US2373395A - Electron discharge device - Google Patents

Description

April 10, 1945- J. R. HEFELE ELECTRON DISCHARGE'DEVICE 2 Sheets-Sheet 1 Filed May 1, 1941 T0 SWL'EP CIRCUITS AAAMA RECT/FIER AND F'/L TER 0 Y G. w TVIII i n i f 2 1 4 .L L M M 5 w w. u m 5W OFT U/ U A asn Non UNE .HR wir New nm IJ OY W NT C wo i H D U P 5 m 2 4 Ffa. 2

Ar ORA/Ev April l0, 1945. J. R. HEFELE 2,373,395

" ELEGTRON DISCHARGE :DEVICE Filed May l, 1941 v 2 Sheets-Sheet 2 EVEN NUMBER 0F AMPLI/'ER STAGES v F/G. 4 scANN/Na BEAM of PR/MARY sLEcTno/vs /NSULA T/NG PART/CL E S r um.. sou/acier" FRAME 54' No LINE J. RHEFELE BV AToRA/Er Patented Apr. 10, 1945 ELECTRON DISCHARGE DEVICE John R. Hefele, Yonkers, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New

, York, N. Y., a corporation of N ew`rYork Application May 1, 1941, serial No. 391,306

12 Claims. ,f (Cl. 250-167) This application relates to target structures for electron space current devices. One Well-known cathode ray transmitter tube is known as the iconoscope. In one form of the iconoscope, the target comprises a metal backing plate, a sheet of mica or :glass on the backing plate and a multiplicity of discrete lglobules of photosensitive material on the insulating material.I Radiations from an olbject or field of View are applied .to the photosentitive mosaic surface to cause the emission of photoelectrons, leaving the various elemental areas ofthe target charged to an amount-dependingon `the light-tone values of the corresponding-elemental areas of the object. A b'eam of electrons from a cathode ray gun so situated as to scan the .globules of the target brings these charges to anequilibrium value and in -so doing causestheformationof an image current througha resistance which is con'- nected to the backing plate. l o

.One of the vdisadvantages of the iconoscope above described is that yphotoelectrons and secondary electrons (caused by the primary beam striking the photosensitive surface of the target) are attracted towards lmore positively charged .elemental areas of thegmosaic instead of being directed to the collecting electrode as intended. This `,gives riseto spurious signals which detract from theappearanceof the received image. t This disadvantage may be largely eliminated by employinga camera tube disclosed and claimed in the copending application of J. R. Hefele, Serial No. -l."1,715,.filed April 4, 1942, which .is a division of this application, which tube utilizes the target structure hereindisclosed. The target structure and its advantages when used in such a tube a'rehereinafter described `with special referencefto this Aparticular use of the invention. In acathode ray television transmission tube employing a target according .to this invention, potential differences between insulating portions of a target and a conducting signal plate or member, which potential differences .are generated by secondary emission `from the insulatedportions of the target-are used to increase the photosensitivity and todecrease thespurious signals common iniconoscopes. In one embodiment of the invention, chosen b'y-way of example vto illustrate the principles of this invention, a-tube is provided having a photoemissive layer or `film (which may be continuous), this layeribeing coated on a conducting backing .or fsignal plate lwhich isv preferably continuous. `On .the photosen'sitive surface, and in Aintimate. contact with it, is placed a discontinuous layer of insulating-material, for example, small glass beads slightly embedded in thek photosensitive surface or vstretched woven glass fabric formed upon it or quartz or other insulating particles sputtered or sprayed upon it, so that the continuous photosensitivesurface is partly exposed through openings .between portions of the layer of insulating material. Preferably vthere are many insulating Particles per picture elemental area (the size of the elemental area being determined bythe size of the scanning spot). Each elemental area therefore comprises a `num-ber Vof insulating particleseach closelyadjacent a photoemissive surface. The insulating material and thebeam strength are So chosen that the secondaryemission, ratio is greater than unity. If desired, metallic particles can be added to the insulating material to increase its secondary emissionn The theory of operationofV this tube is as follows: A focussed electronbeam scans the surface of the mosaic upon which an image of the orbjectis also. izz-rojelcted.` Whenthe beam in its passage over thetargetstrikes they insulating portions of an elemental area of the mosaic target, anoexcess of secondary electronsover primary electronsis formed, which causes the-potential of these insulating portions toreach avalue which is Vpositive -withrespect to the signal plate. The

signal plate cannot appreciably change its po-v `tential since that isdetermined by its connection to v ground or other reference point of yfixed potential through a constantspotential source, the negative pole of the source being connected to the signal plate. `The neld set up between thesuriaces of `the insulating portions of the target struck bythe beam and the signal plate is such as to `cause secondary electrons emitted from said insulating portions to, leavethe target substantially .normal to its surface. These secondaries are .col-r lected by the collector electrode placed ata posi? tive potential with respect .to the potential of the signal plate.y yfIThe-beam of primary electrons also passes through gthe openings in the layer of insulating material and ystrikes v`the photoemissive surface Lof r:the r,signal plate, whichA also ,gives off secondaryelectrons. Dueto their initial velocity and the accelerating Iieldbetween the .collector electrode andA thefsignal plate most of these Seeondary electrons y.are drawn kto the collectorvelectrode, the potential offlthe insulating-surfaces struck vby the yb'ea'rnnot being .sufficient to'more thandeect them.. Some .secondaries are attracted to the insulating surfaces struck by the bea-m but-as thebeam is still .on these surfaces, thesesecondar-ies are eitherredirectedto the collector electrode or take the place of other secondaries driven from these insulating surfaces by the beam. When the beam leaves any particular elemental area to travel over the remainder of the mosaic, the conditions which exist at the elementary area are as follows: there are photoemissive portions of the elemental area which are closely adjacent corresponding elemental insulating surfaces which surfaces are charged to a positive potential with respect to that of the photosensitive plate. Radiations from the object or field of view, an image of which is formed on the target, cause photoelectrons to be emitted from the elemental photoemissive area in an amount which is a function of the light intensity falling on the area under consideration and these photoelectrons are accelerated toward and drawn to the nearest surface of positive potential which is the surface of the adjacent insulating particle. The photoelectrons, having only a relatively small initial velocity, are not directed towards the collector electrode although aV small percentage of them may reach this electrode. Being situated very close to the-area of emission, the field intensity due-to the positive charges on the insulating surfaces struck by the beam is lextremely high at the photoemissive surface; the .photoelectric emission therefore occurs near voltage saturation conditions, that is, substantially all of the emitted photoelectrons are collected by the surfaces of the adjacent insulating portions struck bythe beam. The insulating particles of brightly illuminated elementalareas attract more photoelectrons and therefore the potentials of their vsurfaces become more negative than those near less brightly illuminated elemental areas. When the ybeam again strikes this elemental area of the mosaic, secondary electrons are emitted from the element to such an extent that its insulating particles return to the equilibrium potentiaLwhich is positive with respect to the photoemissive surface. The impulse created by each elemental capacity (between the front surface of the insulating particles and the signal plate) returning to the equilibrium condition produces a current impulse in thefsignal resistor in th external circuit and a succession of these pulses produces the video current, which can. be obtained by filtering the alternating component of the current through this resistor from the average emission current fby a capacitative coupling. This average emission current is an indication of the average brightness of the scene focussed onto the plate and can therefore-be used to determine or adjust the background-control or height of' the blanking impulse of the transmitted picture signals. Such an average current is not produced in the usual iconoscope as its formation vis ob'viated due to the capacitative coupling between the backing plate and the photoemissive globules'on the insulating surface. The element charging pulses may like- Wise Ibe obtained by passing the secondary emission current reaching the collector electrode invention which is suitable for use in the tube shown in Fig. 1;

Fig. 3 is a side view of the target shown in Fig. 2 except that the insulating mesh is replaced by discrete particles; l

Fig. 4 is an .enlarged elevation View of a target in accordance with the invention in another one of its forms with arrowsI superimposed to aid in explaining' the mode of operation ofthe invention; and

Fig. 5 shows an output circuit suitable for use with a tube like that shown in Fig. 1 having a through a resistor to ground, the varying potential i r Fig. 2 is an enlarged front view, with portions v broken away, of a target in accordance with this target structure in accordance with this invention. v

Referring more specifically to the drawings, Fig. 1 shows, Iby way of example, a television'transmitting arrangement including a tube i0 employing an electron beam target of this invention. The tube iii comprises an envelope II enclosing an electron gun I2 for generating, focussing and accelerating a .beam of electrons towards a target I3 and two sets ofY deflecting plates I4, I4 and I5, I5 for causing the beam of electrons to scan every elemental area in turn of a field of view on the target i3.V The electron gun arrangement I2 preferably .comprises a cathode [6, a first anode VI adjacent the cathode comprising a cylindrical -portion having one or more metallic apertured diaphragms therein and a large cylindrical portion near the cathode, and a second anode I8 which is preferably a metallic cylinderof larger diameter than the smaller cylindrical portion Of the first anode II. A filament I9 is pr-ovided to heat the cathode IB. A conducting coating 20 on the inside `walls of the tube between the portion of the tube adjacent the second anode I8 and the bulb portion thereof surrounding the target I3 is placed at the same potential as the second anode I8. The first anode I'I is placed at a relatively high potential with respect to that of the cathode I6, that is, of the order of 3000 volts, while the second anode -I8 is placed-at a'potential which is positive with respect to that of the cathode but which is substantially negative withv respect to the potential of the first anode, as for example from 800 to 900 volts with respect to the potential- 'of the cathode. Due to the fact that the 'second anode I8 is` at a negative potential with respect to that of the first anode II, the secondary electrons emitted by the primary beam impinging upon one or the other of the -cliaphragms of the first anode are repelled by the second anode I8. inasmuch as the electrons in the primary beam are of high velocity, the electrons thereof are decelerated by the second anode I8 but are not prevented thereby from reaching the screen or target. Between the first anode and the cathode is preferably arranged Aa control element 2| for varying the intensity of the primary electron beam, which element in the preferred embodiment constitutes'a cylindrical member having a metal cap in which vis located a circular aperture. This cylinder "2l `is placed directly around the cathode I6. Suitable potentials for thevarious electrode elements are obtained from taps on a potentiometer resistance 22 which is connected across a rectifier or' filter, represented in Fig. 1 by the block 23, which receives power from an alternating current oscillator 24. The electron gun briefly described above producesa beam at the target which is relatively free from secondary electrons and thus prevents distortion resulting from the imperfect focussing of these secondaries. For a more complete description of these distortions and of a of this type, reference may be as'zaaos made to Patent12,217,168 issued October 8, ,194() to John R. Hefele andGordon-K. Teal. Any other suitable electron gun, however, may be used with the v.target of this invention as the present inventionis not limited to any particular type of electron gun. j A

Inorder to cause the electronbeam, generated by the Yelectrongun apparatus described.- above, to scan Aevery elemental area of the imagevor field of view .on .the screen `or targetl3, suitable defiecting means, such as, for example, the two pairs of electrostatic deiiecting plates t4, I4 and I5, I5, `the axes of whichare located at right angles toeachother, are provided. vTo the ,defleeting plates I5, I5v are applied deecting voltages of. frame frequency and having a saw-tooth wave form to produce the vertical deflection of the beam while deflecting voltages of line scanning frequency and yof saw-'tooth wave form are applied to the .deflecting plates I4, I4.to produce the horizontal deilection of the beam. Any suitable sweep circuit (not shown) may be used togenerate these horizontal andzvertical deflecting voltages. For example, reference may be made to. Patent 2,178,464 issued October 3l, 1939, to M. W. Baldwin, Jr., which discloses appropriate balanced sweep circuits for this purpose. Connections maybe made from the balanced sweep circuits to the pairs of Aplates I4, I4 and I5, I5 by means of coupling condensers 30, 3| and 32, 33, respectively. High coupling resistors 34 and 35 arefrespectively connected across the pairs of plates I4, I4 and I5, I5. The mid-points of the resistors 34 and 35 are connected to the second anode I8 inorder that the average potential oi The targets'ishown'in y1igs..2,'3 and 4) may be madein variousways., '.In one-form of target employing a discoizitinuous photosensitive layer, the backing -plate comprises ya sheet of silver lupon which is sprayed aluminum oxide ymixed with a suitable vbinder such `as .althinsolution of gum arab-ic. The binder .is then'burned ofi. The percentage of the ytarget :.'areacovered by the alumithe deiiecting plates is at fall times substantially equal to the potential of the second Vanode I8. For a -full description of the advantages of balanced sweep circuits, reference may be made to the above-mentioned Baldwin patent and also to Patent 2,209,199, issued July 23, 1940, to Frank Gray. y

The screen or target I3 comprises (see vas a rst example Figs. 2 and 3)' a metal backing member 40, which is preferably continuous and which will be designated the signal plate, -this term being considered broad enough to include a conducting meshA or a 'thin metal layer on a supporting member, a continuous photosensitive layer 4I of any suitable material sensitive-to the radiations from the object or field of view `to be electrically transmitted by the tube, such as, for example, caesium-sensitized-silver and a discontinuous layer 42 of insulating material, such as partially embedded glass beads (Fig. l3) stretched woven glass fabric (see Fig. 2) or sputtered quartz (Fig. 3). The discontinuous layer 42 of insulating material is preferably arranged in intimate contact with the photoemissive layer in such a way that 'the photosensitive surface` is partly expcsedthroughthe interstices of the insulating material. An elemental area may cornprise as kmany as a hundred or morel elemental insulated particles. `If desired, the photo-sensitive layer may be discontinuous,v but each element of thesurface must lay on and make contact with the-signal plate. Fig.r Llisa'greatly'enlarged elevationview of another form of target wherein a discontinuous layer of photosensitive particles is used, these particlesbeing placed in Itheapertures of the insulating layer 6I consisting of a multiplicity of `discrete insulating'particles of quartz, aluminum .oxide .or other suitable material. .The hill-and-dale nature of the targetv is'shownin Fig. Y1.1- y- 1 num oxide particles maybe observed through a microscope.'v The target is then placed in the tube, rthe tube yexhausted of airand gases, oxygen admitted and the silver 1 oxidized by a glow discharge oriby any-.other well-known means. Caesiumis evaporatedzfroma pill located in a side tube and .the tube baked.l .Any other satisfactoryrtechnique .'forisensitizingiconoscope mosaics may be used, .if desired. ;In another form of target (see :'Eig. 4), quartz powder .is mixed with a binder .and :sprayed on 'the silver sheet. The quartz particles 16| (beads) are not, strictly speaking, embedded in the silver-'they are held against it .by molecular attraction. The silver is then photo-sensitized to form 'la layer` 60 by any suitablemethod. .Fine particles of magnesium oxide or aJlattice-workJof insulating material may also be used asthe insulating layer. The insulation layer is preferably only one particle thick, this being only a fraction-.of the .diameter of the beam. The beam vcross'section covers a number of insulated particles. Treatment of the surface of the insulator in order to' increasevits secondary' emission ratio,vwith, `forexample, finely divided oxidized metallic particles may be resorted to in order to'increase the `eiiiciency of operation. y yPulses are-generally added to the video signal during the retrace period ofthe scanning cycle of the transmitter tubev during which period no picture signals are generated. Thejamplitude of these pulses furnishesinformation to the receiving circuit asto the average illumination level of the scene whichislbeing transmitted.

The picture signal 'which is obtained from the well-known-iconoscope does not yield any information as to the average illumination of scenes which are focussed onto the mosaic to be trans- A mitted. The signal pulsesgenerated by a line of average gray on a black background will be identical 'with that of a white line on an average gray background. To supply this information of background level, the amplitude of these added pulses is manually'controlled by a monitor operator or automatically-by a photoelectric cell which can be so positioned near the iconoscope that it receives some light from'the scene to be transmitted and sogenerates a current proportional to the average illumination of the image. This: output of the phototube then controls the amplitude of the pulses which areinserted or l`added to the video signal during the blankng period.

In the arrangement according to this invention, a continuous current proportional tothe picture illumination flows from the photocathode 4I or 80 to the insulated'elements-42 or 6I of the mosaic during the entire frame time during which it is exposed toradiations from lthe object O. This current flows through the loutput resistor 43 which is connected betweenthef'signalplate 40 through the source-fof potential 45 'to `ground potential which'in the arrangement vshown is also the potential of-the'rral anode 20 inthe tube I0.

Secondary electronemission Vfrom the photocathode 4I dueto the bombardment of'its surface by -thefelectron lbeam( also liiows through the resistor 43, but since `the secondary emission of'such a cathodeis virtuallyindependent of its illumination, this current shouldfremain'substantially constant .during the wholescanning period. i

The pulses produced by `the discharge of the elemental capacitors of the mosaic when the beam 'scans the surface of the mosaic constitute a signal current which likewise flows through'this output resistor. 'Ihese pulses `are produced'as follows (referring by way of example to Fig. 4) When the beam strikes the' insulated portions 6I of an elemental area of the mosaic, the secondary electrons which are emitted from those-portions cause them to become charged to a positive po'- tential with respect to their surroundings as indicated in Fig. 4.` When-'the beam strikes the photosensitive portions B of the elemental areas of the mosaic through the interstices of the insulating material, the photosensitive layer 60 cannot change its potentialas'itis coated Von and therefore electricallyconnected to the `signal plate 49. As more secondary electrons are emitted from the insulatingportions 6| of the mosaic target i3 than there arefprimary electrons in the scanning electron beam, the insulating portions 6| are brought to a positive equilibrium potential with respect tothe signallplate andthe photoemissive coatings 50 thereon. .Minus signs have been afxed to the elements 50 in the drawing as the potential of these elements is negative with respect to the potential of thelelements 6|. 'I'he field between the insulating portions 6l of the target and the photoemissive portions 60 thereof is such as tocause the ,secondary electrons to leave the target substantially normal to its surface and to be collected lbythe collector electrode 20 which is placed at aA positive potential with respect to the potential yofy the signal plate 40 by the source of potential45. The potential of the source 45 varies from a value near zero to about 100 .volts depending on the surface used, the

geometry of the tube, etc. At the higher voltages, i

however, the focus of the scanning beam becomes slightly impaired. ATubes have worked satisfactorily with this potential difference less than 20 volts. When the beam leaves this particular ele. ment to travel over the remainder of the mosaic, the conditions which exist at the elemental area are believed to be as follows:` There are photoemissive portions 6U in each elemental area which are closely adjacent corresponding elemental insulating surfaces (ilv which surfaces are charged to a positive potential with Arespect to that of the photosensitive elements v(ill. Radiations from the object or eld of lview O cause photoelectrons tobe emitted from the elemental photoemissive surfaces in an amount which is a function -of the light intensity falling on the elemental area and those photoelectrons (as pointed out above) are accelerated towards and drawn to the nearest surface .of positive potential which is the surface of the adjacent insulating particles B l. Being situated Vvery close Vto the area of emission, the field intensity due to the positive charge is extremely high at the emissivel surface; the photoelectric emission therefore occurs near voltage saturation conditions. rThe insulating particles 6l of brightly illuminated elemental areas collect more photoelectrons4 `and are thus negative in potential with respect to those near less brightly illuminated elemental areas. The paths of the photoelectrons are represented by dashed line arrows` in Fig.A 4. Whenl the beam again strikes this elemental area of thev mosaic, secondary electrons are emitted from the insulated elements to such an extent that'the insulating particles of the elementalgarea return :tothe ,equi-A circuit 0f tube 50.

agavaaec f librium potential. As pointed out above, the electric field of the photosensitive surface which is comparatively very negative, extending through the interstices ofthe insulating material, aids in focussing and accelerating the secondary electrons from the insulating portions in a direction normal to the target surface as shown by the full line arrows in Fig. 4. They therefore travel directly away from the target surface since the charge on neighboring elemental areas tends to become nullied by this negative field. .Preferably the insulating particles or elements 6| have portions which have a higher elevation from the surface of the backing plate 40 than dothe photosensitive particles 60 to insure maximum directive elds. The impulse created by each elemental capacity (between the front surface of the insulating particles and the signal plate) returning to the equilibrium potential produces a current impulse in the signal resistor 43 in the external circuit and a succession of these pulses produces the video current. The above is believed to be the correct theory of operation as tubes-constructed as described above have operated satisfactorily in a'manner to indicate the correctness of the theory.

There are obtained across the resistor 43 pulses of potential due to the successive discharge of the vindividual elements, as well as a potential proportional to the average illuminationwhich varies as the average scene illumination varies superimposed upon an unvarying potential which is due to the emission of secondary electrons from the photosensitive plate. This constant potential is quite useless in the operation of the device as a camera tube and can lbe `balanced out before transmission.

Reference will now be made to Fig. 5 which indicates schematically a circuit for utilizing the output currentof the camera tube shown in Fig. ll. The current flowing through the-resistor 43 is impressed through a coupling condenser 46 to the tube 4l the output circuit of which may be connected through` an even number of amplifier stages represented by the box 48 to a tube 49, the output circuitof which is connected in parallel with the output circuit of tube 59, to the input The input circuit of the tube 59 is connected to a source l0 of rectanguar pulses. These pulses are of line 'and frame repef tition frequency and of such a width as to blank out vall of the video signals generated during the return time ofthe scanning cycle of the beam in lthe transmitter tube. The amplifiers 4l, 48 and 49 transmit only the alternating current components of the signal produced by the tube i0 since all of the direct and slowly varying components are removed by the coupling condenser 46. The potentials generated by the tube l0 are alsotransmitted to the grid 55 of a high vacuum tube 54 through the resistance-capacity filter comprising resistance 56 and capacity 53. This resistance-capacity arrangement filters out all but the slowly `varying and constant potential. Connected inthe cathode-anode circuit and also in the cathode-grid circuit of the tube 54 is resistorl 5l having a variabletap 58 thereon which is connected to the input circuit of the tube 50. The bias of the tube 50 is, therefore, dependent on the intensity of the continuous and low frequency components of the current vflowing through the resistor-.43.. The amplitude of the blanking pulses is controlled `by adjusting the grid' potential of the tube 50 which adjustment is performed automatically. bythe directly coupled amplier tubegd, the output voltage of which controls the grid bias of the tube 50. There is then obtained in the output of the tube 50 picture signal pulses due to the scanning of the mosaic by the electron beam and pulses at the end of each line and frame which are indicative of the average illumination of the mosaic. At the receiver, the amplitude of these blanking pulses is utilized, by well-known methods, to set the grid bias of the receiving cathode ray tube which adjusts the background level of the illumination of the receiver tube screen, upon which grid the video signal is superimposed. While a specific circuit arrangement has been described, it is obvious that other circuitarrangements may be used with the tube above described.

The element charging pulses may likewise be obtained by passing the secondary emission current reaching the collector electrode 20 through a resistor to ground, the varying drop across which also constitutes the signal which may be used in connection with a circuit of the type shown in Fig. 5.

The advantages of the arrangement shown in Fig. 1 using a target structure in accordance with the invention may be summed up as follows: (1) Photoemission occursunder saturated voltage conditions from a continuous surface. (2) Secondary electrons have a eld which directs them normal to the surface and away from neighboring elements. wThis eifect would eliminate the shadows and loss of contrast experienced with tubes of the usual iconoscope type. (3) The continuous photoelectric surface presents no difficulty in manufacture. In making photosensitive mosaics for iconoscopes there is great diiculty in keeping the elements discrete, as well as inability to measure the true sensitivity of the surface. (4) A continuous current indicative of the average illumination of the beam focussed on the target of the pick-.up tube flows from the sensitive surface thereof which current can be used to determine the background level of the transmitted picture. (5) The target of this invention employing a caesium-oXygen-silver surface has a much stronger response in the red and related portions of the spectrum than has the ordinary iconoscope target. This is believed due to the increased field acting on the photoemission bringing the sensitivity more nearly in line, with that of the caesium-oXygen-silver photocell than is that of the ordinary icofnoscope Various modifications may be made in the embodiments described above without departing from the spirit of the invention the scope. of which is indicated by the appended claims.

What is claimed is:

1. A target for an electron beam comprising a conducting backing element, and a mosaic surface on the side thereof to face said beam which comprises a multiplicityr of interspersed hills and dalesthe hills containing insulating material in direct contact with said backing element and the dale portions containing photosensitive material in direct contact with said backing element.

2. A target for an electron beam comprising a conducting backing element impervious to light, and a mosaic surface on the side thereof to face said beam which comprises a multiplicity of interspersed hills and dales, the hills containing insulating material in direct contactl with said backing element and the dale portions containing photosensitive material, said photosensitive material being conductively connected to said conducting backing element.

3. A target for an electron beam comprising a conducting backing element, and a mosaic sur-v face on the side .thereof to face said beam which comprises a multiplicity of interspersed hills and dales, the hills containing insulating material in direct contact with said backing element and the dale portions containing photosensitive material in direct contact with said backing element, each of the hills and dales being relatively small compared with the cross-section of' the electron beam.

4. A target for electrons comprising a metal plate, a photoelectric layer on one surface of the metal plate,` and a layer of stretched woven fabric insulating material on the surface of the photoelectric layer remote from the metal plate.

5. A target structure for a beam of electrons comprising insulating portions interspersed with photoemissive portions, and means electrically conductively connecting all of the photoemissive portions together, said means having negligible resistance regardless of the amount of illumination of said photoemissive portions and said insulating portions comprising glass.

6. A target structure for a beam of electrons` -comprising insulating portions interspersed with photoemissive portions, and means electrically conductively connecting all of the photoemissive portions together, said means having negligible resistance regardless of the amount of illumination of said photoemissive portions and said insulating portions comprising quartz particles.

7. A target structure for a beam of electrons comprising insulating portions interspersed with photoemissive' portions, and means electrically conductively connecting all of the photoemissive portions together, said means having negligible resistance regardless of the amount of illumination of said photoemissive portions and said insulating portions comprising stretched woven fabr1c.

8. A`target structure for a beam of electrons comprising insulating portions interspersed with photoemissive portions, and means electrically conductively connecting all of the photoemissive portions together, said means having negligible resistance regardlessvof the amount of illumination of said photoemissive portions and said insulating portions comprising finely-divided oxidized 'metal particles timprove the secondary emitting ratio of the insulating portions.

9. A target for a beam of electrons comprising a single layer of particles of insulating material each particle adherent to a base member consisting of a thin, plate-like element of material. of high electric conductivity having photoemissive material adherent to the surface thereof to face said beam, said particles being on the side of said base member to faceA said beam, being closely spaced and forming valleys `therebetween with photoemissive material at the bottoms of the valleys whereby it is directly exposed to the electron beam.

10. A target for a beam of electrons comprising a single layer of glass particles each particle adherent to a base member consisting of a thin, plate-like element of material of high electric conductivity having photoemissive material adherent to the surface thereof to face said beam, said particles being on the side of said base member to face said beam, bein-g closely spaced and forming valleys therebetween with photoemissive material at the bottoms of the valleys whereby it is directly exposed to the electron beam.

11. A target for a beam of electrons comprising a single layer of quartz particles each particle adherent to a base member consisting of a thin, plate-like element of material of high electric conductivity having photoemissive material adherent to the surface to face said beam, said particles being on the side of said base member to facesaid beam, being closely spaced and forming valleys therebetween with photoemissive material at the bottoms of the valleys whereby it is directly exposed to the electron beam.

12; A target structure for a beam of electrons comprising a base member consisting of a, platelike backing element of material having high electrical conductivity and photoemissive material on the side of said element to face the beam and adherent thereto, and a layer of insulating material adherent to the side of said base member to face: the beam, said layer of insulating material having a multiplicity of minute discrete openings each extending through said layer in a direction generally perpendicular to said base member to photoemissive material on said back ing element whereby the photoemissive material at the bottoms of said openings is directly expsed to the beam.

JOHN R. HEFELE.

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