US2540632A - Television image and pickup tube - Google Patents

Description

Feb. 6, 1951 A. RosE TELEVISION IMAGE AND PICKUP TUBE Filed Sept. 27, 1946 those by thehigh lights. charge within afraine time, but'the resistance anode 4. All these parts may be of well-knownf Y considered a collector for the photoemission from Patented hFeb; 6, 19751 *i d 4,l

UNITED. STATES fPr'J'iaNT olii-ICE TELEVISION IMAGE AND PICKUP TUBE Albert Rose, Princeton, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application september 27, 1946, serial No. 699,7os

. 4 Claims. (Cl. Z50-165) v:

l' This invention relates to image tubes. 'The sons that will appear lateri' The parts 5, Band 11i' invention is not restricted to any particular typeare, of course,4very thin elements and their thick-i.:- of tube, but it has particular importance in the ness has been greatly exaggerated. In front of? orthicon type of cathode ray p ck-up tube in the photo-sensitive material 'I and closely Vadi-f' which the beam lands on the target at zero 5 jacent thereto is locatedv afine mesh screen il.v velocity or thereabouts. In this type of tube," In an intermediate part of the tube is located with a limited signal output and a gamma of the target 9,` which may consistof a thin semi-f. unity, the signals from the low lights may be in conducting vmaterial, such as gass, fully de"=v the noise range. One can bring the low light" scribed in my Patent 2,473,220 issued June 14,-v image charges above the noise by increasing the w 1949, and my application led November 28, 1945, light over. the entire photocathode, but this pro-v Serial No. 631,441, now U. S. Patent No. 2,506,741, portionately increases the high light charges on issued May-7 9, 1950. Reference to those applica-l the target and the beam cannot fully discharge tions is made for a full description of the target them. Y and its action, but it may be said briey that the- It is an object of this invention to provide an target of thin glassis suiiiciently conducting for arrangement for increasing the charges bythe the electrons of the beam B landing thereon to low lights without proportionately'V increasing pass through the target and discharge the image Another object is to provide means forlimiting is sufciently high to prevent the image charges the signal charges of the .high lights without from spreading over the target within a frame limiting those Vof the low lights. l time. Between the target '9 and the photo' g Another object of the invention is to provide cathode is located the iine mesh screen I0, usually means for varying the gamma of a pick-up tube called a signal screen, though I prefer to obtain by a control potential. Y the signal from the return beam B', as will later Other objects of the invention will appear in be referred to. Between the signal screen I0 and the following description, reference being had to the photocathode is positioned the electrode II" the drawings, in which: f for accelerating the photo-electrons ofthe photo- Fig. 1 is an axial section of a pick-up tube cathode to the surface of target 9. Directly in" embodying the invention; and front of the target 9 is positioned the decelerat- Fig.2 is a modification. m 4ing ring electrode I2.

f'Figure 3 is an enlarged View of a portion of Immediately in front of the end of the rstv the tube shown in Figure 1. anode 4 is positioned'an electrode I3, usually'.

Figure 4 is a graphical showing of the relationc'aleda persuader, and between the persuader ship between the light input and signal output electrode and the decelerating electrode I2 is lo of pickup tubes to be described. i" cated the usual `wall coating'anode I4. This? 'Referr ing `to the drawing, the` invention -is` electrode obviously need not'be coated'v on the shown as applied to an image |"orthicon tube, wall of the tube, as the"` name indicates. Itmay' which is essentially the combination of an imageA be' a separate electrode mounted with the other tube with the usual orthicon in which the beam electrodes, as disclosedin the patents of StanleyI lands vat low velocity on the target. In Fig. 1 .lo V'"F01g11, 2,441,315 issued May 11,` 1948, andv the'evacuated envelope I may consist of glass or 2,460,381 issued February 1, 1949, and the otherY other appropriate material. ,At one end of they electrodes disclosed herein may be supported tube is, located the cathode 2 having the usual Within the tube in the Way thedSCOSed. heater, not shown. The cathode is surrounded T0 Scan'he beam BV OVeI the target. the usual b3ta perforated grid 3 and a perforated iirst" 45 defecting unit l5 may be used. This maybe' construction.\v At the other end of' the tube 'isv lr'lndlmayconsist 0f two 'ils having their located the image section, having a photocathodeeldS at vrighi? angles t0 each' other and to the comprising a translucent semi-conducting film `5', axial magetic focusing eld pl'OduCed by C0i1 l5 a translucent conducting iilm electrode 6 and a 50 This 00i1 iS Of Sufficient length 110 DIOVide an photosensitive material 1 applied to the semiaxial focusing iield for the electrons of the beam conductor 5. This photosensitive material should Band the photoelectrons produced by the photonot havesuiiicent conductivitv` 'to increase the cathode. r-.;, Y Y y; conductivity-of `the-semi-condueti ng element-5 'jAs I have-stated;the-signal:#ouldgr'begtairem azlg-th'elsurfacefacing the rBt'anocle, for rea-f 55 oit -tbeconductor-leading to'sclee'irei. but iti' preferable to multiply the signal before transmitting it to the pre-amplifiers (not shown). Various types of multipliers may be used, but I prefer to use the one disclosed in the Patent of Paul K. Weimer, 2,433,941 issued January 6. 1948. This multiplier is not described in detail herein and reference is made to the Weimer application for a full description, but it may be said that it consists of four or five multiplying stages surrounding the first anode. It is indicated herein by reference character l1.

The operation of my improved tube is as follows:

The imaging light L is focused on the photocathode. The rays pass through the translucent lms 5 and 6 and cause emission of photoelectrons from the sensitive surface 7. These photo-electrons are accelerated by electrode ll and bombard secondary electrons from the surface of collector target 9, which are collected by the-screen IvD. These photo-electrons'are focused Thus,

tribution of light and shade of the image on theY photoeathode. As thus far described, the operation is. the same asy in the standard image orthicon.

My inventioncomprises the use of screen 8, connected by `lead 26 to a Voltage -divider 2l and, lio-which I apply a voltage suiciently near the potential of the low light charges onlsensitive surface 1 to limit the high light charges, .so that they cannot build up to the full-potential that would normally be produced by the light. The potential of thehigh lights cannot increase above thatof the screen, because in that case the surface would be more -positive than kthe screen and the electrons would return to the sensitive surface and .bring it back to the limitingA potential of' the screen. Thus I can vary the gamma of thel tube byvarying the potential onscreen 8. Target electrode 6 is connected by lead 25- to a more negative portion of voltage divider-27 than screen 3.

The charge image produced onV the photocathode side of target 9 is discharged by the electrons of the beam B landing thereon and passing through the thin glass filrnvvithin a frame time. The remaining electrons of the beam then are accelerated back towards the end of the first anode 4 as beamV B. anode is the first d-ynode of the `mutiplier and the secondary electrons emitted are directed by the4- persuader electrode-I3 to the remainingstages of the multiplien-indicated by block-diagrain Il; and the output of -the tube is taken' from the nal stage-of this multiplier bya con-- ductor, not shown. It will thus beV seen that,

with -myinvention, I can increase the light L'A focused on the photocathode and thus bring the low light charges out of the noise range, whilev The endv of the fifi 4 emission would be from the areas brightly illuminated by the scene. The presence of the positive grid 8 would have little or no effect on the photoemission from the target surface under these conditions.

Curve 30 of Figure 4 represents the relationship of the light input to photoemission from portions of a photosurface iny direct contact with a signal plate. However, when a semiconducting lm 5 is provided between the conductive plate 6 and the photoemissive surface l, then the current ow, between conductive lm 6 and those portions of the photosurface 'l illuminated by bright light, is greatly reduced and thus portions of the photosurface 'l which are illuminated by bright light will assume a charge equal to the product of the resistivity of film 5 and the current flow between lm 6 and surface 1. The presence of the semiconducting film 5, in this manner, limits the current ow between films 6 and 'l to all portions'of the photosurface 1, but ina -much less degree to portions illuminated by low lights. than those portions illuminated by bright or high lights of the scenes focused upon the photo-v oathode electrode. Thus, more of the total current flow between lms 6- and 'l will go to theN portions of the photosurface illuminated by the,

low lights and will thus increase the signal rfc--y sponse of the tube in the low light area, It is'; seen, then, that with a semiconducting lm 5, the photoemission from each portion of the sur-V- face I is not in direct proportion to the lightv falling upon that surface. Curve 40 of Figure 4g represents the relationship between the light in-f put and photoemission from portions of a, photosurface having a semiconducting film lbetween the, photosurface and a'source of energy.

Furthermore, the screen electrode B is very-jnecessary when this semiconducting film 5 .is used. If the screen ily were not .present, then` those portions of the photoemissive film 1 illu-'- minated by the high lightsv or brightportions of the scene would continue to charge up' towardthe potential of electrodes IB and I I In tubes-V of the ty-pe shown in-liigureV 1, the potential of electrodes Ill and H are in the hundreds of volts;

and long before the high light areas of surface'- the high-lighted portions 0f surface 1 canelle; and as chargedA islimited to this potential,.- described.

Itis desirable, in order to avoid picture dis-l tortion by large potential differences along 4.thee

surface of the photocathode and in order to takel advantage of the velocity distribution of the;-

photo-electrons, to provide a smooth bending of 1 the output versus input curve, to adjust thev re-av sistivity and thickness of the semi-conductor 5,-,

for the range of light intensity employed, so that'` the screen 8 may be heldat only a few volts posi-V tive with respect to the photo-sensitive.surfacel- To provide this condition, the conductive filmv4 (i.t

and the screen 8 are connected to a voltagedi-:

vider 27 by leads 25 and 26 respectively. Asin-V dicated in Figure 1, the source of potential ZP will; provide the screen 8; with a potential positive, be@ few; relie .relative be.,that;.oi.ebneietiri By providing the screen electrode 8+.

screen 6. If the maximum photo current in amperes to be passed by the screen is I, then' the resistivity and thickness in centimeters of the semi-conductor 5 for a screen voltage of V is determined by thequation Where 1' equals the resistivity of the semi-conductor in ohm centimeters, d equals thickness of the semi-conductor in centimeters, A equals the area of the semi-conductor in centimeters squared.

My improvement may also be used as an image tube having an output of light with a desired relation (gamma) to the input and to project the light output onto the collector target of an orthicon, but in this case it is more efcient to combine the orthicon with the image tube, as shown in Fig. 2. In this modification, the potoelectrons emitted by the photocathode produce a light image by being focused on a fluorescent screen S coated on the translucent conductive lm I8 on the back of a target plate I9 which may consist of a thin luminescent sheet of mica. A photosensitive mosaic 24 is formed on the opposite surface of the mica sheet I9 and is formed similar to surface 'I of the tube of Figure 1. other elements shown in this figure are the same as those shown in Fig. 1 and they have been given the same reference characters. By way of variation, the accelerating anode has been shown as acoating on the inside of the tube at 20, in-

stead of as a separate element I l, but the actionis entirely the same.

While certain specic embodiments have been illustrated and described, it will be understood that various changes andmodications may be made therein without departing from the spirit and scope of the invention.

What I claim as new is:

l. A photoelectric device comprising, a photocathode electrode including a photoemissive surface, an electrically conductive sheet parallel to and closely spaced from one side of said photoemissive surface and a material of high electrical resistance conductively connecting portions of said photoemissive surface respectively to adjacent portions of said conductive sheet, a collector electrode spaced from the other side of said photoemissive surface and a conductive screen electrode closely spaced from said other side of said photoemissive surface and between said surface and said collector electrode, means connected to said screen electrode and said conductive sheet for joining said sheet to a first source of potential and for joining said screen electrode to a second source of potentia1 positive relative to said first source in the order of a few volts.

2. A photoelectric device comprising a semiconductive sheet, a photoemissive cathode film on one surface of said semiconductive sheet, and an electrically conductive lm on the other surface of said semiconductive sheet, a collector electrode spaced from said one surface of said semiconductive sheet, a conductive screen electrode closely spaced from said one surface of said semiconductive sheet and between said sheet and said collector electrode, conductive leads connected respectively to said screen electrode and said conductive film for joining said con- The - surface of said sheet, and a conductive film on the opposite surface of said sheet, said sheet being of high electrical resistance and conductively connecting said photoemissive film to said conductive film, a target electrode having an insulating surface and spaced from said photocathode, means between said photocathode and said target electrode for focussing the emission from said photoemissive film onto said target electrode to charge said insulating target surface, means including a scanning device for discharging said target surface and initiating a signal voltage therefrom, a screen electrode closely spaced from said photoemissive film and between said photocathode and said target electrode, and means for maintaining the potential of said screen electrode in the order of a few volts posi- Y tive relative to the potential of said conductive film.

4. An image pickup device comprising a photocathode electrode including a sheet of semiconductive material, a photoemissive film on one surface of said sheet and a transparent conductive lm on the opposite surface of said sheet, said semiconductive sheet being of high electrical resistance for conductively connecting said photoemissive lm to said conductive film, a target electrode comprising a thin glass sheet spaced from said photocathode, electrode means between said photocathode and said target electrode for accelerating and focussing the emission from said photocathode onto said glass target to charge one surface thereof, an electron gun for providingV a low velocity beam of electrons, electrode means for directing the electron beam'to the opposite surface of said target for depositing electrons thereon and for returning the remaining electrons of the beam toward said gun, means for scanning the electron beam over said opposite surface of said glass target to discharge said target, and an electrode within said envelope for collecting the returning electrons of said beam, a conductive screen closely spaced from said photoemissive film and between said photocathode and said target electrode, and vmeans connectedl to said conductive screen and said conductive lm for joining said conductive lm to a iirst source of potential and for joining said conductive screen to a second source of potential positive relative to said rst source in the order of a few volts. l

ALBERT ROSE.

REFERENCES CITED The following references are of record in the le of this patent:

, UNITED STATES PATENTS Number Teal Jan. 18, 1944

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