US2123011A - Electron discharge apparatus - Google Patents

Description

.Fuly 5, 1938. J. E. KEYSTON ET AL 9 3 I ELECTRON DISCHARGE APPARATUS Filed March 31, 1936 3 Sheets-Sheet l INVENTORS John faqar Keysfon Frederick Hermes Nico/l 0H0 Klemp rer EYD'OW ATTORNEY y 5, 1938- J. E. KEYSTON ET AL 9 39 ELECTRON DISCHARGE APPARATUS Filed March 51, 1936 3 Sheets-Sheet 2 52 IN VENTOR-S J h Ed I( f 4 rmzauzizesfi-czz Julyi, 193. .1. E. KEYSTQN ET AL. 9

ELECTRON DISCHARGE APPARATUS Filed March 31, 1936 5 Sheets-Sheet 5 v INVENTORS John Edqar lfeysfon Frederick Hermes Mcol/ Oflo e rer Br I ATTORNEY electron lens.

Patented July 5, 1938 PATENT OFFICE ELECTRON DISCHARGE APPARATUS John Edgar Keyston, Hayes, Frederick Hermes Nicoll, Ickenham, and Otto Klemperer, Hayes, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex, England, a

British company Application March 31, 1936, Serial No. 71,970 In Great Britain March 19, 1935 4 Claims.

The present invention relates to electron discharge apparatus in which a stream of electrons acted on by focusing means, is caused to sweep over a screen.

5 In known types of electron discharge device commonly referred to as cathode ray tubes, electrons are emitted from a cathode and accelerated towards and focussed upon a screen associated with the tube, by means of what is known as an An electron lens may be of the electrostatic or electromagnetic type. In such tubes means may be provided for deflecting 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 deflecting 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 deflected out of a straight path due to the electrostatic field between the plates. Alternatively, the deflecting system may consist of a set of electromagnetic coils, usually placed outside the tube, deflection being effected by passing a suitable current through the coils.

The electron lens system of the tube focuses the beam of electrons to a small spot which is at a fixed distance from the cathode.

The deflecting 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 lie at the centre of the deflecting system. Therefore, in order that the beam may always be 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 deflecting system of the tube.

This is often not a convenient arrangement. For example, in certain cathode ray tubes, in particular thoseused for television transmitting purposes there is often employed a plane screen, the normal to which is set at an angle to the undeflected direction of the electron beam, which angle may be as great as 45. When an electron beam is caused to scan such a screen, it is clear from 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 conventional type without the present invention.

Fig. 2 shows schematically the operative elements of another conventional type of cathode ray tube without the present invention.

This may be more clearly understood by reference to Fig. 1 of the accompanying drawings in which there is shown diagrammatically part of a cathode ray tube of the kind referred to above. In the bulbous part 2 of the tube there is arranged a screen I, which is scanned by a beam of electrons 3 from a cathode ray gun (not shown). In an undeflected position 3a the beam is focused at a point I on the screen 1. Two electrodes 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 deflecting 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 fluorescent screen which is plane or has such curvature that the centre of curvature does not lie in the region of the deflecting system.

It is one object of the present invention to provide means whereby a beam of electrons remains 3o focused on a surface of the tube or screen placed therein during deflection of the beam, though said surface or screen is not shaped in the form of part of the surface of a sphere.

In cathode ray tubes in which the electron lens is of the electrostatic type and the deflecting system comprises deflecting plates to which deflecting potentials are applied, it is found that the electrostatic fleld set up across the deflecting plates, more especially the field across the pair of 40 plates nearest the electron lens, distorts the focusing field 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 deflecting potential. 45

It is a further object of the present invention to provide means whereby the focal length of the electron lens may be made substantially independent of the potential on the deflecting plates.

In Fig. 2 of the accompanying drawings there 5 is illustrated another known form of electron discharge 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 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 diaphragm I! so that the aperture l3 scans the electron image. Only those photo-electrons passing through the aperture ii at any time are operative in developing picture signals for transmission.

A coil I 4 producing a steady and substantially uniform magnetic field in the general direction of electron travel is usually employed for focusing purposes. The electrons may be accelerated towards the diaphragm by maintaining the diaphragm l2 at a suitable positive potential with respect to the photoelectric surface of cathode ID. The aperture I3 is usually formed in the centre of the diaphragm i2 and the cathode I0 and the diaphragm 12 are usually in the form of flat plates arranged parallel and facing one another and the magnetic focusing field due to the coil i4 is then arranged to be normal to the planes of these two plates. A thin cylinder of metal l5 (usually acoating upon the walls of the tube in which the electrodes are mounted) may be provided extending between regions close to the edges of the two plates l0 and I2 and thus surrounding the space throughwhich the electrons travel. This metal cylinder I5 is of high resistance and can be arranged to ensure that there is a uniform potential gradient along the space between the surface In and the diaphragm l2.

The electrons forming the image on the diaphragm I! move in spiral paths, the motion being revoluble into a circular motion around a line of force and a straight line motion in the direction of the electrostatic field between the plates. The time of flight of electrons between the plates HI and I2 is dependent upon the distance and the potential difference between the plates and is independent of the path followed by individual electrons. The time required by an electron to perform one revolution under the influence of the magnetic field is inversely proportional to the line integral of the magnetic field strength along the path between the plates. In the undeflected condition of the electron image this line integral has the same value for all electrons and it can therefore be arranged by suitable choice of magnetic field strength and accelerating potential difference that in the undeflected condition all electrons have rotated through the same angle (for example they may have made one revolution) 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 deflected (for scanning purposes) by suitable means which may be electrostatic or electromagnetic (for example two pairs of "electromagnetic deflecting 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 flight has not changed the image is no longer sharply focused.

It is a further object of the present invention to provide means whereby this change of focus as the result of deflection in devices of this kind can be reduced or eliminated.

According to one feature of the present invention there is provided electron discharge appara- III, on to which an optical image of the object 'to' tus comprising a cathode, an electron lens system.

for focusing electrons emitted from said cathode on to a screen and means for deflecting said electrons over said screen, wherein there are provided means for automatically varying the effective focal length of said lens system in accordance with the deflection of said electrons whereby changes in focus of the operative electrons from said cathode which would otherwise occur as said electrons are deflected over said screen are removed or substantially reduced.

In a modification of the apparatus according to the preceding paragraph, where the electron lens system and the deflecting means operate electrostatically, the means for automatically varying the effective focal length of said lens system in accordance with the deflection of said electrons may be such that changes of the focal length of said lens system which would otherwise occur as a result of the distortion of the electrostatic fleld in said electron lens by said electrostatic deflecting means are removed or substantially reduced.

According to a further feature of the present invention there is provided electron discharge apparatus comprising a cathode, an electron lens system for focusing electrons emitted from said cathode on to the surface of a screen, and means for deflecting the electrons emitted from said cathode over said screen surface, the screen surface not being a spherical surface having its centre in the region of 'the deflecting means, wherein means are provided for automatically varying the effective focal length of said lens system in accordance with the deflection of said electrons in such a manner as to reduce changes in sharpness of focus of the beam with deflection thereof.

According to the present invention in another aspect there is provided cathode ray tube apparatus comprising a cathode, an electron lens' system for focusing electrons from said cathode into a narrow beam upon the surface of a screen associated with the tube and means for deflecting the beam over the screen, the screen surface not being a spherical surface having its centre in the region of the deflecting means, wherein means are provided for automatically varying the effective focal length of said lens system in accordance with the deflection of the ray in such a manner as to reduce changes in sharpness of focus of the beam with deflection thereof.

According to the present invention in a further aspect there is provided apparatus for transmitting images of an object to a distance comprising a photo-electrically active screen, means for projecting upon said screen an optical image of an object to be transmitted, an apertured diaphragm spaced apart from said photo-electrically active screen, an electron lens system for focusing upon said diaphragm electrons emitted from said screen under the influence of light, to form an electron image thereon, and means for sweeping 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 deflection of said image in such a manner that changes in the sharpness of focus of said electron image with deflection thereof are reduced.

Further features of the invention will appear from the following description and appended claims.

The invention will now be described with refergrammatic 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 electrostatic deflecting plates and embodying a further feature of the present invention.

Fig. 5 shows a modification of part of the tube of Fig. 4.

Fig. 6 shows a cathode ray tube having electromagnetic-deflecting means and embodying a feature of the present invention.

Figs. 7 and 9 are explanatory diagrams.

Fig. 8 is a circuit diagram for use in carrying 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 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 shield 2| which may in certain cases be used as a modulator and an electron'lens system comprising a first anode 22 and a second anode 24.

These electrodes may be in the form of cylindrical tubes of equal diameters. The first anode 22 is provided with two apertured diaphragms 22a and 22b the diaphragm 22a being positioned at the cathode end of the cylinder and the diaphragm 22b being positioned about two thirds of the way along the cylinder from the cathode end. The second anode 24 consists of a metal cylinder without any apertured diaphragms.

The form of electrode assembly in the neck portion 23 used in this embodiment of the invention is not of great importance and the arrangement shown is by way of example only.

In the bulbous portion 2 of the tube is arranged a screen I which is to be scanned by a beam of electrons 3. The screen I may consist for example of a mosaic screen comprising a number of photo-electrically active metallic elements disposed upon a mica sheet which is backed by a metal signal plate. In using such a tube, an optical image of an object to be transmitted, is projected upon'the screen, and the screen is scanned by means of the cathode ray beam 3 which is deapparatus of this kind is well known and need 7 not be further described.

In the following description it will be assumed for the sake of convenience, that the view of Fig. 3 is a side view of the tube, and that the deflecting plates 4, 5, serve to deflect the beam 3 vertically up and down the screen I.

In order to focus the beam on the screen in every deflected position, the present invention, in this embodiment, provides an electrode system for varying the effective focal length of the electron lens system existing between the anodes 22 and 24 by the production, in the path of the ray, of an asymmetrical electrostatic field. To this end a sliver coating 21 is formed on the walls of the tube, extending partly into the neck portion 23 and partly into the bulbous portion 2. The silvering 21 extends to a plane normal to the tube axis and located about half way between the Junction of the neck portion 23 with the bulbous portion 2 and the loweredge of .the screeni. This metallic coating may be maintained 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 bulbous portion2, the median plane of this zone 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 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 field between the two coatings is thus asymmetrical and can be arranged to lengthenthe effective focal length of the electron lens system of the tube when the ray is on the upperv part of the screen that is in position b relatively to the eflective focal length when the ray is on the lower part of the screen in position 0.

Clearly the field in which the correction is applied need not be an accelerating field. The ringshaped zone 28 may be maintained at a lower potential than the metallic coating 21 so that the correction is applied in'a decelerating field.

In another arrangement according to the invention a suitable varying potential which maybe derived from the deflecting circuit is'applied to an electron lens of the tube in such a way as to alter the effective focal length of this lens in accordance with the deflecting current or voltage applied to the deflecting 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 arranged a cathode 20 heated by a heater coil 19, a cathode shield or modulator 2|, first and second anodes 22 and 24, deflecting plates 4, 5 and a screen i. These electrodes may have the form described with reference to Fig. 3.

A second pair of deflecting plates indicated at 29 are provided between the plates 4, 5 and the screen I. Surrounding the space between the first and second anodes 22 and 24 is arranged a cylindrical electrode 30. This electrode will be referred to as a compensating electrode.

The first and second anodes 22 and 24 are given suitable positive potentials relative to the cathode 20, and the potential on the second anode 24 is made such in relation to the potential on the first anode 22 that with zero potential on the compensating electrode 30, the beam in the undefiected condition is focused upon the screen.

The second anode 24 is connected to the defleeting plate 5 and to earth. An oscillator indicated at 3| generates saw-tooth oscillations which are taken from a terminal 32 and applied to the plate 4 by lead 33 for the purpose of giving to the beam 3 the vertical component of a scanning motion. Saw-tooth potentials of a higher frequency applied to the plates 29 give the beam the horizontal or line component of scanning. The terminal 34 of the oscillator is connected to earth.

Across the terminals of the oscillator 3! is connected 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 deflecting 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 positive potential, the beam 3 falls on the top of the screen, and for ,the beam to be in focus, it is necessary 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 compensating electrode 30. Similarly when the plate 4 is negative with respect to the plate 5, the focal length maybe at a minimum; the negative potential then applied to the compensating electrode reduces the focal length of the electron 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 limited to the particular means described for applying to the compensating electrode 30 a potential derived from that on the deflecting plate 4. Any other suitable means may be provided for obtaining 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 removed from the cathode potential in order to ensure that substantially no current is collected thereby from the electron stream.

An alternative arrangement of first and second anodes and compensating electrode is shown in Fig. 5. In that figure the electron lens comprises a first 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 cylinder 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 compensating electrode and having a diameter intermediate between the diameters of the first and second anodes 22 and 24 is arranged around the first anode 22 so as to overlap the latter and to project within the second anode 24. In one example, the first anode 22 is inch in diameter, the compensating electrode 30, 3 inch in diameter and the second anode 24, 1 inch in diameter, and the compensating electrode 30 extends 8 millimetres beyond the end of the first anode 22.

' The arrangement of Fig. may be used in the same manner as described with reference to Fig. 4.

The invention may in this embodiment also be applied to tubes in which the beam is deflected over the screen by means of electromagnetic deflecting coils instead of by deflecting plates.

In Fig. 6 is illustrated a plan view of the tube shown in Fig. 4, with the deflecting plates 4, 5

and 29 omitted. Deflection is carried out by means of two pairs of deflecting coils, one pair of which is shown at 40 and 4| and the other pair of which is indicated at 42. The pair of coils 40, 4| which serve to deflect the ray vertlcally up and down the screen, that is in the direction normal to the plane of the figure, are connected in series, and deflecting currents are passed through them from an oscillator indicated 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 accordance 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 electrode 30 biased at cathode potential.

The electrode arrangement described with reference to Fig. 5 may also be applied to the case in which electromagnetic deflecting coils are used.

Where the change of focus of the beam with deflection is due to the use of an inclined plane screen, as described in the preceding examples, the compensation required is a linear function of the deflecting current or potential. If the change of focus is due to the use of forms of screens which are other than spherical (when no correction is required) or planar and inclined to the mean path of the beam, the compensation required may be nonlinear. The desired form of compensation can be obtained by suitable selection of the shape and position of the compensating electrode, and of the bias potential applied 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 containing its axis may be chosen to suit any particular case.

In the arrangement shown in Fig. 5, the com-'- pensating electrode may have the form of an apertured diaphragm the diameter of the aperture being intermediate the diameters of the first and second anodes 22 and 24.

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 defines the beam and the screen because a change in the divergence ofthe 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 compensating electrode should be combined with the final electron lens as it may be arranged to cooperate 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 controlling 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 dependent upon the modulating potential which beam is at the beginning and end of its stroke the focal length of the lens system must be greater than when the beam is half way across the screen. The correction need only be applied during the slower movement of the beam across the screen,

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 to Fig. 7 at (a) is shown the wave form of a scanning potential which is used to deflect the beam in horizontal lines across the screen. Potential is plotted as abscissa against time as ordinate. As the potential follows the lines PQ and RS the beam is moved comparatively slowly across 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 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) where at the points J, L and N the compensating potential is such that the electron lens has a maximum focal length and at the points K and M a 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 of the kind shown at (a) in Fig. 7 are applied to the terminals 5| and 52. The terminal 5| is connected to the control grid of a thermionic valve 7 53, the cathode of which is connected through a bias resistance 54 to the terminal 52. The anode of the valve 53 is connected through a resistance 55 to terminal 55 which is connected to the posi tive terminal of a source of potential (not shown). The terminal 52 is connected through lead 51 to the terminal 58 to which is connected the negative terminal of the source of potential.

Between the anode of valve 53 and the lead 57 is connected a condenser 59. Between the terminal 56 and the cathode of valve 53 is connected a variable resistance 60, by means of which the bias on the grid of valve 53 may be varied.

The operation of the circuit above described is 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 O in that figure. While the valve 53 is nonconducting the condenser 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 tooth'wave form applied across the terminals 5|, 52. Condenser 59 will then change linearly.

When the valve 53 begins to conduct, the potential of the condenser 59 will fall in sympathy with the potential on the grid of valve 53. Thus the 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 a valve 63 which, with its associated circuits comprises an ordinary resistance-capacity coupled phase-reversing stage. The output terminal 66 of this phase reversing stage may be connected to the compensating electrode 30 of Fig. 4, 5 or 6.

In the above described arrangement, the required derived potential may be obtained by replacing the resistance capacity circuit 55. 59 by its equivalent resistance inductance circuit.

When correcting for changes in eflectlve focal length of an electron lens arising from defiecting a beam of electrons over a screen the central region of which is nearer to the deflecting system than the edges, it may be necessary to apply a varying correcting potential which does not change linearly with time. Thus in certain cases it may be necessary to make the lines JK, KL 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 correcting potentials are applied.

Alternatively the potential applied to the compensatiiig electrode 30 may be given a wave form which is other than linearly related to the deflecting potential. The alteration of wave form may be obtained by means of a suitable circuit; for example, a .circuit whereby there may be derived from a saw tooth oscillation an oscillating potential which is substantially proportional to the integral of the saw tooth wave form. Such 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 long compared with the period of oscillation of the saw-tooth oscillation. The integrated wave form then consists of two parabolic arcs for each cycle of saw-tooth oscillation. The major arc corresponds to the slowly changing part of the saw-tooth wave form and this part of the integrated wave form may in suitable circumstances be fed to the compensating electrode 30.

Two connecting potentials may be applied to the compensating electrode 30, each derived from the corresponding deflecting'potential, to correct for changes in focus of the beam in both directions.

As already mentioned in the introduction to this specification, variations in the focus of the beam occur in cathode ray tubes employing electrostatic electron focussing and electrostatic deflection due to the potential on the deflectingplates distorting the focussing field.

This difliculty may be overcome, according to the present invention, by methods similar to those described above for compensating for variation in the distance of different points on the screen. Thus a varying potential derived from the saw-tooth deflecting potentials may be fed to the compensating electrode 30, the amplitude of the potential being suitably chosen so that the efiect of distortion of the deflecting potentials on the focussing field is eliminated.

An alternative arrangement by means of which elimination of distortion may be effected will be described with reference to Figs. 9 and 10.

Referring first to Fig. 10, a cathode ray tube is shown having a neck portion 23 and a frustoconical portion 10. At the base of the frustoconical portion 10 is a fluorescent screen 7!. In the neck portion 23 is arranged an electrode system comprising a cathode 20 heated by a heating coil I9, a cathode shield 2|. an accelerator electrode 61, a modulator electrode 68, a first 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 potentials in the negative direction applied to it the beam due to configuration of the screen need not be considered in describing this embodiment of the invention.

The second anode 24 and the deflecting 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 deflecting plates. It is found as already mentioned, that, given constant voltages on the anodes 22 and 24, the cross sectional area of the beam on the screen II where in the present case it forms a spot of light, varies according 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 the first anode 22 at which the spot size is a minimum. In Fig. 9 is a curve showing the relation between the potentials on the first anode 22 plotted as abscissae against potentials on the plate 4 plotted as ordinates for which the size of the spot on the screen 1| is a minimum. It

.will be seen that the curve has an exponential form.

' series. One end 14 of the combination is earthed and connected to the negative terminal of a source of potential (not shown). The other end 15 is connected to the positive terminal of the source of high potential. The first 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 connected through variable resistance 18 and lead 19 to a tapping on resistance 19. Thus the anode 22 is given a positive potential relative to the cathode 28. The potential on the accelerator 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 complicate the drawings, however, the connections to the accelerator 81 and cathode shield 2| are omitted.

' A screen-grid valve 8| has its anode connected through winding 82 of transformer TI to terminal 83' which is connected to a source of positive potential (not shown) the negative terminal of which is earthed. The screening grid of the valve 8| is connected through a winding 84 of a transformer 85 to a terminal 88 which is connected to a suitable tapping on a source of potential. This may be the same source which is connected to the terminal 83. One end of a 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 connected to this lead through a variable resistance 98. The cathode of the valve 8| is connected directly to the earthed lead 89. One end of a third winding 9| of the transformer 85 is connected 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 deflecting plate 4 of the cathode ray tube. through a condenser and a circuit comprising an inductance 98 shunted by a condenser 91 and resistance 98 in series. The plate 4 is connected to the plate i through a leak resistance 99 of high value.- The anode of the valve 8| is also connected through a condenser |88 and resistance 8| to the lead 19.

The operation of the circuit is as follows.

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 sufllcient to say that the valve becomes alternately conducting and non-conducting. of these changes is controlled by impulses applied across the terminals 92 and 93. These impulses may have the form of square topped pulses which are obtained from the synchronizing impulses of the received television signal.

While the valve BI is non-conducting, the condenser 94 charges up through the impedance of the Winding 82 of transformer 'l'l. When the valve 8| conducts, the condenser94 is rapidly discharged. 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), modified in that the lines PQ and RS conform to an exponential curve. These oscillations are passed to the first anode 22 through winding 88 of transformer 11 and lead 18. The connections to the winding 88 are so arranged that the oscillations passed to the first anode 22 are in phase opposition to those passed to the deflecting 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 filter circuit so designed that the exponential sawtooth wave form is converted into a straight line saw-tooth wave form of the kind shown in Fig. 7 (a). Thus the beam of the tube is deflected in the usual fashion over the screen 1|, and by applying the correcting voltage of exponential form to the first anode 22, variations in focus of the spot on the screen are substantially removed.

Usually a change in voltage on the first anode 22 will result in a change in the current flowing to the second anode 24, and hence in a variation in brightness of the spot on the screen II. To avoid this, a part of the voltage applied to the first anode 22 is also applied, in phase opposition, to the cathode 28. Thisvoltage is fed to the cathode 28 through the condenser 88 and resistance |8|.

Preferably there is connected a decoupling condenser |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 first anode 22 is of exponential 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 first anode. Thus the value of the condenser 94 may be made suf- The frequency ficiently large to cause a linear saw-tooth wave form to be developed across it. In this case the filter circuit 96, 91, 98 may be'omitted. If desired,

to Figs. 4, 5 or 6, the lead I6 being connected thereto instead of to the first anode 22. In such a case, no variations in beam current occur and variation of the potential applied to the cathode then unnecessary. The circuit I00, llll may therefore be omitted.

There will now be described methods of compensating for change of focus due to deflection of an electron image over a screen, as described and illustrated in the introduction to this specification with reference to Fig. 2. Referring again to Fig. 2 one way of doing this is to superimpose upon the magnetic focusing field due to the coil M an inhomogeneous steady magnetic field so that the composite magnetic field is stronger in the central than in peripheral regions.

If the aperture l3 in the diagram 12 is centrally disposed, the centre of the image will be operative upon the aperture l3 in the undefiected condition smaller than in the case of the former electrons.

The time of revolution of the electrons which is inversely proportional to the line integral is therefore greater with deflection than without. By suitable arrangement the distribution of magnetic field strength may thus be made such that, in the case of all operative electrons, the time of flight is substantially equal to the time required for one revolution and the focus is thus maintained during the scanning process.

The inhomogeneous magnetic field may be produced by placing a permanent magnet on the axis of the coil i4 and some distance away from it. The magnet is arranged with its magnetic axis on the axis of the coil M and so placed that its field enhances that due to the coil H. Alternatively the permanent magnet may be replaced bya further coil energized by a suitable current. atively short coil arranged coaxial with the main focusing coil at some distance away from it.

Another way in which the change of focus can be corrected is by making the accelerating electrostatic field between the two plates stronger in peripheral than in central regions. In this way the time of flight is made shorter for electrons which are operative in the deflected condition than for electrons which are operative in the undefiected condition. The required electrostatic field distribution may be obtained by arranging that the surface of the cathode l0 facing the diaphragm l2 or the surface of the This coil may take the form of a reldiaphragm IZ- facing the cathode III, or both these surfaces, are concave.

In both the examples given above, the desired efiect is obtained by providing a suitably inhomogeneous magnetic or electric field. A similar result can also be obtained by producing variations in the field so that the field strength varies in accordance with the part of the image which is active upon the scanning aperture l3. In such cases the field may be homogeneous. The desired variations in the field may be obtained (in the case where the field to be varied is the electrostatic field) by superimposing upon the normal steady potential difference between the cathode l0 and the diaphragm 12 a compensating potential difference varying in a suitable manner in accordance with the deflection. For example where the deflection is effected in known manner with the aid of two electrical oscillations of saw-tooth wave form, one at line scanning frequency and the other at picture frequency, the varying potential differences re-' quired for focus compensations may be derived from these two saw-tooth oscillations. These derived corrective potential difierences will normally be arranged to be constituted by components of twice line frequency and twice picture frequency, and may have the wave form indicated in Fig. 7 (b). They may be derived from the saw-tooth scanning currents by means of circuits of the form illustrated inFig. 8. Clearly, if correction is to be applied in both scanning components, one such circuit will be needed to derive compensating potentials from the scanning oscillations at line frequency, and another to derive compensating potentials from the scanning 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 oscillations.

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 system cemprising two cylindrical lens electrodes juxtaposed co-axially and being associated with means for maintaining a potential difference between said lens electrodes, means for deflecting 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 potential thereon relatively to that of said lens electrodes according to one or both of the two varying oscillations which, in operation, produce the deflection of the electrons, thereby changes in focus of the operative electrons from said cathode which would otherwise occur as said electrons are deflected over said screen are removed or substantially reduced.

2. Electron discharge apparatus as claimed in claim 1, wherein means are provided for generating and applying, to effect deflection of the electrons, a deflecting saw-tooth wave form, and wherein said compensating means for varying the potential on said compensating electrode comprise a circuit for developing from the sawtooth oscillation and feeding to the said compensating means a potential of substantially symmetrical zig-zag wave form having a frequency equal to that of the saw-tooth oscillations.

' 3. Electron discharge apparatus as claimed in Ierent diameters, and the compensating electrode claim 1 wherein said lens electrodes are equal in overlaps one of said lens electrodes and projects diameter and said compensating electrode is anwithin. the other of said lens electrodes. nular and surrounds the space between said lens 5 electrodes. JOHN EDGAR KEYSTON.

4. Electron discharge apparatus as claimed in FREDERICK HERMES NICOLL. claim 1 wherein said lens electrodes are of dif- OTTO KIEMPERER.

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