US2227015A - Picture transmitter - Google Patents

Description

Dec. 31, 1940. K. SCHLESINGER EIAL PICTURE TRANSMITTER Filed Jan. 25, 19s? 4 Sheets-Sheet 1 Mym\ paw MW 1940- K. SCHLESINGER ETAL. 2,227,015

PICTURE TRANSMITTER Filed Jan. 25, 1937 4 Sheets-Sheet 2 D 3 0- K. SCHLE-SINGER EIAL 2,227,015

PICTURE TRANSMITTER Filed Jan. 23, 1937 4 Sheets-Sheet 5 D 31, 0- K. SCHLESINGER ETAL 2,227,015

PICTURE TRANSMITTER Filed Jan. 25, 19s? 4 Sheets-Sheet 4 Patented Lee. 31, 1940 UNITED STATES PATENT OFFICE PICTURE TRANSMITTER of New York Application January 23, 1937, Serial No. 122,032 In Germany January 29, 1936 2 Claims.

necessary to secure the aid of very powerful artificial light sources. In the case of weak light sources the practicability of the use of an ikonoscope is limited by the noise of the amplifying tubes themselves.

On the other hand a new amplifying method has become known by the researches of Zworykin and others, the electron multiplier method, which is distinguished by a considerably smaller natural noise, as in this method any feed current is missing. If it were possible to combine this method of direct current amplification with the principle of the chronological storage of the ikonoscope, an apparatus would thus be obtained, the operation of which would be in turn to the extent of one order of magnitude more sensitive than that of the normal ikonoscope associated with a conventional amplifier. A combination of this kind of the two apparatus known per se is impossible in their known form. The known ikonoscope is a voltage generator, i. e., upon the scanning of the picture receiving area it supplies potential oscillations to a free terminal, and accordingly can only be employed in conjunction with the grid of a normal amplifying tube but is not suited to an electron current amplifier. For a device to be so suited it is necessary to act not as a voltage generator but as an electron current generator.

The following invention by Kurt Schlesinger and Gerhard Liebmann relates to corresponding modifications of the ikonoscope in which, in contradistinction to the known ikonoscope, electrons are liberated upon the scanning of an accumulating light-electric surface, and in which these electrons are drawn directly on to an anticathode, at which there occurs a current amplification so as known from the multiplier," due to a powerful secondary emission. The currents thus amplified may then be passed further to an additional anticathode and maythus be additionally amplified in the manner already known. The complete operation takes place in the tube according to the invention in a common high vacuum.

Various forms of embodiment of the invention will hereinafter be described by way of example, reference being to be made to the accompanying drawings,of which-- Figs. 1 and 2 are diagrammatic sectional elevations of electron tubes according to the invention together with a diagrammatic partial showing of the optical and electrical equipment thereof, whereas Fig. 3 is a plan view of a detail of the tube shown in Fig. 2, and

Fig. 4 is a diagrammatic elevation of a modification of this detail and its circuits.

Further modifications of a tube such as shown in Fig. 1 are illustrated by further showings of this same type in Figs. 5, 6 and '7.

Fig. 8 is a plan view of a special mosaic structure according to the invention, of which structure Figs. 9 and 10 show a part on an enlarged scale substantially in plan view (9) and in sectional elevation (10).

Figs. 11 and 12, again, are showings of a similar type as Figs. 1 and 2 and illustrate certain modifications and improvements according to the invention.

Fig. 13 is a plan view of a mosaic structure for use in a tube as shown in Fig. 12.

In Fig. 1 is illustrated a form of embodiment of the idea according to the invention, in conjunction with which the essential explanations may be given. In the original ikonoscope the image receiving area itself consists of insulated photo-electric particles. These particles lose electrons during the storage period and have the same compensated for by the scanning electron ray In this device, therefore, no new electrons are to be gained during the scanning, but electrons are consumed from the ray. In the arrangements according to the invention these operations, therefore, are modified accordingly. The storage plate must'recover electrons during the period of rest, and these must be liberated point-by-point upon the scanning operation. For this purpose it is proposed first electronoptically to produce on a mosaic surface an electron image of the image-receiving area. In Fig.

1 the image-receiving area I is a coherent, i. e., undivided photo-electric plate. On to this plate is projected through the objective 2 by way of a small mirror 4 an image of the object 3. Of this light-optical image 3' an electron image is projected on to a plate I by means of a simple electrical lens comprising the electrodes and 6/ This plate I in turn consists of an insulating sheet of mica 8, which is furnished on the side directed towards the interior of the tube with a layer Ia divided in mosaic fashion and comprising minute photo-cells, and on the opposite side with a coherent. counter-electrode 1b.. The latter is earthed. The primary electrons, represented by the rays 9, are caused to traverse magnetic field H), which is to be imagined at right angles to the paper. They are then deflected in the manner shown. By means of an electrode 5a, which may be raised to about the same potential as 5, the electrons 9 are slowed before impinging on the mosaic and reach the mosaic with a potential of less than 100 volts. At this low speed they are no longer capable of liberating secondary electrons from the layer la, but merely charge negatively to different potentials the particles of this layer. The layer la is illuminated from an optical scanning line screen II by way of a lens l2 and thereby discharged point by point. There then result per image element electron currents of varying intensity, corresponding to the number of electrons 9 which have successively reached the image element in question during the storage period. Since these electrons leave the plate, i. e., have an opposite direction of movement to the primary electrons 9,

they will be circularly deflected in the opposite sense by the magnetic field I0. The path is represented by iii. The electrons are liberated from the layer by an anode l4. They then meet against a first anticathode I5 in the manner known from the "electron multiplier and liberate there a multiple secondary electrons, the

paths of which are designated l6 and which now impinge on a new anticathode l1, and so forth.

Each additional stage of this nature is furnished with a new, appertaining suction anode, the suction anode in respect of the paths It being designated l8. The potentials of these successive anodes consecutively increase by the same amount respectively in relation to one another.

Each accelerating anode is connected with the appertaining plate, i. e., the anode l4 with the anticathode IS, the anode IS with the anticathode l1, etc. The final anticathode, in this case I1, is connected by way of a coupling resistance IS with the final accelerating potential 20, so that to this resistance there may be connected an end amplifier 22.

In Fig. 1 a magnetic field has been employed for spacially separating the primary and secondary currents. As already known, there may also be employed in place thereof an electrostatic electron-optical system. Further, in place of a straight-view primary reproducing electronoptical system 5, 6, 5a with a built-in optical deflecting mirror 4, a reproduction may be made use of with magnetically or electrically curved rays, wherein such deflecting mirrors may be dispensed with. Further, the screen area ll consisting of luminous points may be produced by a mechanical decomposing machine in conjunction with a technical light source or by the luminous screen of a Braun tube.

All of these different forms of embodiment, to which others may also be added, have in common the feature of the invention consisting in the fact that of the image original 3' there is first projected an electron image on to a storing mosaic surface, that the storing mosaic is then discharged point by point by light, and that the stored electron currents thus liberated are amplified by secondary emission by way of direct current amplification to above the noise level of tube amplifiers by which the electric excitations are dealt with afterwards.

A tube for converting a light image into an electron image has already been described in a practical form of embodiment in the application Ser. No. 111,815. The described operation of the point-by-point discharge of the stored charges of the photo-cells occurs only in the manner described if there is no appreciable secondary emission from these cells in the dark condition. As a rule, 1. e., with electron rays having speeds of between 100 and 2,000 volts, this requirement is not complied with, but the cells are charged to the approximate potential of the incoming ray and remain at this potential independently of the intensity of the ray impinging thereon. According to a feature of the invention, therefore, either the primary ray must be slowed, so that the electrons meet against the layer at a speed of less than 100 volts. This may be performed by the cylinder electron lens as described, or for instance by coating the wall of the tube 8 with a conductive layer. e. 'g., in the form of a graphite spiral and applying the desired terminating potentials to the ends of this spiral (afterretardation). The desired effect may also be accomplished by the use of rays of more than about 2,000 volts, since from a certain speed up wards the secondary emission no longer suflices to cover the current balance between incoming and leaving electrons and a point-by-point charging then occurs in proportion to the intensity of the primary current.

A particularly simple embodiment of the optically scanned ikonoscope in conjunction with a current amplifier is shown in Fig. 2. Herein 23 is a plate having perforations of image-point size, and 24 is a two-dimensional structure which is situated opposite thereto and consists, so to speak, only of image points, and in which points of silver are arranged on an insulating supporting plate 25. An arrangement of this kind has already been described in a previous application Ser. No. 111,815. Both the perforated plate 23 as well as the point plate 24 are activated with caesium and made photo-sensitive only on the side facing each other. The light from the object 3 to be transmitted passes by way of a reproducing lens 2 through the spaces between the image points 24 onto the activated layer side of the perforated plate 23. From the latter there are liberated photo-electrons, which, by reason of their low natural speed, impinge on to the oppositely disposed image points 24 and charge these negatively to an extent which depends on their number. Should the initial energy not be sufiicient, there may be applied between the perforated plate and the anode, by the introduction of a very fine-mesh anode 26, a small bias 'of' approximately 10 volts, which accelerates the electrons in both directions.

Fig. 3 is a magnified front view of a portion of the perforated plate, showing that a wire net 26 may further be introduced in such fashion between the apertures in the perforated plate 23 that the wires thereof are finer than the connecting pieces between the apertures of 23, so that when looking from the left in Fig. 2 merely the connecting pieces of the perforated plate 23 and when looking from the right merely the image points of 24 are visible, whilst the anode maximum produced picture frequency is connected between the perforated plate 23 and an auxiliary electrode 21 and is operated by a highfrequency generator at 28. The electrostatic field then provides for the liberation of the electrons from the perforated plate. The same highfrequency generator may also release electrons from the mosaic plate 24 in the opposite direction by way of a push-pull circuit, as illustrated in Fig. 4. For this purpose a counter-electrode 21a is required.

The mosaic plate 24, having thus been charged negatively point by point during the storage period, is then illuminated in a successsive manner by a scanning ray of light proceeding from the right hand through a lens i2. As scanning light source there is again employed a mechanical system oi the Nipkow disc type or a Braun tu generally speaking a surface ll over which t ere moves a luminous image point. In this way the electrons which have accumulated at the mosaic 24 are photo-electrically liberated, conveyed outwards towards the perforated plate by means of a fine-mesh net 30' with a positive bias 29, acting as anode, and then concentrated on to an anticathode l5 by means of an electron-optical system, which in its simple form comprises two cylinders 30, 3|, of which 3| is raised to a still higher positive bias 2|. At 15 secondary electrons are liberated in the manner known per se and are concentrated by a further and smaller electron-optical system 33/34 upon a final anode FL The electron-optical system shown is an electrostatic one. A battery 20 is applied with its full voltage of approximately 200 volts relatively to IE to the cylinder 34 and the intercepting plate I! by way of the coupling resistance 13, whilst a medium potential of 20 is adjusted at the reproducing electrode 33, thus effecting focusing. 22 is the input tube of the associated image amplifier.

The double-plate arrangement 23/24 contains at least as many image points as indicated by the square of the desired number of lines. The arrangement is preferably produced mechanically, by first stamping a perforated plate 23. With a size of approximately 20 cm. square, and 400 lines, an individual aperture in this plate still has a diameter of 5 mm., which may be readily produced. If there is employed the precision art of producing finest apertures of approximately 50;: in diameter as developed in the production of Nipkow discs, a 400-line screen may even be produced in a plate of 4 cm. square.

In Fig. 5 is shown an arrangement similar to that of Fig. l. A deviation in direction is assumed amounting to 45. The object 3 to be televised ls reproduced by a lens 2 on the primary cathode I. The cathode l is a coherent photoelectric layer. The image 3 is reproduced by the lens 50-52, the sharpness of which is adjusted at 5|, on the mosaic Ia. A magnet field l0 perpendicular to the plane of the paper deflects all of the reproducing rays 9a to at the point of their maximum construction by the samev angle (45 have beenshown), and accordingly acts as an electron reflector. The reproduction remains sharp in the plane la. A slowing system formed of three rings 53 in conjunction with the resistance l slows the electrons down about 0 volts.

In this way secondary emission is avoided, and there results on la a charge image having charges which are the more negative the brighter the corresponding points of the image on I happen to be. A lens l2 reproduces a luminous scanning area, for example the surface of a television cathode ray tube, II, on la. Since la is photo-activated, the illuminated points discharge electrons until they have entirely lost their negative charges. The thus produced oathode rays 13a to I30 are deflected by the same magnetic field ill in the opposite direction and enter into a tubular member 30, being accordingly completely separated from the primary currents.

Following on the positive tubular member 33 there may be employed any of the known multiplication systems for a direct amplification of electronic currents. There is shown in the drawing an embodiment with an electrostatic multiplier. As well known, such a multiplier consists of a tubular member so and a tubular member 3i which is positive in relation to 30. The anticathode l5 ismet at the point 59 by the primary rays l3, and the electrons 60 liberated in excess at that point under certain known conditions are concentrated by the tubular member 34, and finally collected by the anode H.

A further embodiment of the idea of the invention is set forth in Fig. 6. The image to be transmitted is reproduced on the photo-cathode l by means of the lens 2. In the example shown the photo-cathode is represented as being perpendicular to the axis of the tube and the direction of the light falling thereon, and must accordingly be assumed to be transparent. It may, however, also be inclined to the axis of the tube (with correspondingly modified arrangement of the lens2) in Fig. 7. It is possible to impart to the photo-cathode, in the manner known per se, a curvature as shown, which eliminates the reproduction fault of the electron-optical reproducing system 6|, 6|" 50, 62, which reproduces on the net electrode 63 the electrons liberated from the photo-cathode. The electronoptical reproducing system, in the manner known per se, may consist of a system of rings 6|, 61", etc., a diaphragm 5lland an anode cylinder 62. By adjusting the bias of the diaphragm 50 the scale of enlargement of the system may be varied within certain limits. The potentiometer which serves to impart to the said electrodes the potentials necessary for obtaining a good reproduction is not shown in the drawings. The electronoptical reproduction, in the manner known per se, can also be performed by magnetic means or by a combination of electrostatic and magnetic means. The electrode 63 represents a mosaic composed of electrically conductive metallic pellets embedded in a highly insulated fashion in a conductive net. The production of this electrode will be described later. It separates the aforesad electron-optical reproducing system mechanically and electrically from the scanning system 64. The side of the mosaic electrode facing the photo-cathode I is preferably furnished with a coating due to which the metallic pellets insulated one against the other, assume the property that the number of secondary electrons liberated per oncoming primary electron is considerably smaller than i. It has been found that certain carbon modifications, for example finely as the ratio between the number of liberated secondary electrons and the number of primary electrons amounts to about 0.5 in the case of practically all potentials concerned. This layer of soot must naturally be applied so thinly that no conductive bridges are formed between the insulated metallic pellets. Instead of applying the mentioned coating of a substance giving off a poor supply oi\secondary electrons the total accelerating potential of the electron-optical reproducing system 6|, 6|", etc., may be so chosen that the yield factor of secondary electrons remains below 1.

Under the stated conditions each of the insulated metallic pellets is charged to a potential which is somewhat more negative than the potential of the supporting conductive net, which is the same as that of the final electrode of the reproducing system 6|, 6|" and the same as that of the final anode of the scanning system 64, which is still to be described. The potential up to which the metallic pellets are charged is proportional to the light intensity values of the light that impinges on the corresponding points of the primary cathode. By the embedding of the insulated metallic pellets in a conductive net the individual pellets are to a high degree screened off electrically one against the other; in addition the capacity is considerably greater than in the case of the known mosaic screens deposited on mica, which reduces the danger of self-discharge, the potential of each element remaining comparatively low.

The side of the mosaic screen 63 facing away from the photo-cathode l, or in other words the rear sides of the negatively charged metallic pellets, are scanned in the example shown by means of a cathode ray, in the manner known per se. For producing the cathode ray there is employed the electrode system 64, and for per- 40 forming the scanning the deflecting system 65,

which may be an electrostatic, a magnetic or a mixed electrostatic-magnetic deflecting system.

To avoid distortions of the image the axis of the cathode ray system 64 is disposed vertically to the 5 mosaic screen 63. If the photo-cathode I is inclined in relation to the axis of the tube the axis of the scanning system is preferably inclined in such fashion that the resulting scanned image is again undistorted.

The accelerating potential of the cathode ray scanning system is so chosen that each electron of the scanning ray liberates more than one secondary electron. This eifect may be further improved upon by sensitizing the metallic pellets on the side facing the cathode ray, for example by means of caesium. In this case the accelerating potential may be adjusted to 2,000 volts. This potential, however, as all potentials given in the drawings, are merely quoted by way of example.

0 passes thereover. vA higher positive charge than corresponds to the equilibrium potential cannot take place, since, as well known, the higher potentials thus resulting would reduce the yield factor of the secondary electrons to a value below 1.

The secondary electron current impulses released upon the scanning by the cathode ray and the discharging of the pellets to the equilibrium potential correspond in their strengths to the charges of the scanned metallic pellets, and accordingly to the light intensities of the image elements of the image projected on to the photocathode I. These secondary electronic impulses are now amplified in a current multiplier of the known kind, applied somewhat obliquely to the main tube, and are conducted by way of the terminal anode l1 and the resistance ii! to the final amplifying tube 22. As current multiplying means there may be employed any of the known arrangements, for example those with magnetic or electrostatic deflection of the secondary currents supplied by diiferentplates, or' those with a superimposed auxiliary oscillation of ultra-high frequency, which hurls the electrons repeatedly against suitable secondary electron cathodes. The most suitable for thepresent embodiment of the invention, however, is the use of a multiplier which consists of a series of grids 6B, 66", etc., suitable for supplying secondary electrons, which are provided with increasing potentials.

In an electron multiplier of this kind, nearly the entire amplification of the image impulses which may be necessary may be performed.

Instead of scanning the mosaic with an electron ray it is, also in this arrangement, possible to perform the scanning by means of a ray of light, which compensates the negative charges of the metallic pellets by the liberation of photoelectrons.

Instead of the electrons which are liberated upon the scanning there may also be multiplied, in accordance with the invention, those electrons by means of which the photo-cathode, on to which the image of the object to be transmitted is projected, is reproduced on the mosaic electrode. An arrangement of this kind is shown in Fig. 7, which at the same time illustrates the aforementioned modification in the position of the photocathode in such fashion that the light meets against the same at an angle and the photoelectrons leave the same on the side on which the light impinges. The use of an inclined frontview photo-cathode has the advantage that the primary sensitivity is considerably higher than in the case of the transparent-view cathodes as employed up to now.

In Fig. '7, I is the photo-cathode, which is inclined at an angle of 45, and on to which the image 01 the scene to be transmitted is projected by means of the lens IS. The electrons which are liberated at the photo-cathode l in correspondence with the light intensity of the individual points of the image, and constituting the current Ip, are projected by the electronoptical reproducing system 61', 61", on to the metallic plate 68, which is likewise inclined at an angle of 45.

The reproducing system consists in the manner known per se of rings 61', 61", 61", 61 to which, by means of a potentiometer (not shown) there are imparted suitable potentials which increase in the direction from the cathode to the metallic plate 68. In this arrangement the first ringiil may be adapted in its form to the inclined position of the photo-cathode I, and in accordance with the invention may consist of a thin-wire metallic fabric in order not appreciably to weaken the image light possibly projected through the same. The final reproducing electrode 61 of the said system is preferably maintained at the same potential as the metallic plate 68 and the first ring of the electron-optical reproducing system 69', 69", This potential in relation to the photo-cathode l is so chosen potential each liberate a multiple number of secondary electrons. These form the current Is, and produce the image on the mosaic screen 63 owing to the electron-optical system consisting of the rings, 69', 69", 69, 69. The total difference in potential which is maintained between the secondary cathode 68 and the last ring 69, and accordingly approximately (with the exception of the fluctuations caused by the generated impulses) also the coherent conductor (metallic net) of the electrode 63, corresponds in turn to the most favourable compromise between as good as available a reproduction and the liberation of secondary electrons, and may again amount, for example, to 800 volts.' Owing to the fact that the secondary current Is now liberates tertiary electrons at the mosaic screen 63, the individual mosaic elements are charged to a considerable positive potential in relation to the coherent conductor of the mosaic electrode 63. The tertiary electrons are picked up by the final ring of the reproducing system 69', 69", 69. The charges of the mosaic elements correspond to the light intensity of the individual corresponding points of the photo-cathode I, and accordingly of the original image. By the scanning of the mosaic 63 by the cathode ray IA, which may be caused by the deflecting coils 65, the charges of the mosaic 63 are compensated 'in the manner known per se, and image impulses are accordingly generated, which flow from the coherent conductor by way of the resistance l9 to earth and accordingly control the grid of the input tube 22 of the subsequent amplifier. The mosaic screen 63 is so inclined in relation to the scanning electron ray that the image distortion is compensated which is caused by the inclined position of the photo-cathode i.

It is naturally also possible in the arrangement according to Fig. 7 to repeat the described liberation of secondary electrons. The possible number of reproductions, however, is limited, as with each additonal electron-optical reproduction the lack of sharpness of the reproduction increases, the more so as secondary elec- -tronic currents always contain a considerable proportion of comparatively rapid electrons, which results in faults in reproduction corresponding to the chromatic errors of light-optical systems.

The image transmission tube set out in Fig. 6 is free from these disadvantages, as only the primary photo-electrons, which have a very low starting speed, are once reproduced electronoptically. The multiplication by secondary electron liberation does not take place before, by means of an electron ray or a ray of light, electron impulses have been produced, the strength of which corresponds to the local light intensity of the image. The multiplying device then no longer requires to have point by point reproducing properties.

An essential component part of the present invention is represented by the embodiment of the two-sided mosaic employed. Two-sided mosaics have already been proposed previously, which consist of an arrangement of small aluminium bars coated electrolytically with aluminium oxide.

It has been found, however, that this kind of mosaics or multiple grids can only be employed in tubes with thermionic cathodes, above all, therefore, for image receiving tubes, as the percentage of gas owing to these mosaics is too large to permit of the introduction of such mosaics into vessels containing highly sensitive photo-cathodes. This gas cannot be sufliciently expelled even by a several days preliminary treatment in vacuum at high temperatures.

According to a further feature of the invention, therefore, the mosaic consists of a metallicpreferably tungsten-net, whichis coated with aluminium oxide. Silver pellets are fused into the meshes thus insulated.

For producing a mosaic of this kind the procedure is preferably as follows: Preliminarily burnt, ground, pure aluminium oxide, which is mixed with collodion and amyl acetate, is sprayed on to the tungsten net from both sides by a. spray gun, which requires to have a very fine nozzle, so that, while the meshes are not yet closed, all wires are well imbedded. After drying the net mately 1,800 C. In this way the aluminium oxide is sintered to form a firmly adhering, highly insulating coating on the wires of the net. Following thereon the pores are filled out with powdered silver, or a certain amount of silver amalgam is painted into the meshes and the mercury expelled by heating. The net is then heated in vacuum to a temperature above the melting point of the silver. Owing to the surface tension the silver'forms into small pellets, so that there is one silver pellet in each mesh. This operation is repeated until the pellets have assumed such a size that they completely fill out the pores.

Fig. 8 shows the finished mosaic 03 fitted into the tube. The net is stretched over small metal plates 10 and insulating plates II, which after the production may be metallizeda They are held together by the rivets 12. In this manner the mosaic fills out the entire cross-section of the tube and separates the discharge spaces. Fig. 9 shows a piece of the mosaic, Fig. 10 a sectional elevation of this piece. In these figures 13 are the individual tungsten wires of the net, 14 is the surrounding insulating aluminium oxide coating, and 15 are the fused in silver pellets. 16 are particles of soot, which are applied to the one side in order to suppress there the emission of secondary electrons, and 11 are coatings which are applied to the opposite side and favour the emission of secondary and photo-electrons.

It is naturally possible to replace the tungsten, for example, by molybdenum, to employ in place is annealed in a hydrogen furnace at approxiof aluminium oxide another, preferably ceramic 05 63 facing the photo-cathode I is coated with 75 carbon, it is desirable, pending termination of the sensitization of the photo-cathode I, to close the opening in the electrode 50 by means of a piece of mica, which is subsequently removed, in the same manner as the intermediate spaces between the other electrodes of the reproducing system 6|, 6|", 62, the absorption by the carbon of the vaporized caesium being so strong that otherwise the sensitization of the photocathode I could only be performed with difllculty.

Certain improvements and simplifications in respect of transmission tubes as described in the tained, preferably by means of a regulable potentiometer, at the potential of that next adjacent net cathode which, in the particular system concerned, assumes the function of the cathode, or at a somewhat more negative potential. By means of the potential pot thus formed there occurs a concentration and directing of the liberated secondary electrons towards the next secondary cathode, which in this system acts as the anode. This concentration is of importance in view of the space charge efiective in the higher stages and also on account of the numerous secondary electrons with high lateral speeds which are present in every stage. Between the mosaic plate and the first secondary cathode net of the multiplier there may also be provided in accordance with the invention a concentration ring," which preferably consists of a comparatively large-mesh, extremely thinwire metallic fabric (both in the case of light ray as well as electronic ray scanning). It is apparent that this measure of intermediate concentration is also important quite generallv in electron multiplying systems.

An additional improvement consists in the fact that the axis of the multiplying system coincides with the axis of the electron-optical system causing the reproduction of the photo-cathode on the mosaic plate. In this way favourable conditions are provided in respect of the sucking ofi of the electrons liberated at the mosaic plate and entering the multiplier. The bundle of cathode rays or light rays necessary for liberating these electrons falls on to the mosaic plate in an inclined direction and the spacing between the first net of the multiplier and the mosaic electrode is so large that there is no interference with the scanning bundle of cathode or light rays. When using a scanning ray of light the portion between the mosaic plate and the first secondary cathode may be furnished with a semi-conductive transparent coating. which connects the mosaic electrode with the first secondary cathode. In this way a more even suction field is ensured at the mosaic electrode. The axes of the reproducing bundle of light and the scanning bundle of cathode or light rays'again form such an angle that thtei resulting image distortions compensate each ot er.

A simplification, which is independent of the aforesaid improvements, which are capable of being employed separately, consists in the fact that there is not employed an electron-optical reproducing system for reproducing the photo-cathode on the mosaic plate, but that a conductive net is provided just in front of the mosaic electrode, and is photo-electrically sensitized on the side facing away from the mosaic screen. The weak field caused by the discharge of the mosaic points upon the scanning operation extends through the meshes of this net and sucks the liberated photo-electrons on to the storage elec-" trode.

An exemplary embodiment of the stated improvements and simplifications is set forth-in Fig. 11. Herein 18 is the mosaic electrode, 19 the photo-cathode net situated closely in front of the same, 86, 80, I1 the multiplying system, and 84 the scanning cathode ray system. The scanning movement of the cathode ray is produced by the coils 65. The electron-multiplying system consists of the nets 66, 66', 66", disposed one behind the other with intermediately disposed concentration rings 80, 80, 80" and the pick-up electrode H. The potential provided by a potential source 8| is applied in a suitable manner to the multiplier stages by means of the potentiometer 82. The current impulses developed are conducted to the end amplifying tube 22 by way of the grid resistance IS. The reproduction on the photo-cathode 19 of the image 3 to be transmitted is produced by the lens system 2. The light ray and cathode ray axes are preferably inclined at an angle of 45 in relation to the mosaic screen, in the manner shown.

A further embodiment of the invention is 11- lustrated in Fig. 12. Herein, 8 is the mica plate of an ikonoscope, la the mosaic which in the known manner is illuminated from the object 3 through the optical device 2 and is scanned by means of the cathode ray system 64. According to this embodiment of the invention there is provided on the rear side of the mica sheet 8, not a coherent collector plate as heretofore, but a second mosaic 'lc. To this mosaic there is applied in close contact with its surface a coherent grid which consists of a fine network 83. Behind this first net 83 there follow a plurality of other nets 66, 86' etc. By means of a terminal anode II, which may also be a wall coating of the tube, the amplified electron current is picked up, to be converted at the resistance l9 into fluctuations in potential which excite the amplifier 22. The rear mosaic 1c is illuminated by a constant light source 84, possibly with a condensing lens 85. The grids 86, 86'l1 are given increasing positive potentials in a manner such as known from the multiplier designed by Weiss.

The operation of this arrangement is as follows: By the illumination of the primary mosaic 1a by the image there is produced in the known manner an electrical charge image, which contains positive residual charges at the illuminated points and no residual charges at the unlighted points. Upon the scanning by the cathode ray the positive residual charges are neutralised. This results in a negative potential impulse at that one of the capacitatively coupled points of the rear mosaic To which is situated immediately opposite the point just scanned.

By the illumination of the rear mosaic by the light source 84 all particles of the mosaic were previously charged positively owing to their photo-electric emission until their potential had become equal to the potential of the net 83. The

contents of the meshes and the wires thereof accordingly had the same potential. If new by reason of the said scanning of 1a the said negative potential impulse is developed in the content of any of the meshes, the content of the particular mesh is able to emit electrons, the surroundings 83 being positively biased in respect of this part of the mosaic. These electrons might be directly collected and amplified in the circuit of 83. It is also possible, however, first to cause the electrons to impinge on the nets 66 and 66' and, in the known manner according to Weiss, liberate at these nets secondary electrons in comparatively large number. If this is done, there is ultimately obtained at the final anode I! a considerably amplified effect of the ikonoscope.

This final efiect has the advantage that it becomes zero in the case of black points of the image. There is no feed current which might produce any interfering shot noise, since the primary cathode, viz., the contents of the meshes 10, consists of insulated particles which continue to discharge until no further electrons can be emitted.

In the practical embodiment there are naturally several other practical points which require to be considered:

l. The illuminating light 84 should liberate electrons not from the nets 83, 66, 66' but only from the mosaic 1c. Since the anticathodes 66, 66 etc., also consist of silver and caesium, the light is to be so guided that it does not meet against the meshes, i. e., enters obliquely. By the way, the emission of subsequent anticathodes is not dangerous, as it is not multiplied.

2. The sheet of mica 8 must cause an optical and an electrical separation of the two discharge spaces. It is made larger than the scanned area. In place of mica there may also be employed black glass and the like.

In place of a laid-on net 83 a network which is applied by atomization may be employed with advantage.

For-the sake of clear comprehension Fig. 13 shows a plan view of the rear mosaic 1c consisting of insulated particles, which are photo-electrically activated and of the net 83 which is preferably made of a noble metal, gold, platinum and the like, so that it will notemit electrons by itself.

We claim:

1. A television system for translating an optical image into one electrical representation thereof comprising a substantially planar mosaic formed of a multiplicity of electrically isolated individual elements, a foraminate grid electrode element positioned in a plane parallel to the mosaic and relatively adjacent thereto, the opposed surfaces only of both the mosaic and the grid element being light responsive, optical means for projecting and focusing an optical image through the mosaic onto the light responsive surface of the grid element to release therefrom electrons and to produce thereby an electronic current replica of the optical image, means including a source of high frequency for producing an alternating high frequency electrostatic field between the mosaic and the grid element for directing the electronic current replica onto the mosaic to produce a substantially complete electrostatic image thereof over the surface of the mosaic, a beam of light of substantially elemental crosssectional area, means to scan the light responsive surface of the mosaic according to a pre-established scanning pattern by directing the beam of light through the foraminate grid element to liberate, as a result of scanning, electrons from the individual elements of the mosaic, the number of electrons liberated being proportionally related to the variance of the electrostatic charge image produced on the individual elements of the mosaic, and means to collect the liberated electrons.

2. A television system for translating an optical image into one electrical representation thereof comprising a substantially planar mosaic formed of a multiplicity of electrically isolated individual elements, a foraminate grid electrode element positioned in a plane parallel to the mosaic and relatively adjacent thereto, the opposed surfaces only of both the mosaic and the grid element being light responsive, optical means for projecting and focusing an optical image through the mosaic onto the light responsive surface of the grid element to release therefrom electrons and to produce thereby an electronic current replica of the optical image, means including a source of high frequency for producing an alternating high frequency electrostatic field between the mosaic and the grid element for directing the electronic current replica onto the mosaic during each half cycle of a predetermined polarity, to produce a substantially complete electrostatic image thereof over the surface of the mosaic, a beam of light of substantially elemental cross-sectional area, means to scan the light responsive surface of the mosaic according to a pre-established scanning pattern by directing the beam of light through the foraminate grid element to liberate, as a result of scanning, electrons from the individual elements of the mosaic, the number of electrons liberated being proportionally related to the variance of the electrostatic charge image produced on the individual elements of the mosaic, the liberated electrons being directed through the grid element during each half cycle of the opposite polarity, means to collect the liberated electrons, and a load circuit for utilizing the collected electrons.

' KURT SCHLESINGER. GERHARD LIEBMANN.

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