BACKGROUND OF THE INVENTION
[0001] The present invention relates to a color display system and particularly to a cathode
ray tube having improved resolution over the entire phosphor screen and a color display
system provided with this cathode ray tube.
[0002] The resolution of a color cathode ray tube depends on the size and shape of beam
spots on the phosphor screen.
[0003] If the beam spot formed by impingement of an electron beam emitted from an electron
gun onto the phosphor screen and resultant luminescence of the phosphor screen is
small in diameter and close to a true circle, it provides a good resolution.
[0004] The electron beam emitted from the electron gun is deflected horizontally and vertically
on the way to the phosphor screen and reaches the phosphor screen. The central area
and peripheral area of the phosphor screen are different in the distance from the
center of deflection from each other, so that as the deflection of the electron beam
increases, the shape of the beam spot elongates vertically for the most part.
[0005] In a so-called in-line electron gun emitting three electron beams, the two side electron
beams are displaced from the tube axis, so that their convergence is degraded in the
peripheral area of the phosphor screen and the resolution deteriorates.
[0006] Fig. 1 is a schematic cross sectional view illustrating a structure example of a
color cathode ray tube to which the present invention is applied. Numeral 1 indicates
a panel portion, 2 a funnel portion, 3 a neck portion, 4 a phosphor screen, and 5
a shadow mask which is a color selection electrode. Numeral 6 indicates a third electrode,
7 a fourth electrode, 8 a shield cup, 14 a deflection yoke, 15, 16, and 17 center
axes of electron beams, and 18 and 19 centers of the side electron beam passage apertures
of the fourth electrode 7.
[0007] Cathode portions K1, K2, and K3, a first electrode 10, and a second electrode 20
constitute a so-called triode portion.
[0008] As shown in the figure, the color cathode ray tube comprises an evacuated envelope
formed of the panel portion 1 and the neck portion 3 joined to the side wall of the
panel portion 1 via the funnel 2, an electron gun incorporated in the neck portion
3, the deflection yoke 14 mounted on the outer wall of the funnel portion 2 and the
neck portion 3 in the neighborhood of their junction, and the multi-apertured shadow
mask 5 in predetermined spaced relation adjacent to the phosphor screen 4.
[0009] Striped or dotted phosphors of red, green, and blue are coated on the phosphor screen.
[0010] Three electron beams emitted from the electron gun are color-selected by the shadow
mask 5, impinge on the phosphors associated with the respective electron beams and
cause the phosphors to luminesce.
[0011] The electron gun comprises an electron beam generation portion for generating, accelerating,
and controlling three parallel electron beams of in-line arrangement from the cathode
portions K₁, K₂, and K₃, a prefocus lens portion for focusing the electron beams slightly,
and a main lens portion for focusing the electron beams on the phosphor screen 4 and
the three electron beams are deflected by the magnetic deflection yoke 14 so as to
scan the beams in a rectangular raster over the phosphor screen 4.
[0012] The constitution shown in the figure is an example and a variety of electron guns
are known in terms of the number of electrodes constituting the electron gun, the
shapes of electron beam apertures in the electrodes, and the structures of the electrodes.
[0013] Fig. 2 is an illustration of the magnetic deflection field by the deflection yoke
acting on electron beams. The magnetic deflection field by the magnetic deflection
yoke has, as shown in the figure, a pin cushion shaped distortion 14H in the horizontal
deflection field and a barrel shaped distortion 14V in the vertical deflection field.
[0014] Figs. 3A and 3B are illustrations of the deflection and shape distortion of an electron
beam spot by the magnetic deflection field. An electron beam B deflected to the periphery
of the phosphor screen is subject to diffusing force fh in the horizontal direction
and focusing force fv in the vertical direction as shown in Fig. 3B in addition to
the force Fh for deflecting the electron beam as shown in Fig. 3A and forms a distorted
spot shape.
[0015] Fig. 4 is an illustration of the beam spot shapes on the phosphor screen. Although
the beam spot OO in the center area of the phosphor screen 3 is circular, the beam
spots generated in the peripheral area of the phosphor screen are distorted to a non-circle
comprising a core BC of high intensity and a halo BH and particularly the large vertical
elongation of the halo BH affects adversely the focus characteristic.
[0016] As a countermeasure for degradation of the focus characteristic, for example, an
art disclosed in Japanese Patent Application Laid-Open 62-58549 may be cited.
[0017] Fig. 5 is a cross sectional view illustrating the constitution of the electron gun
disclosed in the aforementioned prior art. Symbols K1, K2, and K3 indicate cathodes,
numeral 10 a control grid, 20 an accelerating electrode, 30 a first focus electrode,
40 a second focus electrode, 48 a rim electrode, 50 a third focus electrode, 60 an
anode, 11, 12, 13, 21, 22, 23, 31, 32, 33, 41a, 42a, 43a, 41b, 42b, 43b, 51a, 52a,
53a, 51b, 52b, 53b, 61, 62, and 63 respective electron beam passage apertures thereof,
44, 45, 46, and 47 vertical plates, and 54 and 55 horizontal plates. Symbol C indicates
an electron gun axis (coincides with the tube axis), S1 a displacement of each of
the side electron beams from the electron gun axis C, and S2 a displacement of each
of the side electron beam passage apertures 61 and 63 of the anode 60 from the electron
gun axis C.
[0018] Fig. 6 is a plan view of the accelerating electrode 20 in a direction of the arrow
100 shown in Fig. 5, and Fig. 7 is also a plan view of the second focus electrode
40 in a direction of the arrow 101, and Fig. 8 is also a plan view of the third focus
electrode 50 in a direction of the arrow 102.
[0019] As shown in Fig. 6, slits 24, 25, and 26 elongated in the inline direction of the
three electron beams are superposed on the three circular electron beam passage apertures
21, 22 and 23 on the first focus electrode 30 side of the accelerating electrode 20.
[0020] As shown in Fig. 7, the second focus electrode 40 has the circular electron beam
passage apertures 41b, 42b, and 43b on the side of the third focus electrode 50, opposes
the third focus electrode 50, and furthermore has a first plate electrode (vertical
plate) comprising the four vertical parallel plates 44, 45, 46, and 47 which are attached
on the opposite sides of each aperture so as to extent toward the third focus electrode
50.
[0021] The second focus electrode 40 has the rim electrode 48 which surrounds the first
plate electrode and extends a predetermined distance from ends 44a, 45a, 46a, and
47a of the parallel plates toward the third focus electrode 50.
[0022] As shown in Fig. 8, the third focus electrode 50 has the three circular electron
beam passage apertures 51a, 52a, and 53a on the side of the second focus electrode
40 and has a second plate electrode (horizontal plate) comprising a pair of horizontal
parallel plates 54 and 55 which are attached so as to sandwich the three circular
electron beam passage apertures vertically and to extend toward the second focus electrode
40. The ends 54a and 55a of the horizontal parallel plates constituting the second
plate electrode extend into the rim electrode 48 of the second focus electrode 40
and are spaced a predetermined interval L from the ends 44a, 45a, 46a, and 47a of
the vertical parallel plates of the second focus electrode 40 along the electron gun
axis.
[0023] The anode 60 has the three circular electron beam passage apertures 61, 62, and 63
on its end face. Between the displacement S2 of the side electron beam passage apertures
61 and 63 from the electron gun axis and the displacement S1 of the cathodes K₁ and
K₃ and the side electron beam passage apertures of the control grid 10, the accelerating
electrode 20, the first focus electrode 30, the second focus electrode 40, and the
third focus electrode 50 preceding the anode 60, a relation of S2>S1 is held, a main
lens is formed between the third focus electrode 50 and the anode 60, and the side
electron beams SB1 and SB2 are converged at a point on the phosphor screen.
[0024] In operation of the electron gun, 50 to 170 V is applied on the cathodes K₁, K₂,
and K₃, 0 to-150 V on the control grid 10, 400 to 800 V on the accelerating electrode
20, 5 to 8 kV on the second focus electrode 40 as a focus voltage Vf, 23 to 30 kV
on the anode 60 as an anode voltage Eb, and a dynamic voltage DVf which varies in
synchronization with the horizontal and vertical deflections of the electron beams
on the first focus electrode 30 and the third focus electrode 50.
[0025] When the electron beams are undeflected, there exists no potential difference between
the first focus electrode 30, the second focus electrode 40, and the third focus electrode
50. Therefore, the presence of the parallel plates (vertical plates) 44, 45, 46, and
47 in the second focus electrode 40 and the parallel plates (horizontal plates) 54
and 55 attached to the third focus electrode 50 exerts no influence on the beams and
the cross section of the electron beams are elongated horizontally by a quadrupole
lens formed by the slits 24, 25, and 26 elongated in the inline direction of the three
electron beams on the side of the first focus electrode 30 of the accelerating electrode
20 but the electron beams are brought into an optimum focus on the phosphor screen
by the main lens between the third focus electrode 50 and the anode 60.
[0026] Fig. 9 is an illustration of an electron beam bundle emitted from the accelerating
electrode 20 under the aforementioned operating voltage condition and Fig. 10 is a
schematic diagram expressing the electron beam trajectories electron-optically.
[0027] The electron beams leaving the slits 24, 25, and 26 of the accelerating electrode
20 are subjected to a strong vertical focusing action and the cross section of each
electron beam is elongated horizontally on the phosphor screen as shown in Fig. 9.
In this case, the H portion of high current density is formed in the center of each
cross section and the L portions of low current density are formed on both sides thereof.
[0028] When the electron beam is undeflected, the electron trajectories are as shown in
Fig. 10, and the electron beam is overfocused horizontally indicated with Ph and underfocused
vertically indicated with Pv, due to spherical aberration and the focus voltage is
adjusted for focus within the shown range W on the phosphor screen.
[0029] The beam spot on the phosphor screen at this time has a vertically elongated shape
comprising the H portion of high current density.
[0030] Fig. 11 is an illustration of an effect on beam spots by the parallel plates (vertical
plates) 44, 45, 46, and 47 in the second focus electrode 40 and the parallel plates
(horizontal plates) 54 and 55 attached to the third focus electrode 50 and Fig. 12
is an illustration of an effect on a beam spot by the parallel plates (horizontal
plates) 54 and 55 attached to the third focus electrode 50.
[0031] When the deflection amount of each electron beam is increased, the potentials of
the first focus electrode 30 and the third focus electrode 50 is made higher than
the potential of the second focus electrode 40. Therefore, a strong horizontally focusing
lens action (Fv<Fh) by the parallel plates (vertical plates) (44), 45, 46, and (47)
in the second focus electrode 40 as shown in Fig. 11 and a strong vertically divergent
lens action Fvv by the parallel plates (horizontal plates) 54 and 55 attached to the
third focus electrode 50 as shown in Fig. 12, constitute a quadrupole lens electric
field and the cross section of the electron beam is shaped to be elongated vertically,
and at the same time the potential difference between the third focus electrode 50
and the anode 60 is reduced, and the focusing action by the main lens is weakened,
and the electron beams are brought into an optimum focus in the peripheral area of
the phosphor screen.
[0032] The aforementioned quadrupole lens action acts so as to cancel the effect on the
electron beams by the magnetic deflection aberration, so that the electron beams are
brought into an optimum focus on the screen. However, the entrance angle of the electron
beam into the main lens formed by the third focus electrode 50 and the anode 60 and
the beam diameter are different between the horizontal direction and the vertical
direction, and it is impossible to make the shape of the beam spot closer to a circle
because the lens magnification in the main lens is different between the horizontal
direction and the vertical direction.
[0033] Figs. 13A and 13B are illustrations of a light-optical equivalent of the quadrupole
lens action by the second and third focus electrodes and the electron beam trajectories
when the electron beams are deflected horizontally, and Fig. 13A is a horizontal cross
sectional view, and Fig. 13B is a vertical cross sectional view. Numeral 70 indicates
a crossover point of an electron beam equivalent to an object of the lens system,
72 a convex lens representing the horizontal focusing action by a quadrupole lens
electric field formed between the second focus electrode and the third focus electrode,
73 a main lens, 74 a concave lens representing the horizontal diverging action by
the magnetic deflection field, 75 a phosphor screen, 76 an electron beam trajectory,
78 a concave lens representing the vertical diverging action, 79 a convex lens representing
the vertical focusing action by the magnetic deflection field, and 80 a beam impinging
point on the phosphor screen.
[0034] As shown in the figure, the electron lens system can be represented by a light-optics
equivalent of a sequential arrangement of the convex, convex, and concave lenses in
a horizontal cross section from the object 70 side and a sequential arrangement of
the concave, convex, and convex lenses in a vertical cross section. When the lens
system is adjusted for horizontally and vertically optimum focuses, the horizontal
and vertical entrance angles of the beam impinging on the phosphor screen 75 have
a relation of αH<αV.
[0035] Assuming that an electron beam leaves the object 70 at an exit angle α and impinges
on a position 80 at the entrance angle α0 on the phosphor screen via the lens system,
and the potentials at the object 70 and the phosphor screen are V and V' respectively,
the electron lens system magnification M can be generally expressed by

, and the horizontal magnification MH of the lens system can be expressed by

and the vertical magnification MV can be expressed by

.
[0036] As mentioned above, the horizontal and vertical entrance angles of impinging on the
phosphor screen 75 have a relation of αH<αV, resulting in the relationship of the
lens magnifications MV<MH, and the beam spot diameter becomes elongated horizontally.
[0037] To correct the horizontal and vertical lens magnifications, the slits 24, 25, and
26 are formed in the accelerating electrode 20 as shown in Fig. 6.
[0038] Figs. 14A and 14B are illustrations of light-optics equivalents representing a correction
of the horizontal and vertical lens magnifications by the slits of the accelerating
electrode, and Fig. 14A is a horizontal cross sectional view, and Fig. 14B is a vertical
cross sectional view.
[0039] As shown in Figs. 14A and 14B, the quadrupole lens electric field generated by the
slits of the accelerating electrode produces a convex lens 71 having a weak focusing
action in the horizontal direction and a convex lens 77 having a strong focusing action
in the vertical direction.
[0040] An electron beam emitted from the object 70 at an angle of α enters the convex lens
71 in the horizontal direction the focusing action of which is weaker than that in
the vertical direction, so that the exit angle in the horizontal direction becomes
α' close to α and the exit angle in the vertical direction becomes α'' smaller than
α. In this case, the object position viewed from the electron beam having passed the
convex lens 71 or 77 generally moves backward from the object 70. However, since the
accelerating electrode is at the crossover position, this shift is small and can be
ignored.
[0041] The exit angle of the electron beam in the vertical direction is made smaller than
that in the horizontal direction by the quadrupole lens electric fields (convex lenses)
71 and 77 generated by the slits of the accelerating electrode. As a result, the vertical
entrance angle α'V of an electron beam which passes through the electron lens system
and strikes the beam impinging point 80 on the phosphor screen will not become excessively
larger than the horizontal entrance angle α'H and α'V can be considered to be nearly
equal to α'H. Namely, the vertical and horizontal lens magnifications MV and MH can
be considered nearly equal to each other.
[0042] By doing this, an optimum focus characteristic can be obtained over the entire phosphor
screen.
SUMMARY OF THE INVENTION
[0043] According to the aforementioned prior art, when electron beams are undeflected, the
quadrupole lens by the slits of the accelerating electrode operates so that the electron
beams are elongated horizontally. Therefore, the beam spots on the phosphor screen
are elongated vertically from the relation with the aforementioned current density
distribution and the cross section of the electron beam is increased by correction
of the difference between the horizontal and vertical focal lengths, accordingly the
horizontal resolution is easily degraded.
[0044] In the prior art electron gun, for a large beam current operation the quadrupole
lens formed by the slits of the accelerating electrode produces a stronger effect
on the electron beam. When the beam is undeflected, the vertical diameter of the beam
spot increases further, and when the electron beam is deflected to the corners of
the phosphor screen, the quadrupole lens action (horizontal elongation of the cross
section) on the beam is stronger and the horizontal diameter of the electron beam
inside the main lens increases and consequently the spherical aberration affects more
adversely, and increases the horizontal diameter of the electron beam.
[0045] These degrade uniformity of the beam spot over the entire phosphor screen depending
upon the amount of the beam current.
[0046] The current density of an electron beam is unevenly distributed so that it is high
in the center and low at the peripheries, and the current density distribution is
easily imbalanced due to the physical variations of the electrodes and the assembly
errors thereof of the electron gun. When the electron beam is deflected to the corners
of the phosphor screen, the portion of low current density is imbalanced further due
to the magnetic deflection field and the image quality is degraded.
[0047] An object of the present invention is to solve the aforementioned problems with the
prior art and to provide a color cathode ray tube having an electron gun which can
produce a satisfactory resolution over the entire phosphor screen and a color display
system using it.
[0048] According to one aspect of this invention there is provided a color cathode ray tube
having an electron gun comprising at least a cathode, a control electrode, an accelerating
electrode, a focus electrode and an anode spaced axially in the order named, wherein
the focus electrode compries at least a first focus electrode, a second focus electrode
and a third focus electrode spaced in the order named, the first focus electrode faces
the accelerating electrode, a first quadrupole lens structure is formed by at least
one of a portion of the first focus electrode facing the second focus electrode and
a portion of the second focus electrode facing the first focus electrode, and a second
quadrupole lens structure is formed by at least one of a portion of the second focus
electrode facing the third focus electrode and a portion of the third focus electrode
facing the second focus electrode.
[0049] According to another aspect of this invention there is provided a color display system
including a color cathode ray tube having an electron gun comprising at least a cathode,
a control electrode, an accelerating electrode, a focus electrode and an anode spaced
axially in the order named, wherein the focus electrode comprises at least a first
focus electrode, a second focus electrode and a third focus electrode spaced in the
order named, the first focus electrode faces the accelerating electrode, a first quadrupole
lens structure is formed by at least one of a portion of the first focus electrode
facing the second focus electrode and a portion of the second focus electrode facing
the first focus electrode, a second quadrupole lens structure is formed by at least
one of a portion of the second focus electrode facing the third focus electrode and
a portion of the third focus electrode facing the second focus electrode, and a dynamic
focus voltage varying with deflection of an electron beam to a voltage higher than
a voltage applied to the second focus electrode is applied to the first and third
focus electrodes so that the first quadrupole lens structure produces horizontally
diverging and vertically focusing actions on the electron beam and the second quadrupole
lens structure produces horizontally focusing and vertically diverging actions on
the electron beam.
[0050] According to the present invention having the aforementioned constitution, when the
electron beam is undeflected the horizontal and vertical lens magnifications can be
made equal to each other in the main lens formed between the third focus electrode
and the anode, and an electron beam emitted from the cathode produces almost a truly
circular and small beam spot.
[0051] When the deflection amount of an electron beam is increased, the electron beam is
initially elongated horizontally by horizontally diverging and vertically focusing
actions produced by the quadrupole lens formed between the first focus electrode and
the second focus electrode and subsequently by vertically diverging and horizontally
focusing actions produced by the quadrupole lens formed between the second focus electrode
and the third focus electrode, the imbalance between the vertical and horizontal lens
magnification is corrected. Furthermore, the amount of correction is varied with the
deflection amount of the electron beam, and correction in the lens magnifications
can be designed as desired, and the current density distribution in the horizontally
elongated electron beam bundle becomes almost uniform unlike that when the accelerating
electrode 20 shown in Fig. 6 is used, and the imbalance amount in halo due to the
assembling errors of the electron gun is reduced.
[0052] When an electron beam is undeflected, the electron beam emitted from the cathode
can provide a truly circular and small beam spot by the main lens formed between the
third focus electrode and the anode.
[0053] Furthermore, according to the present invention, when an electron beam is deflected
and the voltage applied to the first focus electrode is raised, the electric field
strength in the spacing between the accelerating electrode and the first focus electrode
increases (the lens magnification increases) and the angle of divergence of the electron
beam leaving the accelerating electrode is reduced.
[0054] This reduction of the divergence angle of the electron beam decreases the beam diameter
within the preceding one of the two quadrupole lenses and the beam diameter within
the main lens when the electron beam is deflected, suppresses the horizontal spreading
of the electron beam at a large current and reduces the influences of spherical aberration
of the main lens and those of deflection aberration produced by the magnetic deflection
field. The reductions in these two aberrations improve uniformity of the shapes of
the beam spots over the entire phosphor screen in a range of small to large currents.
BRIEF DESCRIPTION OF THE DRAWINGS
[0055] Fig. 1 is a schematic cross sectional view illustrating a structural example of a
color cathode ray tube to which the present invention is applied.
[0056] Fig. 2 is an illustration of magnetic deflection fields acting on an electron beam
generated by a deflection yoke.
[0057] Figs. 3A and 3B are illustrations of deflection of an electron beam and a distortion
of the shape of the electron beam spot by a magnetic deflection field.
[0058] Fig. 4 is an illustration of shapes of the beam spot on the phosphor screen.
[0059] Fig. 5 is a cross sectional view illustrating the constitution of an electron gun
of the prior art.
[0060] Fig. 6 is a plan view of the accelerating electrode in a direction of the arrows
100 shown in Fig. 5.
[0061] Fig. 7 is a plan view of the second focus electrode in a direction of the arrow 101
shown in Fig. 5.
[0062] Fig. 8 is a plan view of the third focus electrode in a direction of the arrow 102
shown in Fig. 5.
[0063] Fig. 9 is an illustration of the beam spot shape on the phosphor screen under the
operating voltage condition shown in Fig. 5.
[0064] Fig. 10 is a schematic diagram expressing a lens action on an electron beam.
[0065] Fig. 11 is an illustration of effects of parallel plates (vertical plates) in the
second focus electrode and parallel plates (horizontal plates) attached to the third
focus electrode on a beam spot.
[0066] Fig. 12 is an illustration of an effect of parallel plates (horizontal plates) attached
to the third focus electrode on a beam spot.
[0067] Figs. 13A and 13B are illustrations of electron beam trajectories when an electron
beam is deflected horizontally by using light-optics equivalents.
[0068] Figs. 14A and 14B are illustrations of corrections of the horizontal and vertical
lens magnifications by the slits of the accelerating electrode by using light-optics
equivalents.
[0069] Fig. 15 is a cross sectional view illustrating the constitution of an embodiment
of an electron gun for a color cathode ray tube of the present invention.
[0070] Figs. 16A and 16B are a front view of the first focus electrode in a direction of
the arrow 103 shown in Fig. 15 and an illustration of an action thereof on an electron
beam respectively.
[0071] Fig. 17 illustrates a lens action on an electron beam in the neighborhood of the
accelerating electrode shown in Fig. 15.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0072] The embodiments of the present invention will be explained in detail hereunder with
reference to the accompanying drawings.
[0073] Fig. 15 is a cross sectional view illustrating the constitution of an embodiment
of an electron gun for a color cathode ray tube of the present invention.
[0074] Fig. 16A is a front view of the first focus plate electrode in a direction of the
arrow 103 shown in Fig. 15 and Fig. 16B is an illustration of an action of the electrode
shown in Fig. 16A on an electron beam.
[0075] In Figs. 15, 16A, and 16B, symbols K1, K2, and K3 indicate cathodes, numeral 10 a
control electrode, 20 an accelerating electrode, 30 a first focus electrode, 35 a
first focus plate electrode, 40 a second focus electrode, 48 a rim electrode, 50 a
third focus electrode, 60 an anode, 11, 12, 13, 21, 22, 23, 31a, 32a, 33a, 31b, 32b,
33b, 41a, 42a, 43a, 41b, 42b, 43b, 51a, 52a, 53a, 51b, 52b, 53b, 61, 62, and 63 electron
beam passage apertures thereof, respectively, 36, 37, and 38 vertically elongated
rectangular apertures, 44, 45, 46, and 47 vertical plates, and 54 and 55 horizontal
plates.
[0076] Symbol C indicates an electron gun axis (coincides with the tube axis), S1 a displacement
of each of the side electron beams from the electron gun axis C, and S2 a displacement
of each of the side electron beam passage apertures 61 and 63 of the anode 60 from
the electron gun axis C.
[0077] The first focus electrode 30 has the circular beam passage apertures 31a, 32a, 33a,
31b, 32b, and 33b. The first focus plate electrode 35 has the vertically elongated
rectangular apertures 36, 37, and 38 and is electrically connected to the first focus
electrode 30.
[0078] The second focus electrode 40 has a first plate electrode (vertical plate) formed
of the four vertical parallel plates 44, 45, 46, and 47 attached on the opposite sides
of each of the three circular electron beam passage apertures 41b, 42b, and 43b on
its end face on the side of the third focus electrode 50. The second focus electrode
40 has the rim electrode 48 which surrounds the first plate electrode and extends
a predetermined distance from ends 44a, 45a, 46a, and 47a of the parallel plates toward
the third focus electrode 50.
[0079] The third focus electrode 50 has the three circular electron beam passage apertures
51a, 52a, and 53a in its end face on the side of the second focus electrode 40 and
has a second plate electrode (horizontal plate) formed of a pair of horizontal parallel
plates 54 and 55 attached thereon and extending toward the second focus electrode
40 so as to sandwich the electron beam passage apertures vertically.
[0080] The ends 54a and 55a of the parallel plates 54 and 55 constituting the second plate
electrode extend into the rim electrode 48 of the second focus electrode 40 and are
spaced a predetermined interval L from the ends 44a, 45a, 46a, and 47a of the vertical
parallel plates of the second focus electrode 40 along the electron gun axis.
[0081] In the end face of the anode 60, the three circular electron beam passage apertures
61, 62, and 63 are formed. Between the displacement S2 of the side electron beam passage
apertures in the anode 60 from the electron gun axis and the displacement S1 of the
cathodes K₁ and K₃, and the side electron beam passage apertures of the control grid
10, of the accelerating electrode 20, of the second focus electrode 40, and of the
third focus electrode 50, a relation of S2>S1 is maintained, a main lens is formed
between the third focus electrode 50 and the anode 60, and the side electron beams
SB1 and SB2 are designed to converge on the center electron beam CB on the phosphor
screen.
[0082] In operation of the electron gun, 50 to 170 V is applied on the cathodes, 0 to -150
V on the control grid, 200 to 1000 V on the accelerating electrode, 4 to 10 kV on
the second focus electrode 40 (hereinafter V
f), 23 to 30 kV on the anode (hereinafter E
b), and a dynamic voltage DVf which varies in synchronization with the horizontal and
vertical deflections of the electron beams on the first focus electrode 30, the first
focus plate electrode 35, and the third focus electrode 50.
[0083] When the electron beams are undeflected, there exists no potential difference between
the first focus electrode 30, the first focus plate electrode 35, the second focus
electrode 40, and the third focus electrode 50. Therefore, the presence of the vertically
elongated rectangular apertures 36, 37, and 38 in the first focus plate electrode
35, the parallel plates (vertical plates) 44, 45, 46, and 47 in the second focus electrode
40, and the parallel plates (horizontal plates) 54 and 55 attached to the third focus
electrode 50 exerts no influence on the electron beams and the electron beams from
the cathodes form circular and small beam spots on the phosphor screen by the main
lens formed between the third focus electrode 50 and the anode 60.
[0084] When the deflection amount of an electron beam increases and the potential of the
first focus electrode 30 increases as shown in Fig. 17, the potential difference between
the first focus electrode 30 and the accelerating electrode 20 increases further and
the equipotential lines E1, E2, and E3 indicated by solid lines between the first
focus electrode 30 and the accelerating electrode 20 change to more sharply curved
equipotential lines E1', E2', and E3' indicated by dotted lines.
[0085] The electron beam at this time is subjected to a stronger focusing action than that
when the magnetic deflection field is 0, and the angle of beam divergence of the electron
beam trajectory Bc in the aperture 31b of the first focus electrode 30 is reduced
as indicated by the trajectory Be, and the electron beam enters between the first
focus plate electrode 35 and the second focus electrode 40, is horizontally elongated
in its cross section by the quadrupole lens action, and then enters the lenses between
the second focus electrode 40 and the third focus electrode 50 and between the third
focus electrode 50 and the anode 60, successively.
[0086] A dynamic voltage varying by an amount (200 to 800 V for the useful scanned area
of the phosphor screen, for example) close to a voltage applied to the accelerating
electrode 20 (200 to 1000 V, for example) is applied to the first focus electrode
30 facing the accelerating electrode 20 supplied with a comparatively low voltage
(200 to 1000 V, for example) synchronised with deflection of the beam, resulting in
effective dynamic focus variation.
[0087] It is preferable that a dynamic differential focus voltage Dv for the useful scanned
area of the phosphor screen of the color cathode ray tube and a voltage Av applied
to the accelerating electrode 20 measured with respect to the control electrode 10
satisfy the following inequality,

where a dynamic differential focus voltage Dv is a voltage difference between
a dynamic focus voltage when the beam is at the center of the phosphor screen and
a dynamic focus voltage when the beam is at the extreme right or left edge and the
top or the bottom of the useful scanned area on the phosphor screen.
[0088] In the present invention, when an electron beam is deflected, the spreading of the
electron beam due to an increase in the current can be suppressed by the enhanced
focus lens action by the accelerating electrode 20 and particularly the effect of
the spherical aberration due to the horizontal spreading in the main lens by the quadrupole
lens formed between the first focus plate electrode 35 and the second focus electrode
40 can be suppressed.
[0089] Although the effect of the quadrupole lens formed between the first focus plate electrode
35 and the second focus electrode 40 is reduced because the ratio of the diameter
of the electron beam to the diameter of the quadrupole lens is reduced, the beam diameter
in the magnetic deflection field is also reduced, aberration caused by the magnetic
deflection field (quadrupole lens) is reduced, and a correction of the imbalance between
the horizontal and vertical lens magnifications can be maintained.
[0090] When the deflection amount of an electron beam increases, the potentials of the first
focus electrode 30, the first focus plate electrode 35, and the third focus electrode
50 become higher than the potential of the second focus electrode, a vertically elongated
divergent lens is formed as shown in Fig. 16B by the vertically elongated slits 36,
37, and 38 of the first focus plate electrode 35 and the electron beam is subjected
to a diverging action stronger in the horizontal direction than that in the vertical
direction (Fh>Fv) and horizontally elongated in its cross section.
[0091] The aforementioned quadrupole lens electric field for elongating an electron beam
vertically is formed by the parallel plates (vertical plates) 44, 45, 46, and 47 in
the second focus electrode 40 and the parallel plates (horizontal plates) 54 and 55
attached to the third focus electrode 50, and the potential difference between the
third focus electrode 50 and the anode 60 is reduced, and the focusing action by the
main lens is weakened.
[0092] Since the diameter of the quadrupole lens is large compared to the bundle of horizontally
elongated electron beams shaped by the quadrupole lens between the first focus plate
electrode 35 and the second focus electrode 40, the current density distribution becomes
uniform. An imbalance of the lens magnifications for the horizontally elongated electron
beams is corrected between the second focus electrode 40 and the third focus electrode
50 and between the third focus electrode 50 and the anode 60.
[0093] According to the present invention, a satisfactory resolution can be obtained over
the entire phosphor screen.
[0094] As mentioned above, according to the present invention, an electron beam emitted
from the cathode is subject to the equal horizontal and vertical lens magnifications
of the main lens between the third focus electrode and the anode when the electron
beam is not deflected, so that the electron beam spot becomes almost truly circular
and small.
[0095] When the deflection amount of an electron beam is increased, the electron beam is
elongated horizontally by the quadrupole lens exerting horizontally diverging and
vertically focusing actions formed between the first focus electrode and the second
focus electrode and then an imbalance between the vertical and horizontal lens magnifications
is corrected by the quadrupole lens exerting vertically diverging and horizontally
focusing actions formed between the second focus electrode and the third focus electrode.
[0096] According to the present invention, a satisfactory resolution can be produced over
the entire phosphor screen of from high to low brightness.