US2290581A - Light valve - Google Patents

Description

J. S. DONAL, JR

LIGHT VALVE Filed Feb. 17, 1939 2 Sheets-Sheet 1 NYJ INVEN TOR. JOHN S. DON/IL, JR.

ATTORNEY.

Juy 2l, 1942.

o 4 .fn/4414.77

y 2l, 11942. .1. s. poNAL, .JR 2,290,581 LIGHT VALVE `Filed. Feb. 17, 1939 2 Sheets-Sheet 2 INVEN TOR.

NAL JR.

ATTORNEY.

Patented July 2l, 1942 UNITED STATES PATENT y OFFICE LIGHT VALVE i John Scott Donal,l Jr., East Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware v Application February 17, 1939, Serial No. 256,833

My invention relates to television receiving tubes and ysystems and particularly to tubes of the cathode ray light valve type and method of operation.

In the art of television certain types of light Figure 1 is a diagrammatic view of a cathode roy light valve or tube embodying my invention; t

Figure 2 is a cross-sectional view of a portion of the envelope structure of the tube shown valves utilizing the electron beam of a cathode 5 in Figure 1; ray tube to control the illumination of the pio- Figure 3 is a curve indicating the secondary ture by a valve action on light from an auxilelectron emission characteristics of certain of 'Lary light source have been proposed, but have the electrode structure shown in -Figure 1; not been altogether feasible because of the in- 10 Figure 4 is a schematic representation of cerrbility. of the structure to utilize the control tain of the tube structure shown in Figure 1 to Lotion of the beam to the fullest advantage, so illustrate certain aspects of the operation of my hat the eifect of the electron beam in controldevice; ing the lightl from the auxiliary source has been Figure 5 is a curve showing secondary electron nadequate. Other 'diiiculties have been en- '15 velocity distribution during the operation of the ountered because the structure scanned by the device shown in Figure 1; lectron beam and responding thereto would not Y Figure 6 is a partial Viewv of a modified strucapidly return to a condition of equilibrium to ture incorporating my invention; and, e in a condition to be rescanned. 1 Figure 7 is a view of a modified target elec- The principal object of my invention is to pro- 2o trode which may be used in my invention. ide a device of the cathode ray light valve type Referring to Figure 1 which illustrates one em- `herein the response is adequate to satisfy opbodiment of my invention the cathode ray tube ating conditions in modern television transmitor light valve includes a highly evaculated enng and receiving systems. It is also an obvelope or bulb l of cylindrical shape with a tubukct to provide a device which is responsive to the 3 lar arm or neck section enclosing a conventional ntrol action of a cathode ray beam to a greatetlectron gun. The cylindrical portion of the 'degree than heretofore; Itis a further object bulb I`is provided at one end thereof with a provide an improved television receiving tube window 2 of optically uniform material such as 1d a method of operation for such tubes whereglass so that light from a substantially constant the above diiculties with the prior art deintensity light source 3 may be formed into par- :es are overcome. It is a still further object so alle] rays of light by the lens system 4 and provide a'tube which may be easily fabricated projected through the cylindrical portion of the fh respect to sealing of the envelope which is bulb I. The opposite end of the cylindrical porechanically strong, although portions thereof tion of the tube is provided with a closure, likee of very thin material. wise of optically transparent material which will [n accordance with my invention a local .light 35 hereinafter be referred to as the target 5 being Arce of substantially constant intensity is deoped at a. television receiving station and the ht is utilized to illuminate a screen or target ich is scanned by an electron beam to produce ctrostatic charges which control the transpar- :y or opacity of a light transmitting or absorbmedium between the target and the screen which an image is to be developed. Further accordance with my invention I provide a thod of operation of a device of the type de- `be'i wherein an electron beam serves simul-` enusly to control the illumination from the iliary light source and to re-establish condi- Is of operation suitable for the continuation he operation steps. These and still other obs. features. and advantages of my invention become apparent and will at once suggest nselves to those skilled in the art from the Winar description taken in connection with accompanying drawings in which;

positioned so as to be scanned by an electron beam originating from the gun structure in the neck of the bulb I.

The electron gun assembly `is of the conventional type and comprises a cathode 6 from which an electronvstream may be drawn, a control electrode l, and a first anode 8 maintained positive with respect to the cathode 6. The electron '45 stream leaving the rst anode 8 is accelerated and concentrated into an electron scanning beam focused on the surface of the target 5 facing the electron gun by a second anode 9 which is preferably a conductive coating on the inner sur- 50 face of the envelope l over a portion of the neck 'section and also overthe inner surface of the so positive potentials with cylindrical section with-the exception of the window 2 and the target 5. The first anode 8 and the second anode'S are maintained at the desired respect to 'the cathode 6 I tive with respect means for sweeping electrode and the cathode in serieswith a source 5 of control potential or vido receiver I2 such as used inconventional television receiving systems. Adjacent the end of the rst anode 8 furthest removed from the control electrode 1 I provide the electron beam over target 5.' For this purpose I have shown the deflection coils H and V for deecting theV electron beam in mutually perpendicular directions such as in a horizontal anda vertical direction the 1o drical glass bulb usingmica as mica is sealed. The method of heating the seal may be any which accomplishes the desired end of fusing the intermediate vitreous material without damaging the other components of the seal. It is necessary only that coefficients of expansion of the component parts of the seal shall be of such magnitude that rupture will not result. I have constructed vacuum seals between a mica disc' 2 inches in diameter and a cylinthin as 0.005 of an inch. Cathode ray tubes provided with mica faces constructed in the foregoing manner have never failed of operation from causes attributable to the seal in question and have withstood respectively. The coils aresuppled With the atmospheric pressure indefinitely.

proper currents for sweeping the beam over the target 5 by conventional horizontal and vertical sweep oscillators, that supplying the coils H being suitably controlled for keystone compensation as well known in tudinal axis of the electron gun. It is understood that conventional deflection plates may be substituted for either one or both of the deflection coils if desired.

In accordance with my invention the target 5 is composed of high electrical resistance materialfhaving the desired quality of being optically transparent and sealed to the bulb I by the use of an intermediate vitreous material Ia double Wall for the as best shown inl Figure 2. I have found it necessary to provide a physically rugged target of y material having the desired characteristics but which is nevertheless sufficiently thin to meet the operating requirements of my device. the preferred modifications of my device I prefer mica as a target material because even when suciently thin, such as a few thousandths of an inch, it is suciently strong to withstand normal atmospheric pressures. terial Ia is preferably of powdered glass having a coefficient of expansion similar to that of the bulb I and target 5. Thus if' the bulb I is of soft glass such as lead glass the material Ia is preferably of nely divided powdered lead glass having a melting point considerably lower than that of the glass of the bulb 'I and such that it is reasonably fluid at moderate temperatures such as 600 C., and having a coefficient of linear expansion of about 87 l0'1/ C. Thus the coefficient of expansion of the glass envelope and the mica are effectively matched by the intermediate glass. I prefer to form a paste of the low melting point 'powdered glass with water which is then applied to the edges of the bulb I '55 and the target 5. The two parts are then pressed together.' placed in an oven and heated to about 600 for several minutes or until the powdered glass is fused whereupon the assembly may be cooled and annealed to reduce strains in the seal. Itis preferable to make this seal subsequent to the introduction of electrodes into the cylindrical portion of theenvelope as later described. However, this seal should preferably be made prior to the introduction of the electrodes forming the electron gun assembly. In addition to several lead glasses of slightly different composition I have sealed mica to lime glass wherein the viterous material Ia was also the art since the plane of the 20 target 5 is at an angle with respect to the longi- In eter of the electron beam from The vitreous mao yThe electrode Further in accordance with my invention I provide a container or reservoir I3 positioned exterior to the envelope or bulb I and adjacent the target 5, the target 5 preferably forming one wall of the container. In the wall of the reservoir I3 and opposite the target 5 I provide a window I4 of glass or other transparent material axially aligned with the target 5 and also with the window 2 so that light from the light source 3 may be projected through the cylindrical section of the bulb'l and the reservoir I3 and focused on a distant light intercepting o! veiwing screen I5 by the lens system I6. The target 5 and window I4 constitute in effect e envelope or bulb I. Th1 distance between the adjacent surfaces of thi target 5 and the window I4 is not critical although it should be small. Preferably this dis tance should be substantially equal to the diam the electron gun I have made satisfactory tubes 0f this type wher this distance was less than one millimeter. Oi one surface of the reservoir window I4, prefer ably the outside surface thereof, I provide a sub stantially transparent electrically conductiv electrode I1, preferably an exceedingly thin fill of metal, such as gold or platinum, which ma be sputtered, condensed from the vapor phas or otherwise applied as well known to those fe millar with the art of making thin conductii films. While I have shown the electrode I on the'outside surface of the window I4, it mz be deposited on the opposite or inside surfal and still meet the requirements of my devic I1 is preferably connected to tl second anode 9 and the battery I0 through low resistance noninductive path although have operated valves of the type to be describi wherein the electrode I1 was isolated from oth parts of the device. Within the reservoir I provide a suspending medium or liquid I8 ca rying in suspension a great number of light i tercepting or absorbing particles I9, which m be oriented to vary the light transmitting che acter of the device. The characteristics a functions of the liquid I8 and the particles will be considered more fully below. Betwe the target 5 and the window I4, I provide mechanical agitator which is maintained in n tion during operation of the device.

Preferably the agitator may comprise a tl sheet 20 of mica or other substantially trai Y parent material having an area equivalent or greater than that ofthe target 5, which powdered lead glass having a coefficient of exsupported by a mechanical link or rod 2l z pansion of 92 l07/ C. Powdered glasses of somewhat different melting point and coefficients of expansion may be employed although the melting point should be considerably lower than that of the glass of the envelope to which the constrained by gravity to hang vertically shown in the drawings. The rod 2| pa: through a flexible diaphragm 22 so that wi the free tip of the rod I0. is moved in a ci:

the sheet of mica 20 moves so that its ec' remain parallel to themselves and any point on its surface moves in a circular path in the plane of the mica sheet. l 1 Further. in accordance with my invention I make the target 5 of such a material that when bombarded by energy such as by a beam of electrons off relatively low velocity thecharge produced thereon is negative whereas when bombarded with higher velocity electrons the charge developedfthereon is `positive in that more secto produce a picture produced in a manner well known in the art but instead of the picture being rendered visible by the cooperation of a iluorescent material deposited on the target the electrons produce an ture, which electrostatic image varies in intensity of charge from area to area in accordance with the light and shade areas from which the signals applied to the control electrode ,were derived. The electrostatic image on the target 5 orients the particles I9 in the suspension in a manner to be described `and thus permits varylng amounts of light from the=source 3 to pass through the suspension and be focused upon the viewing screen I5, the energy of the viewed picture being controlled but not generated by the electron beam.

AFurther in accordance with my invention the suspending medium or liquid I8 contained in the reservoir I3 may be any liquid having the desired characteristics as regards electrical resistance, transparency, vapor pressure and viscosity. Thus the suspending medium preferably has very high electrical resistance and transparency and low vapor pressure and viscosity. I have found a number of materials suitable for use as the suspending medium, these materials including liquids such as namyl-sebacate, ethyl-hexylphthalate, ethyl-hexyl-acetate, and tetrabromoethane. I have found that the performance of the light valve is largely dependent upon the nature of the suspended particles employed and I have been able by altering this nature, to effect a great improvement in the change in light transnission produced by the electrostatic image on the target 5 and in the eiilciency of the light con- ;rol mechanism. Thus, I have used particles of graphite of a size larger than colloidal, particles vhich miscroscopic examination have shown to :onsist of relatively large at plates. I have also itilized commercial aluminum foil having a hickness less than l/2 micron, the foil being sublivided into particles that are very thin comared to their other dimensions. The ratio of he areas of the particles normal to the light ays passing through the Ilight valve in the deriented and oriented condition should be as arge as possible. This ratio may be termed the bape factor of the particle when taken with repect to the interposed particle area in the two onditions. I have found that with particles of ny average shape factor the change in light 'ansmission of the suspension with the applicaon of the iieldand the efficiency with which 1e light is controlled may be varied ln a predictelectrostatic image of the Ypicable manner byl changing the number of particles contained in a unit volume of the suspending medium. Thus, for television purposes, for

example, maximum contrast 'of the optical image projected as a picture on the viewing screen I5 vcan be adjusted to the desired value when the light valve is constructed and the average brightness of the picture can be adjusted at any time by changingithe intensity of the light source 3. l0

As indicated above, I have found that a suspending mediumA having low viscosity is desirable so that the suspended Particles may readily orient themselves and may likewise be de-oriented, such as by methods later to be described. Therefore with the suspending media having relatively high viscosity I have found it advantageous to heat the liquid to reduce its viscosity, such asv by passing current through the coil 23 which is immersed in the liquid. Since the allowable viscosityv of thesuspending medium is determined by the electrostatic field strength existing between the electrostatic image produced on the vtarget under scansion and the electrode I'l, itis impractical to designate an upper viscosity limit. However, I prefer to use a suspending medium having the lowest Iviscosity compatible with the other desired characteristics such as high resistance and' high transmission and low vapor pressure.`

Referring to Figure 3 the curve represents the emission of secondary electrons from the scanned surface' of the target 5 as ordinates for values of the second anode potential as abscissas. Line A represents unity secondary electron'emission, a condition which exists when the secondary electron emission from the target is equivalent in quantity to the number of primary electrons impinging on the target. It will be observed that at second anode potentials below the point marked B on the curve the ratio of secondary electrons to primary electrons is less than unity, whereas for second anode potentials between the points B and C on the curve the secondary emission is greater' than unity and for higher potentials than that represented by point C this ratio is again less than unity. It may, therefore, be observed that the curve crosses the line A at two points B and C. The points B and C will therefore be referred to as rst and second cross-over respectively. In practicing my method of operation I utilize a target material having .a secondary electron emission characteristic of the general configuration to that shown in Figure 3 and operate the tube with a second anode potential exceeding the potential of the second cross-over C. This second cross-over is that potential assumed by an insulated surface when bombarded by an electron beam of such high velocity that for this and all higher velocities of the beam the surface has a secondary emission ratio of less than unity and hence is capable of collecting electrons. I shall, therefore, refer to such a high velocity beam as one which has a volt velocity, that is, velocity in electron volts, greater than the second cross-over of the surface scanned. My YInode of operation implies that an electrostatic field of controllable magnitude or time of existence or both is applied to the suspending medium in the reservoir I3 at all 'stsajer'ogsstheres/sgatremaniements--- f consortiav .'or@1:imi.. nv l potential ofgthej second? e; l y be increase 'lanriotibe pot "rosseoverpoten y p that of the second anode, thus in the example t1 given, from 3000 to 5000 volts. produced by the second anode 9 for collecting It should'be emphasized that the' suspension Secondary electrons from the scanned areas is should be kept at its desired orientation for as much improved Sinoe the Collector iS at a Considlong as possible consistent with its readiness to el'abiy higher Potential than the SUIfa-ee of the be re-actuated by the next scan, for it is precisely target 5 dllline SCanSionby this persistence effect that the valve offers a It iS neeeSSalY that the response 0f the light considerable improvement over mechanical scanvalve be susceptible of continuous modulation or ning by a factor equal to the number of picture so graded response in order to permit the rePrOdileelements. Thus while the discharge and detion of half-tones. In the operation of my device,

orienting mechanisms should not be so slow as to this graded response arises from that fact that cause undue highlight persistence, neither should over an easily determined range of beam current. they be so fast as to prevent orientation of the depending upon the Capacity 0f the Valve. the particles in the suspension, or so fast as to darken Potential belOW eolleetOr t0 Which an area iS a scanned element before itis necessary, charged is a reproducible function of the beam I have found that when'using volt velocities of i Current and hence the light passed by the valve is a function of the beam current. Thus I have the beam below second cross-over that is, a secl Ondary emission ratio greater than one at a given found that when the trace of the beam is passed target the brightness of the volt velocity of th'e beam, not only isvthe surface 60 rapidly aCl'oSS the l being scanned charged up to second anode poten- ,y line Obtained may be Varied smoothly from Zero tial by the scanning beam but secondary electrons t0 a maximum by Varying the Current n the liberated from the target at the scanned points Scanning bealny tendto charge the scanned points as well as the During th'e operation of devices such as showr adjacent areas toward collector potential, resultin Figure 1 I have found that. fithe eieotlioa ing in a detrimental spreading of the charge resistance of the suspending medium is less that desired at any point. In my present method of the resistance of the target 5 polarization occur: operation, however, when the volt Velocity of the Which may' interfere With the desired Operatie! beam is above second cross-over for the surface of the device.- Thus if the resistance of the susi being scanned, adjacent areas while held at a pending medium is too low with respect to th potential approaching that of the second anode resistance of the target, the electrostatic fiel( are not at the same potential as the scanned areas across th'e suspending medium falls to zero to' for the latter are held at the second cross-over quickly and the condition of orientation of th` potential. Thus if the electrode I1 is held at suspended particles is changed. It may, there second anode potential, no effective electrostatic fore, be desirable to utilize a target materia having. the same resistance asl that of the suspending medium. In the present device using mica as a material for the target and one of the liquids mentioned above as the suspending medium, this polarization l eii'ect is not serious. However, it will be found advantageous to match the resistance of the target material with that of the suspending medium as closely as possible.

In the operation of the device shown in Figure 1 I provide a step inthe operation of discharging the scanned areas which are at or near the potential of the second cross-over back to second anode potential. The necessity of this step may be seen from an examination of Figure 4 which shows a fragmentary cross-section of the light valve elements in Figure 1. Assume that the electrode I'I and the second anode 9 are maintained at 5000 volts with respect to the cathode of the tubeand that the second crossover of the surface of the target 5 is at 3000 volts which is representative of a material such as v mica. Referring to Figure 4, the surface of the target 5 scanned by the electron beam is indicated at C, the opposite surface as E, the surface of the window I4 adjacent the suspending medium as F, and the electrode I'l on the opposite surface of the window as G. Potentials of a given area on these four surfaces under various conditions of operation are shown in the following tabulation:

D E F G Volts Volts Volta lf olts (a) Before scanning 5, 000 5, 000 5, 000 5, 000 (b) Immediately after scanning. 3, 000 3, 500 4, 500 5, 000 (c) After polarization 3, 000 4, 000 4, 000 5, 000 (d) After discharge 0 6, 000 4, 000 5, 000 (e) After reverse polarization.. 5,000 5,000 5, 000 5, 000

It is assumed that the structure of Figure 4 is at a potential of 5000 volts before scanning. Immediately after scanning, there is a eld across the whole valve and that across the suspending medium causes the suspended particles to be oriented. These two conditions are shown in the above tabulation by (a) and (b). Polarization then takes place and the electrostatic field across the suspension falls to zero as shown by (c). If there is no discharge mechanism, the scanned area remains at 3000 volts and when th'e beam returns a second time for rescanning of this area it can have no further effect upon the area.

Thus, I have found for a representative-tube of the type described that for a weak discharging mechanism a line scanned thirty times per second maintains the suspended particles in an oriented condition and maintains the lillumina- `:ion of this lineon the viewing screen substan- ;ially constant but when the line was scanned ibout 13,000 times per second the line of light on zhe viewing screen faded out since in the latter :ase the scanned area remained at second crossver indefinitely and,due to polarization, there vas no further field available to keepthe susended particlesoriented. If however, as in d) of the above tabulation, the scanned area of he target 5 is discharged back to' collector potenial the potential of the opposite surface of the argot is carried momentarily above that of the irface scanned andy the'suspension is subjected i a reversed field which' tends toreorient the ispended particles or maintain thestate of oriitation. Finally after a second polarization :presented by (e) the eld across the device :ain falls to zero and the given area of the yrget is ready for another scanning and a repethere are primary electrons'arriving. When` the number of secondary electrons is equal to the` number of primary electrons the scanned area assumes the potential at which the secondary emission ratio is unity. Referring to Figure 5 which shows the velocity distribution of secondary electrons emitted by the target 5, it may be seen that the majority of the electrons have an initial velocity of only a few volts represented as group K. Let it be assumed that an area has been scanned and left at a potential of 3000 volts and that an adjacent area is then scanned and chargedvto the same potential. The high velocity groups J and L reach the area to be discharged with relatively high velocity and they tend to charge it to a potential approaching the second anode potential such as 5000 volts since these higher velocity secondary electrons J and L arrive at the latter area with a. velocity corresponding to a secondary emission ratio greater than one. Leakage from adjacent areas of (the target 5 at higher potential to areas at lower potential aids in raising the potential of the area to be discharged. Since the potential difference between these areas is quite high the leakage is suicient to materiallyhelp the discharge action. After the charged areas have risen in potential by about volts by reason of leakage all of the groups of electrons including group K at lowy initial velocity are accelerated to the area to be discharged and tend to charge it toward second anode potential.

vI have found that it may be advantageous to depend not only on the initial velocity of the secondary electrons liberated from the target under scansion but to provide means for producing a more copious supply of secondary electrons to be used for discharge purposes. 'Therefore in accordance with one modication of my invention shown in Figure 6 I provide a secondary electron emitting grid-like electrode 24 in a plane parallel to the target 5 and between the second anode 9 and the target. In this type of structure I prefer to operate the second anode at a somewhat lower, potential than previously indicated that is below the second cross-over potential of the target 5 such as at 2500 volts and apply the higher potential such as 5000 volts to a third anode 25 which may be a conductive coating on the wall of the tube between the electrode 24 and the target 5. 'Ihe electrode 24 is operated at a potential between that of the second and third anodes and s' htly below second crossover potential such as 2700 volts. I have found that the electrode 24 when made of a line wire mesh bright nickel screen having a hole opening 50-60% with respect to total area is satisfactory for good operation. However the hole opening may be increased if materials having higher sec- Vondary emission than bright nickel are used.

' could be reduced to r that of the second 25. with suc'h an electrons liberated from the electrode 24 supplement those liberated from the target so that,

lconsidering the curve of Figure 5 as applying to liberated from the electrode the group K, may be secondary electrons 2.4, the electrons such as accelerated to the recently scanned areas of the that my present discharge the effective repetitive development of potentials which diifer from that of the second anode by more than the electron volt velocity corresponding to the second crossover, for the adjacent areas can up until electrons from the scanned spot reach them with this velocity, at which point the secondary-emission ratio is reduced to unity. 'Ihus for my device as described, the preferred maximum second anode potential is about 6000 volts, or, in general, twice the potential of the second cross-over of the scanned area, A higher second anode potential would not be harmful but would merely result in a bias across the valve, uhich zero by reducing the potential of the electrode I1 below that of the second anode. In .certain cases it might be advantageous to operate with a second anode potential above (in this case) 6000 volts, for then the surface potential after discharge is reduced below anode and the collecting field for the tertiary electrons produced by second- Aaries reaching adjacent areas is thereby increased.

Ihave found that while the use of the electrode 24 and third anode 25 shown in Figure 6 is advantageous in producing the required number of `secondary electrons for effectively discharging the scanned areas of the target, it nevertheless vinterposes light absorbing or reflecting material between the light source 3 and the viewing screen I5 thereby eiectively reducing illumination on the viewing screen and interfering with the ideal operation of the device. Therefore, in' accordance with a further teaching of my invention I may generate additional secondary electrons directly in the plane of the target without interposing any light absorbing medium between the light source 3 and the target 5. As shown in Figure 7 I may utilize an electrode structure surrounding and preferably in the plane of the target 5 for the purpose of generating secondary electron emission with which to discharge the target. Referring to Figure 7 which shows only the target 5 and certain associated structure which may be substituted for the target shown in Figure l, I provide, a secondary electron emitting electrode 26 which surrounds the target 5 preferably lying in a plane substantially coplanar therewith. I .prefer to form the electrode 26 by depositing a film of metal around the edge or edges of the target 5. The metal may be in suspension andpainted thereon to form the electrode 26. This electrode 26 is connected to a somewhat lower potential than that of the second anode 9 so that secarrangement the secondary' only be charged p ondary electrons from the electrode 26 may be accelerated to the target '5. should be of a material having a high secondary emission ratio at the particular voltage applied thereto and may be made of a metal such as bright nickel or platinum or of an alloy known to have high secondary electron emission characteristics. The metal of which the electrode The electrode 26 2,290,581 `4 l 26 is made, such as bright nickel or platinum,

has a considerably higher secondary emission second cross-over than the target 5 so that the number of secondary electrons emitted by this electrode when bombarded by the beam from the electron gun will greatly exceed and supplement the number of secondary electrons emitted by the target proper. While I have disclosed the electrode 26 as being of metal, it may be of any material which has a high secondary emission second cross-over or if of material having a second cross-over equivalent to or lower than that of the target proper, it may be operated at a somewhat lowerv potential than at second anode potential.

During the operation of the device incorporating a target such as shown in Figure '7 the target is scanned bythe electron beam from the gun throughthe action of the coils H and V andis also caused to over-scan the target 5 and impinge on the electrode 26 at the beginning and end of each line sweep. Also in accordance with my invention the intensity of the electron beam may be increased during thetime or times it impinges upon the electrode 26 so that even greater quantities of secondary electrons may be emitted. This desired action may be obtained by removing or reducing the bias applied to the control electrode 'l at the beginning and end of each line sweep. c

f' From the foregoing it will be evident that I 'have provided a tube or light valve suitable for D Therefore, while I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specic -application for which my invention may be employed, -it will be apparent that my inventor 0 is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structurl used and the purpose for which it is employe( without departing from the scope of my inven t". on as set forth in the appended claims.

What I claim as new is:

l. In a `system for television reception a cath ode ray tube including a light transmitting dou ble end wall of electrically insulating materie having an area at least equal to the summatio4 of elemental picture point areas for which rece'p tion is desired, an electron gun Within said tui". to generate an electron beam of elemental pictui area and of variable intensity, a suspension t light intercepting non-magnetic particles in liquid carrier between and coextensive with tl said area of said double wall, means to scan tl inner surface of one wall of said double wall wit I said electron beam to produce thereon an elel trostatic image over elemental picture areas re] resentative of the intensity of said' beam, ar means to impress an electrostatic field corr` sponding in spatial intensity to said electrostat image across said suspension said means inclu ing an electrode adjacent and coextensive wi pension lying between said one wall and said electrode of said double end wall whereby an electrostatic eld may be developed through said suspension and the light transmitting properties of the said suspension may be varied in accordance with the intensity of said beam.

2. In a system'for television reception, a cathode ray tube "having a light transmitting target of a material which liberates fewer secondary electrons than incident electrons when bom barded with a high velocity cathode ray beam, a container joined to said tube said container having a light transmittin'gwvindow opposite and axially aligned with said target, a suspension of light absorbing particles in a liquid carrier in said container and between said target and said window, means including an electron gun within said tube to generate an electron beam having such a high velocity that when. incident upon said target fewer secondary electrons than incident electrons are generated, whereas a beam of lower velocity generates more secondary electrons,vand a beam of still lower velocity generates fewer secondary electrons than the electrons incident on said target, means to vary the intensitly of said beam and means to scan said beam over said target to vary the light transmitting properties of said suspension in accordance with the intensity of said beam.

3. In a system for televisionI reception, a cathode ray tube having a mica target, a container joined to said tube said container having a light transmitting window opposite and in alignment with said mica target, a suspension of light absorbing particlesin a liquid carrier in said container and between said target and said Window, means within said tube to generate an electron beam, means to vary the intensity of said beam, means to accelerate said beam to a volt velocity in excess of 3000 to produce such a high beam velocity that when incident upon said target fewer secondary electrons than incident electrons are generated, whereas a beam of lower velocity generates more secondary electrons, and a beam of still lower velocity generates fewer secondary electrons than lthe electrons incident on said target, and means to scan said beam over said target to vary the light transmitting properties of said suspension in accordance with the intensity of said beam.

4. In a system for television reception a cathode ray tube having a light transmitting target of a material which liberates fewer secondary electrons than incident electrons when bomaarded with a high velocity cathode ray beam, in electron gun for producing a variable in- ;ensity electron beam, an anode for accelerating ind focusing said beam on said target, means to :can said beam over said target to produce theren an electrostatic image having a charge from. Lrea to area thereof representative of the inensity of said beam, a perforated screen interuosed in the path of said beam to produce secndary electrons, an electrode between said perorated screen and said target to accelerate secndary electrons toward said target, means to iaintain said anode, said screen. and said elecrode at progressively increasing positive poten- Lals with respect to said electronl gun, a coniiner adjacent said target the target forming ne wall of said container, a light transparent all in said container opposite and in alignment ith said target and a suspension of light ab,-

tainer and between said said particles being opaque and of irregular shape and having the property of individually rotating to present a smaller area in a direction perpendicular to said walls to intercept a minimum of light when subjected to an electrostatic eld between the target and said wall.

5. Apparatus for television reception comprising a cathode ray light valve device having an evacuated envelope, an electron gun in said envelopefor developing an electron beam of elemental cross-section picture area. and of variable intensity, an electrically insulating target of extended picture area forming one wall of said envelope, a light transparent window exposed to and in axial alignment with said target, an electrode adapted vto liberate secondary electrons adjacent said target and exposed to said electron gun, means to scan said beam over said target to generate an electrostatic image on said target representative of the intensity of said electron beam and over said electrode to develop secondary electrons, a container joined to said envelope having the said target as one wall thereof, a window in said container forming another wall thereof opposite said target and in axial alignment with said rst mentioned window, a suspension of light intercepting particles in a liquid carrier lling the space between the walls of said container and an electrode adjacent the said other wall to subject the particles in said liquid carrier to the said electrostatic image to thereby vary the light transmitting properties of the said suspension in accordance with the intensity of said beam.

6. Apparatus for television reception comprising a cathode ray light valve device having a light transmitting target forming one wall of an envelope, a container adjacent said target, a light transmitting element forming a closure for said container opposite to and. in alignment with said target, a light transmitting electrode over one surface of said light transmitting element opposite the target, a suspension of light'intercepting particles in a liquid carrier in said container, means to generate an electron beam, means to vary the intensity of said beam, means to scan the surface of said target opposite that in contact with said suspension with the variable intensity beam of electrons to produce an electrostatic eld between said target and said electrode for aligning' the light absorbing particles in the suspension, and substantially transparent means wholly interposed between the target and light transmitting element for maintaining theV suspension in agitation.

7. Means for receiving television signals comprising a tube having a double end wall, a suspension of light intercepting particles in a liquid carrier between the said double end wall, electronic means to vary the light transmitting properties of said suspension in accordance with the received television signal, means immersed in said liquid carrier and wholly interposed between the walls of said double end wall for maintaining the suspension in agitation and means for varying the viscosity of said liquid carrier during operation of the tube.

rbing particles in a liquid carrier in said con- 75 8. Means for television reception comprising a cathode ray tube having an envelope, -a mica target sealed to said envelope and forming one wall thereof, a light transmitting window in `alignment with said mica target and forming another wall of said envelope, a 'container adjacent said Yenvelope having a light transparent wall oppotarget and said wall,

8 2,290,581 site said target and in axial alignment with said stantially transparent material, means to move target and said other Wall of said envelope, a `said mentioned means in a plane parallel to the suspension o1 light absorbing material. in a liquid plane of the target and means within the encarrier in said Container, means wholly intervelope to scan said target with a variable inposed between said v/tar'get and the wall of said 5 tensity high velocity electron beam.

container for maintaining said suspension in agitation, said means comprising a sheet of sub- JOHN SCOTT DONAL, JR.

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