Патент USA US2124270код для вставки
July 19, 1938. ' L. F. BROADWAY 2,124,270 'CATHOD’E RAY'TUBE Filed Nov. 5,’ 19:55 2 Sheets-Sheet 1 July 19, 1938. L. F. BROADWAY ' 2,124,270 CATHODE RAY TUBE Filed Nov. 5, 1955 ' J; c Q rd fa 2 I 3/51 I 2 Sheets-Sheet L2, I8 4 I P4 (8 [-21 F2: 1 . Z *“v/ ‘w / 1%,. a. km K ff 43- ?ak.” 2,124,270 Patented July 19, 1938 UNITED STATES PATENT ~ orrics 2,124,270 CATHODE BAY TUBE Leonard Francis Broadway, Hillingdon, England, assignor to Electric 8; Musical Industries Lim ited, Hayes, England, a British company i Application November 5, 1935,, Serial No. 48,348 In Great Britain November 8, 1934 16 Claims. (Cl. 250-275) The present invention relates to improvements which would produce perfect focusing of a conical beam of electrons. in cathode ray tubes and the like. Fig. 3 shows lines of force between several elec A cathode ray tube is known which comprises a sealed envelope having disposed within it, in the trodes of a known form of cathode ray tube. Fig. 4 shows equipotential lines between the order mentioned, an indirectly heated cathode, electrodes arranged similarly to that of Fig. 3. a cathode shield, an accelerating electrode (or ac Fig. 5 shows equipotential lines in the neighbor celerator) , a grid or modulating electrode, a first hood of an electrode constructed in accordance and a second anode and a screen such as a ?u with the present invention. crescent screen. Usually the shapes and dis Fig. 6 shows another form of electrode con 10 10 positions of the electrodes and the potentials ap plied to them are such that a roughly conical structed in accordance with the present invenon. beam of electrons emitted from the cathode is Fig. 7 shows the form of a cathode of a cathode brought to a focus in the neighborhood of the modulating electrode and to another focus on the ray tube constructed in accordance with the pres ent invention, 15 screen. Any such arrangement of electrodes which by Figs. 8 and 9 showparts of the electrode system virtue of the electrostatic field existing between in a cathode ray tube constructed in accordance the electrodes exerts a focusing action upon the conical beam of electrons is known as an electron 20 lens and it is an object of the present invention to with the present invention, ' Figs. 10, l1, and 12 show further forms of elec trodes constructed in accordance with the pres provide an improved lens of this kind. According to one feature of the present inven tion there is provided an electrostatic electron ent invention, and Fig. 13 illustrates the application of the pres ent invention to the formation of electron im lens system comprising two co-operating elec ages. 20 - 25 trodes having apertures through which an elec tron beam to be acted upon by the lens system can Referring to Fig. 1, in which there is shown an optical lens I whose thickness is small com of potential and hence a focusing ?eld between the two electrodes, wherein the electrodes are so 30 shaped that at any point in a region within and extending to the boundary of at least one of said apertures the component of the electric ?eld strength in directions perpendicular to an axis which passes through the centres of said aper 35 tures is substantially proportional to the distance of the said point from said‘ axis. According to a. further feature of the present invention there is provided an electrode, for use in an electron lens, said electrode having an'aper 40 ture bounded by a surface or by surfaces of which sections in planes normal to said surface or sur faces satisfy or substantially satisfy the equa tion V=2:|:L-y3, where V is a constant and :r and all rays of light from a point 2 should pass through, that is to say, come to a focus at, a point 3 is that the refraction or bending occur 30 ring at any point within the lens should be pro portional to the distance of that point from the be passed and means for establishing a difference - pared with its focal length, the condition that 11 represent distances measured along suitably 45 chosen mutually perpendicular co-ordinates. Further features of the invention will appear from the following description and the appended claims. _ The invention will now be described, by way of 60 example, with reference to the accompanying dia grammatic drawings in which:'-— Fig. 1 shows the path of a conical beam of light through a convex optical lens. Fig. 2 shows the shape of equipotential curves axis 2, 3 of the lens provided that the aperture of the lens is small compared with its distance. from the points 2 and 3. A similar condition must hold in the case of an electrostatic ?eld functioning as an electron lens, the condition in this case being that the ?eld strength perpendic ular to the axis of the electron‘lens at any point within the lens must be proportional to the dis tance of that point from the axis of the lens. This condition holds only in the case of a thin electrostatic lens, since if the lens is thick com pared with its focal length, the inertia of the elec trons passing through the lens complicates the 45 problem. ' The shape of the electric ?eld which satisfies the above condition will now be determined. after which there will be described arrangements of electrodes designed to produce such a ?eld. Throughout the following calculation it will be assumed that the electron beam itself has no per turbing effect on the ?eld set up by the lens. The theorem of Gauss states that if any closed surface (S) is taken in an electric ?eld and if N 55 amaaro denotes the component of electric intty at any, point in this surface in the-direction of the out ward normal, then J'Nds = 413 where the integration is taken over the whole surface and E is the total charge enclosed by the surface. . the constant 0 does not a?’ect the shape of the family 0! curves represented by the equation but merely determines their position in relation to the origin of coordinates, whilst the constant k ' merely alters the scale of the curves. These con stants d, c and It may therefore be omitted with out loss of generality and equation VIII reduces ' From this equation it may be shown that with 10 in a surface enclosing no charge, the potential V within the surface satis?es a di?erential equa tion which is independent of the charges, lying outside the region, which produce the potential. Taking rectangular co-ordinates this may be expressed by the equation 15 V=2a?~y+r____________ __ IX It now V be given values 0, 1, 2 - - - - and -i, -2, - - - - etc., the ?eld represented by equation IX is obtained as a series of equi potential curves, some of which are shown in Fig. 2. These curves belong to two families of hyperbolae and with the potential gradients in 10 15 dicated in the ?gure, represent the ideal con I which is kno u as Laplace’s equation. verging lens. The corresponding ‘diverging lens is obtained by reversing the sign of the poten eld is to focus a conical beam of ' Since the electrons " it must" obviously be a cylindrically tials. In practice this would be done by revers ing the sign of the potential on the electrode pro etrical ?eld. Assuming the axis of sym metry to be the :r-axis (and it will be seen eventually that the .r-axis is the axis of the lens 25 and an axis of the electrode co-operating in pro ducing the required ?eld) we have to both families of hyperbolae. enn bs” f by’ 80 20 ducing the ?eld, relative to the potentials of adjacent electrodes. It will be noted that one of the hyperbolae de generates into two straight lines intersecting on the X-axis and this pair of lines is asymptotic 25 - If the origin be taken as the point of inter section of the asymptotes equation IX reduces to whence equation I becomes V=2a:2—y2 ____________ __s X Dav 962V @1- 3;r=° -------------- -- H It has been stated above that the intensity (F) of the ?eld at any point within the lens must 35 be proportional to the distance of the point from the axis of the lens. Assuming the axis of the lens to coincide with the :c-axis, we have 80 since the e?ectpi' the ?rst order terms is to de termine the position of the curve with respect to the origin, without changing the shape of the curve. The angle between the asymptotes may be 35 found by considering the particular case when V is zero. Putting this value of V in equation X Zea-312:0 where Fy represents the ?eld strength in direc tion 1! and It represents a constant. Di?erentiat ing equation III, we have b’V 45 and b_‘-ya = ‘— _ _ _ - - _ _ _ . _ - - _ - IV and substituting for 922 by; _ so in equation H biV Biz-2' '- 2k — 0 ____________ _ _ Integrating equation V 55 V ‘ V=kx2+cx+d ____________ .... VI where c and d are constants so far as a: is con 40 and the asymptotes are given by a+\/§r=0 e—\/§r=0 45 The angle between the two straight lines repre sented by these equations is 70°32’ and it should be noted that no alteration of the constants of the ?eld equation which has been derived can alter this angle, it being a fundamental 50 property of any cylindrically symmetrical ?eld of the mud under consideration. It can be proved generally, in fact, that in any cylindrically symmetrical ?eld of this type, the ?eld near the axis has two asymptotes which 55 intersect at the angle 70°32’. An arrangement of electrodes which will pro duce a ?eld of the kind shown in Fig. 2 may cerned but may be functions of 1!, Integrating equation m now easily be round. _ A known arrangement of electrodes in a cath where e is a constant so far as y is concerned but may be a function of z. Combining equations VI and VII 4 V=k(2x2——y’)+cx+d ____ __. VIII where c and d are constants. It can be seen by di?'erentiation that equation VIII is a solution of equation 11 and that it 70 satis?es equation III, therefore it must represent the ?eld required. ' The constant it is obviously an added potential which does not a?ect the shape of the ?eld but merely admits of the solution being ?tted to the 35 boundary conditions of a given problem. Also 60 ode ray tube is shown in Figs. Sand 4; in both of these ?gures it represents a cathode, 5 a. cathode shield, '6 an accelerating electrode, 1 a grid or modulator and 8 a ?rst anode. Throughout the following description the cathode shield 5, cath 65 ode tl and modulator ‘i will be assumed to be at substantially zero potential and the accelerator 6 and the ?rst anode 8 at positive potentials. In Fig. 3 the lines of force between the various elec trodes are shown and in Fig. 4 the equipotential 70 lines in the neighbourhood of the accelerator and modulator are shown. Referring to Fig. 3, an electron approaching the Y ‘accelerator and being slightly oil the axis of the tube is accelerated by the ?eld away from the axis I 3 amaavo and on passing through the accelerator is again accelerated away from the axis, so that a conical beam of electrons whilst passing through the ac celerator experiences a diverging effect. At the modulator the component of the?eld normal to the axis is opposite to that at the ac celerator and a conical beam approaching and passing through the modulator is converged. . That such is the case can also be appreciated from an inspection of Fig. 4. In the neighbour hood of the aperture in the accelerator 8 the potential is increasing from the axis outwards to the accelerator so that electrons are urged of! the ing the accelerator and has a semi-vertical anglev , of 54°.44', so that it forms‘an asymptote to the equipotential curves. The cathode shield v5 sur rounding the cathode has a diaphragm portion 20 which is shaped to form a continuation of the surface IQ of the cathode l. With the electrodes shaped in this manner, the ?eld between them will be of the form deduced above. up to the sur-' face of the electrodes. Instead of being in frusto conical form, the cathode surface l9 and the por 10 tion 20 of the cathode shield 5 may be curved to the shape of the equipotential surfaces of the ?eld, for instance those marked a or b in Fig. 7. Fur axis, whilst the reverse takes place in the neigh ther, the accelerator 6 may have the form illus bourhood of the modulator. trated in Fig. 5. ' Now near the centres of the apertures in the accelerator and modulator the equipotential curves approximate roughly to the ideal curves which have been calculated above and which are shown in Fig. 2, but near the edges of the aper tures the equipotential curves depart widely from - 15 Fig. 8 shows the electrode arrangement of Fig. 4 in which the curved electrodes according to the ' present invention are substituted for the dia phragm electrodes of Fig. 4. The lines between electrodes represent the equipotential lines in 20 the electrostatic ?elds. With the electrodes shaped as. shown, the equipotential lines conform to the system given by equation X above through out the electrostatic ?eld; the asymptotes c, d and e, ]‘ intersect in the centre of the electrode. With 25 the calculated ideal curves since the outermost equipotential curve is represented by the section of the surface of the electrode itself and this sec tion clearly does not belong to either of the fami lies of hyperbolae shown in Fig. 2. This is ob ‘ the potentials of the electrodes as stated above, the lens between the cathode 6 and the ac viously a serious defect in known focusing ar celerator B will be diverging, and the lens between rangements especially in the case of an aperture, . the accelerator 6 and the modulator ‘i will becon such as the accelerator, which is ?lled with elec with a suitable choice of the strength of 30 trons, since in this case if the paraxial region verging; potentials on the electrodes, and their relative of the beam is correctly focused the marginal the dispositions, the net result of the two lenses may region is not. ' In order to obtain a field which is correct up to the edge of the aperture, the electrode must be of such shape that its surface, which is an equipo tential surface in the ?eld, forms part of the sys tem calculated above. s ' be made converging,'the conical beam of electrons being focused at a point near the aperture of the modulator, electrode ‘I. The beam diverging from 35 this point may be focused upon a screen as sociated with the tube by a suitable lens system, ' for instance by the ?eld between two tubular elec One such electrode is shown at 9 in Fig. 5; any trodes usually termed the ?rst and second anodes, section of this electrode in a plane containing the , the ?rst anode being held at a high positive po—' \ tube axis Iii comprises two straight lines (neglect tential relative to the modulator electrode, and ing the discontinuity at the aperture) intersect the second anode being for example held at a ing on the axis at an angle of 70".32'. These high positive potential relative to the‘?rst anode. lines thus form- a part of the asymptote oi‘ the These electrodes may be ‘arranged in the manner two families of hyperbolae calculated above. described in co-pending application No. 745,838, The electrode can be made of two frusto-conical ?led September 28, 1934, by I. Shoenberg, et al., diaphragms II and I2 inserted into a tube, the entitled “Cathode ray tubes". In Fig. 8 a part of diaphragm ll nearer the cathode having its apex the ?rst anode 8 is shown, provided with a dia turned away from the cathode and the other i2 phragm 2|, which is placed su?lciently far in the having its apex turned towards the cathode. The tube from the modulator 'I that it does not distort semi-vertical angle, i. e. the angle lying between the ?eld existing ‘in its neighbourhood. Either of the surface II and the axis 10, of each diaphragm the electrodes 6 or ‘I may have the form illus is 54°.44'. . trated in Fig. 5, instead of, as shown, that illus In Fig. 6 there is illustrated an electrode l3 trated in Fig. 6. which is of theoretically better shape: any sec In a preferred arrangement the ?rst and second tion of this electrode lying in a plane containing anodes may have the form shown in Fig. 9. As the axis l4 approximates to one curve of the ideal ' shown in this ?gure two tubular electrodes 8 and family of hyperbolae, so that the field produced is 22 are provided with two frusto-conical portions theoretically correct right up to the electrode. 23 and 24 arranged baseto base as shown in the Such an electrode may be pressed out of sheet ?gure. The angle 9 between their surfaces is ar-‘ ‘ ranged to be 70°32’, so that they form the ) metal. The cathode also may be shaped to conform to asymptotes of the system of curves calculated the system of equipotentials calculated above. In above. The ?eld between the electrodes will then Fig. '1 is shown a cathode I heated by a heater be of the required form according to equation X, coil l8 supplied by alternating current from the - up to the surfaces of the frusta 23 and 24. The secondary winding ll of a transformer 40. The electrodes are preferably provided with dia~ cathode 4 is surrounded by a cathode shield 5. phragms 2| and '25, and 26 respectively, which In front of the cathode is arranged an accelerator should be arranged sufficiently far from the electrode 6 of the type illustrated in Fig. 6. The focusing ?elds to prevent them from distorting cathode and the cathode shield 5 are connected ' to the centre part of the secondary winding 4| of the transformer 40 and to the negative terminal. l2 of the battery '43. The positive terminal 44 of the battery 43 is connected to the accelerator electrode 6. The emitting surface ill of the cath ode is i’rusto-conical in‘shape, with its base fac these ?elds. A In Fig. 10 is shown an alternative form of elec trode which may be used in any of the systems described above. The electrode is formed of a tubular portion l8’. and carries a number of an 40 45 50 55 60 65 70 nular diaphragms 3|, the internal diameter of 75 aieasro which decreases towards the centre of the tube the screen in number proportional to the light It’. falling on that point. These electrons are then focused by means of the field between two elec trodes 2t and hi on to the screen 28 to form an inverted electron image thereon. To obtaincor rect focusing, the ?eld should have the form The internal edges of the annular dia phragms are so positioned that they all touch a surface which is a surface of revolution of one member of the system of curves shown in Fig. 2. Thus this diaphragm is in e?ect similar to that shown in Fig. 6 and electrodes built up, as in given by Equation K above, and to this end the Fig. 10, of a suitable number of diaphragms ar ranged to simulate a thicker diaphragm having 10 continuous boundaries are regarded in this speci ?cation and in the claims as the equivalent of the electrodes is and 35 are provided with frusto conical portions 3d and 32 respectively, the semi vertical angle of the frusta being‘ preferably was. If the electrodes are then held at suit latter diaphragms. ' able different potentials (the potential of the In Fig. 11 is shown a form of electrode similar to that shown in Fig. 5, but which conforms more 15 closely to the theoretical system of curves. The dotted line 35 indicates the theoretically correct shape of the'electrode. The electrode shown in Fig. 11 is made of two frusto-conical portions 36 and 3'5 arranged with their smaller diameter 20 ends adjacent one another. A tubular portion 38 is arranged between the two frusto-conical electrode as being usually more positive) the ?eld between the screens 2‘? and 28 will cause substan tially all electrons emitted from a point on the; screen N to be focused on to a corresponding point on the screen 28. Clearly if desired the shape of the portions 80 and $2 of electrodes 29 and 88 respectively may be curved to conform more exactly with the required form of equi El potentials in the focusing held. In any of the above described examples, where electrodes in the form of conical frusta having semi-vertical angles of 54‘244’ have been de scribed, such electrodes may be replaced by elec portions 36 and 3?, and ?xed to the tubular por tion 38 is arranged an annular diaphragm 39, the aperture boundary or inner edge of which is ar 25 ranged to lie'on the theoretical curve 35. This form of electrode can be arranged to simulate trodes having the form of the surface of revo lution of one of the hyperbolae of the form given fairly closely the theoretically correct form de scribed with reference to Fig. 6. An alternative structure for the electrode of 30 Fig. 11 is shown in Fig. 12. Here the tubular portion 38 is omitted, and the diaphragm por tion 39 is attached to the main supporting tube IS’. The surfaces of the frusta 36 and 8? and the inner edge of the diaphragm portion .89 all 35 lie on a surface approximating ‘to the theoretical ly correct surface deduced above, which is shown in dotted lines at 35. by Equation X, and shown in Fig. 2. the optical case of a cylindrical or other lens . having different forcusing powers in di?erent planes. Such electron lenses may clearly be formed by electrodes shaped in cross section in 35 the same way as those described above (for ex ample in Figs. 5 and 6), but having the aper ture in the electrodes other than circular, for Slight departures from the theoretical shapes described above may be made in order to allow for 40 disturbing conditions such as the space charge example in the form of a rectangle having one side longer than the other. A lens system 40 has properties analogous to those possessed by an optical cylindrical lens. The above description has been‘concerned for the most part with electron lenses formed be 45 tween two apertured electrodes. ‘In Figures 7 and within the tube, for example. Thus the optimum semi-vertical angle of the diaphragms of the elec-_ formed by two or more electrodes of this nature trode 9 shown in Fig. 5 may be either greater or Similarly the electrode it shown in Fig. 6 may depart from the theoretical ‘ less than 54°44’. 45 shape at the position IS in order to avoid an edge at the join of the electrode it to the tube It in which it is mounted. It has been found however that the divergences from the theoretical value need only be’slight in order to correct for dis 8 there are shown arrangements in which elec tron lenses are formed between electron emitting surfaces it and apertured electrodes 8. It is to be understood that the present invention is also applicable to electron lenses formed ‘between 50 50 turbing conditions. The invention is not limited to the production of electron lens systems and electron focusing ?elds for the purpose of focusing the electron beams in cathode ray tubes of ‘the usual type 55 alone. an apertured electrode and an electron receiving screen, such for example as a mosaic or a duo rescent screen. The electron receiving surface is then shaped in the same way as the emitting 55 surface it of Figs. 7 and 8. It is also applicable to all cases where electron focusing is desired. For example, certain known methods of transmitting images of‘ an ob ject to a distance‘ include the step of projecting an optical image of the object to be transmitted upon a photo-electrically active screen. From each point on the screen electrons are emitted which are proportional in number ‘to the in tensity of the light falling on the respective points on the screen. These electrons are then focused 65 or directed on to a mosaic screen of mutually in sulated elements, to form an electron image thereon, and to charge the elements according to the number of incident electrons. These charges are then utilized to form picture signals for trans 70 mission. The present invention provides suitable means for effecting this focusing. As shown in Fig; 13, an optical image of an object is formed by means of a lens 33. on a semi-transparent photo-electrically active screen 75 2'5. Electrons are emitted from each point on ' In certain cases it may be desirable to produce an electron focusing‘ ?eld which is other than 30 circular in shape, i. e. one which corresponds to i I claim: 1. An electrostatic electron " lens system com prising two co-operating electrodes of hyper bolical cross-section, at least one. of which is ' apertured to admit the passage of electrons, said 80 electrodes being adapted to have different po tentials‘ applied thereto to provide a focusing ‘?eld therebetween, said electrodes being so shaped that at any point in a region within and extending to the boundary of the aperture in 65 said apertured electrode the component of said focusing held in directions perpendicular to the axis of symmetry of said aperture is substan tially proportional to the distance of said point 70 from said axis. ‘ 2. An electrostatic electron lens system com prising two co-operating apertured electrodes of hyper-helical cross-section, through which an electron beam to be acted on by the Ienssystem can he passed, said electrodes being adapted to 5 2,124,270 trode, a cross section of said apertured electrode have different potentials applied thereto to form a focusing ?eld therebetween, said electrodes be being bounded by two frusto-conical portions, ar. ranged with their smaller diameter ends adia ing so shaped that at any point in a region with in and extending to the boundary of at least one cent one another, the surfaces of said frusta ap proximating to the surfaces of revolution of a line of said apertures, the component of said focusing ?eld in directions perpendicular to a hne which passes through the centres of said apertures is substantially proportional to the distance of the said point from said axis. satisfying the equation V=2a:*-y’, said frusta having between them an annular portion, the inner edge of which lies substantially on the curve V=2:r"-y=, where V is a constant, :1: represents distances measured from a fixed point along the axis of said frusta, and 1! represents distances from said axis. 12. A cathode ray'tube comprising an indirectly heated cathode having an emitting surface, the emitting surface of said cathode having the form 15 of a surface of revolution formed by rotating 3. An electrostatic electron lens system com 10 prising two tubular electrodes, said electrodes be ing adapted to have different potentials‘ applied thereto, said electrodes having surfaces facing one another of frusta-conical shape, the angles 15 between said surfaces of said frusta and the axis .of symmetry of the lens system beingysubstantial ' 1y equal to 55°. about an axis a line substantially satisfying the equation V=2:c*—y’, where V is a constant, :c is the distance along said axis from some ?xed ' 4. An electrostatic electron.lens system com prising two tubular electrodes and being adapted to have different potentials applied thereto, said point, of any plane perpendicular to said axis and 20 intersecting said line and 1! is the distance from said axis of the intersection of said line with said electrodes having surfaces facing one another of frusto-conical shape, said electrodes lying wholly without each other. 5. An electrostatic electron lens system com ' prising two tubular electrodes and being adapted to have different potentials applied thereto, said electrodes having surfaces facing one another of frusto-conical shape, the larger diameter end of each of said frusta being arranged in reg 30 ister with one another and lying in spaced‘ par allel planes. 6. An electron lens electrode combination com prising an apertured electrode, the aperture in said apertured electrode being bounded by at 35 least one surface of which sections in planes plane. - 13. A cathode ray tube as claimed in claim 12 and comprising in addition a .tubular cathode 25 shield surrounding the indirectly heated cath ode, said shield having a diaphragm portion sep arated from but forming a continuation of the surface of said cathode. 14. Cathode ray tube comprising a ?rst screen 30 adapted to receive electrons, a second screen adapted to emit electrons under the in?uence of light, and surrounding the space between said screens, means for forming on said ?rst screen an electron image of said second screen, said 3.5 V=,2:rL-u1, where V is a constant and a: and 1/ means comprising two mutually insulated elec trodes having frusto-conical portions, the frusto represent distances measured along suitably chosen mutually perpendicular co-ordinates. base to base. normal thereto substantially satisfy the equation '7. An electron lens electrode combination com conical portions of said electrodes beingarranged - 15, Cathode ray tube comprising a ?rst screen adapted to receive electrons, a second screen prising an apertured electrode" the aperture in said apertured electrode being bounded by a sur-v adapted to emit electrons under the influence of face of revolution formed by rotating around an light, and surrounding the space between said axis a line substantially ‘satisfying the equation screens, means for forming on said ?rst screen V=2z¢--z/*, where V is a constant and z and 1! are distances measured along mutually per pendicular co-ordinates, the a: co-ordinate being constitutedby said axis. 8. An electron lens electrode combination comprising an apertured solid of revolution elec trode, a cross-section of said apertured electrode being bounded by two surfaces having the form of conical. frusta and being placed with their smaller diameter ends facing one another. 9. Cathode. ray tube comprising two coaxial tubular electrodes ?aring out into frusto-conical portions, said electrodes being placed coaxially with the bases of the frusto-conical portions fac ing one another. 10. An electron lens electrode combination comprising an apertured electrode, the aperture in said apertured electrode being bounded by a pluralityv of annular diaphragms of such number and spacing as to simulate substantially an ap erture bounded by a continuous surface, of which sections in planes normal to said surface substan tially satisfy the equation V=2r1—il‘. where V is a constant and a: and v represent distances measured along suitably chosen mutually per pendicular co-ordinatee. 10 11. An electron lens electrode combination comprising an aperturedqsolid of revolution elec an electron image of said second screen, said means comprising two apertured electrodes, each ‘aperture of said apertured electrodes being bounded by a surface of revolution formed rotat ing around an axis a line substantially satisfying the equation V=2z1-y=, where V is a constant 50 and a: and 1! are distances measured along mutu ally perpendicular co-ordinates, the :r co-or dinate being constituted by said axis. 16. Cathode ray tube comprising an electron receiving surface, an apertured electrode through 55 the aperture of which an electron beam to be focused on said electron receiving surface may be‘ passed, said surface and said electrode being adapted to have different potentials applied thereto to form a focusing ?eld therebetween, 60 both the bounding surface of said aperture and said electron receiving surface having the form of a surface of revolution formed by rotating about an axis a line substantially satisfying the equation V=2:r’—u', where V is a constant, a: is the distance along said axis from some ?xed point, of any plane perpendicular to said axis and intersecting said line and y is the distance from said axisof the intersection of said line with said plane. 70 woman FRANCIS BROADWAY.