US2118186A - Image receiving tube - Google Patents

Description

My 24, 1938. P 'T. FARNswoRTH 2,118,186

IMAGE RECEIVING TUBE Filed July 15, 1935 s sneeuw-sheet 1y .eo F 1. ,im fl] 9 7, I I\ I l 2 Jl l 1.,', Il 172 i 4 6 I, I --I 1 l "4 f g .I g r 37 l0 77 CAEsIuM ON SILvEIz 0mm'.

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May 24, 19318. P. T. FARNswo'RTH 2,118,186

IMAGE RECEIVING TUBE Filed July` l5, 1935 ,3 Sheets-Sheet 2 fullurl INcANDEscANT LAMP 32 @fao v INsuLAl-oe 72 w BAS: Wm:

BY v agaqd ATT RNEYS.

fllllallllllhlli May 24, 1938. P. T. FANswoRTH I 2,118,186

IMAGE RECEIVING TUBE Fi-ledJuly 15, 1955 3 Shee'cs-Sheei;i 3

INVENToRY PH/o 7: FARNSWORTH.

' ATTORNEYS Patented May 24, 1938 'UNITED STATES lPA'rrN'r orrlcl:l I l 'nuca morons I Philo T. Farnsworth, ssn signor to Farnsworth San Francisco, Calif.,

forni:

Application July l5,

4 Claims.

My invention relates to athermionic device, and more particularly to a. cathode ray tube adapted to produce an electron image of greater intensity than can be ordinarily produced di- 5 rectly by the impact of4 an optical image upon a photoelectric surface.

Y The present application is a continuation in part of my invention disclosed and claimed as to system and method ln my application, Serial No. 29,242 led July 1, i935v for an Electron image amplifier, this application dealing solely with tube structure.

Among the objects `of my invention are: To provide a photoelectric tube having a high output; to provide-a photoelectric tube which when energized vvillproduce an output current greater than can be produced by the impact of lightl upon the photoelectrlc surface alone; to provide an image: amplifying tube having an effective photoelectric emitter of greater sensitivity than has hitherto been produced; and to provide a simple and efcient electron image amplifying tube particularly suitable for use in television systems for the creation of 'a train of television signals. n

Other objects oi my invention will be apparent or will be specically pointed out in the description forming a part oi this specincation, but I do not limitl myself to the embodiment of the'invention herein described, as various iorms may be adopted within the scope of the claims.

In my prior application above referred to, I have described and claimed an embodiment ci a method of electron image amplification employing the tube of the instant application. in previous applications and patents of mine I have described television transmitting apparatus and systems wherein an optical image of the object or Y picture held is' thrown upon a photosensitive cathode and the emitted electrons are accelerated and focused to form au electron image. By electron image i mean a plane through which the electron stream passes, the electron density of which ovaries spatially across the stream in the same manner as the illumination density varies across the optical image. En other words, the electron values represent spatially, the illumination ci the picture field.

l The electron stream iorming this image may be deected by means weil known in the art, out preferably by magnetic means to pass over an aperture in .auch e. manner as to eect the scan` ning of the image. Selected portions of the electron stream passing through the aperture are collected to form a picture current or train oi picture signals which may be amplified and mod- Francisco, Calif., as- Television Incorporated, a corporation of Cali- 1935, Serial N0.'31,410

(Cl. Z50-27.5)

@ated upon a radio wave, or. transmitted by wire. This method o'f television transmission offers the advantage of having no moving parts and of being suitable for the electrical projection of pictures having any desired flneness of detail.

The principal weakness of this method lies in the fact'that only a relatively small portion of the electrons emitted from the total photoelectr'ic area is used at any given instant and at the present time photoelectric emission is relatively small 1 in intrinsic value. Therefore, the highest possible sensitivity must be obtained from the pho-- toelectric surfaces and even then high gain ampliliers are necessary in order that satisfactory picture currents can be obtained. With small output currents, attempts to amplify the signals above a certain level bring in background noise, Shottke eect and other ordinarily negligible factors which tend to make the amplified picture currents unsatisfactory and distorted, and the received picture lacking in the detail which it would have if such interference were not present.

The tube oi the present invention enables the fundamental principle of my previous inventions to be retained While other desirable features are added. An electron image corresponding to the optical image is formed and is thereafter dissected as before. In the present tube, however, the image has a considerably higher average value than the previous device because of the fact that a space charge is formed, the electrons in the space charge being released by the ac. tion of the light in the optical image. I am therefore able to produce electron images with the present device which are far more powerful than the electron images which are heretofore produced. Under these circumstances, when the picture is scanned, picture currents of much greater amplitude are obtained in the tube out put thereby eliminating high gain amplifiers with their objectionable features.

Describing my invention in broad terms, l pre ier to utilize an envelope containing a source of electrons, these electrons being preferably low ve lccity electrons. in certain cases, Il may use a thermionic emitter ior this source but in other cases, i prefer to utilize e. photoelectric surface and dood the surface with long wavelength light in order to obtain a stream of low velocityv electrons therefrom. I then position in the path of this stream an epertured electrode or grid capable of having a. charge ilxed thereon in accordance with the spatial distribution of illumination intensities existing in an optical image.

At the other end of the tube, I prefer to position an apertured electrode, the aperture being of elemental area and positioned to allow electrons to pass therethrough and I place, immediately be-A hind the aperture, a vcollecting anode to inter-V cept electrons passing through the aperture. The tube when energized is supplied with moving magnetic fields which scan the electron image past the aperture so that successive elementary areas thereof may be collected to produce a train of television signals.

Various other modications and applications of my invention will be apparent to those skilled in the art andfor other broad aspects of my invention lI prefer to refer to a detailed description of several preferred embodiments of my invention as shown in the drawings, of which Figure 1 is a longitudinal sectional view of a television dissector tube provided with focusing and scanning coils, together with means for projecting an optical image therein.

Figure 2 is a diagrammatic circuit reduced to lowest terms showing how the device of Figure 1 may be operated to produce a train of television signals.

Figure 3 is a detailed sectional view of one form of grid.

Figure 4 is a detailed view in section of another form of grid.

Figure 5 is a cross sectional view of another form of dissector tube showing diiferential illuf. mination of the photosensitive surface therein.

Figure 6 is a cross sectional view of a portion of the control grid of the tube showing in Figure 5.

Figure 7 is a diagrammatic circuit showing how the tube of Figure 5 may be operated.

VFigure 8 is a longitudinal sectional view of another embodiment of my invention.

Figure 9 is a cross sectional view which is identical with cross sectional views of the device shown in Figures 1, 5 and 8, looking toward the charge storage electrode.

Figure 10 is a perspective view of a mesh mosaic.

Figure 11 is a diagrammatic view of apparatus used in forming a silver mosaic.

Figure 12 is a longitudinal sectional view of another modification of my invention.

Figure 13 is a cross sectional view of a composite grid having two photoelectric surfaces.

Describing the apparatus in detail, without reference to the operation thereof, which will be taken up later, and referring directly to the modiilcation shown in Figure 1, an envelope I is provided at one end and with a transparent window 2 in front of which is placed an optical system 4 in such a manner as to focus an optical image of an object upon acharge storage electrode or grid 5 positioned adjacent the opposite end of the tube. On the other side of this grid there is positioned a wide angle electron gun comprising a coneshaped apertured anode 6, a control grid 1, and preferably an indirectly heated cathode 9. 'I'his assembly is preferably mounted on a stem I0 in an extension arm II of the envelope.

Adjacent the window end of the tube an apertured anode assembly is providedcomprising a g A -finger tube I2 haying an aperture I 4 facing the ly back of the aperture I 4,

vides a longitudinal magnetic field between the vgrid 5 and the scanning aperture I4 for the purpose of maintaining the electron image in proper spatial relationship during its passage therebe tween.

The tube is also provided with exterior coils 22 and 23 positioned substantially at right angles to each other for moving the electron image in two directions over the scanned aperture I4, the magnetic fields of these coils being formed by energization by scanning oscillators 25 and 24 respectively.

Several different types of grid structures may be used with this device, but in any case the grid member 5 is apertured preferably with an area devoted to the apertures equal to that of the supporting material. For example, I may make, in certain cases, `my grid entirely'of insulating material as shown in Figure 3. Here the insulator 26 is provided with apertures 2l and is also provided with a layer of caesium on silver oxide 29. f This layer is formed in the usual manner for forming photoelectric surfaces, the silver being deposited thereon in such a manner that it congeals in droplets so that a mosaic is formed with the caesium deposited thereon, leaving photoelectric islands more or less uniformly'insulated one from the other, as is well known in the art.

Another form of grid is shown in Figure 4 where the base material 30 is a conducting material such .as nickel wire, for example, which has deposited thereon a layer of insulating material 3i and over' the insulating material the mosaic 29 of caesium on silver oxide is formed, as described before. 'Ihe preferred way of forming the grid of this latter construction is to utilize a mesh screen of nickel, for example, having rather larger spaces between the grid wires than the area of the wires themselves and smoking the entire screen with burning magnesium until the wires are covered with magnesium oxide to aV point where the spaces are approximately equal to the area of the composite member. Caesium on silver oxide is then formed upon the magnesium base to form the mosaic, as will be later described.

When I desire to form a mosaic on an insulating surface of a mesh grid fabric, I have found that it is-not necessary to follow the complicated and uncertain process above referred to.

In Figure 11 I have shown a preferred apparatus for forming a mosaic on the mesh grid. The grid is covered with insulating material, at least on the side facing the optical image, preferably by smoking with magnesium oxide as above described, and silver is evaporated thereon preferably from substantially a. point source |00, a convenient means being the application of eddy currents from a coil IOI energized by an oscillator |02 to a container |03 having an aperture |04 facing the grid and enclosing silver metal |05.

'I'he silver, being evaporated n vacuo, travels in straight lines and therefore casts sharp shadows. As the individual wires inthe Vmesh alternately pass over and under the wires at right angles to them, the silver deposit is separated into small rectangles the size of the mesh. No silver is deposited under the overhang of the wires, and not only do I obtain substantially -perfect insulation between the silver islands, but I am able to yaccurately control the size of the islands by changing the mesh of the'fabric. A mosaic of this type is shown in Figure 10 in perspective and. 1

in Figure 13 in cross section.

when the tube is formed this im is completely oxidized, utilizing, as is customary, a radio-frequency discharge in pure oxygen.

I also prefer to completely clean up the Vexcess caesium after the tube is formed. There are a number of ways of accomplishing such a cleanup but I prefer to include within the tube, either connected to the cathode or the anode, a fairly large surface of pure silver which may be oxidized with a fairly heavy coating. After the tube is formed, therefore, this surface is capable of absorbing a large amount of caesiurn,A even after the thin film of silver on` the grid has taken up its maximum.

The preferred procedure, therefore, after the surfaces it is desired to sensitize have reached maximum sensitivity, is to lower the temperature somewhat and bake the tubeout -in this lower temperature for a sufiiciently long time until the excess of 'caesium within the tube isall taken upv by the accessory oxidized silver surface.

Another modification of my invention is shown in Figure 5 and in this case a photoelectric emitter has been substituted for the electron gun, the photoelectric emitter in this case comprising preferably a continuous photoelectric surface 32 formed on a base member 34, preferably a silver plate. 'Ihis plate is supported on the stem I0 in any customary manner. The grid 5 is then positioned immediately in front of.) and parallel to the photoelectric surface, and in this case the grid 5 comprises preferably a conductor having an insulating surface. While the entire --grid structure may be an insulator, I prefer to utilize the conducting base 30 provided with a relatively thin layer of magnesium oxide formed thereon as above described, and in this case I d0 not place upon the insulator any photoelectric material.

Referring directly to the modification shown in Figure 1 connected as in Figure 2 and assuming that it has a grid as shown in Figure 3; in other words, a grid which is completely an insulator,

`the anode 6 of the electron-gun is connected to the' cathode in series with an anode battery 35 and the cathode 9 is energized in the usual mannerso that the entire grid structure 5 will be bombarded with a suitable amount of 100 to 300 volt electrons. The grid will thenassume a negative charge just suiilcient to prevent these electrons from striking it, thus forming a space charge immediately back of the grid. The potential of the grid, that is, the normal uniform potential of the grid, will be .largely determined by a small number of electrons having lthe highest velocity and due to leakage in the grid structure, the entire grid will assume a uniform charge equilibrium.

inasmuch as the grid in this instance is entirely formed of insulating material, the uniform grid charge in the structure shown in Figure 3 will be slightly higher than in the structure shown in Figure 4, where the insulating layer is formed on a base wire 30, as I prefer to connect this base wire to the anode 6 by aconnection 38. In this case, the vnegative charge, because of greater leakage opportunity will be slightly' less than when the entire grid is formed of insulating material.

After the grid has assumed a uniform negative charge and the space charge has been formed y behind the grid due to thisnegative charge, an

optical image of'an object 31 is focused by means of optical system 4 on the side of grid 5 opposite to that being bombarded. This optical image falls on the mosaic photoelectric layer 29 and causes emission therefrom,l the electrons being drawn toward anode I2, those of some particular elementary area entering aperture I4 and being collected on collecting plate I6. In order to create electron traversal of the tube, anode I2 is maintained at a positive potential by means of an anode battery 39 and the collecting plate.Y I6 is maintained at a potential positive to anode I2 by collecting source 40. 'I'he difference between the number of electrons collected by the collecting plate I6 and finger I2 passing through output resistor I4 I creates a potential available for further use in output leads 42.

When electrons are emitted vfrom the mosaic surface due to the action of the image light, they will of course be emitted in proportion to the intensity of the light falling on the individual mosaic islands and the islands will become more or less positive due to the loss of electrons in l Adifferent portions of the grid to a new point of equilibrium 'and results in the formation of a charge image on the grid representing in intensity the light intensity of the image. Electrons from the space charge developed back of the grid 5 are thus able to be drawn through the grid toward theV anode I2 and due to the fact that the number drawn through at any particular elemental area is controlled by the charges on elemental areas of the grid, there will be formed in space between the grid and the anode, an electron image. This electron image is maintained in spatial relationship by the focusing coil I9 and is scanned across aperture I4 by the moving magnetic fields developed by the scanning coils and generators positioned as previously described.

If desired, lthe output of theelectron gun can be modulated at radio frequencies by means of the gun grid 1 supplied with a radio frequency modulation voltage through input lead 44 and blocking condenser 45. This allows output amplication with this type of 'amplifier and in far less amplification and with consequent quietness and freedom from interference and distortion.

Another apparatus for producing electron storage for the purposeof providingv an amplified electron image is shown inv-Figure 5vand connectedas shown in Figure 8. In this case, I do not use an electron gun for the source of eleotrons forming a uniform electron stream, but I prefer to use a nat photoelectric cathode 32.l

This cathode is preferably formed as is customary in the art on a silver plate 34 by oxidation and the deposition of caesium until maximum sensitivity to light is obtained and is preferably not a mosaic. 'Ihe cathode is then flooded with infra-red and red light from lamps 46 until the cathode develops in a specific instance for example, 100 microamperes current. Grid 5 in this case is not photoelectric, but is preferably composed of a base wire 30 of nickel having an insulating coating thereon preferably of magnesium oxide formed as above described.

,In operation, the object 31 is illuminated solely by sunlight, for example, or by incandescent or arc lamps l1, care being taken that none of the white light reaches cathode 32 except that which is reflected from the object 31 and focused on the cathode I2 by lens I. This is accomplished in practice by the use oi. reflectors 43 on the white lights and as it is also desirable that no red light illuminate object 31 reflectors Il are used on the red lights, these reflectors being so placed that the light is directed in one c'ase on the cathode alone and in the other case on the object alone. The tube of Figure 5 may be hooked up as shown in Figure 7, the scanning and focusing coils beingomitted from the sketch in the interests of simplicity, it being understood that they are to be used in the operation of the device.

Cathode 32 is connected with anode finger I2 in series with the anode source 39. 'I'he usual collecting source 40 is connected to the collecting plate IB, the output appearing across output resstor 4I in output leads 42 exactly asin the previous instance. Thus, lthere will fall upon the cathode surface 32'two different illuminations. One, a low wavelength uniform illumination from the red lamps 46 which causes a uniform emission of low velocity electrons from the cathode. The other illumination is an optical image of light having a wavelength range containing short wavelengths which will produce electrons from cathode 32 falling into a different velocity catefA gory and having higher average velocities.

' Thus there will be-emitted from cathode 32 electrons falling into two velocity categories. One category, made up of a uniform component of low velocity electrons, the other category, a

non-uniform electron image of higher velocity electrons. The grid, assuming that no optical image is thrown upon the cathode,will assume a negative charge because electrons reaching the grid collect upon the insulating layer and leak on' to the base wire, thus reaching an equilibrium value at perhaps in the neighborhood` ofthreequarters of a volt. This will form a space charge back of the grid.

When this equilibrium is reached, that is, the equilibrium ldue to the bombardment by low velocity electrons, the charge on the grid will be uniform throughout. When, however, theoptical image reaches the cathode surface, electrons of higher velocity are emitted, which, .due to their higher velocity can reach the grid and thus charge l the grid more negatively at those .points wherethey do reach it and by an amount in proportion to the numbersreaching it. Ina'smuchas the number of high velocity electrons reaching the grid at any elementary area thereof will be dependent upon the illumination of the cathode on corresponding elementary areas thereof by the Y optical image, it can be seen that a charge pattern is built up upon the grid, this charge pattern modulating the electrons Apassing through the grid dueto the pun of the anode lz, thus :orm-

ing in the space between the grid and the anode I a new electron image of higher intensity which In this instance, the grid becomes more negative due to the.v action of the optical image emission, and modulation is downward.`

Ihave found that by the building up of a space charge behind the grid and then'forming a charge pattern on the gridlin accordance with an optical image pattern, that I have increased the overall sensitivity of the dissector tube over one thousand times, thus decreasing enormously the amount of amplification necessary in the, circuits utilized to vhandle and make useful train produced by scansion.

In the'embodimentshown in Figure 8, the source of the uniform electron stream is a lament Il and the lament is surrounded by a cylindrical electrode 52 which tends to form a luni-potential space around the cathode emitter so that the electrons will be uniformly accelerated toward the grid i, in this case provided with a photoelectric mosaic and formed as shown in Figures 3 and 4 or other charge storage electrodes .such as are described and claimed in a companion applfca'tiomSerial No. 30,118, led July 6, 1935 fo'r a Charge storage dissector.

In Figure iil the end wall 2 is left free from any.

iilm so that the 'optical image'is projected through ontothe charge storage electrode 5, the filament assembly being so .small that no appreciable amount oi.' light is intercepted thereby. Electrons from the filament charge the mosaic surlface 2O a uniform potential and the emission of electrons from the mosaic surface due to the ligh't from .the optical image' falling thereon causes the formationi'l f a charge image. This charge image modulates the electrons from the filament which are l:being drawn through the apertures in the charge storage electrode 5, and the modulated stream'is then accelerated toward the anode aperture III selected areas thereof the signal passing through tobe collected by the collecting plate Il.

vThe space between the charge storage electrode or grid l and the collecting anode is partially surrounded by a illmjil on the inside of the envelope which acts asl an electrostatic focusing aid and which is usually maintained at the,

` to a movement ofthe object occurs .during scansion, the picture produced toward the end of the scanningl cycle is 4simply an intermediate picture between that which was shown at the beginning of the scanning cycle` and that which is to come on the following scanning cycle. Even when objects move with the highest speeds customarily occurring withinthe perception of the human eye there is plenty of time for the charge pattern

Images