US2840755A

US2840755A - Large storage low noise image tube

Info

Publication number: US2840755A
Application number: US277780A
Authority: US
Inventors: Richard L Longini
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1952-03-21
Filing date: 1952-03-21
Publication date: 1958-06-24
Anticipated expiration: 1975-06-24

Description

June 24, 1958 R. L. LONGINI I 2,840,755

LARGE STORAGE LOW NOISE IMAGE TUBE Filed March 21, 1952 Secondary Electron Ratio Primary Electron Energy Fig. l.

WITNESSES: INVENTOR 9/41 4, AL; I :ichurd L. Longini ATTORN EY United States Patent "ice,

V I 2,840,755 7 LARGE STORAGE Low NOISE IMAGE TUBE Richard L. Longini, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa.', a corporation of Pennsylvania Application March 21, 1952, Serial No. 27 7 ,7 80 6 Claims. (Cl. 315-11 Patented June 24,1958

apict ure pickup tube of the storage type which operates with relatively high potential gradients between difierent points on its screen. 7 ,Anotherobject is to provide an improved type of im-' agepickup tube. Another object is to of storage. V I Still another object is 'to provide an elect'ric image storage device ofhighenergy gradients. e Yetanother object is to mode a device for storing provide a tube with large capacity negative charge images of high electric gradient.

, Another object is to provide a picture pickup in which the picture current is largely made up of low-energy sec; ondary electrons.

Still another object is to provide an image storage control tube in which the noise will be greatest at points corresponding to greatest image intensities.

' Another object is'to provide means by which low en-' ergy secondary electr'onsem itted from the scanned sur-f face of a target, are separated from reflected primary and high energy secondary'electrons;

charge distribution, point-by-point. over the screen, by

knocking out secondary electrons. This distribution is scanned by an electron beam from which electrons are abstracted instant by-instant to neutralize the stored charge at the point of. the screen on which the electron beam is incident at themomeut. The electrons remaining in the beam are returned to the beam cathode as a current in an external. path which controls the picture transmitter. 1 p

However, the ability of'such image pickup tubes as now operated is limited in the following respects: (1) they have. limited energy storing capacity,and (2) they have a high electron noiselevel.

Whenthe number of secondary. electrons knocked. out

of th e glass screen per incident primary electron is meas.- ured for various potential diflerencesbetween the photoelectric screen -and the glass screen, 'a curve of the form shown in Fig.7 1 results. I Thus, asenergy of the'primary electrons is increased, ',the ratio ofsecondary. electron to primary electrons is first less than 'unity,'but soon rises above that value. This ratio rises to a maximum and then falls away until it finally crosses the value unity. The point where the ratio first rises above the value is called the first crossover point and where it again falls below unity is .the second crossover point. For accelerating energies; between the two points, the screen is left with a positive stored charge when electron bombardment ceases; for other energy values the stored charge is negative. f

Conventional Image. Orthicons have glass screen potentials relative to the photoelectricinput screens such that they operate in the energy range not far above the first crossover point in Fig. 1, and the electrical charge distribution on these screens are positive. In contrastwith this, I proposed to operate glass screens in my tube be- I,

yond thesecond crossover point so that primary electrons emitted from the photoemissive screen enter theglass and distribute a negative charge over the surface ofthe glass screen. Because the charge image on the screen 7- is due, not to secondary electrons, but to primary electrons several hundred volts of potential 'diflierence may be produced between points corresponding to dark and bright areas" in. the picture with my tube, whereas the low-energy secondary electrons limit conventional Image Orthicons to potential differences of theo'rder of one to two volts between such areas.- 4 7 r One object of my invention is accordingly to provide Other objects of my invention w1ll become apparent upon reading the following description taken in connection with the drawings in which: i Figure 1 i's'a' graph showing certain featuresof secondary electron'emission which is useful in explaining my invention; and a Fig. 2 is a schematic showing partly in section of a picture pickup tube embodying'the principles of my in-' vention. 1

Referring in detail to Fig. 2 of the drawings, a picture pickup tube embodying the principles of my invention comprises the Image Qrthicon which comprises an evacuatedienclosure 1, which may .be of glass, having an image chamber 2 a scanning'cham-ber 3 and an electron multiplier section/t; The image chamber 2 is of a struc-' tu're'well known in the artand needs no descriptionhe're beyond the statement 1 that a conventional photoelectric coating forms a screen Son which the picture to be trans-. mitted "isfocussed by a suitable optical system (not shown) outside the'tube. A thin -glass membrane 6 faced with a metallic screen 7 is supported parallel to screen 5. The screens-5 and 7 are provided with leads so that desired voltages maybe impressed upon them by potentials derived from taps on a direct current sourceSA,

' and the inner wall of the chamber is preferably given a conducting coating '8 which is electrically connected to screen 5. The voltage between screen 5 and 7'is such that the electrons have energyabove the second cross over of the material in the membrane 6. In the case of glass,.the range difference is of at least 5000 or -mor'e volts. The scanning chamber 3, which is cylindrical, is likewise provided" with a conductive coating 9 and is similar to present-day ImageOrthicon structure. At the region remote from imagecha mber 2, the scanningc'ham: her 3 is provided witha separate coating 11, the electrical potential of which may-be fixed by an in-1eacl'12. In "the central axis. of coating 11 is positioned the end-- disc 13 of the electron gun. -Beyond=the disc '13"arelo-" cated dynodes 14, 15, 16 and an output electrod'e 1-7of conventional structure-which it is believed superfluous to describe, each provided with an in-lea-d by whichits potential maybe fixedat will. A focussing coil 18 is provided for image chamber 2, and a set of deflecting -elec trodes-19, 20 are arranged to cause an electron beam issuing from the orifice in 'disc.13 to scari'screen 6 with a centeredtrace of small area. A separating coilh25 having its axis inclined to the axis of tube 1 produces a magnetic field inclined to thattube'axis for 'a" purpose pointed out :below.

* The scanning beam for screen 6' is producedfby a s conventional electron gun comprising an electron-emisleast for low signal intensity.

sive cathode 21 and electron lens elements 22, 23 of a well known type terminating in endrlisc 13, from the contracted orifice 24 in the center of which the electron beam issues.

Since the structure so far'described1s convent1onal today, it is believed suflicientin further description of.

storage of electric charges on screen 6 which duplicate in space distribution the light field incident on the photoelectric screen 5. r 7

As was stated above, the potential of glass screen 6 is the same as that applied to the metallic screen with no electron bombardment and relative to photo-emissive screen 5, is made to be above the second cross-over point of Fig. 1 so that the charges on screen 6 .due to photoelectron bombardment from photoemissive screen are negative. The screen 6 is scanned point-by-point with the electron-beam emanating from the orifice 24 in electron-gun end-disc 13, and the electron-gun cathode 21 is made negative relative to screen 6 by an amount less than the voltage corresponding to the second cross-over point and greater than first crossover on Fig. 1. Consequently, the impact of the electron-beam on the latter sets free secondary electrons and imparts to screen 6 a positive charge which neutralizes the negative chargeimage existing on screen 6 at the point of impact. At any particular instant, electrons coming from screen 6 are repelled by the electric field aboutthe point on screen 6 upon which the electron-beam. is then incident and return'toward thedynode14 where they generate secondary electrons which flow by external circuits to the electron-gun cathode. The number of these repelled electrons varies with the charge at the point on screen 6 where the electron-beam is incident at any time and this results in a current which varies in time at output electrode 17 in correspondence with'the space,distributionof charge on screen 6, and hence with the lightdistribution in the picture focussed. on photoelectric input screen 5. The use of a current varying intime in correspondence with light distribution on, a, picture input screen to control a television transmitter is conventional in present day television systems, and is believed t require no further description here e V The secondary electrons generated at glass screen 6 by the electron beam from electron gun cathode 21 will be of relatively low energy and these alone will vary in time incorrespondence with the charge-distribution on screen .6. On the other hand any reflected primary electrons from that beam willbe of relatively high energy. The separating coil 25 acts to separate the low energy and high energy electrons since dynode 14 may be positioned so that it is shielded from the only slightly curved paths of high energy electrons reflected from screen 6, but will be struck by low-energy electrons whose paths are sharply curved by the field of the above-mentioned separating. coil 25. The signal-to-noise ratio of my present arrangement will thus be higher than that of conventional Image Orthicons. Furthermore the advantage results that noise is greatest when the signal is greatest, and

I claim as my invention:

. 1. A television picture pickup tube comprising aphotoelectric screen on which said picture may be focussed, a screen of glass whichemits secondary electrons when primary electrons are incident upon it, meansto accelerate electrons from said photoelectric screen into incidence with said glass screen with an energy greater than the second cross-over of the curve relating electron energy with secondary electron-emission for. said glass; means for producing an electron beam andimeans to cause it to scan said glass screen withelectronshaving anenergyless than said secondcross-over, electron mul iplier means laterally displaced from the, origin of said electron beam, and means to produce a magnetic field having a substantial component transverse to the undeflected path of said electron beam and in the region adjacent the input of said electron multiplier.

2. A pickup tube system comprising an'input screen of a type which emits electrons when irradiated, a target member of insulating material "which emits secondary electrons when primary electrons are incident upon it, a

first current source and connections from it to said screen and said target to accelerate electrons from said first screen into incidence with said target with an energy greater than the second crossover value on the curve relating electron energy to secondary electron emission for said target member to produce a charge image on the bombarded surface, 'a'second current source, an electron gun positioned on the opposite side of said target with respect to said input screen, a second current source connected to said, gun and said target for scanning said target with a beambf electrons from said gun having an energy less than second crossover value and greater than said first crossover value and means to collect secondary electrons which are emitted from the surface of said target during said scanning operation.

. 3. A pickup tube'system comprising an input screen of a type which emits electrons when irradiated, a target member of insulating material which emits secondary electrons from the bombarded surface when primary electrons are incident upon it, means for impressing a field between said input screen and target toac'celerate electrons'ifrom said input screen into incidencewith one surface of said target with an energy greater than the second crossover valueonflthe curve relating electron ener'g'y to'secondary' electron emission for said target to produce a charge image representative of the radiation on said surface of said target, said charge image setting up a-potential pattern on the opposite surface of said target, an electron gun provided on the opposite side of said target with respect to said input screen for generating an electronbeam to scan the potential patternonthe surface of said target, said electron beam having electrons of an energy less than said second crossover value and greater than said first crossover value and means to collect secondary electronsemitted from the scan surface of said target in response to bombardment of the target bysaid electron beam. i

4. A pickup tube system comprising an input screen of a type which emits electrons when irradiated, a target member of insulating materialwhi'ch emits secondary electrons from the bombarded surface when primary electrons are incident uponit, means for impressing a field between said input screen said target to accelerate electrons from said input 'screen'into incidence with one surface of said targetto produce a negative charge im:

age representative of the radiation on said surface of said target, 'said charge image setting up a potential pattern on the opposite surface'of' said target with respect to said input screen for generating an electron beam to scan the potential pattern on the surface of saidtarget with electrons of an energy less than saidsecond crossover value and greater than said first crossover value on the curve relating electron'energyto secondary electroniemission' for saidtarget'and means to collect'secondary electrons which are emitted from'the scannedsurface'of sa'id target during said' scanning operation and an electron multiplier means having its input responsive to said collected secondary electrons.

5. A pickup tube system comprising an input screen of a type which emits electrons when irradiated, a target member of insulating material which emits secondary electrons from the bombarded surface when primary electrons areincident upon it,"me'ans for impressinga field between;said'inputjscreen and said target to aceelerate electrons from said input sereeninto' incidence with one surface of said target with an energy greater than the second crossover value on the curve relating electron energy to secondary electron emission for said target to produce a charge image representative of the radiation on said surface of said target, said charge image setting up a potential pattern on the opposite surface of said target, an electron gun provided on the opposite side of said target with respect to said input screen for generating an electron beam to scan the potential pattern on the surface of said target having electrons of an energy less than said second crossover value and greater than said first crossover value, means to collect secondary electrons emitted from the scanned surface of said target during said scanning operation, said means comprising an electron multiplier, and means to produce a magnetic field having a substantial component transverse to the undeflected path of said secondary electrons to defiect said secondary electrons of low energy onto said collecting means.

6. A pickup tube system comprising a tube having a photocathode screen of a type which emits electrons when irradiated, a dielectric target positioned to intercept electrons emitted from said photocathode, means for accelerating the electrons emitted from said photocathode into incidence with one surface of said dielectric layer at an energy greater than the first and second crossover value on the curve relating electron energy to secondary electron emission for said dielectric material to produce a negative charge image on the surface of said dielectric layer facing said photocathode, said charge image setting up a potential pattern on the opposite surface of said di electric layer, means provided on the opposite side of said dielectric layer with respect to said photocathode for generating an electron beam having electrons of an energy less than second crossover and less than first crossover on the curve relating electron energy to secondary electron emission for said dielectric material, means for deflecting said electron beam to scan the potential pattern on the surface of said dielectric layer facing said electron gun, and means for collecting low energy secondary electrons emitted from the surface of said dielectric layer facing said electron gun in response to bombardment by said electron beam.

References Cited in the file of this patent UNITED STATES PATENTS 7 2,537,250 Weimer Jan. 9, 1951 2,544,753 Graham Mar. 13, 1951 2,544,754 Townes Mar. 13, 1951 2,723,360 Rotow Nov.'8, 1955 Freeman Jan. 17, 1956