Note: Descriptions are shown in the official language in which they were submitted.
6~
ACCELERATING AND SCAN
EXPANSION ELECTRON LENS SYSTEM
Technic~l Field
This invention relates to electrostatic
post-deflection accelerating and scan expansion lens
systems, and in particular, to such a lens system
included in a cathode ray tube in which the lenses
provide linear magnification of the elec~ron beam
deflection angle and corrected geometry of the image
displayed on the fluorescent ~creen of such tube.
Backqround of the Invention
A post-deflection accelerating and scan
expansion lens system is included in a cathode ray
tube to perform two distinct functions. The lens
system magnifies the amount of electron beam
deflection produced by the deflection means to
provide an image of the desired size on the
fluorescent screen. The lens system also increases
the velocity of the electrons in the electron beam by
means of a high intensity electric field to raise the
energy of tha electrons and thereby produce a
brighter image on ~he fluorescent screen.
A number of accelera~ing and scan expansion
lens systems make use of a focusing lens of the
.
3~:1
quadrupole type. For an electron beam traveling
toward a fluorescent screen in the Z direction and
deflected horizontally in the X direction and
vértically in the Y direction Qf a three dimensional
Cartesian coordinate system~ a quadrupole lens
conver~es with respect to its principal axis the beam
in one of the X-Z and Y-Z planes and diverges it in
the other one of the planes. ~he particular planes
of convergence and divergence are determined by the
distribution of the voltages applied to the
quadrupole lens electrodes. Thus, the paths of
deflected electron beams traveling in the Z direction
and converging in the Y-Z plane are brought to a line
focus parallel to the X axis. To obtain a point
focu~ of the electron beam on the fluorescent screen
and thereby produce an image in sharp focus and great
detail, a post-deflection lens system incorporating a
quadrupole lens operating in the manner described
requires the use of a second quadrupole lens which
converges the electron beam in the Y-Z plane and
diverges it in the X-Z plane.
Two distinct distortion mechanisms
associated with guadrupole accelerating and s~an
! expansion lens systems deform the image displayed on
the fluorescent screen. These include nonlinear
l magnification of the beam deflection angle and the
I "pincushion" type of geometry distortion. Nonlinear
magnification of the beam deflection angle is
produced by the nonuniform influence of the electric
field flux lines of the lens system on the dieection
of beam travel. The electric field expands the scan
deflection angle of the beam to produce a
corresponding light image of the desired size on the
fluorescent screen. Generally, in lens systems of
this type, an electron beam which is deflected to a
37~
great degree is not magnified in the same proportion
relative to a beam de1ected to a substantially
lesser degree. For example, a low voltage sine wave
represented in the time domain would appear on a
fluorescent screen as being frequency modulated at
the end points of the display because the nonlinear
effects of scan expansion on the time base sweep
would produce zero crossings at the ends of the sign
wave image which would ~ different from the uniform
spacing of the zero crossings at the center portion
of the display.
~ eometry distortion of the displayed image
is caused by aberrations in the shape of the electric
field flux lines in the space above and below the
lens axis. The elactric field flux line pattern
developed in the X-Y plane in a quadrupole lens
system is characterized generally as having a
plurality of parallel horizontal lines disposed
transversely of an electron beam traveling along the
z axis. Moderate fluctuations in the flux lines
produce geometry distortion of the image. Such
distortion is characterized by deformations in an
image of intended rectangular form which is displayed
as having stretched vertices and concave side
portions. An intolerable degree of geom~try
distortion of an image generally is introduced in
short length cathode ray tubes wherein the quadrupole
accelerating lens with a short focal length provides
a high intensity electric field for electron beam
deflection angle magnification.
A short length cathode ray tube is a tube
that performs compara~ly to a tube of standard length
but has an overall length of about five centimeters
less than the standard length.
The basic principles underlying the
.
~3637::1
operation of electrostatic lenses for focusing
electron beams produced in electron discharge devices
are described in U.S. Patent No. 2,412,687 of
Klemperer. ~he Xlemperer patent teaches the
S formation of an electron lens that makes use of a
pair of aligned tubular electrodes maintained at
different potentials to converge an electron beam
toward the electrode which is held at a more positive
potential. One lens system described by the
Klemperer patent includes two overlapping coaxial
cylindrical electrodes with portions projecting from
the end of the smaller diameter electrode being
embraced by the larger diameter electrode The
Klemperer patent does not suggest the use of such
lens systems for acc~leration of a deflected electron
beam and, therefore, does not disclose compensating
means for correcting nonlinear magnification of the
amount of deflection of the beam and geometry
distortion of the corresponding light image displayed
on a fluorescent screen.
A quadrupolar accelerating electrostatic
lens system which includes a "lippad" cylindrical
tube protruding into a wider tube, such as the
conductive wall coating on the neck of a cathode ray
tube envelope, is described in O. Xlemperer, Electron
Optics, 100-106 (3d ed. Cambridge University Prass,
1971). In Electron Optics, Klemperer discusses in
general the parameters associated with focusing an
electron beam for accomplishing scan magnification to
produce increased deflection of an electron beam, but
does not address the problems of nonlinear scan
expansion of the electron beam or geometry distortion
of the image displayed on a fluorescent screen.
U.S. Patent No. 3,496,406 of Deschamps
describes a cathode ray tube having an electrostatic
~ ~Ç;371
lens system that includes a quadrupole scan expansion
lens disposed within a dome-shaped post-deflection
acceleration electrode having a slot at its apex.
~he dome-shaped electrode is positioned to enclose
the portion of the guadrupole lens facing the funnel
portion of the cathode ray tube envelope that bears a
conductive coating to which is applied the
accelerating potential. The dome-shaped electrode is
held at ground potentialJ thereby providing a shield
to isolate the quadrupol~ lens from the effects of
the intense electric field developed between the
dome-shaped electrode and the conductive coating on
the inner surface of the tube. This combination of
the scan expansion quadrupole lens and the
dome-shaped electrode constitutes a lens system which
causes the electron beam paths to cross over in the
vertical plane and the electrons to be accelerated
toward the fluorescent screen after they exit the
slot in the dome-shaped electrode.
A discussion of the operation of and the
mathematical expressions relating to a short length
cathode ray oscilloscope tube having a quadrupole
scan expansion lens in conjunction with a dome-shaped
electrode accelerating system of the type disclosed
in the Deschamps patent is described in A. Martin ~
J. Deschamps, A Short Length Rectangular Oscilloscope
Tube With H~gh Deflection Sensitivi~ By Using an
Original Technique, 12 Proceedings of the Society for
Information Display 18 (lst Qtr. 1971). However,
there is no disclosure of a compensating means for
correcting distortion of the image displayed on the
screen.
U.S. Patent No. 3,792,303 of Albertin, et
alO describes a modification of the Deschamps Lens
system in an attempt to correct for distortion of the
;~ i3'~l
~ 6 --
displayed image. Albertin, et al. increases the
length of the sides of the dome-shaped electrode to
cover all bu~ one side of the quadrupole scan
expansion lens. A single disk-shaped slot electrode
is disposed perpendicular to the electron beam axis
on each end of the guadrupole lens. The first slot
electrode is positioned inside the dome-sha~ed
electrode heyond the quackupole lens and in front of
the slot in the dome-sha~d electrode. This slot
electrode is physically and electrically connected to
the dome-shaped electrode so that both elec~rodes are
at ground potential. The second slot electrode is
positioned adjacent the edge of the base portion of
the dome-shaped electrode in front of the quadrupole
lens and is electrically isolated so that it can be
raised to a voltage which is different fro~ ~hat of
the first slot and dome-shaped electrodes.
A disk-shaped electrostatic screen or shield
electrode having a conventional rectangular slot is
held at ground potential and is placed immediately in
front of the second slot electrode of Albertin, et
al. The screen electrode and dome-shaped electrode
substantially fully enclose the guadrupole lens
within an equipotential space at ground potential.
Albertin, et al. describes a compensation
technigue which is said to correct for image
distortion by separating into horizontal and vertical
components the combined effects of sc:an nonlinearity
and geometry distortion. The geometry of the
aperture of the first slot electrode included within
the dome-shaped electrode is determined
experimentally to correct for distortions ~hich
appear on the screen and are introduced by the
horizontal deflection of the electron beam during
scanning. The geometry of the aperture of the second
71
-- 7 --
slot electrode positioned in front of the quadrupole
lens is determined experimentally in conjunction with
a suitable applied potential to correct for
distortions which appear on the screen and are
introduced by the vertical deflection of the ~lectron
beam during scanning. In addition, the voltage
applied to the second slot electrode affects the
performance of the first slot electrode. Thus, the
geometry of the aperture of the first slot electrode
corrects for distortion introduced by not only the
horizontal electron bea~ trace, but also the presence
of the second slot electrode.
This compensation technique suffers from a
disadvantage in that experimental adjustments for the
lS horizontal and vertical image distor~ion components
are not independent. Thus, the aperture size and the
voltage applied to the second slot electrode affect
the aperture size and compensating electric field
produced by the first slot electrode.
U.S. Patent Nos. 4,137,479 and 4,188,563 of
Janko describe a cathode ray tu~e having a
post-deflection quadrupole lens system which, unlike
the quadrupole lenses disclosed in Deschamps and
Albertin, et al., simultaneously expands the
deflection scan of the electron beam and accelerates
the electrons toward the fluorescent screen. The
Janko lens system includes a pair of aligned tubular
entrance and exit electrodes of the same diameter,
the adjacent ends of which are spaced apart with an
air gap therebetween and have interdigitated sections
that describe complementary curvilinear courses along
the peripheries of the electrodes. The accelerating
electric field is produced by connecting the entrance
electrode to ground potential and applying the
acceleration voltage to the exit electrode which is
3'7~
electrically connected to the conductive ~oating on
the funnel portion of the tube. An octupole lens
system is positioned in front o~ and adjacent to the
entrance electrode to correct for both nonlinear scan
magnification of the electron beam and geometry
distortion of the displayed image. Janko also
~uggests an alternative embodiment which utilize a
pair of coaxial tubular electrodes of different
diameters with the outer elec~rode encompassing the
curvilinear edge portions of the inner electrode.
The Janko lens system of aligned tubular
electrodes of the same diameter preceded by an
octupole distortion correction lens is of limited
utility as an accelerating lens because of a tendancy
of dielectric breakdown to occur in the air gap
between the electrodes. Since a potential difference
of about 23 kv is applied between the electrodesr the
adjacent edges of each electrode must be smoothed to
eliminate sharp edge points of microscopic dimensions
which tend to produce excessively high electric field
strengths that cause field emission of electrons
which form an electric arc between the electrodes.
The coaxial lens system of the alternative
embodiment suggested by Janko is less susceptible to
25 dielectric breakdown, but it has the inherent charac-
teristic of producing a weaker lens which provides
less scan magnification of the electron beam deflec-
tion angle. In general, for a given applied voltage,
the focal length of such a lens is directly propor-
30 tional to the diameter of the inner electrode. Thus,an inner electrode of relatively small diameter is
required to produce a strong lens with a short focal
length. A strong scan expansion lens enhances geo-
metry distortion effects, and it has been determined
35 empirically that the Janko coaxial
37~
g
accelerating lens operating with an octupole
correction lens produçes geometry distortion to an
unacceptable degree for inner electrode diameters of
less than 1.905 centimeters. Thus, the Janko coaxial
lens system is not suitable for use in short length
cathode ray tubes, in which strong scan expansion
lenses with short focal lengths are re~uired.
U.S. Patent No. 4,124,12B of Odenthal
describes a cathode ray tube provided with a
rectangular box-shaped scan expansion lens which
includes at least four t~bular elements disposed
end-to-end and spaced apart to isolate them
electrically. The tubular elements have bias
voltages o different values that change the lens
characteristics. Distortion of the image due to
nonlinear scan expansion is corrected by ~he
incluqion of additional side plates to which a
differential bias voltage is applied.
U.S. Patent No. 3,023,336 of Frenkel
describes a cathode ray tube in which post-deflection
acceleration and scan expansion is accomplished by a
combination of an electrostatic accelerating and
converging lens with a magnetic converging lens which
create spherical aberrational effects that compensate
for each other to project an image in sharp detail on
a fluorescent screen.
Summary of the Invention
One object of this invention is to provide a
post-deflection accelerating electrostatic lens
system which accelerates an electron beam after
deflection of such beam and provides deflection scan
expansion with low distortion.
Another object of this invention is to
provide such a lens system including a pair of
overlapping coaxial lens electrodes forming an
;3~7~
10 -
accelerating and scan expansion lens and two
compensating slot lenses employed for cooperation
with the accelerating and scan expansion lens to
independently provide for linear magnification of the
electron beam deflection angle and corrected geometry
of the image.
A further object of this inven~ion is to
provide such a lens system in a cathode ray tube
~hich operates at a relatively low gun voltage to
accomplish strong scan magnification of an electron
beam and sufficient electron acceleration to produce
a bright, distortion-fre~ light image of the desired
size on the fluorescent screen of such tube.
Still another object of this invention is to
provide in a short length cathode ray tube such a
post-deflection acceleration and scan expansion lens
system which includes a pair of coaxial accelerating
and scan expansion electrodes of different diameters
and produces with ~he inner electrode of relatively
small diameter an electron image which is free of
geometry distortion.
Yet another object of the invention is to
provide such a lens system which r~duces the tendency
of field emission of electrons from the lens
electrodeS.
The presen~ invention relates to an
electrostatic lens system which may be included in an
electron discharge device, such as a cathode ray
tube, having an electron gun that produces a beam of
electrons directed along a beam axis in the tube and
deflection means for de1ecting such beam. The lens
system is positioned afl:er the deflection means along
the beam axis and comprises slot lens means includin~
a plurality of apertured slot electrodes having slots
symmetrically aligned about the beam axis. An
37~
accelerating and scan expansion lens means including
two aligned cooperating ~ubular electrodes o~
different diameters is supported downstream of the
slot lens means to provide in cooperation with the
slot lens means a linear magnification of the amount
of electron beam deflection produced by the
deflection means. An exit lens means including an
exit lens electrode supported adjacent the output of
the accelerating and scan expans~ion lens means and
having a slot aperture with a pair of opposed notches
provides corrected geometry of the image on the
display screen of the cathode ray tube.
The particular accelerating and scan
expansion lens system disclosed herein by way of
example operates at relatively low voltage and
produces a bright image of the desired size in sharp
focus and great detail on a fluorescent screen.
Conventional cathode ray tubes having post-deflection
acceleration and scan expansion lens systems
typically reguire a potential difference of about 23
kv measured between the cathode of the electron gun
and the conducting layer on the fluorescent screen to
obtain a sharp focused image of high brightness
comparable to that achieved in the present invention
;25 with a lower potential difference of 16 kv. The lens
lsystem of the present invention utilizes a pair of
coaxial electrodes of different diameters including
an inner electrode having a relatively small diameter
of less than or equal to 1.905 centimeters to produce
-30 a lens of the quadrupole type with increased electron
beam magnification and ~l short focal length. The
advantages of a lens system of this type include its
- suitability for use in short length cathode ray tubes
and a reduced tendency or field emission of
electrons from such electrodes causing dielectric
;371
- 12 -
breakdown in the air gap between the electrodes. The
latter advantage is due to the ov2rlapping coaxial
positioning of the electrodes and the operation at a
lower voltage. It will be understood that the
present invention can also be incorporated in
standard length cathode ray tubes.
Accele ating lenses that have overlapping
coaxial electrodes heretofore have operated
successfully only with inner electrodes having
relatively large diame~ers. As the diameter of the
inner electrode is reduced below 1.905 centimeters,
geometry distortion effects on the image cannot be
adequately corrected. Thus, a coaxial tubular
electrode lens with an octupole distortion
compensating lens of the type disclosed by the Janko
patents is useful for only cathode ray tubes of
relatively long lengths because of the constraint of
a minimum diameter for the inner electrode.
An interdigitated electrode lens of the type
disclosed by ~anko that includes two tubular
electrodes of the same diameter is not suitable for
use in short length cathode ray tubes. The
interdigitated electrode lens requires a higher
voltage to increase lens magnification and thereby
shorten the focal length. The increased voltage,
however, enhances to an unacceptable degree the
likelihood of field emission of electrons.
The compensating lens system of the present
invention allows an accelerating and scan expansion
lens having a pair of overlapping coaxial electrodes
of different diameters to operate successfully in
short length cathode ray tubes. The compensating
lens system operates in communication with the
accelerating and scan expansicn lens means and
comprises first and second compensating lenses. The
37 ~
- 13 -
~irst lens is positioned adjacent the front end of
the acceleratin~ and scan expansion lens means and
includes six closely spaced-apart wafer-like slot
lens electrodes having apertures symmetrically align-
ed about the beam axis. A DC bias voltage is appliedand distributed to certain ones of the slot lens elec-
trodes to produce electric field flux lines that in-
fluence the vertical direction of travel of an elec-
tron beam deflected to a great degree by the deflec-
10 tion means so as to provide a linear vertical magnifi-
cation of the amount of beam deflection. The second
lens is positioned adjacent the rear end of the ac-
celerating and scan expansion lens means and com-
prises a single exit electrode having an elongated
slot aperture with a pair of opposed arcuate cu~out
portions or notches in the longer edges of the slot
on opposite sides of the beam axis. This second lens
produces an electric field pattern having a flux line
shape and distribution which is similar to that of
20 the accelerating and scan expansion lens means but
with oppositely phased fluctuations~ The superposi-
tion of these two electric fields effectively elimin-
ates geometry distortion of the displayed image.
Each of the two compensating lenses is
physically separate from and functionally independent
of the other lens. Therefore, each compensating lens
provides correction for a different ~ype of image
distortion and is adjusted for optimum performance
without affecting the adjustment or performance of
the other lens.
The lens system of the presen~ invention is
different in both function and implementation from
that described in the Albertin, et al. patent.
Albertin, et al. uses a quadrupole lens which
3~ performs only scan expansion. A separate dome-shaped
;3'7~
- 14 -
electrode is added to accelerate tha elec~rons toward
the display screen. On th~ other hand, in the
present invention, an electrostatic lens of the
quadrupole type having coaxial tubular electrodes
wherein a larger diameter electrode overlaps the
contoured end of a smaller diameter electrode
performs simultaneQusly both functions of electron
acceleration and electrosl beam scan expansion.
The compensating lens mechanisms of
Albertin, et al. and the present invention utilize
entirely different methods to correct image
distortion introduced by the entirely different lens
systems. Albertin, et al. makes use of slot lens
electrodes on the front and rear ends of the scan
' 15 expansion quadrupole lens to correct for the combined
! effects of scan nonlinearity and geometry distortion
in the vertical and horizontal, respectively,
dimensions of the display. The compensating lens
mechani~m performs its task by converging in the
vertical direction the electron beam at ~he aperture
of the first slot electrode prior to its emergence
from the dome-shaped acceleration electrode.
Further, interaction between thé two lens makes
adjustment and performance of one lens dependent on
the other lens.
In the present invention, each compensating
lens is dedicated to a particular distortion
mechanism and is separate from and imdependent of the
other compensating lens. In addition, unlike the
compensating mechanism in Albertin, et al., it is
essential that the compensating lens mechanism of the
present invention converge the electron beam prior to
- its entry into the aperture of the exit lens
electrode to provide for corrected geometry of the
image.
-
1~ ;37~
- 15 -
Additional objects and advantages of the
present invention will be apparent from the following
detailed description of a preferred embodiment
thereof which proceeds with reference to the
accompanying drawings.
Brief D~scri~tion of Drawings
FIG. 1 is a schematic longitudinal section
view of a cathode ray tube incorporating the
post-deflection accelerating and scan expansion lens
system of the present invention;
FIG. 2 is an exploded view showing the
components of ~he lens system in the cathode ray tube
of Fig. l;
FIG. 3 is an enlarged side elevation view of
the lens system of Figs. 1 and 2 with portions of the
inner and outer tubular electrodes shown in phantom;
FIG. 4 is a vertical section view taken
along line 4--4 of Fig. 3 with the mounting rods
shown in phantom;
FIG. 5 is an enlarged vertical section view
taken along line 5--5 of Fig. 2 showing one of the
pair of concave sections on the inner tubular
electrode;
FIG. 6 is an enlarged horizontal section
view taken along line 6--6 of Fig. 2 showing one of
the pair of protrusions on the inner tubular
electrode; and
FIG. 7 is an end elevation view of the right
end of Fig. 3 showing the slot and cutout portions
along the edges of the exit lens electrode.
Preferred Embodiment of the Invention
With reference to Fig. 1, an electron beam
accelerating and scan expansion lens system 10 in
- accordance with the present invention is contained
within the evacuated envelope of cathode ray tube 12
- 16 -
~or a cathode ray oscilloscope. The envelope
includes tubular glass neck 14, ceramic funnel 16,
and transparent glass face place 18 sealed together
by devitrified glass seals as taught by U.S. Patent
No. 3,207,936 of Wilbanks, et al. A layer 20 of a
phosphor material is coated on the inner surface of
face plate 18 to form the fluorescent screen for the
cathode ray tube.
Electron gun 2',' including cathode 24,
10 control grid 25, and anode 26 is supported inside
neck 14 at the end of the tube opposite the
fluorescent screen to produce a beam of electrons
directed generally along beam axis 28, which is
coincident with the central longitudinal axis of the
15 tube, toward the fluorescent screen. A DC voltage
source of approximately -2 kv is connected to cathode
'~ 24 and the electrode beam emitted from such cathode
E is accelerated toward anode 26 which is connected to
ground potential. Cathode 24 is supported within and
20 is electrically isolated from the control grid
electrode 25 by ceramic spacer member 32. Grid 25 is
!, at a more negative voltage of about -2.1 kv than the
cathode to control the number of electrons passing to
i the anode 26 and thereby vary the intensity of the
! ~5 electron beam.
The electron beam passes through an aperture
) in anode 25 toward stigmator lens 34, which is
-, connected to the movable contact of potentiometer 36
i to provide a beam astigmatism correc:tion adjustment
30 of between 0 and +50 v.
A focusing lens system preferably of the
type disclosed in U.S. Patent Nos. 4,137,479 and
4,188,563 of Janko is disposed adjacent the output of
stigmator lens 34 and includes first quadrupole lens
35 38 and second quadrupole lens and 40. Quadrupole
;371
- 17 -
lens 38 converges the electron beam in the X-Z plane
and diverges it in the Y-Z plane; whereas, quadrupole
lens 40 diverges the electron beam in the X-Z plane
and converges it in the Y-Z plane. The coordinate
5 axes X, Y, and Z to which reference herein is made
are shown in Fig. 2 and include a horizontal axis X,
a vertical axis Y, and a beam axis Z. The movable
contacts of potentiometers 42 and 44 are connected ~o
quadrupole lenses 38 an~ 40, respectively, ~o provide
10 the focus adjustment of between 0 and + 60 v.
The electron beam strikes the fluorescent
- screen and forms a light image thereon after the beam
is deflected by the deflection means which changes
, the position of the beam on the screen. The
15 deflection means includes vertical deflection plates
46 and 48, preferably of the type described in U.S.
Patent No. 4,093,891 of Christie, et al., and
i horizontal deflection plates 50 and 52. Deflection
t plates 46 and 48 deflect the beam in the vertical
! 20 direction in response to a vertical deflection
signal, which is applied to neck pins 54 and 56.
Deflection plates 50 and 52 deflect the beam in the
horizontal direction in response to a horizontal
deflection signal, which is the ramp voltage output
- 25 of a conventional time base sweep circuit and is
applied to neck pins 58 and 60.
A third quadrupole lens 62 preferably of the
type disclosed in U.S. Patent Nos. 4,137,479 and
4,188,563 of Janko is interposed between the vertical
and horizontal deflection plates 46, 48 and 50, 52,
respectively, along the path of the deflected
electron beam to provide a scan expansion lens which
converges the electron beam in the X-Z plane and
diverges it in the Y-Z plane. This lens increases
the amount of vertical deflection produced by the
~.~"3~
- 18 -
deflection plates 46 and 48 in response to the
applied vertical deflection signal. The movable
contact of potentiometer 64 is connected to thir~
quadrupole lens 62 to change the deyree of
5 magnification or scan expansion produced by the lens
by adjusting its voltage between 0 and -200 v.
The accelerating and scan expansion lens
system 10, which comprises three separate lenses, is
positioned adjacent to and downstream of horizontal
10 deflection plates 50 and 52. The first lens o lens
system 10 includes slot lens means 66 which comprises
six slotted electrodes, each having a slot aperture
positioned symmetrically about beam axis 28.
Potentiometer 68 provides on its movable contact an
~' 15 adjustable voltage of between 0 and -900 v which is
'; applied to certain electrodes of lens 66 to produce
electric field flux lines that extend through the
electrode apertures. Adjusting the voltage on the
~ movable contact of potentiometer 68 changes ~he shape
t 20 and distribution of the flux lines which influence
the direction of electron beam travel as will be
further hereinafter described.
The second lens of lens system 10 is
accelerating and scan expansion lens means 70 that
25 includes inner tubular electrode 72, which is in
coaxial relation with and enclosed in par~ by outer
tubular electrode 74. Both electrodes 72 and 74 have
generally cylindrical shapes in the preferred
embodiment of the invention. Cross-shaped support
30 ring 76 is mounted at the front end of inner
electrode 72 and is positioned immediately adjacent
the output electrode of slot lens means 66. Ring 75
is secured to four glass mounting rods 77 (Fig. 4)
and provides the support for inner tubular electrode
35 72 so that the axis of the electrode is aligned
;37~
-- 19 -
coincident with beam axis 28.
The third lens of lens system 10 is an exi~
lens means 78 that includes a single apertured
electrode of a cap shape which has a slot aperture 79
~ymmetrically disposed with re~pect to beam axis 28.
Lens means 78 is mounted on and extends over a
portion of the rear end of outer tubular electrode 74
to provide corrected geometry of the image displayed
on the fluorescent screen.
The electrons in the electron beam are
accelerated by a high potential electrosta~ic ield
and strike the display screen at a high velocity.
This post-deflection acceleration field is produced
between inner tubular electrode 72 and outer tubular
electrode 74, as well as envelope wall coatings B0
and 82. One such wall coating i~ a thin, electron
; transparent aluminum film 80 that overlays phosphor
layer 20. Film 80 is connected to an electrically
conductive layer 82 deposited on the inner surface of
funnel 16. Conductive layer 82 terminates just
beyond exit lens means electrode 78 and is connected
to such electrode by conduc~ing lead 84. Conduc~ive
layer 82 is connected through feed-through connector
86 to an external high voltage DC source of
approximately ~14 kv relative to inner electrode 72
which is connected to ground potential tbrough
, cross-shaped ring 76. The potential difference
developed across inner electrode 72 and outer
electrode 74 changes the direction of the electron
beam traveling through the opening in inner electrode
72 to converge the electron beam in the Y-Z plane and
diverge it in the X-Z plane. Thus, the coaxial
- electrode~ 72 and 74 perform the dual functions of
accelerating the electzons in and expanding the
deflection angle of the electron beam.
371
~ 20 -
With reference to Figs. 2-4, slot lens means
66 includes six slot electrodes 88, 90, 92, 94, 96,
and 98 which are closely spaced-apart, substantially
flat wafers having apertures symmetrically aligned
about beam axis 28. Tab portions 99 of the slot
electrodes are bonded into glass rods 77 to main~ain
the spacing between adjacent electrodes and the
alignment of the apertu~es. Tab portions 99 also
extend from outer electrode 74 and ring 76 and are
bonded into glass rods 77 to maintain tAe coaxial
alignment of electrodes 72 and 74. Input slot
electrode 88 has a vertically disposed oblong slot
88a which receives the electron beam after it has
been deflected by horizontal deflec~ion plates 50 and
52. The remaining five slot electrodes 90, 929 94,
96, and 98 have horizontally disposed slo~s, the
opposite short sides of which are concave as viewed
from the interior of the slot. Slot electrodes 90
and 92 have identical apertures, 90a and 92a,
respectively; and the remaining slot electrodes 94,
96, and 98 have slot apertures 34a, 96a, 98a,
respectively, of varying vertical widths. The
graduated size openings in slot lens means 66
produced by the varying wid~hs of the aligned slot
electrode apertures is best seen in Fig. 4.
Electrode 96 is connected to the movable
contact of potentiometer 68 which provides a negative
potential thereto. Electrodes 88, 92, 94, and 98 are
connected to ground potential. Electrode 9o is con-
30 nected to a l50 volt source (not shown). The dimen-
sions of the apertures of the slot electrodes and the
magnitude and distribution of the voltage applied to
certain ones of the electrodes determine the charac-
teristics of the electric field flux lines so as to
35 provide a force directed slightly toward beam axis 28
7:~
- ?1 -
on an electron beam which has been deflected to a
great degree. Since the electric field flux lines in
the space proximate the central portion of the open-
ing of slot lens means 66 are approximately parallel
to beam axis 2a, electron beams experiencing a moder-
ate amount of or no appreciable deflection are not
influenced by the electric field of lens means 66.
Lens means 66 constitutes a first compensating lens
that in communication with accelerating and scan ex-
pansion lens means 70 produces a compensating elec-
tric field to linearize the vertical scan expansion
provided by accelerating and scan expansion lens
means 70.
The electron beam exits output slot electrode 98
of lens means 66 and enters at the front end of inner
tubular electrode 72 of accelerating and scan expan-
sion lens means 70. The very high electrostatic poten-
tial applied between electrodes 72 and 74 attracts
the electrons in the beam toward outer tubular elec-
trode 7~, thereby magnifying the amount of electronbeam deflection or scan produced by the deflection
means. Since the electric field flux lines near the
inner surface of electrode 72 are not uniformly
shaped, the scan magnification is not linear for elec-
tron beams deflected to a great degree. However, ac-
celerating and scan expansion lens means 70 and slot
lens means 66 cooperate to provide linear magnifica-
tion of the deflected electron beam by applying com-
pensating forces on electron beams deflected into the
region.
It should be understood that the coaxial acceler-
ating and scan expansion lens is not a pure quadru-
pole lens. In addition to its predominant quadrupole
action, lens means 70 has an octapole moment that
produces defocusing of the electron beam in a verti-
cal plane. The effect of this defocussing is to pro-
duce a horizontal ellipse rather than a circular spot
371
-- 22 -
at the fluorescent screen, with the length o the
ellipse increasing as the beam is deflected away from
beam axis 28. The distortion produced by the octupole
moment of lens means 70 i5 corrected by electrode 90
which, together with input slot electrode 88, pro-
vides a compensating octapole action at the entrance
to slot means 66. The magnitude of the correction
depends on the potential applied to ~lectrode 90 rela-
tive to adjacent electrodes 88 and 92, which are at
ground potential. Addit;ional correction may be pro-
vided by operating quadrupole lens 62 in such a man-
ner that it has a compensating octapole component.
With reference to Figs. 2-6, the end of the
inner electrode 72 which is enclosed in part by outer
electrode 74 has a pair of opposed projections 100
and 102 of substantially similar shape on opposite
sides of beam axis 28. Opposed projections 100 and
102 are separated on either side by and aligned trans-
versely of a pair of opposed curvilinear portions
104, 106 of substantially similar shape. Thus, the
partly enclosed end of electrode 72 is characterized
as having twofold symmetry. The first region of sym-
metry lies on either side of the Y-Z vertical refer-
ence plane coincident with beam axis 28. The portion
lying on the right side of the plane is shown in Fig~
5. The second region of symmetry lies on either side
of the X-Z horizontal reference plane coincident with
beam axis 28. The portion lying belo~ the plane is
shown in Fig. 6.
Each projection 100 and 102 has a pair of
lobes 108 and 110 which are separated by a concave
portion 112 to provide symmetrical projections. The
lobes are formed by mac:hining away portions of the
end of a cylindrical tubular member; therefore, the
inner and outer surfaces are shaped in accordance
with the contour of the cylindrical surface of elec-
trode 72.
;37~
- 23 -
Curvilinear portion 104 separates ~he two
lobes 108, an~ curvilinear portion 106 separates the
two lobes 110 of the opposed projections 100 and 102.
Each curvilinear portion is comprised of three con-
cave sections 114, 116, and 118 including two sidesections 11~ and 116 separated by a middle section
118. The precise shape of the contour of the lobes
and curvilinear portions is determined empirically
and is dictated by the electric field flux line pat-
tern required to perform linear magnifications of thedeflected electron beam. In general, however, the
hori~ontal linearity of the lens system is determined
by the shape and length of lobes 108 and 110. (As
mentioned above, the slot lens 66 corrects vertical
linearity.) It will thus be appreciated that the lens
system of the invention allows independent correction
of horizontal and vertical linearity.
The diameter of inner electrode 72 deter-
mines the extent of magnification produced and there-
by the focal length of the accelerating and scan ex-
pansion lens. Therefore, the required electric field
flux line pattern is controlled primarily by the diam-
eter of electrode 72. The particular shape of the con-
toured rear end of electrode 72 shown in the drawings
pertains to an irner electrode having an inner diam-
eter of 1.65 centimeters and an outer diameter of
1.905 centimeters.
Sufficient spacing between the outer sur-
face of inner electrode 72 and the inner surface of
outer electrode 74 is critical to the extent that
dielectric breakdown in the air gap therebetween
does not occur at operating voltages and that the
electric field flux lines near inner electrode 72
remain undisturbed. In both instances, a greater sep-
a~ation is required as the potential difference be-
tween the two electrodes is increased.
As shown best by the phantom lines in Fig.
5, the contoured edge of inner electrode 72 including
;3~71
- 24 -
the opposed projections and curvilinear portions are
beveled to smooth its surface and eliminate sharp
edge points thereby preventing field emission of elec-
trons therefrom. A thickness of 0.150 centimeters for
inner electrode 72 is adequate to provide an accept-
able beveled edge.
~ ith re~erence to Figs. 2, 3, and 7, theexit lens means formed by electrode 7~ has a horizon-
tally disposed slot aperture 79~ the opposite short
sides of which are concave as viewfrom the interior
of -the aperture. Opposed arcuate cutout portions or
notches 122 and 124 are provided in the longer top
and bottom sides symmetrically aligned on opposite
sides of central longitudinal axis 28. Electrode 78
is mounted directly to the rear end of outer elec-
trode 74 and connected by conducting lead a~ to theconductive wall coating 82 which is raised to a poten-
tial of +14 kv. Notches 122 and 124 alter the charac-
ter of the electric field flux lines developed be-
tween inner electrode 72 and outer electrode 74 so as
to provide corrected geometry for the resultant dis-
played light image. The notches 122 and 124, which in
the preferred embodiment are of arcuate shape but can
be formed in other shapes, produce electric field
flux lines that flùctuate moderately to compensate
for the similar but oppositely phased fluctuation
characteri~ing the electric field f~ux lines produced
by the potential difference between electrodes 72 and
74. The dimensions and shape of slot aperture 79 and
the notches 122 and 124 therein are the parameters
which are adjusted to eliminate pin cushion geometry
distortion of the image displayed on the fluorescent
screen. Thus, the exit lens means 78 constitutes a
second compensating lens that in communication with
accelerating and scan expansion lens means 70 pro-
vides a compensating electric field flux line pat-
tern to eliminate geometry distortion of the dis-
played image.
3~7~
- 25 -
It will be obvious to those having skill in
the art that many changes may be made in the above-
described details of the preferred embodiment of the
present invention. Therefore, the scope of the pres-
ent invention should be determined only by the follow-
ing claims.
