US2460093A - Cathode beam transmitter tube - Google Patents

Description

Jan. 25, 1949. H. B. LAW 2,460,093

CATHODE BEAM TRANSMITTER TUBE Filed April 19, 1945 Mm amm/a jpg l @i Patented Jan. 25 1949 f 'i cATHoDE BEAM TRANSMITTER TUBE Hai-old B.'La1w,rPrnceton, N171., assignorto Radio Corporation of America, acmrlloxatonoDpcla.`

Ware

Application April 19, 1945, Serial No. 589,241-` 1 This invention relates to television pick-up tubes having two- 'ded targets in which one side of the target is scanned by a cathode beam to discharge an electrostatic Vimage on the other side set up by light or electrons passing througha collecting screen adjacent thereto. In one type of this kind of tube, the two-sided target has heretofore comprised-a sheet of glass'or other material having. such thinness and conductivity as to permit the discharge within a frame time of the electrostatic-image by .passage .of the beam electrons transversely through the thin sheet, but havingsufcient resistivity to. prevent substantial spreading of theimage charges or electrons longitudinally ofthe sheet.A 1

' In pick-up tubes ofthe type referred to, it has been thought necessarytoplace the screen very close to thesheet of glass tov obtain the necessary capacitance toprovide a usable signal. Satisfactory signals` have thus been obtained with spacings between target Yand screen in the neighborhood'of 2 mils. In this position .the screen cast a shadow or formed an image onthe target that was visible in the transmitted picture. It is yanobject of this invention to prevent the formation of the image of the screen on targets of this type.

Another object of the inventionis to increase the spacing between screen and target sufficiently to defocus the screen without substantial decrease ofthe maximumV output signal from the screen and targets Y Y Y Another object is to increase the range of high light scene brightness for cathode beam pick-up tubes with nite gamma throughout the range.

-Another object vis to increase the scene brightness for cathode beam-pick-up tubes without reducing the gamma to zero.

Another object of the invention is to provide a cathode beam tube giving improved half-tone discrimination, especially under high lights.

Other objects will appear inthe following descrption, reference being had to the drawing, in

which: f l

-Figure 1 is a sectional elevation ofa tube having a.4 target and screen associated in accordance with my invention. f

vFigure 2 is a section of the lm and screen in the supporting'frame.

Figure 3 is a greatly enlarged representation of a screen-target area for explaining the operation ofthetube."` lv Figure f4 is a diagrammatic Vrepresentation of the film andvscreen.foreiiplaining thetheory of the operation. v

4 claims.` (C1. vr.25o- 164) Figure 5 contains two curves showing the signal current characteristics for close and wide spacing of the screen and target.

In prior art tubes referred to, the screen for collecting the secondary electrons emitted by the target, comprising a semi-conducting glass lm of about two ten-thousandths of an inch in thickness, was spaced from the target surface less than the diameter of a picture element. The capacitance C between two plates (glass lm and collector screen in this instance) was thought to be derivable from the usualformula in Vwhich lc is a constant, A the area of one plate therefore considered to be gr t in which V is the potential swing of the target and, tristhe time the beam takes to discharge the target', or the Aframe time. Therefore,the extremely close spacing of 2 mils was thought necessary to provide-sumcient capacity between the screen and iilm for a satisfactory signal.

In these prior art tubes, it was at iirst thought advisable to place the screen in contact with the glass film to obtain the desired capacitance. This, however, did `not prove to be entirely satisfactory, as the target had too high a capacitance tobe discharged properly by the beam in a frame time. The screen was then spaced from the lm about two mils, or thousandths vof an inch, and it was found that sui'cient capacitance remained to provide the required signal jand that the target could be discharged by the beam in the frame time. With this spacing, the image of the signal screen could be seen in the transmittedpicture, because it was close enoughto the target to be in focus, asv already referred to. y

It was determined that at least 60 and preferably 80 or more milsspacing would be required to place the screen in good unfocused position. Since the capacity was thought toV vary inversely with the spacing, this abnormal spacing was considered su'icient to render the signal Wholly unsatisfactory and the image of thescreen in the transmitted "picture was toleratedas being unavoidable. To determine how much the screen image could be reduced'without ruining the signal, I tested a tube withv'wide spacing between up the target by impinging photoelectrons andA secondary emission to a uniform potential ofr aboutY 2 Volts positive. InthcsegtestsV the potential was uniform over the target because the light was projected directly onto the photocathode and not through a picture.A As thevbeam scanned the target, it would thus pass over a few elemental areas with the excitinglight on and over the rest of theV areas with it 01T. Surprisingly, it'was `found that the signal waspractically the same for all spacings while the beam was passing over the areas under excitation (light on). For the remaining equally charged but unexcited yareas (light off) the signal varied inversely with the: :spacing` Only thev elemental areas of the target scanned when the exciting light was oi gave the reduced signal one would expect from thecapacitycalculated by the formula Y A f kA t It therefore appeared that some hitherto unknown phenomena accounted.V for additional storage o picture potentials on a target in the neighborhood of the beam. The reason forv this isl not yetcompletely understood, but it is believed'that in the neighborhoodof the beam the capacitance of a picture element' under excitation cannot be correctly calculated by the for-V mula e y where C is the capacitance of the'target and N is the `number of elements. It appears that the capacitance-of such element is greaterY than this and will approach that of a disc in free space, expressed by Ythe well-known formulaV where dr is the diameter of the element. The capacitance, calculated in this manner, Canaccount for the high signal in vmyV inprovement.

The parallel plate capacitance'of an element oi the .target surface approximates the capacitanceY 'of a disc in free space. A simple argument for this relation may be ,derived from the capacitance expression `for two concentric spheres, the inner sphere beingcomparable Ato the picture element .disc and the outer sphere to the collector screenv behind the targetq, As known, the capacitance C for concentric spheres is obtainable from the equation 717'2 :C K Y Y Y where r1 is the radius ofthe inner-sphere, r2 that of the outer sphere and 7c is a constant. If r1 is nearly the same as n, the arrangement is similar to lthat vof a closely spaced-,target and the formula becomes Y 4 where 1 has a mean value between r2 and 11. It will be seen that this expression is, in form, like .that used for determining the capacitance of a parallel plate condenser, that is,

a Y D l If,on the otherhand, rz ismuch larger than r1, the expression k TIT?, Y l d Y 72-71 becomesv approximately Y V` or ich. Y This arrangement is similar to that of a V.widely spaced target and screen and therefore Y Ich may be regarded as expressing the approximate capacity between the screen and anfelemental target area `under .beam action.Y This expression is, in formy like that for the capacitance of a disc in free space, namely, n

rThe capacitanceV thus is Vsubstantially independent of therspacing and isproportional to the diameter .rather than` to the area of theV mosaic element.` This would seem `to' explain why with Yhighlights Iam able to defocus the screen by Y moving it awayfromA the target any distance without substantial. decrease of the .signah Thevphenomena Vcan also be -denronstrated'by means. of Fig. 4, Where .the parallel capacitances 1 of elementalV areas .a tog of a small Ypartl ofA the target'i `are indicated by C1 to C7 with capaci- Vtances therebetween represented by c1 to. ce.

40 The capacitance betweenelemental area aand space.

the screen 3 d oes not result Ysolely from its area andthe perpendicular :distance between it. iand thescreenf It results Vfrom all paths between ,af and the screen 3. ThisV contemplates a path through the Vintervening capacitance c1 to area b and thence through capacitance C2 to the screen, Another through intervening,r capacitances c1 and 'c2v through C3 tothe screen, and `so on. The summationof all theY parallel and series capacitances constitutes the total capacitance of an elemental area. Thus, the capacitance approaches the capacitance of a -disc in free The target in my improvement comprises a very thin impertorate lm or vsheet-l of homogeneous electrically semi-conducting material, such as glass. Associated with the target sheet on the sidetoward the photocathode 2 isarranged'the collecting screen 3l `The semi-conducting nlm may have a vspeciiic resistance lying within the range of 109 te 101.2V ohm-centimeters, as this has been found satisfactory. vIt has been found that a lthin sheetof glass, known inthe art as Corning G-8 glass, has the desired specific 'resistance of approximately 5 l01" ohm-.centimeters when formed .in :very thin'iilms, such as two-tenthousandths of an inchin thicknessbut other glasses of suitable resistivity may .be used.,VIK The thin Vglass film may be'rconstructed'rand stretched out across ringV 4, forexample, .by the method disclosed Y in theY application of Albert yRose, filed September 20, 1940, Serial No. 357,543or which v'application Serial No. 631,441 was led VNovember 28,1945, as a continuation, after which application Serial No. 357,543 was abandoneiibut-knowl.,

andere;

edge of these details is not necessary for an understanding of my invention and the description eednot be repeated here. i, f y u The wire mesh screen 3 isnasseinbledin spaced non-focused po'sitln from thefilm I in ring 4, such as-by the metal ringsii and t (Fig. 2). The spacing betvveen the screen and lm should preferably b e lmore than 60 to 80 mils to defocus the SQI'CDJPTQPGTW' I In the use of my invention I prefer to employ the tube having a multiplier construction, for eX- ample, like that shown in the application of Paul K. Weimer, led September 16, 1944, Serial No. 554,494 now United States Patent 2,433,941, issued January 6, 1948, to which reference is made for various details of construction not required for an understanding of my invention. With this tube, the operation of my Widely spaced targetscreen unit may be explained as follows:

Assume that the left-hand surface of the target Sheet I in Figs. l and 3 is scanned by the electron beam 'I (greatly enlarged in Fig'. 3) and that photoelectrons, as at 8, are projected from the photocathode 2 by light rays L from an object focused thereon. These photoelectrons are focused through screen 3 by coil 9 on .the righthand surface of the sheet I, such as over the elemental area B, and accelerated by :the electrostatic fleld of anode 9' at such velocity as to charge it positively by the secondaries emitted therefrom. The secondaries are collected by screen 3, which is, as usual, maintained at or slightly above the voltage of the gun cath-ode It. The coil 9 maintains a magnetic focusing eld for the beam electrons, as well as the photoelectrons. Since the area B is made positive by the photoelectrons, the space around B, including an area of the screen 3, will also be positive by electrostatic iniluence. Points very near B, such as the area A, Will have almost the same potential as B. A portion of the electrons of the scanning beam "I, in passing over the area A, will land and deposit enough charge to reduce the potential on the area A Ito the potential of the gun cathode and the remainder II (Fig. l) will return toward the cathode. yIhe posi-tive charge at B and the negative charge at A on target I unite by conduction through the -thin sheet of semi-conducting material and both areas A and B return to their original uncharged state in a frame time, thus producing Ia signal by modulation of the returning beam II, the electrons of which land on the rst stage I2 of the multipliers around the gun, as shown and described in detail in said Weimer application, but it may be briefly said that the secondaries bombard other -secondaries from the surfaces I3 through shield screens I4 successively from the i'lrst stage I2 to the second -stage i5, third stage I6, four-th stage II and the nal disc stage I8. The secondarles bombarded from this nal stage are collected by signal electrode i9 of negligible capacitance With the target. The signal vthen passes at 20 to suitable amplier-s and other utilization devices, not shown.

In Fig. 5, signal vs. light curves A and B are for closely and Widely spaced screens, respectively. The ordinates are the signal output values in microamperes at .the target.' The abscissas are the signal input in microamperes or total photocathode current, the illumination on the photocathode (10 microarnperes/lumens sensitivity) in foot candles and the high light scene brightness in candles/ft?, using la lens of f/2.3. At very low illumination and photocathode current the two curves are the same, but at 2I the curves sep- 6 arate. At this point the parallelplate capaci'- tance l has been saturated and further increases in signalin curve B are due to the disc or elemental area capacitance kd Ialone, as hereinbefore described. The signal continues to increase with :Finite gamma up to -22, at which point gamma becomes zero, gamma being the exponent in the equation S=lcL^/, Where S is the signal output at the target and L is the light on the photocathode. With close spacing (curve A), the gamma is finite only up to 23 and is zero therebeyond. Thus, by increasing the spacing between the target and screen, I remove the screen from focusing position and also have finite gamma at high light. or scene brightness, Where it would be zero with the closely spaced target and screen (curve A). In curve A the -signal output saturation occurs at 10-1 foot candles of illumination yon the photocathode, but in the Widely spaced screen and target (curve B) signal output saturation does not occur until 10 foot candle illumination is reached. This gives greater half tone discrimination at high lights.

In accordance with my invention, moving the screen from the previously employed close distance of 2 mils to beyond a distance of 60 to 80 mils, enables me to entirely eliminate the image of the screen 3 in the picture Without any appreciable reduction of signal strength and with finite gamma at high light. I have advanced a, theory explaining this seeming paradox, but the invention `is not limited to this or any other theory or explanation.

Having described my invention, what I claim 1. A television transmitting tube comprising means for producing a uniform magnetic focusing field, an envelope containing a cathode beam gun, a glass lm target in said eld, a collecting screen spaced more than lapproximately 60 mils from the target on the side remote from the gun and means for forming an electrical picture image on said target through said screen.

2. A television transmitting tube comprising means for producing a uniform magnetic focusing field, an envelope containing a cathode beam gun, a glass nlm target in said eld, a collecting screen spaced more than approximately 60 mils from the target on the side remote from the gun, means for forming an electrical picture image on said target through said screen and a signal electrode of negligible capacitance with said target.

3. A television transmitting tube comprising means for producing a uniform magnetic focusing eld, an envelope containing a cathode beam gun, a glass lm target, a collecting screen spaced more than approximately 60 mils from the target on the side remote from the gun, means for forming a desired electrical picture image on said target through said screen, means for scanning the beam of said gun over said target to discharge said electrical picture image and a signal electrode of negligible capacitance with said target for collecting the electrons of said beam after discharge of said image.

4. A television transmitting tube comprising an evacuated envelope containing a cathode ray beam gun, a photocathode, a target consisting of a thin sheet of semiconducting material, eld producing means for focusing electrons from said genome gun on one surface of said tzai'rgetarzid electrons f REIPEREIXCES CI'YIED from said photocathode on the other surfaceV thereof, said target being positioned in a focalV plane of said electrons, a screen positioned out- *me of thm patent Y side said focal plane between the photocathode 5 UNITEDSTATES PATENTS and the target and a, signalA electrode spaced Y Number" x 'Name Y Y, Date Widely from said target having negligible capaci 2 25,7 942 Farnswrth Oct 7 1941 tance thewlth- 2,288,402 rams June so, 1942 HAROLD B. LAW. 2,377,972 schade Y V June 12, 1945 The folowng references are offxrecord: in. the

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