Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED CRT FOCUS UTILISING MAGNETnI.C MEANS
This invention relates to a method and apparatus for
improving the focus oE an electrostatically focused cathode ray
tube ( CRT ) .
A conventional electrostatically focused CRT comprises
an e3ectron gun comprising a cathode for emitting electrons,
accelerating electrodes and focusing electrodes the CRT also
includas a means such as a magnetic yoke to deflec-t an electron
beam, and a florescent screen. If the cathode, electrodes and yoke
are not co-axial so that the electron beam does not travel along
the axis of the focusing electrodes, focus will be degraded.
Further, if the yoke is off-axis or there are symmetry imperfect-
ions in the magnetic fields produced by the yoke, especially within
the low beam velocity region of the gun, focus will be degraded.
It is known to glue permanent magnets about the periphery
of the yoke in order to compensate for imperfections in the yoke.
~owever, such permanent magnets produce stray magnetic fields which
influence the electron beam in the vicinity of the cathode with a
consequent degradation ln focusc
U.S. Patent No. 41801,843 issued January 31, 1989 to
Mensies discloses two magnetic rings fixed on the focusing
electrodes inside the evacuated neck of a CRT. The magnetic rings
are magnetised as a dipoles in order to provide flux lines parallel
to the axis of the focusing electrode. In a well understood
manner, these parallel Elux lines serve to bunch the electron beam
on the axis of the focusing electrodes which improves the focus of
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the CRT. However, the electron beam will retain focusing problems
due to stray fields from the correction magnets on the yoke.
Accordlngly, there remains a need for a method and
apparatus to improve the focus of a CRT which avoids drawbacks of
known methods.
In accordance with the present invantion there is
provided a method for improving the focus of a CRT comprising the
step of: placing magnetic means around the neck of a cathode ray
tube and translating and orienting said magnetic means untll the
focus of the CRT improves.
In another aspect, the present invention comprises a
magnetic means for use in improving the Eocus of a cathode ray tube
comprising the Eollowing:
a support for translatable reception by the neck of a
cathode ray tube proximate the cathocle of said cathode ray tube,
said support having means to carry a plurality of magnets in a
selectable orientation; and
at least one magnet carried by said support~
whereby an electron beam emitted by said cathode may be bunched
alon~ the axis of the focusing electrode and orienting said at
least one magnet to provide a magnetic Eield making an angle with
the path of said electron beam and the degree of bunching of said
electron beam may be controlled by selec-ting the number of magnets
carried by said support.
In the figures which disclose example embodiments of the
inven-tion,
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~ igure l is a side schematic view oE a CRT made in
accordance with this invention,
~ igure 2 is an end view of a magnetic means made in
accordance with this invention,
Eigure 2a is a plan view of a magnet used in the magne-tic
means of figure 2, and
figures 3 and 4 are a schematic views of a portion oE a
CRT.
Turning to figure l, a CRT with electrostatic focusing
is illustrated schematically at lO. The CRT comprises a neck 12
and a screen 14. The neck contains a cathode 16, a control
electrode 18, accelerating electrodes 20 and 22, and a focusing
electrode 24. A yoke 26 surrounds the neck 12. The yoke 26
comprises two coils oriented so as to produce magnetic fields at
right angles to each other and to the axis of the CRT when the
coils are energised, as is well understood by those skilled in the
art. Permanent magnets 23a and 28b, which are correction magnets,
may be glued to the yoke A voltage source 32 may supply a
variable d.c. supply to one of the coils of the yoke 26. This d~c.
voltage is superimposed on the a.c. voltage which is applied to the
coils to deflect an electron beam about the screen. The remainder
of the electrical control system for the yoke is conventional and
is not shown. A magnetic means made in accordance with this
invention is shown at 30.
Figure 2 illustrates the magnetic means 30. This is seen
to comprise a support 34 which is a split ring of resilient
material. Three pos-ts 36a, 36b, and 36c extend radially outwardly
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from the ring 3~; these posts may support ring magnets of the type
shown in figure 2a at 38. In figure 2, posts 36a and 36c are shown
supporting ring magnets 38a and 38b, respectively. The ring
magnets are polarized across their diameter. The inner annulus of
the ring magnets 38 and the diameter of the posts are sized to
pro~ide an friction fit between the magnets and the posts 36a, 36b,
and 36c of the split ring; consequently, the magnets will hold any
position to which they are set on the posts and in particular may
be rotated on the posts to any desired rotational position.
Because of the resilience of -the split ring 30, it may
be snapped over the neck of a CRT and will have a snug fit with the
neck so that it will maintain any position on the neck to which it
is moved. Thus, the ring may be rotated and translated on the neck
of the CRT.
In general -terms, the operation of the CRT of figure 1
absent a consideration of magnetic means 30 and d.c. source 32 is
as follows. Electrons emitted from the cathode 16 drift toward the
control electrode 18 as a beam of electrons. The beam tends to
diverge due to the forces of repulsion between the electrons of the
beam. As the beam passes through the aper-ture oE the control
electrode 18 it is constricted by the negative voltage on this
electrode. The beam is then accelerated by the accelerating
electrode 20. The focusing electrode 24 applies a concentrating
Eorce to the electron beam. More particularly, the focusing
electrode imparts to the electrons of the beam a velocity componen-t
directed radially inwardly toward the a~is of the focusing
electrode. After the beam leaves the focusing electrode it is
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accelerated by acceleratlng electrode 22 and anode 21 and is then
deflected by the coils oE yoke 26. The deflected beam impinges on
the screen 14.
The forces of repulsion be-tween the electrons of the beam
impart a radially outwardly directed acceleration to the electrons.
The focusing electrode imparts a radially inwardly directed
component of velocity to the electrons in the beam, and converges
the beam to a point called the image point. It is intended this
image point lies on the screen~ Since the distance from the
focusing electrode to the screen varies with the angle of
deflec-tion, dynamic focusing is necessary to ensure that the beam
is focused at the screen irrespective of the angle of deflection.
More particularly, this image distance from the focusing electrode
varies directly with the voltage on the focusing electrode,
consequently, by varying the voltage of the focusing electrode
depending on the angle of deflection, the beam may be focused on
the screen at any angle of deflection. Correction magnets 28a and
2~b are glued to the yoke in an attempt to compensate for
aberration~ in the image on the screen resulting from imperfections
in the yoke.
The focus of the CRT may be improved by placing a split
ring 34 on the neck of the CRT, judiciously choosing the number of
ring magnets for the split ring 34, appropriately translating the
ring with ring magnets and rota-ting the magnets on the posts of the
ring, for reasons which will now be explained. Once the ring is
in the desired position on the neck of the CRT it may be glued in
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place and an appropriate d.c. bias voltage applied to the coils oE
the yoke in order to centre the image back on the screen.
As is well understood by those skllled in the art, iE
electrons are injected into a magnetic ~Eield which is perpendicular
to their direction of travel, the electrons will move in circles
with a radius inversely proportional to the magnetic flux density.
Consequentlyr an electron entering a magnetic field at an angle
other than nine-ty degrees will follow a helical trajectory wi-th the
axis of the helix parallel to the field lines of the field. A beam
of electrons entering a field at an angle will accordingly follow
the direction of the field with electrons which were divergent from
the direction of the field following a helical trajectory. These
helical trajectories keep the beam from spreading radially (so that
the beam is "bunched") and hence improve the electron density
distribution within a cross-section of the beam.
Figure 3 illustrates a portion of a C~T wi-th a cathode
11~ having an axis 140 ~hich is not aligned with the axis 142 of
the focusing electrode 124. ~ccordingly, absent correction, an
electron bezm emitted from the cathode will follow the axis 140 oE
the cathode and hence will be off the axis of the focusing
electrode, which will result in degraded focus. Since the ring
magnets of this invention are polarized across their diameter, the
magnetic means of this invention may be used to produce a local
magnetic field which makes any desired angle with the axis 140 of
the cathode, such as the -field lines 150 of figure 3. The strength
of this magnetic Eield may be controlLed by choosing the strength
of the magnets on the posts of the ring. If ring magnets are
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placed on the ring and rotated and the split ring translated so as
to produce field lines 150, the electrons in the beam will tend to
~ollow the direction of Eield lines 150 toward the axis 142 of the
focusing electrode. Thus the magnetic means 30 of the subject
invention may be used to correct for axial mis-alignment of the
cathode and Eocusing electrode.
Figure 4 illustrates a portion of a CRT showing stray
field lines 200 from correction magnets on the yoke of the CRT in
the vicinity of the cathode 216 and focusing electrode 224 of the
CRT. It may be noted that CRT's are frequently manufactured with
short necks which exacerbates the problem of stray fields since the
end of the yoke is proximate the cathode area. Since the
illustrated stray field induces the electron beam to follow its
path, it will be apparent that the stray field moves an electron
beam which is on the axis 242 of the focusing electron beam off-
axis. In addition, the stray field illustrated by field lines 200
reduces the velocity component oE the beam in a direction toward
the screen and parallel with axis 242. Both of these effects
degrade focusing. More particularly, the second effec-t will
develop local degradation of the image, as the image distance
depends on the velocity of the electrons in the beam in a direction
-toward the screen. Thus, a slower velocity will shorten the image
distance.
The magnetic means of the subject invention may eliminate
the degradation in focus caused by these stray fields by applying
a field in opposition to the stray field as illustrated in do-tted
lines at 2~0. Again this is achieved by appropriately orienting
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a judiciously chosen number o~ ring magnets on the split ring oE
the magnetic means.
The subject magnetic means may simultaneously correct for
both described defocusing efEects by applying resultant magne-tlc
fields which resolve -to the components 150 and 250 of figures 3 and
4.
In practice, the ring and the magnets thereon are
appropriately positioned by trial and error. That is, the screen
of a CRT may be watched while the ring is translated and rotated
and the magnets thereon rotated until focus has improved
acceptably.
Application of the fields from the magnetic means of the
subject inven-tion have the effect of displacing the entire image
on the screen. This may be corrected by applying an appropriate
d.c. bias to the yoke by means of variable d.c. voltage source 32.
The efEects illustrated in figures 3 and 4 are small and
have been exaggerated in these figures for the purpose of
explanation.
