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.Fuly 5, 1938.
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I
J. E. KEYSTON ET AL
29123311
ELECTRON DISCHARGE APPARATUS
Filed March 31, 1936
3 Sheets-Sheet l
INVENTORS
‘
John fa’qar Keysfon
Frederick Hermes Nico/l
0H0 Klemp rer
EYD'OW
ATTORNEY
July 5, 1938-
J. E. KEYSTON ET AL
29123911
ELECTRON DISCHARGE APPARATUS
Filed March 51, 1936
3 Sheets-Sheet 2
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Julyi, 193.
.1. E. KEYSTQN ET AL.
2,1239%“
ELECTRON DISCHARGE APPARATUS
Filed March 31, 1936
5 Sheets-Sheet 5
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INVENTORS
John Edqar lfeysfon
Frederick Hermes Mcol/
O?o
Br
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I
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ATTORNEY
_ Patented July 5, 1938
2,123,011 '1
~ 'UNITED STATES
PATENT OFFICE
2,123,011
ELECTRON DISCHARGE APPARATUS
John Edgar Keyston, Hayes, Frederick Hermes
Nicoll, Ickenham, and Otto Klemperer, Hayes,
England, assignors to Electric & Musical In
dustries Limited, Hayes, Middlesex, England, a
British company
Application March 31, 1936, Serial No. 71,970
In Great Britain March 19, 1935
4 Claims. (Cl. 178-72)
The present invention relates to electron dis
ments of another conventional type of cathode
charge apparatus in which a stream of electrons ray tube without the present invention.
acted on by focusing means, is caused to sweep
This may be more clearly understood by ref
over a screen.
erence to Fig. 1 of the accompanying drawings in
5
In known types of electron discharge device‘ which there is shown diagrammatically part of a
commonly referred to as cathode ray tubes, elec
cathode ray tube of the kind referred to above.
trons are emitted from a cathode and accelerated In the bulbous part 2 of the tube there is ar
towards and focussed upon a screen associated
with the tube, by means of what is known as an
‘10 electron lens. An electron lens may be of the
electrostatic or electromagnetic type. In such
tubes means may be provided for de?ecting the
beam out of a straight path to any desired degree,
so that it falls on any desired spot of the sur
16 face of the tube or screen. The de?ecting means
may comprise two pairs of parallel plates between
which the beam passes, the plates of one pair
being at right angles to the plates of the other
pair. By applying suitable potential differences
20 between the plates, the beam is de?ected out of a
straight path due to the electrostatic ?eld be
tween the plates. Alternatively, the de?ecting
system may consist of a set of electromagnetic
coils, usually placed outside the tube, de?ection
25 being effected by passing a suitable current
. through the coils.
ranged a screen I, which is scanned by a beam
of electrons 3 from a cathode ray gun (not
shown). In an unde?ected position 3a the beam pa 0
is focused at a point ‘I on the screen 1. Two elec
trodes 4 and 5 are provided for sweeping the beam
of electrons over the screen in directions parallel
to the plane of the figure. ' The beam of electrons
may be considered as being pivoted at the centre 15
of the de?ecting plates 4, 5 and the focused point
of the beam 3 will describe an arc of a circle
shown by the dotted line 6. In the positions 31)
and 30 it is clear that the beam will not be in
focus on the screen I, but will fall on a consider- 20
able area of the screen as shown at 8 and 9. Thus
as the beam moves over the screen the area of
the scanning spot will vary.
A similar disadvantage is present in cathode
ray tubes in which a beam of electrons is focused 25
upon a ?uorescent screen which is plane or has
The electron lens system of the tube focuses
the beam of electrons to a small spot which is at
a ?xed distance from the cathode.
30
The de?ecting plates or coils sweep the focal
point of the beam over part of the surface of a
sphere, the centre of which may be considered to
therein during de?ection of the beam, though
lie at the centre of the de?ecting system. There
fore, in order that the beam may always be
said surface or screen is not shaped in the form
of part of the surface of a sphere.
35 focused on the surface of the tube or screen, the
latter must also be in the form of part of a sphere
with its centre at the centre of the de?ecting sys
In cathode ray tubes in which the electron lens
is of the electrostatic type and the de?ecting sys
tem of the tube.
This is often not a convenient arrangement.
40‘For example, in certain cathode ray tubes, in
particular those- used for television transmitting
purposes there is often employed a plane screen,
the normal to which is set at an angle to the un
de?ected direction of the electron beam, which
45 angle may be as great as 45°. When an electron
beam is caused to scan such a screen, it is clear
2from what has been said above that the cross
sectional area of the beam where it strikes the
screen will vary as it moves over the screen, and
consequently the size of the scanning spot on the
screen will vary.
Fig. 1 shows a cathode ray tube of one conven
tional type without the present invention.
Fig. 2 shows schematically the operative ele
such curvature that the centre of curvature does
not lie in the region of the de?ecting system.
It is one object of the present invention to pro
vide means whereby a beam of electrons remains 3o
focused on a surface of the tube or screen placed
tem comprises de?ecting plates to which de?ect
ing potentials are applied, it is found that the
electrostatic ?eld set up across the de?ecting
plates, more especially the ?eld across the pair of 40
plates nearest the electron lens, distorts the focus
ing ?eld in the electron lens, with the result that
_the focal length of the electron lens varies, as the
beam moves over the screen, in accordance with -
the magnitude of the de?ecting potential.
45
It is a further object of the present inven
tion to provide means whereby the focal length of
the electron lens may be made substantially inde
pendent of the potential on the de?ecting plates.
In Fig. 2 of the accompanying drawings there 5
is illustrated another known form of electron dis
charge device in which electrons, acted on by
focusing means, are caused to sweep over a screen.
Referring to Fig. 2, this known device comprises a
transparent photo-electrically sensitive surface 55
2
III, on to which an optical image of the object 'to'
be transmitted is thrown by means of a lens II.
The photo-electrons emitted from the surface are
focused upon an apertured diaphragm l2 to form
an electron image thereon. The electron image is
caused to sweep over the aperture I: in the dia
phragm I! so that the aperture l3 scans the elec
tron image. Only those photo-electrons passing
through the aperture ii at any time are operative
10 in developing picture signals for transmission.
A coil I 4 producing a steady and substantially
uniform magnetic ?eld in the general direction of
electron travel is usually employed for focusing
15
20
25
30
35
40
tus comprising a cathode, an electron lens system.
for focusing electrons emitted from said cathode
on to a screen and means for de?ecting said elec
trons over said screen, wherein there are provided
means for automatically varying the effective fo
cal length of said lens system in accordance with
the de?ection of said electrons whereby changes
in focus of the operative electrons from said
cathode which would otherwise occur as said elec
trons are de?ected over said screen are removed l0
or substantially reduced.
In a modi?cation of the apparatus according to
the preceding paragraph, where the electron lens
purposes. The electrons may be accelerated to
system and the de?ecting means operate electro
wards the diaphragm by maintaining the dia
statically, the means for automatically varying
phragm l2 at a suitable positive potential with the effective focal length of said lens system in
respect to the photoelectric surface of cathode ID. accordance with the de?ection of said electrons
The aperture I3 is usually formed in the centre may be such that changes of the focal length of
of the diaphragm i2 and the cathode I0 and the - . said lens system which would otherwise occur as’
diaphragm 12 are usually in the form of ?at a result of the distortion of the electrostatic ?eld
plates arranged parallel and facing one another in said electron lens by said electrostatic de?ect
and the magnetic focusing ?eld due to the coil ing means are removed or substantially reduced.
i4 is then arranged to be normal to the planes
According to a further feature of the present
of these two plates. A thin cylinder of metal l5 invention there is provided electron discharge ap
(usually acoating upon the walls of the tube in paratus comprising a cathode, an electron lens
which the electrodes are mounted) may be pro
system for focusing electrons emitted from said
vided extending between regions close to the cathode on to the surface of a screen, and means
edges of the two plates l0 and I2 and thus sur
for de?ecting the electrons emitted from said
rounding the space through‘which the electrons cathode over said screen surface, the screen sur
travel. This metal cylinder I5 is of high re
face not being a spherical surface having its
sistance and can be arranged to ensure that there centre in the region of 'the de?ecting means,
is a uniform potential gradient along the space wherein means are provided for automatically
between the surface In and the diaphragm l2.
varying the effective focal length of said lens
The electrons forming the image on the dia
system in accordance with the de?ection of said
phragm I! move in spiral paths, the motion being electrons in such a manner as to reduce changes
revoluble into a circular motion around a line of ' in sharpness of focus of the beam with de?ection
force and a straight line motion in the direction thereof.
of the electrostatic ?eld between the plates. The
According to the present invention in another
time of ?ight of electrons between the plates HI aspect there is provided cathode ray tube ap
and I2 is dependent upon the distance and the paratus comprising a cathode, an electron lens'
potential difference between the plates and is in
system for focusing electrons from said cathode
dependent of the path followed by individual
into a narrow beam upon the surface of a screen
electrons. The time required by an electron to
perform one revolution under the in?uence of
associated with the tube and means for de?ecting
the beam over the screen, the screen surface not
45 the magnetic ?eld is inversely proportional to the
line integral of the magnetic field strength along
the path between the plates. In the unde?ected
condition of the electron image this line integral
has the same value for all electrons and it can
50 therefore be arranged by suitable choice of mag
netic ?eld strength and accelerating potential
. difference that in the unde?ected condition all
electrons have rotated through the same angle
(for example they may have made one revolu
55 tion) in the time taken to reach the diaphragm
i2.
Under these conditions a focused electron
image will be formed on the diaphragm l2.
,
When however, the electron image is de?ected
(for scanning purposes) by suitable means which
60 may be electrostatic or electromagnetic (for ex
ample two pairs of "electromagnetic de?ecting
coils, one pair of which are shown in section at
l6 and I1, and one member of the other pair of
which is shown in dotted lines in side view at
l8) , the line integral changes in value and hence
the angle of revolution of electrons during their
passage between the plates l0 and I2 changes.
Since the time of ?ight has not changed the im
age is no longer sharply focused.
It is a further object of the present invention to
70
provide means whereby this change of focus as
the result of de?ection in devices of this kind can
be reduced or eliminated.
According to one feature of the present inven
tion there is provided electron discharge appara
being a spherical surface having its centre in the
region of the de?ecting means, wherein means
are provided for automatically varying the effec
tive focal length of said lens system in accordance
with the de?ection of the ray in such a manner
as to reduce changes in sharpness of focus of
the beam with de?ection thereof.
According to the present invention in a further
aspect there is provided apparatus for transmit
ting images of an object to a distance comprising
a photo-electrically active screen, means for pro
jecting upon said ‘screen an optical image of
an object to be transmitted, an apertured dia
phragm spaced apart from said photo-electrically
active screen, an electron lens system for focus
ing upon said diaphragm electrons emitted from
said screen under the influence of light, to form
an electron image thereon, and means for sweep
ing said electron image over said aperture in such
a manner that said aperture scans said electron
image, wherein there are provided means for
varying the effective focal length of said lens
system in accordance with the de?ection of said
image in such a manner that changes in the
sharpness of focus of said electron image with
de?ection thereof are reduced.
,
Further features‘ of the invention will appear
from the following description and appended
claims.
The invention will now be described with refer
70
‘
2,128,011 '
ence to Figs. 3 to 10 of the accompanying dia
grammatic drawings in which
Fig. 3 shows a cathode ray tube embodying one
feature of the present invention.
Fig. 4 shows a cathode ray tube having elec
trostatic de?ecting plates and embodying a fur
ther feature of the present invention.
'
Fig. 5 shows a modi?cation of part of the tube
of Fig. 4.
Fig. 6 shows a cathode ray tube having electro
magnetic-de?ecting means and embodying a fea
ture of the present invention.
Figs. 7 and 9 are explanatory diagrams.
Fig. 8 is a circuit diagram for use in carrying
15 out one feature of the present invention, and V
Fig. 10 is a circuit associated with a known
form of cathode ray tube for use in carrying out
the present invention.
Referring to Fig. 3, the envelope of a cathode
20 ray tube consists of a neck portion 23 joined to
a bulbous portion 2. In the neck portion 23 are
arranged a heater coil is, a cathode 20, a cathode
the Junction of the neck portion 23 with the
bulbous portion 2 and the loweredge of .the
screen- i. >This metallic coating may be main
tained at a. potential of say 500 volts positive
relative to the cathode 20, and may, if desired, be
connected to the second anode 24 of the main
electron lens system 22, 24.
_
'
'
A further ring-shaped metallized zone 20 of
relatively small area is provided upon the bul
bous portion2, the median plane of this zone 10
being‘ inclined to the normal to the axis of the_
tube so that portionsthereoi'lie approximately
opposite the upper and lower edges of the screen
i. The ring-shaped zone 28 may be maintained
at a positive potential of say 1000 volts relative 15
to the cathode 20. The lower part of the zone is
much closer than the upper part thereof to the
edge of the 500 volt’ coating 21. The electrostatic
?eld between the two coatings is thus asymmetri
cal and can be arranged to lengthenthe effective 20
focal length of the electron lens system of the tube
when the ray is on the upperv part of the screen
shield 2| which may in certain cases be used as a
that is in position b relatively to the e?’ective
modulator and an electron'lens system compris
focal length when the ray is on the, lower part '
of the screen in position 0.
.
25
25 ing a ?rst anode 22 and a second anode 24.
These electrodes may be in the form of cylindri~
cal tubes of equal diameters. The ?rst anode 22
is provided with two apertured diaphragms 22a
and 22b the diaphragm 22a being positioned at
30 the cathode end of the cylinder and the dia
Clearly the ?eld in which the correction is ap
plied need not be an accelerating ?eld. The ring
shaped zone 28 may be maintained at a lower
potential than the metallic coating 21 so that
The second anode 24 consists of a metal cylinder
the correction is applied in'a decelerating ?eld. 30
In another arrangement according to the in
vention a suitable varying potential which maybe
derived from the de?ecting circuit is'applied to
without any apertured diaphragms.
The form of electrode assembly in the neck
3.5
an electron lens of the tube in such a way as to
alter the effective focal length of this lens in ac 35
phragm 22b being positioned about two thirds of‘
the way along the cylinder from the cathode end.
portion 23 used in this embodiment of the inven
tion is not of great importance and the arrange
ment shown is by way of example only.
In the bulbous portion 2 of the tube is arranged
40 a screen I which is to be scanned by a beam of
electrons 3. The screen I may consist for ex
cordance with the de?ecting current or voltage
applied to the de?ecting means.
Such an arrangement is illustrated in Fig. 4
of the accompanying drawings.
'
Referring to Fig. 4, which represents a side
view of a cathode ray tube, in the tube are ar
ample of a mosaic screen comprising a number of
ranged‘ a cathode 20 heated by a heater coil 19,
photo-electrically active metallic elements dis
a cathode shield or modulator 2|, ?rst and second
posed upon a mica sheet which is backed by a
45 metal signal plate. In using such a tube, an opti
cal image of an object to be transmitted, is pro
jected upon'the screen, and the screen is scanned
by means of the cathode ray beam 3 which is de
?ected over the screen i by means of two pairs
50 of de?ecting plates, one pair of which is shown
at 4, 5. A second pair of de?ecting plates (not
shown) at right angles to those shown at 4, 5
may be provided between the latter and the screen
I.
Picture signals for transmission are taken
The
55 from the signal plate by a lead not shown.
manner of operation of television transmission
anodes 22 and 24, de?ecting plates 4, 5 and a
screen i. These electrodes may have the form
described with reference to Fig. 3.
A second pair of de?ecting plates indicated at
29 are provided between the plates 4, 5 and the
screen I. Surrounding the space between the
?rst and second anodes 22 and 24 is arranged a 50
cylindrical electrode ‘30.
This electrode will be
referred to as a “compensating electrode”.
The ?rst and second anodes 22 and 24 are
given suitable positive potentials relative to the
cathode 20, and the potential on the second anode 55
24 is made such in relation to the potential on
apparatus of this kind is well known and need 7 the ?rst anode 22 that with zero potential on
not be further described.
In the following description it will be assumed
60 for the sake of convenience, that ‘the view of
Fig. 3 is a side view of the tube, and that the
de?ecting plates 4, 5, serve to de?ect the beam 3
65
the compensating electrode 30, the beam in the
unde?ected condition is focused upon the screen.
The second anode 24 is connected to the de 60
?eeting plate 5 and to earth.
An oscillator in
dicated at 3| generates saw-tooth oscillations
which are taken from a terminal 32 and applied
vertically up and down the screen I.
In order to focus the beam on the screen in
to the plate 4 by lead 33 for the purpose of giv
every de?ected position, the present invention,
ing to the beam 3 the vertical component of a 65
scanning motion. Saw-tooth potentials of a
in this embodiment, provides an electrode system
for varying the effective focal length of the elec
tron lens system existing between the anodes 22
and 24 by the production, in the path of the ray,
70 of an asymmetrical electrostatic ?eld. To this
end a sliver coating 21 is formed on the walls
- of the tube, extending partly into the neck por
tion 23 and partly into the bulbous portion 2. The
silvering 21 extends to a plane normal to the
75 tube axis and locatedv about half way between
higher frequency applied to the plates 29 give the
beam the horizontal or line component of scan
ning.
The terminal 34 of the oscillator is con
nected to earth. ,
Across the terminals of the oscillator 3! is con-_
nected a resistance 31, and the compensating
electrode 30 is connected through a condenser 36
to a variable tapping point on the resistance 31.
In this way a voltage, derived from the de?ecting
70
4
9,188,011
potential, of wave form similar to the latter and
of the same sign, though of smaller amplitude, is
applied to the compensating electrode 34.
Now when the plate 4 is at its maximum posi
tive potential, the beam 3 falls on the top of the
screen, and for ,the beam to be in focus, it is neces
sary for the focal length of the lens to be a
maximum. This is brought about by the positive
potential applied at this time to the compensat
ing electrode 30. Similarly when the plate 4 is
negative with respect to the plate 5, the focal
length maybe at a minimum; the negative po
tential then applied to the compensating elec
trode reduces the focal length of the electron
15 lens between the two anodes 22 and 24. The
amplitude of the derived potential applied to the
compensating electrode 30 is adjusted ‘to give
correct compensation by adjusting the position of
the tapping on the resistance 31.
The invention in this embodiment is not lim
ited to the particular means described for apply
ing to the compensating electrode 30 a potential
derived from that on the de?ecting plate 4. Any
other suitable means may be provided for ob
taining the derived potential.
The compensating electrode 30 may be given
any suitable bias potential. As shown in Fig. 4,
the electrode 30 is connected to the cathode 20
through a leak resistance 38, and thus the mean
potential of the electrode 30 is the potential of
the cathode 20. The electrode 30 may however,
be given any desired bias for example by means
of a battery inserted in series with the leak
resistance 38. The potential of the electrode 30
is preferably given a bias potential not far re
moved from the cathode potential in order to
ensure that substantially no current is collected
thereby from the electron stream.
An alternative arrangement of ?rst and sec
ond anodes and compensating electrode is shown
in Fig. 5. In that ?gure the electron lens com
prises a ?rst anode in the form of a cylinder 22
having its end remote from the cathode disposed
near to but usually not quite extending to the
plane of the nearer end of a larger cylinder
constituting the second anode. This latter cylin
der has the form of a metallic coating 24 on the
tube walls and may extend to a region close to
the screen which may be arranged as shown in
Fig. 4. A third cylinder 30 to act as the com
pensating electrode and having a diameter in
termediate between the diameters of the ?rst and
second anodes 22 and 24 is arranged around the
?rst anode 22 so as to overlap the latter and to
project within the second anode 24. In one ex
ample, the ?rst anode 22 is 1/2 inch in diameter,
the compensating electrode 30, 33/; inch in diam
eter and the second anode 24, 1 inch in diameter,
and the compensating electrode 30 extends 8
millimetres beyond the end of the ?rst anode 22.
' The arrangement of Fig. 5 may be used in the
same manner as described with reference to
Fig. 4.
direction normal to the plane of the ?gure, are
connected in series, and de?ecting currents are
passed through them from an oscillator indi
cated at 43. Across the terminals 44 and 4! of
the oscillator 43 is connected a resistance 44.
The compensating electrode 30 is connected to
a tapping point on this resistance 46 through
a condenser 41. A leak resistance 48 is connected
between the compensating electrode 30 and .the
cathode 20. Thus a potential varying in accord
ance with the currents in the coils 40 and 4|
is applied to the compensating electrode 30. The
grid leak 48 serves to keep the compensating elec
trode 30 biased at cathode potential.
The electrode arrangement described with ref 15
erence to Fig. 5 may also be applied to the case
in which electromagnetic de?ecting coils are used.
Where the change of focus of the beam with
de?ection is due to the use of an inclined plane
screen, as described in the preceding examples, 20
the compensation required is a linear function of
the de?ecting current or potential. If the change
of focus is due to the use of forms of screens
which are other than spherical (when no correc
tion is required) or planar and inclined to the
mean path of the beam, the compensation re
quired may be nonlinear. The desired form of
compensation can be obtained by suitable selec
tion of the shape and position of the compensat
ing electrode, and of the bias potential applied 30
to it.
The cross section of the compensating
electrode 30 in planes normal to its axis is usually
circular, but the cross section in planes contain
ing its axis may be chosen to suit any particular
case.
35
~
In the arrangement shown in Fig. 5, the com-'
pensating electrode may have the form of an
apertured diaphragm the diameter of the aper
ture being intermediate the diameters of the ?rst
and second anodes 22 and 24.
'
40
It is usually desirable to arrange that the focus‘
‘compensating means shall not alter the current
in the electron beam and where this is the case
the compensation should be effected between the
last aperture which de?nes the beam and the 45
screen because a change in the divergence of
the beam will affect the number of electrons
passing through an aperture arranged in the
path of the beam of changing divergence.
It is however not necessary that the compen
sating electrode should be combined with the‘
?nal electron lens as it may be arranged to co
operate with another electron lens nearer the
cathode. For example, a part of the modulating
cylinder usually provided, for example in cathode
ray tubes used for television reception, for con
trolling the ray intensity may be insulated from
the remainder and used for focal length control.
The various structures above described may be
used to compensate for a change in focus irre
spective of the cause of the change.
'
For example where the focus changes with
change in beam current, a voltage or current
The invention may in this embodiment also be , dependent upon the modulating potential which
applied to tubes in which the beam is de?ected produces the change in beam current may be
over the screen by means of electromagnetic
applied to the compensating means above de
de?ecting coils instead of by de?ecting plates.
scribed so as to correct for the change in focus.
In Fig. 6 is illustrated a plan view of the tube
In certain cases it may be desirable to correct
shown in Fig. 4, with the de?ecting plates 4, 5 for the variation in distance from the centre of
29 omitted. De?ection is carried out by the de?ecting system to points on a horizontal
70 ‘and
means of two pairs of de?ecting coils, one pair line across the screen. If thebeam is de?ected
of which is shown at 40 and 4| and the other across the screen by means of a saw tooth poten
pair of which is indicated at 42. The pair of tial or current it will move relatively slowly in one
coils 40, 4| which serve to de?ect the ray ver-. direction and rapidly back again. When the
tlcally up and down the screen, that is in the
beam is at the beginning and end of its stroke the 75
5
2,128,011
focal length of the lens system must be greater
quired derived potential may be obtained by re
than when the beam is half way across the screen.
placing the resistance capacity circuit 55. 59 by
its equivalent resistance inductance circuit.
slower movement ‘of the beam across the screen, '
When correcting for changes in e?ectlve focal
The correction need only be applied during the
since this is the only time when the beam has
any useful function in this kind of apparatus.
During the quick return stroke the accuracy of
focus of the beam is immaterial. Referring now
length of an electron lens arising from defiect~
ing a beam of electrons over a screen the central
region of which is nearer to the de?ecting system
than the edges, it may be necessary to apply a
to Fig. 7 at (a) is shown the wave form of a varying correcting potential which does not
scanning potential which is used to de?ect the change linearly with time. Thus in certain cases 10
beam in horizontal lines across the screen. Po- _ it may be necessary to make the lines JK, KL
Vtential is plotted as abscissa against time as or
dinate. As the potential follows the lines PQ and
RS the beam is moved comparatively slowly across
15 the screen, and over the lines QR and ST the
beam is made to return quickly to the other side
of the screen. Now as explained above, the focal
length of the electron lens must be: greatest at
the points P, Q, R and S, and shortest midway
20 between PQ and RS, that is at points V and W.
Thus the compensating potential must have a
symmetrical zig-zag wave form as shown at (b)
etc. of the wave form of Fig. 7 (b) curved. This
may be done by inserting a suitable correcting
circuit between the terminal 66 of the circuit
of Fig. 8 and the electrode to which the correct 15v
ing potentials are applied.
'
Alternatively the potential applied to the com
pensatiiig electrode 30 may be given a wave form
which is other than linearly related to the de
?ecting potential. The alteration of wave form 20
may be obtained by means of a suitable circuit;
for example, a .circuit whereby there may be de
where at the points J, L and N the compensating
rived from a saw tooth oscillation an oscillating
potential is such that the electron lens has a maxi
25 mum focal length and at the points K and M a
potential which is substantially proportional to
minimum focal length, the latter corresponding
to the central position of the beam on the screen.
A circuit suitable for deriving a wave form of
the kind shown at (b) from a saw tooth potential
as at (a) is illustrated in Fig. 8.
Referring to that figure, saw tooth potentials
the integral of the saw tooth wave form. Such 25
an oscillation may be obtained by feeding a saw
tooth oscillation to a resistance and condenser
in series. The integrated oscillations are taken
from the terminals of the condenser. The time
constant of the condenser and resistance is made 30
long compared with the period of oscillation of
the saw-tooth oscillation. The integrated wave
the terminals 5| and 52. The terminal 5| is con
form then consists of two parabolic arcs for
nected to the control grid of a thermionic valve 7 each cycle of saw-tooth oscillation. The major
53, the cathode of which is connected through a arc corresponds to the slowly changing part of the 35
bias resistance 54 to the terminal 52. The anode saw-tooth wave form and this part of the inte
of the valve 53 is connected through a resistance grated wave form may in suitable circumstances
55 to terminal 55 which is connected to the posi— be fed to the compensating electrode 30.
tive terminal of a source of potential (not shown).
Two connecting potentials may be applied to
40 The terminal 52 is connected through lead 51 to the compensating electrode 30, each derived from 40
the terminal 58 to which is connected the nega
the corresponding de?ecting'potential, to correct
tive terminal of the source of potential.
for changes in focus of the beam in both direc
Between the anode of valve 53 and the lead 57 tions.
is connected a condenser 59. Between the ter
As already mentioned in the introduction to
45 minal 56 and the cathode of valve 53 is con
this speci?cation, variations in the focus of the
nected a variable resistance 60, by means of which beam occur in cathode ray tubes employing elec
the bias on the grid of valve 53 may be varied.
trostatic electron focussing and electrostatic de
The operation of the circuit above described is flection due to the potential on the de?ecting~
. of the kind shown at (a) in Fig. 7 are applied to
as followsz-
..
The bias of the grid valve 53 is so adjusted that
- _' the valve passes no current when the potential
of the applied wave form (a) of Fig. 7 falls below
a value denoted by the line 0 O in that ?gure.
While the valve 53 is nonconducting the conden
55 ser 59 is charged through the resistance 55, and it
- is arranged that the time constant given by the
product R1C1 where R1 is the value of resistance
55 and C1 the capacity of condenser 59 is large
compared with the oscillation period of the saw
60 tooth'wave form applied across the terminals
‘ 5|, 52. Condenser 59 will then change linearly.
When the valve 53 begins to conduct, the poten
tial of the condenser 59 will fall in sympathy with
the potential on the grid of valve 53. Thus the
65 potential of the condenser 59 will follow a wave
form as shown at (b) in Fig. 7. This wave form
is in antiphase to the potential applied to the
grid of the valve 53. To reverse the phase of the
potential on the condenser 59, there is provided
70 a valve 63 which, with its associated circuits com
prises an ordinary resistance-capacity coupled
phase-reversing stage. The output terminal 66
of this phase reversing stage may be connected to
75
the compensating electrode 30 of Fig. 4, 5 or 6.
In the above described arrangement, the re
plates distorting the focussing ?eld.
This di?iculty may be overcome, according to 50
the present invention, by methods similar to
those described above for compensating for varia
tion in the distance of different points on the
screen. Thus a varying potential derived from
the saw-tooth deflecting potentials may be fed to 55
the compensating electrode 30, the amplitude of
the potential being suitably chosen so that the
e?ect of distortion of the de?ecting potentials
on the focussing ?eld is eliminated.
An alternative arrangement by means of which 60
elimination of distortion may be effected will be
described with reference to Figs. 9 and 10.
Referring ?rst to Fig. 10, a cathode ray tube is
shown having a neck portion 23 and a frusto
conical portion 10. At the base of the frusto 65
conical portion 10 is a ?uorescent screen 7!. In
the neck portion 23 is arranged an electrode sys
tem comprising a cathode 20 heated by a heat
ing coil I9, a cathode shield 2|. an accelerator
electrode 61, a modulator electrode 68, a first 70
anode 22 and second anode 24. The accelerator
electrode is given a potential between that of
‘the anode and cathode. The modulator may be
biased at cathode potential, and modulating po
tentials in the negative direction applied to it 75
6
2,198,011
by a suitable circuit not shown serve to modulate
the intensity of the beam. The modulating po
tentials may consist of received television‘ sig
nals. It will be assumed that the screen ‘II is
formed on part of a spherical surface the centre
of which lies in the region of the de?ecting sys
tem 4, 5, 29, in order that variations of focus of
the beam due to configuration of the screen need ,
'10
‘is
not be considered in describing this embodiment
of the invention.
The second anode 24 and the de?ecting plate
5 are connected together and maintained at,
say, 2000 volts positive relative to the cathode 28.
In order to cause the cathode ray beam to scan
the screen, saw tooth potentials, one at frame
frequency and the other at line frequency, are
applied to the de?ecting plates. It is found as
already mentioned, that, given constant voltages
on the anodes 22 and 24, the cross sectional area
20 of the beam on the screen ‘II where in the pres
ent case it forms a spot of light, varies accord
ing to the potential applied to the electrode 4.
It is also found that for any potential on the
plate 4, there is a corresponding potential of
25 the ?rst anode 22 at which the spot size is a mini
mum. In Fig. 9 is a curve showing the relation
between the potentials on the ?rst anode 22
plotted as abscissae against potentials on the
plate 4 plotted as ordinates for which the size of
30 the spot on the screen 1| is a minimum.
It
.will be seen that the curve has an exponential
' form.
It will be clear that by applying to the ?rst
anode 22 of the tube of Fig. 10 a potential which
varies with the potential of the plate 4 according
to the curve of Fig. 9, the beam may be focussed
on the screen as a spot of substantially constant
size. The circuit of Fig. 10 by means of which
this varying voltage is applied will now be de
40 scribed.
Two resistances 12 and 13 are connected in
' series. One end 14 of the combination is earthed
directly to the earthed lead 89. One end of a
third winding 9| of the transformer 85 is con
nected to lead 81 and to terminal 92, and the
other end to terminal 93.
Across the anode and cathode of the valve 8|
is connected a condenser 94. The anode is also
connected to the de?ecting plate 4 of the cathode
ray tube. through a condenser 95‘ and a circuit
comprising an inductance 98 shunted by a con
denser 91 and resistance 98 in series. The plate 10
4 is connected to the platevi through a leak
resistance 99 of high value.- The anode of the
valve 8| is also connected through a condenser
|8‘8 and resistance | 8| to the lead 19.
The operation of the circuit is as follows.
15
The valve 8| and associated screening grid and
control grid circuits form a normal blocking
oscillator, the operation of which is well known
and need not be fully described. It is su?lcient
to say that the valve becomes alternately con
ducting and non-conducting. The frequency
of these changes is controlled by impulses ap
_
plied across the terminals 92 and 93. These im
pulses may have the form of square topped pulses
which'are obtained from the synchronizing im
pulses of the received television signal.
While the valve BI is non-conducting, the con
denser 94 charges up through the impedance of
the Winding 82 of transformer 'l'l. When the
valve 8| conducts, the condenser94 is rapidly dis
charged. The impedance of the winding 82 and
the capacity of the condenser 94 arepreferably
made such that the curve showing the potential
across the condenser 94 against time during the
charging process has the exponential form of the
curve shown in Fig. 9. Thus the wave form
of the oscillations generated in the anode circuit
of the valve 8| have the form shown in Fig. 7
(a), modi?ed in that the lines PQ and RS con
form to an exponential curve. These oscilla
tions are passed to the ?rst anode 22 through
winding 88 of transformer 11 and lead 18. The
source of high potential. The ?rst anode 22 of
the tube is connected through lead ‘I6 and one
winding 88 of a transformer 11 to a tapping on
resistance 12. The cathode 28 of the tube is con
connections to the winding 88 are so arranged
that the oscillations passed to the ?rst anode 22
are in phase opposition to those passed to the
de?ecting plate 4. ,The oscillations also pass
through condenser 95 and the circuit 96, 91, 98
to the plate 4. The circuit 98, 91, 98 is a ?lter
circuit so designed that the exponential saw
nected through variable resistance 18 and lead
tooth wave form is converted into a straight line '
and connected to the negative terminal of a
source of potential (not shown). The other end
45 15 is connected to the positive terminal of the
Thus the
saw-tooth wave form of the kind shown in Fig.
anode 22 is ‘given a positive potential relative
7 (a). Thus the beam of the tube is de?ected in
the usual fashion over the screen 1|, and by ap
19 to a tapping on resistance 19.
to the cathode 28. The potential on the acceler
ator electrode 61 may be derived in a similar way.
The cathode shield may also be connected to a
tapping on resistance 13. In order not to com
plying the correcting voltage of exponential form
to the ?rst anode 22, variations in focus of the
spot on the screen are substantially removed.
' A screen-grid valve 8| has its anode connected
Usually a change in voltage on the ?rst anode
22 will result in a change in the current ?owing
to the second anode 24, and hence in a varia
tion in brightness of the spot on the screen ‘II.
through winding 82 of transformer TI to terminal
To avoid this, a part of the voltage applied to
plicate the drawings, however, the connections
to the accelerator 81 and cathode shield 2| are
omitted.
83' which is connected to a source of positive
the first anode 22 is also applied, in phase op
potential (not shown) the negative terminal of
which is earthed. The screening grid of the
position, to the cathode 28. Thisvoltage is
fed to the cathode 28 through the condenser
valve 8| is connected through a winding 84 of a
transformer 85 to a terminal 88 which is con
| 88 and resistance |8|.
nected to a suitable tapping on a source of po
tential. This_ may be the same source which
is connected to the terminal 83. One end of a
70 winding 81 of transformer 85 is connected to the
control grid of the valve 8| through a condenser
88, and the other end is connected to an earthed
lead 89. The grid of the valve 8| is also con
nected to this lead through a variable resistance
98.
The cathode of the valve 8| ‘is connected
_
‘Preferably there is connected a decoupling con
denser |82 between the tapping on resistance
12 and the earthed lead 89. In the operation
' of the above described arrangement, the voltage
applied to the ?rst anode 22 is of exponential ' 70
saw-tooth wave form. However, a considerable
improvement in the constancy of spot size will
be obtained if a linear saw-tooth wave form
voltage is applied to the ?rst anode. Thus the
value of the condenser 94 may be made suf 75
2,128,011
?ciently large to cause a linear saw-tooth wave
form to be developed across it. In this case the
diaphragm IZ- facing the cathode III, or both
‘?lter circuit 96, 91, 98 may be'omitted. If desired,
in either of these arrangements the resistance
In both the examples given above, the desired
e?’ect is obtained by providing a suitably inhomo
these surfaces, are concave.
_
llll may be connected to the cathode shield 2!,
geneous magnetic or electric ?eld. A similar re
or in cases where a grid is placed between the
cathode and ?rst anode, the resistance IDI may
sult can also be obtained by producing variations
in the ?eld so that the ?eld strength varies in
be connected thereto. The time constant of the
circuit comprising the condenser I00 and re
10 sistances llll and 18 should be su?lciently large
to prevent any phase distortion of the impulses
is active upon the scanning aperture l3. In such
passed through it.
,
.
. If desired the circuit of Fig. 10 may be used
with a cathode ray tube having a compensating
15 electrode of the form described with reference
_ to Figs. 4, 5 or 6, the lead ‘I6 being connected
thereto instead of to the ?rst anode 22.
In
accordance with the part of the image which
cases the ?eld may be homogeneous.
The de
sired variations in the ?eld ‘may be obtained (in
the case where the ?eld to be varied is the elec
trostatic ?eld) by superimposing upon the nor
mal steady potential difference between the
cathode l0 and the diaphragm 12 a compensat 16
ing potential difference varying in a suitable
manner in accordance with the de?ection.
For
such a case, no variations in beam current occur
example where the de?ection is effected in known
and variation of the potential applied to the
The circuit I00, llll
manner with the aid of two electrical oscilla
tions of saw-tooth wave form, one at line scan
may therefore be omitted.
There will now be described methods of com
ning frequency and the other at picture fre
quency, the varying potential differences re-'
quired for focus compensations may be derived
20 cathode then unnecessary.
pensating for change of focus due to de?ection of
an electron image over a screen, as described and
illustrated in the introduction to this speci?ca
~ tion with reference to Fig. 2.
Referring again
20
from these two saw-tooth oscillations. These de
rived corrective potential di?erences will nor 25
mally be arranged to be constituted by compo
nents of twice line frequency and twice picture
frequency, and may have the wave form’ indi
cated in Fig. 7 (b). They may be derived from
the saw-tooth scanning currents by means of 30
to Fig. 2 one way of doing this is to superim
pose upon the magnetic focusing ?eld due to
the coil‘ M an inhomogeneous steady magnetic
?eld so that the composite magnetic ?eld is
stronger in the central than in peripheral regions. circuits of the form illustrated inFig. 8. Clearly,
If the aperture l3 in the diagram 12 is centrally ’ if correction is to be applied in both scanning
disposed, the centre of the image will be operative ‘components, one such circuit will be needed to
. upon the aperture l3 in the unde?ected condition
derive compensating potentials/from the scan
35 and corners of the image will be operative on
ning oscillations at line frequency, and another 85
the aperture l3 with maximum de?ection. Thus to derive compensating potentials from the scan
v30
with no de?ection the operative electrons travel
in_a relatively strong magnetic ?eld and with
maximum de?ection the operative electrons
travel from a region of relatively weak magnetic
?eld into the stronger region. The latter elec
trons therefore travel in a path along which the
line integral of -the magnetic ?eld strength is
‘ smaller than in the case of the former electrons.
The time of revolution of the electrons which is
45 inversely proportional to the line integral is there
fore greater with de?ection than without. By
suitable arrangement the distribution of mag
netic ?eld strength may thus be made such that,
in the case of all‘ operative electrons, the time
50. of ?ight is substantially equal to the time re
' quired for one revolution and the focus is thus
maintained during the scanning process.
The inhomogeneous magnetic ?eld may be
produced by placing a permanent magnet on the
55
axis of the coil i4 and some distance away from
it. The magnet is arranged with its magnetic
60
05
70
~75
ning oscillations at frame frequency. The two
compensating potentials are then combined and
applied to the cathode ID or diaphragm H in
suitable phase relationship to the scanning os
cillations.
‘
We claim:
1. Electron discharge apparatus comprising a
cathode, a screen, an electrostatic electron lens
system for focussing electrons emitted from said
cathode on to said screen, said electron lens sys
tem cemprising two cylindrical lens electrodes
juxtaposed co-axially and being associated with
means for maintaining a potential difference be
tween said lens electrodes, means for de?ecting 50
said electrons over said screen, a compensating
electrode positioned in the neighbourhood of the
region of juxtaposition of said lens electrodes,
said compensating electrode being associated
with compensating means for varying the poten
tial thereon relatively to that of said lens elec
trodes according to one or both of the two vary
axis on the axis of the coil M and so placed that ing oscillations which, in operation, produce the
its ?eld enhances that due to the coil H. Al
de?ection of the electrons, thereby changes in
ternatively the permanent magnet may be re
focus of the operative ‘electrons from said cath 60
placed bya further coil energized by a suitable ode ‘which would otherwise occur as said elec
current. This coil may take the form of a rel- ‘ trons are de?ected over said screen are removed
ativelyv short coil arranged coaxial with the main or substantially reduced.
focusing coil at some distance away from it.
2. Electron discharge apparatus as claimed in
Another way in which the change of focus can claim 1, wherein means are provided for generat
be corrected is by making the accelerating elec
ing and applying, to effect de?ection of the elec
trostatic ?eld between the two plates stronger trons, a de?ecting saw-tooth wave form, and
in peripheral‘ than in central regions. In this wherein said compensating means for varying
way the time of flight is made shorter for elec
the potential on said compensating electrode
trons which are operative in the de?ected condi
comprise a circuit for developing from the saw .70
tion than for electrons which are operative in tooth oscillation and feeding to the said com
the unde?ected condition. The required elec
pensating means a potential of substantially
trostatic ?eld distribution may be obtained by symmetrical zig-‘zag wave form having a fre
arranging that the surface of the cathode l0 quency equal to that of the saw-tooth oscilla
facing the diaphragm l2 or the surface of the tions.
76
8
~
2,190,011
' 3. Electron discharge apparatus as claimed in
claim 1 wherein said lens electrodes are equal in
diameter and said compensating‘electrode is annular and surrounds the space between said lens
5 electrodes.
4. Electron discharge apparatus’ as claimed in
claim 1 wherein said lens electrodes are of dif-
Ierent diameters, and the compensating electrode
overlaps one of said lens electrodes and projects
within. the other of said lens electrodes.
'
JOHN EDGAR KEYSTON.
FREDERICK HERMES NICOLL.
OTTO KIEMPERER.
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