively.
The electron gun shown in the figure is a so-called large aperture single electron gun used for a projection type cathode ray tube in particular and has a large diameter portion
4
F at the edge region of the second cylindrical focusing electrode
4
T of the fourth electrode
4
. The large diameter portion
4
F is inserted to the inside of the fifth electrode
5
, i.e., the anode electrode. It is noted that the cathode is not shown in the figure.
Since focusing electrode have a gap between the first cylindrical focusing electrode
4
B and the second cylindrical focusing electrode
4
T as shown in
FIG. 14
, the magnet
Nakayama Toshio
Suzuki Nobuyuki
Tanaka Yasuo
Tobe Akiyoshi
Hitachi Ltd
Krishnan Sumati
Milbank Tweed Hadley & McCloy LLP
O'Shea Sandra
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