US2124270A - Cathode ray tube - Google Patents

Description

July 19, 1938. L. F. BROADWAY 2,124,270

'CATHODE 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 Q 2 Sheets-Sheet 2 c d a I 3/51 L2, I I8 4 I Z 1%,. a. *v/ w [-21 F2: m 1 K ff fl k.

Patented July 19, 1938 UNITED STATES PATENT orrics CATHODE BAY TUBE i Application November 5, 1935,, Serial No. 48,348

In Great Britain November 8, 1934 in cathode ray tubes and the like.

A cathode ray tube is known which comprises a sealed envelope having disposed within it, in the order mentioned, an indirectly heated cathode, a cathode shield, an accelerating electrode (or accelerator) a grid or modulating electrode, a first and a second anode and a screen such as a fiucrescent screen. Usually the shapes and dispositions of the electrodes and the potentials applied to them are such that a roughly conical beam of electrons emitted from the cathode is brought to a focus in the neighborhood of the modulating electrode and to another focus on the screen.

Any such arrangement of electrodes which by virtue of the electrostatic field existing between the electrodes exerts a focusing action upon the conical beam of electrons is known as an electron lens and it is an object of the present invention to provide an improved lens of this kind.

According to one feature of the present invention there is provided an electrostatic electron lens system comprising two co-operating electrodes having apertures through which an electron beam to be acted upon by the lens system can be passed and means for establishing a difference of potential and hence a focusing field between the two electrodes, wherein the electrodes are so 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 field strength in directions perpendicular to an axis which passes through the centres of said apertures 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 aperture bounded by a surface or by surfaces of which sections in planes normal to said surface or surfaces satisfy or substantially satisfy the equation V=2:|: -y where V is a constant and :r and 11 represent distances measured along suitably 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 example, with reference to the accompanying diagrammatic 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 which would produce perfect focusing of a conical beam of electrons.

Fig. 3 shows lines of force between several electrodes of a known form of cathode ray tube.

Fig. 4 shows equipotential lines between the electrodes arranged similarly to that of Fig. 3.

Fig. 5 shows equipotential lines in the neighborhood of an electrode constructed in accordance with the present invention.

Fig. 6 shows another form of electrode constructed in accordance with the present inven-- Fig. 7 shows the form of a cathode of a cathode ray tube constructed in accordance with the present invention,

Figs. 8 and 9 showparts of the electrode system in a cathode ray tube constructed in accordance with the present invention,

Figs. 10, l1, and 12 show further forms of electrodes constructed in accordance with the present invention, and

Fig. 13 illustrates the application of the present invention to the formation of electron images.

Referring to Fig. 1, in which there is shown an optical lens I whose thickness is small compared with its focal length, the condition that 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 occurring at any point within the lens should be proportional to the distance of that point from the 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 field functioning as an electron lens, the condition in this case being that the field strength perpendicular to the axis of the electronlens at any point within the lens must be proportional to the distance 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 compared with its focal length, the inertia of the electrons passing through the lens complicates the problem.

The shape of the electric field which satisfies the above condition will now be determined. after which there will be described arrangements of electrodes designed to produce such a field. Throughout the following calculation it will be assumed that the electron beam itself has no perturbing effect on the field set up by the lens.

The theorem of Gauss states that if any closed surface (S) is taken in an electric field and if N denotes the component of electric intty at any, point in this surface in the-direction of the outward normal, then J'Nds 4 3 where the integration is taken over the whole surface and E is the total charge enclosed by the surface.

From this equation it may be shown that within a surface enclosing no charge, the potential V within the surface satisfies a difierential equation 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 which is kno u as Laplaces equation. I Since the electrons it must" obviously be a cylindrically etrical field. Assuming the axis of symmetry to be the :r-axis (and it will be seen eventually that the .r-axis is the axis of the lens and an axis of the electrode co-operating in producing the required field) we have ennbs f by whence equation I becomes D v 6 V 1- 3;r= H

It has been stated above that the intensity (F) of the field at any point within the lens must 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 where Fy represents the field strength in direction 1! and It represents a constant. Difierentiating equation III, we have bV b -y IV and substituting for 922 y; in equation H b V 2k V Integrating equation V V=kx +cx+d VI where c and d are constants so far as a: is concerned but may be functions of 1!, Integrating equation m VIII eld is to focus a conical beam of amaaro the constant 0 does not aflect 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 V=2afl-y+r IX It now V be given values 0, 1, 2 and -i, -2, etc., the field represented by equation IX is obtained as a series of equipotential curves, some of which are shown in Fig. 2. These curves belong to two families of hyperbolae and with the potential gradients indicated in the figure, represent the ideal converging lens. The corresponding diverging lens is obtained by reversing the sign of the potentials. In practice this would be done by reversing the sign of the potential on the electrode producing the field, relative to the potentials of adjacent electrodes.

It will be noted that one of the hyperbolae degenerates into two straight lines intersecting on the X-axis and this pair of lines is asymptotic to both families of hyperbolae.

If the origin be taken as the point of intersection of the asymptotes equation IX reduces to V=2a: y s X since the eflectpi' the first order terms is to determine the position of the curve with respect to the origin, without changing the shape of the curve.

The angle between the asymptotes may be found by considering the particular case when V is zero.

Putting this value of V in equation X and the asymptotes are given by a+\/ and The angle between the two straight lines represented by these equations is 7032 and it should be noted that no alteration of the constants of the field equation which has been derived can alter this angle, it being a fundamental property of any cylindrically symmetrical field of the mud under consideration.

It can be proved generally, in fact, that in any cylindrically symmetrical field of this type, the field near the axis has two asymptotes which intersect at the angle 7032.

An arrangement of electrodes which will produce a field of the kind shown in Fig. 2 may now easily be round.

A known arrangement of electrodes in a cathode ray tube is shown in Figs. Sand 4; in both of these figures it represents a cathode, 5 a. cathode shield, '6 an accelerating electrode, 1 a grid or modulator and 8 a first anode. Throughout the following description the cathode shield 5, cathode l and modulator i will be assumed to be at substantially zero potential and the accelerator 6 and the first anode 8 at positive potentials. In Fig. 3 the lines of force between the various electrodes are shown and in Fig. 4 the equipotential 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 field away from the axis 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 accelerator experiences a diverging effect.

At the modulator the component of thefield normal to the axis is opposite to that at the accelerator 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 neighbourhood 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 axis, whilst the reverse takes place in the neighbourhood of the modulator.

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 apertures the equipotential curves depart widely from the calculated ideal curves since the outermost equipotential curve is represented by the section of the surface of the electrode itself and this section clearly does not belong to either of the families of hyperbolae shown in Fig. 2. This is obviously a serious defect in known focusing arrangements especially in the case of an aperture, such as the accelerator, which is filled with electrons, since in this case if the paraxial region of the beam is correctly focused the marginal 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 equipotential surface in the field, forms part of the system calculated above. s

One such electrode is shown at 9 in Fig. any section of this electrode in a plane containing the tube axis Iii comprises two straight lines (neglecting the discontinuity at the aperture) intersecting on the axis at an angle of 70".32'. These lines thus forma part of the asymptote oi the two families of hyperbolae calculated above.

The electrode can be made of two frusto-conical diaphragms II and I2 inserted into a tube, the diaphragm ll nearer the cathode having its apex turned away from the cathode and the other i2 having its apex turned towards the cathode. The semi-vertical angle, i. e. the angle lying between the surface II and the axis 10, of each diaphragm is 54.44'.

In Fig. 6 there is illustrated an electrode l3 which is of theoretically better shape: any section of this electrode lying in a plane containing the axis l4 approximates to one curve of the ideal family of hyperbolae, so that the field produced is theoretically correct right up to the electrode. Such an electrode may be pressed out of sheet metal.

The cathode also may be shaped to conform to the system of equipotentials calculated above. In Fig. '1 is shown a cathode I heated by a heater coil l8 supplied by alternating current from the secondary winding ll of a transformer 40. The cathode 4 is surrounded by a cathode shield 5. In front of the cathode is arranged an accelerator electrode 6 of the type illustrated in Fig. 6. The

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 cathode is irusto-conical inshape, with its base facing the accelerator and has a semi-vertical anglev of 54.44', so that it formsan asymptote to the equipotential curves. The cathode shield 5 surrounding 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 field between them will be of the form deduced above. up to the sur-' face of the electrodes. Instead of being in frustoconical form, the cathode surface l9 and the portion 20 of the cathode shield 5 may be curved to the shape of the equipotential surfaces of the field, for instance those marked a or b in Fig. 7. Further, the accelerator 6 may have the form illustrated in Fig. 5.

Fig. 8 shows the electrode arrangement of Fig.

4 in which the curved electrodes according to the the potentials of the electrodes as stated above,

the lens between the cathode 6 and the accelerator B will be diverging, and the lens between the accelerator 6 and the modulator i will beconverging; with a suitable choice of the strength of the potentials on the electrodes, and their relative dispositions, the net result of the two lenses may 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 this point may be focused upon a screen associated with the tube by a suitable lens system, for instance by the field between two tubular electrodes usually termed the first and second anodes,

, the first anode being held at a high positive po' tential relative to the modulator electrode, and the second anode being for example held at a high positive potential relative to thefirst anode. These electrodes may be arranged in the manner described in co-pending application No. 745,838, filed September 28, 1934, by I. Shoenberg, et al., entitled Cathode ray tubes". In Fig. 8 a part of the first anode 8 is shown, provided with a diaphragm 2|, which is placed sufilciently far in the tube from the modulator 'I that it does not distort the field existing in its neighbourhood. Either of the electrodes 6 or I may have the form illustrated in Fig. 5, instead of, as shown, that illustrated in Fig. 6.

In a preferred arrangement the first and second anodes may have the form shown in Fig. 9. As shown in this figure two tubular electrodes 8 and 22 are provided with two frusto- conical portions 23 and 24 arranged baseto base as shown in the figure. The angle 9 between their surfaces is arranged to be 7032, so that they form the asymptotes of the system of curves calculated above. The field between the electrodes will then be of the required form according to equation X,

- up to the surfaces of the frusta 23 and 24. The

electrodes are preferably provided with diaphragms 2| and 25, and 26 respectively, which should be arranged sufficiently far from the focusing fields to prevent them from distorting these fields. A

In Fig. 10 is shown an alternative form of electrode 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 annular diaphragms 3|, the internal diameter of less than 5444.

which decreases towards the centre of the tube It. The internal edges of the annular diaphragms 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 efiect similar to that shown in Fig. 6 and electrodes built up, as in Fig. 10, of a suitable number of diaphragms arranged to simulate a thicker diaphragm having continuous boundaries are regarded in this specification and in the claims as the equivalent of the latter diaphragms.

In Fig. 11 is shown a form of electrode similar to that shown in Fig. 5, but which conforms more 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 ends adjacent one another. A tubular portion 38 is arranged between the two frusto-conical portions 36 and 3?, and fixed to the tubular portion 38 is arranged an annular diaphragm 39, the aperture boundary or inner edge of which is arranged to lie'on the theoretical curve 35. This form of electrode can be arranged to simulate fairly closely the theoretically correct form described with reference to Fig. 6.

An alternative structure for the electrode of Fig. 11 is shown in Fig. 12. Here the tubular portion 38 is omitted, and the diaphragm portion 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 lie on a surface approximating to the theoretically correct surface deduced above, which is shown in dotted lines at 35.

Slight departures from the theoretical shapes described above may be made in order to allow for disturbing conditions such as the space charge within the tube, for example. Thus the optimum semi-vertical angle of the diaphragms of the electrode 9 shown in Fig. 5 may be either greater or Similarly the electrode it shown in Fig. 6 may depart from the theoretical 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 beslight in order to correct for disturbing conditions.

The invention is not limited to the production of electron lens systems and electron focusing fields for the purpose of focusing the electron beams in cathode ray tubes of the usual type alone. It is also applicable to all cases where electron focusing is desired. For example, certain known methods of transmitting images of an object to a distance include the step of projecting an optical image of the object to be transmitted upon a photo-electrically active screen. each point on the screen electrons are emitted which are proportional in number to the intensity of the light falling on the respective points on the screen. These electrons are then focused or directed on to a mosaic screen of mutually insulated 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 transmission. 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 2'5. Electrons are emitted from each point on From aieasro the screen in number proportional to the light falling on that point. These electrons are then focused by means of the field between two electrodes 2t and hi on to the screen 28 to form an inverted electron image thereon. To obtaincorrect focusing, the field should have the form given by Equation K above, and to this end the electrodes is and 35 are provided with frustoconical portions 3d and 32 respectively, the semivertical angle of the frusta being preferably was. If the electrodes are then held at suitable different potentials (the potential of the electrode as being usually more positive) the field between the screens 2? and 28 will cause substantially 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 and $2 of electrodes 29 and 88 respectively may be curved to conform more exactly with the required form of equipotentials in the focusing held.

In any of the above described examples, where electrodes in the form of conical frusta having semi-vertical angles of 54244 have been described, such electrodes may be replaced by electrodes having the form of the surface of revolution of one of the hyperbolae of the form given by Equation X, and shown in Fig. 2.

In certain cases it may be desirable to produce an electron focusing field which is other than circular in shape, i. e. one which corresponds to the optical case of a cylindrical or other lens having different forcusing powers in difierent planes. Such electron lenses may clearly be formed by electrodes shaped in cross section in the same way as those described above (for example in Figs. 5 and 6), but having the aperture in the electrodes other than circular, for example in the form of a rectangle having one side longer than the other. A lens system formed by two or more electrodes of this nature has properties analogous to those possessed by an optical cylindrical lens.

The above description has beenconcerned for the most part with electron lenses formed between two apertured electrodes. In Figures 7 and 8 there are shown arrangements in which electron 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 an apertured electrode and an electron receiving screen, such for example as a mosaic or a duorescent screen. The electron receiving surface is then shaped in the same way as the emitting surface it of Figs. 7 and 8.

i I claim:

1. An electrostatic electron lens system comprising two co-operating electrodes of hyperbolical cross-section, at least one. of which is apertured to admit the passage of electrons, said electrodes being adapted to have different potentials applied thereto to provide a focusingfleld therebetween, said electrodes being so have different potentials applied thereto to form a focusing field therebetween, said electrodes being so shaped that at any point in a region within and extending to the boundary of at least one of said apertures, the component of said focusing field 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.

3. An electrostatic electron lens system comprising two tubular electrodes, said electrodes being adapted to have different potentials applied thereto, said electrodes having surfaces facing one another of frusta-conical shape, the angles between said surfaces of said frusta and the axis .of symmetry of the lens system being substantial- 1y equal to 55.

4. An electrostatic electron.lens system comprising two tubular electrodes and being adapted to have different potentials applied thereto, said electrodes having surfaces facing one another of frusto-conical shape, said electrodes lying wholly without each other.

5. An electrostatic electron lens system comprising 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 register with one another and lying in spaced parallel planes.

6. An electron lens electrode combination comprising an apertured electrode, the aperture in said apertured electrode being bounded by at least one surface of which sections in planes normal thereto substantially satisfy the equation V=,2:r -u where V is a constant and a: and 1 represent distances measured along suitably chosen mutually perpendicular co-ordinates.

'7. An electron lens electrode combination comprising an apertured electrode" the aperture in said apertured electrode being bounded by a sur-v face of revolution formed by rotating around an axis a line substantially satisfying the equation V=2z--z/*, where V is a constant and z and 1! are distances measured along mutually perpendicular co-ordinates, the a: co-ordinate being constitutedby said axis.

8. An electron lens electrode combination comprising an apertured solid of revolution electrode, 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 flaring out into frusto-conical portions, said electrodes being placed coaxially with the bases of the frusto-conical portions facing one another.

10. An electron lens electrode combination comprising an apertured electrode, the aperture in said apertured electrode being bounded by a plurality of annular diaphragms of such number and spacing as to simulate substantially an aperture bounded by a continuous surface, of which sections in planes normal to said surface substantially satisfy the equation V=2r il. where V is a constant and a: and v represent distances measured along suitably chosen mutually perpendicular co-ordinatee.

11. An electron lens electrode combination comprising an apertured solid of revolution electrode, a cross section of said apertured electrode being bounded by two frusto-conical portions, ar. ranged with their smaller diameter ends adiacent one another, the surfaces of said frusta approximating to the surfaces of revolution of a line 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 plane.

13. A cathode ray tube as claimed in claim 12 and comprising in addition a tubular cathode shield surrounding the indirectly heated cathode, said shield having a diaphragm portion separated from but forming a continuation of the surface of said cathode.

14. Cathode ray tube comprising a first screen adapted to receive electrons, a second screen adapted to emit electrons under the influence of light, and surrounding the space between said screens, means for forming on said first screen an electron image of said second screen, said means comprising two mutually insulated electrodes having frusto-conical portions, the frustoconical portions of said electrodes beingarranged base to base.

15, Cathode ray tube comprising a first screen adapted to receive electrons, a second screen adapted to emit electrons under the influence of light, and surrounding the space between said screens, means for forming on said first screen 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 rotating around an axis a line substantially satisfying the equation V=2z -y=, where V is a constant and a: and 1! are distances measured along mutually perpendicular co-ordinates, the :r co-ordinate being constituted by said axis.

16. Cathode ray tube comprising an electron receiving surface, an apertured electrode through 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 field therebetween, 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:ru', where V is a constant, a: is the distance along said axis from some fixed 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.

woman FRANCIS BROADWAY.

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