US2277246A

US2277246A - Electron discharge device

Info

Publication number: US2277246A
Application number: US238794A
Authority: US
Inventors: Mcgee James Dwyer; Lubszynski Hans Gerhard
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1937-11-04
Filing date: 1938-11-04
Publication date: 1942-03-24
Anticipated expiration: 1959-03-24
Also published as: FR845626A; GB510696A

Description

March 24, 1942,;

J. D. M GEE ET AL ELECTRON DI SCHARGE DEVI C E Filed Nov. 4, 1938 2 Sheets-Sheet 1 INVENTORS HANS G. LUBSZY/VSKI JAMES D. C655 ATTORNEY -2 Sfieets-Sheet 2 INVENTORS 'HANS G.LUBSZYNSKI OOOO/v OJOOOOOQ J D M GEE ET AL ELECTRON DISCHARGE DEVICE Filed Nov. 4, 1938 Patented Mar, 2%, rate U l? E STATES Pars Writs ELECTRON DISCHARGE DEVICE James Dwyer McGee, Ealing, London, and Hans Gerhard Lubszynski, Hillingdon, England, as-

signors to Electric &

ited, Hayes, Middlesex,

Great Britain Musical Industries Lim- England, a company of Application November 4, 1938, Serial No. 238,794

W In Great Britain llovember 4, 1937 3 Claims.

Proposals have heretofore been made in which picture signals are generated by causing a con stant beam of electrons to scan a fine grid upon which and insulated therefrom is a photo-sensitive mosaic which on projection of an optical image thereon becomes charged positively owing to the emission of photo-electrons. When the scanning beam traverses the grid the primary electron beam is modulated owing to the electrostatic charges accumulated by the grid, the modulated beam being employed for generating picture signals. In another proposal a primary beam of electrons is caused to scan a mosaic screen and to release secondary electrons some of which serve to restore the elements of the mosaic screen to a datum value and thereby generate picture signals whilst the remaining secondary electrons are collected by an adjacent electrode.

These prior proposals are not altogether satisfactory partly on account of their poor sensitivity; that is to say, the signal voltage obtained for each lumen of illumination is very low, and partly on account of spurious signals such as that known as tilt which are generated and require elaborate associated circuit equipment for their elimination and/or compensation.

It is the object of the present invention to provide an improved electron discharge device suitable for use in the conversion of an image into signals suitable for television transmission having improved sensitivity compared with the known devices and in which the spurious signals are reduced or substantially eliminated.

According to one feature of the invention an electronfdischarge device is provided comprising an evacuated envelope, a grid structure comprising an a'pertured conducting element having on one side only a storage surface and so constructed and arranged that an optical or an electron image can be applied to said storage surface so as to produce difierences of potential between opposite sides of said grid structure and means for projecting onto the side of said structure opposite to that adapted to receive said image, an electron stream, the arrangement being such that in operation the stream of electrons which passes through the apertures in the grid structure becomes modulated by said potential differences.

According to another feature of the invention an electron discharge device is provided comprising an evacuated envelope, a grid structure comprising an apertured conducting element associated with a storage surface, and constructed and arranged so that an optical or an electron image can be applied to. said storage surface so as to produce difierences of potential between opposite sides of said grid structure and means for projecting onto the side of the grid structure op- .posite to that adapted to receive said image, an electron stream which is such as to cause the release of secondary electrons from said grid structure, the arrangement being such that the stream of these secondary electrons which passes through the apertures in the grid structure becomes modulated by the said potential differences.

In one form of the invention one side of the grid-like structure is provided with a layer of insulating material and upon the insulating material a multiplicity of mutually-insulated photosensitive elements are arranged. In this form of the invention a light image is projected onto the photo-sensitive surface of the grid-like structure causing the release of photo-electrons which serve to cause a potential difierence to be established between opposite sides of the grid-like structure. In another form, the, grid-like struc-, ture may be provided on one side with a multiplicity of photo-voltaic cells. In this case also when an optical image is projected onto the photo-voltaic cells a difference of potential is established between opposite sides of the grid structure. Other constructions of storage screens will be hereinafter described. Some of the secondary electrons which are released from the electron permeable electrode are drawn through the latter by the said potential difference serving, in the case where the photo-sensitive mosaic elements are employed to restore the elements to a datum value, the secondary electron stream being thus modulated, the modulated stream being employed to produce, for example, picture signals suitable for television transmission. In one form of the invention the modulated electron stream may be projected onto a screen adapted to be rendered luminous under the impact of electrons so that a visible image can be produced. In a further form of the invention, where an electron image is projected onto the grid-like structure, the latter may be provided on one surface with an insulating layer which may or may not be provided with a multiplicity of substantially nonphoto-sensitive mosaic elements, the projection of the electron image onto said surface establishing a potential difference between opposite sides of the structure, the potential difference being the electrons emitted by the ,the surface of which is tem',

' The means for applying teries l as shown.

restored to a datum value by the secondary electrons released from the opposite side of the structure. The grid-like structure may be arranged to be scanned by a scanning beam of electrons to release said secondary electrons or, alternatively a high velocity spray of electrons may be continuously projected onto the grid-like structure to release the secondary electrons.

In order that the said invention may be clearly understood and readily carried into effect it will now be more fully described with references to the accompanying drawings in which:

Figure 1 illustrates the invention as applied to a television transmitting tube Figure 2 is an enlarged view of the grid structure employed in Figure 1, and p Figures 3 to 7 illustrate modified forms of the invention.

As shown in Figure 1, the device comprises an evacuated envelope 3 having at one end a conventional electron gun comprising a cathode a screen 5 and a first anode 5, these elements being arranged in known manner. Focussing of cathode a is also effected by a second anode l which may be formed by silvering the required portion of the interior of the envelope 3. The cathode ray beam is caused to scan a mosaic screen indicated generally at 8, and hereinafter more fully described, by the provision of electromagnetic deflecting coils 9 or electrostatic deflecting plates, in known manner. The mosaic screen as shown in detail in Figure 2, comprises a grid structure such as a mesh of wire or other suitable material or a grille provided on the side opposite to the side. scanned by the'prim'arybeam of'elec-' trons with a layer of insulating material II) on formed photo-sensitive mosaic elements II. The insulating material may be evaporated on the surface of the grid and may comprise lithium borate, sodium fluoride, an enamel or other suitable insulating The mosaic elements may be formed.)

material.

-.meter will be created,

by evaporating silver on to the insulating layer M, the silver layer being aggregated if desired and subsequently oxodised and photo-sensitised with a layer of suitable material, such as caeslum. -An optical image of an object for transmission is projected on to the photo-sensitive side of the grid structure throughan optical sysindicated diagrammatically at l2. On either side of the grid structure' are disposed collecting electrodes l3 and I4, these'electrodes be I trode nation will be as large as Figure 2, on striking the screen will cause the latter to emit a large number of secondary electrons most of which will be drawn back to the collecting electrode l3, as indicated at $1, but a' small fraction of the secondary electrons indicated at S: will pass through the apertures in the screen and will reach the mosaic elements and serve to maintain them at approximately" the same potential as the metal structure of the screen. On the projection of an optical image on the mosaic elements, photo-electrons will be liberated from the screen according to the intensity of theindividual areas of theprojected image so that a distribution of positive potentials will be accumulated on the mosaic screen corresponding to the distribution of light in the image, the photo-electrons released from the mosaic screen being absorbed by the electrode it. The capacity of the mosaic elements to the metal of the screen may be made as small as possible, consistent with the capacity of each element to the screen remaining larger than the way the voltage to which an element will rise during one frame period under a given illumipossible, since the photo electric charge lost is independent of the capacity of the mosaic elements. Suppose that the most brightly illuminated element rises to a potential of +10 volts and the pitch of the grid on which the mosaic elements are applied is 1 of an inch, then an electrostatic field of the order oflOOO volts per centibetween the most brightly illuminated mosaic elements and the mesh of the screen. This electrostatic field will penetrate to the side of the screen facing the elecl3 and'willbe suflicient to draw through the interstices in the mesh a considerable pering maintained at positive potentials with respect to the grid structure oil-about 100 volts. the requir'ed potentials to the. electrodes l3 and lt'areiskown conven-l iently as batteries IS, the electrode 13 being connected to the positive pole of its associated batscribed and applied to an amplifier, in known manner. In the drawings the electrode 13 is connected to the second anode 1. The grid structure is connected to the negative poles of the bat.- If desired,- the electrode 13 may comprise the second anode 1, but where separate electrodes are employed for the second anode and the collecting electrode l3, the secand anode I will be maintained at a few volts negative with respect to the electrode l3. The grid structure has a large shadow ratio, that is to say, the apertures in the screen may be only about 10 to 25 per cent of the total area of the screen.

, at S: will reach I will; due to their from the screenwhen' it is scanned by the primary beam; Some of the secondary electrons which are drawn vthrough the screen indicated thflmo'saic elements and discharge them "whilsL-therest of the secondary drawn through the screen nientiim, pass from the reelectrons which 'ar j-gion of the. high field gradients in the immediate-vicinity of theimosaicscreen and will then be accelerated towards" and collected by the electery through a resistance l6. across which picture signals are developed as hereinafter detrode l4, these secondary electrons being indica'i d at $2- In the partslof the screen which have received no light, no field will be established,

- tending to draw thesecondary electrons through elemental areas of the screen capacity of the elementsto one another. In this which are drawn through the screen the remain-' ing portion of the secondary electron stream passing through the mesh being collected by the electrode Ill. The secondary electrons which are collected by the electrode l3 are returned to the cathode of the electron gun and the metal of thegrid through the signal resistance It which, as above mentioned, an amplifier, as shown. It is clear therefore that some of-the secondary electrons serve to discharge the mosaic elements whilst the rest are drawn to the electrode l3 and electrode I 6 and may serve independently to generate picture signals in opposite phase and if a signal resistance included in each of the leads of the said electrodesof push-pull output can be obtained. It is evident however, that the resistance It may be disposed at any convenient point in the circuit which connects the electrode I3 with the screen 8 and the electrode M.

Figure 3 of the drawings illustrates a modified form of the invention in which the secondary electron stream SI instead of being drawn to the electrode i3 is caused to be projected onto one or more target electrodes for the purpose of obtaining electron multiplication. As shown in this figure, there is provided a lateral extension 3A of the envelope 3 in which is arranged in known manner a number of

target electrodes

28, 24, 22, 23, and a final electrode 24 connected to the signal resistance it. The secondary electron stream Si is caused to impinge on the target electrode Sill and thence in turn on the

electrodes

2i, 22, 23 and 2%, these electrodes being maintained at increasing potentials and arranged so as to cause electron amplification by releasing a larger number of electrons compared with the incident electrons.

Instead of projecting an optical image onto the mosaic screen 8, an electron image may be projected thereon. Such an arrangement is shown in Figure d of the drawings in which at the end of the envelope 3 facing the side of the screen 8 remote from the cathode 4 there is provided a photo-sensitive cathode 25 onto which an optical image is cast by the lens 92, this optical image causing the release of photo-electrons which areiocussed by an electrostatic lens comprising the electrodes 26, 2'7 and 28 onto the mosaic screen, which, in this case is substantially non-photosensitive. If desired, as shown in Figure 4, before projecting the electron image onto the mosaic screen 8 the latter is subjected to one or more stages of so called picture multiplication." For this purpose three

grids

29, 30 and 39 are provided for amplifying the electron image in known manner, the electron image being focussed by a focussing coil 32. In such an arrangement the electrode 25 may be maintained at a positive potential of 100 volts the electrode Z'lat a positive potential of 250 volts and the electrode 28 at a positive potential of 200 volts. In this manner the electron image is focussed onto the grid structure 8 and the secondary electrons which are released when the structure B is scanned are drawn to the electrode 21 which is the most positive electrode of the structure shown in Fig. 4. The use of these additional stages of amplification is more feasible with. the type of device illustrated in the drawings, since the geometrical arrange-- ment of the electrodes is more convenient and since the focussing and scanning fields can be well separated.

If it is found that a non-uniform secondary emission is obtained from the mosaic screen on is connected to the grid of scanning with the primary beam, this can be avoided by providing the surface which the primary beam scans with a fairly thick layer of pure metal, such as silver evaporated on the surface during exhausting of the tube.

If it is found that the photo-electric mosaic screen is not restored sumciently quickly to its equilibrium potential during a frame scanning period restoration of the screen may be facilitated by spraying the mosaic elements with a constant subsidiary beam of electrons or by applying the mosaic' elements to high resistance dielectric.

In a further form of the invention as shown in Fig. 5 the electrode I4 is omitted and the end of the envelope remote from the cathode 4 is provided with a screen 33 which is constructed so as to be rendered luminous under the impact of electrons. The secondary electron stream S2 which is an electron image of the light image projected onto the mosaic screen is focussed by a coil 35 and accelerated by an electrode 35 to produce a visible image on thescreen 33. In this arrangement the original optical image may be projected obliquely onto the mosaic screen through the lens 12.

Fig. 6 of the drawings illustrates the invention as applied to a so-called image dissector, the secondary electron image S2 being scanned over an apertured electrode it beyond which is arranged a collecting electrode 31. The electron image is accelerated by electrode 35 focussed by a coil 3% and deflected over the electrode 36 by scanning coils 38.' If desired, the collecting electrode 3! may be associated with one or more stages of electron multiplication in known manner. I

In a further modification, as shown in Figure 7 of the drawings, the secondary electron image S2 is accelerated by an accelerating electrode 35 and focussed by a coil 36 onto a substantially non-photo-sensitive mosaic screen 39 associated with a signal plate 39 The mosaic screen 39 acquires electrostatic charges which are periodically restored to a datum potential by scanning with a cathode ray beam generated by an electron gun indicated generally at to and contained within alateral tube M. It is clear that the mosaic 39 may be of the double sided type and scanned from the side opposite to that on which the electron image is formed. Also one or more stages of electron image multiplication may be provided between the grid structure 8 and screen 39.

In some cases the scanning beam of electrons or a spray of high speed electrons may be projected obliquely on the mosaic screen so as to reduce the possibility of the primary electron passing through the interstices of the mosaic screen. Where a high speed spray of electrons is applied to the screen this spray may be generated by providing one or more thermionic emitters in front of the mosaic screen.

Although in the specific description of the invention it is stated that the electrons which are modulated by the potential differences. between opposite sides of the mosaic screen are secondary electrons, it will be understood that if desired the electrons which become modulated may be primary electrons. For example, one or more there mionic emitters may be provided suitably disposed with respect to the mosaic screen and arranged to be projected towards the grid structure so as to form a virtual cathode in the plane of the grid structure. When an image is proment the electron beam is jected onto the grid structure, the electrostatic charges or potential differences accumulated by said structure cause varying numbers of the electrons from the virtual cathode to be drawn through the interstices of the grid structure. Further, instead of producing a virtual cathode, the grid structure may be scanned by a scanning beam of electrons which is not of a sufiicient velocity in the vicinity of the mosaic screen to release a substantial number of electrons. For example, a high velocity scanning beam may be used the beam being decelerated in the vicinity .of the mosaic screen. This decelerating may be accomplished, for example, by maintaining the grid structure of the mosaic screen at substantially the same potential as the cathode of the electron gun and in providing adjacent to the grid structure an auxiliary grid maintained, for example, at the potential of the second anode l in Figure 1, that is to say, positive with respect to the grid structure. With such an arrangemaintained substantially in focus up to the auxiliary grid, the beam being then decelerated to a velocity which is such as not to cause the release of a substantial number of secondary electrons.

The present invention is not limited to applying to the mosaic screen a, composite electron image or .a composite light image. For example, in some cases where the apparatus is designed for amplifying a picture in, for example, a television receiver, the storage surface of the mosaic screen may be arranged to be scanned by a modulated electron beam so as to accumulate charges according to the modulation of the beam producing difierences of potential between opposite sides of the screen, this potential difference, in turn, causing modulation of the electron stream incident on the other side of the mosaic screen. Such an arrangement may be used where the storage surface of the mosaic screen is adapted to receive an electron image and in cases where the storage surface is adapted to receive an optical image, the optical image may be applied by scanning the storage surface of the screen with a modulated optical beam. The modulated electron or optical beam may be applied obliquely to the grid structure.

In the above-described embodiments Where an optical image is projected onto the photo-sensitive mosaic screen the mosaic screen may, in these cases, be replaced by a mosaic screen composed of a multiplicity of photo-yoltaic cells. The bells maybe made by depositing on the grid of the screen a layer of selenium or cuprous oxide'or other suitable material and on such layer .a transparent metal layer of moisaic formation is provided. When an optical image is projected onto the mosaic screen so formed the elements of the screen assume different potentials according to the incident light and these potentials serve to modulate electrons as above described.

Furthermore, the storage surface of the screen may comprise a photo-conductive layer or a semiconducting layer, these layers being employed and disposed in the manners well known in the art. The term storage layer is therefore defined in accordance with the present invention as comprising a photo-sensitive mosaic or a substantially non-photosensitive mosaic which may,

' in some cases, merely comprise a layer of incomprises three grid-like suiating material or a photo voltaic mosaic or a mosaic comprising a semi-conductor or a photoconductor.

We claim:

1. An electron scanning tube comprising a photoelectric means adapted to release a current image in response to the impingement of an optical image thereon, a double sided mosaic member having interstices therein, means interposed between said photoelectric means and said mosaic for multiplying the value of substantially each element of the current image, means for moving said multiplied image onto and in imon the side of said mosaic remote from said photoelectric means, means for directing said scanning beam onto said mosaic member whereby at -leest a portion thereof is modulated by the sage of the electron image from the photoelectric means to the mosaic member is substantially without impedance thereby.

2. Apparatus in accordance with .claim 1. wherein said electron image multiplying means comprises a plurality of grid-like members.

3. Apparatus in accordance with claim 1, wherein said electron image multiplying means members.

JAMES DWYER McGEE. HANS GERHARD LUBSZYNSKI.