US2683832A - Image pickup electron tube - Google Patents

Description

y 1954 B. J. EDWARDS ET AL IMAGE PICKUP ELECTRON TUBE Filed Oct. 7, 1949 +2 70 MFA/H52 Inventor BADEN JOHN EDWARDS DAV/D MALCOLM JOHNSTONE Attorneys Patented July 13, 1954 UNITED STATE TENT OFFICE IMAGE PICKUP ELECTRON TUBE Application October 7, 1949, Serial No. 120,103

10 Claims. 1

This invention relates to photo-electric tubes, such as are used for electronic reproduction of optical images and for electronic generation of video signals from optical images for television transmission, having a photo-emissive cathode and a target arranged so that an optical image may be focussed upon the photo-cathode and the electrons emitted therefrom may be projected as an electron image on to the target. The nature of the target depends upon the purpose which the tube is to serve. One genus of tube of the general character referred to is the image converter, in which the electron image is projected on to a fluorescent screen so as to reproduce thereon an optical replica of the original optical image. Image converter tubes have various applications, such as for image intensification and for converting invisible images into visible images. Another genus of tube of the general character referred to is the image analyzer (or television pick-up) tube of the type in which a video signal is developed by scanning with an electron beam a target plate or mosaic on which is stored a charge image formed by projecting onto the target the electron image from the photo-cathode. Examples of such image analyzer tubes are those known as the Image Iconoscope and the Image Orthicon. The present invention has application equally to both the image converter and image analyzer classes of tubes above discussed.

An object of the present invention is to pro vide an improved photo-electric image-forming tube of the general character referred to, which may be constructed in either of the image converting and image analyzing versions and which possesses substantially greater sensitivity compared with tubes of corresponding types known heretofore.

Another object is to provide an improved photoelectric tube in which a stage of gain is inserted prior to the conversion of an electron image into an optical image in the case of a tube constructed as an image converter, and prior to the scanning process in the case of a tube constructed as an image analyzer. Another object is to obtain image multiplication or gain by the application to such tubes of a novel principle, to wit, the use of a normally insulating substance as a current amplifier by bombarding the same with electrons.

Another object is to provide a novel imagemultiplier target for a photo-electric image reproducing or analyzing tube.

It has been discovered that certain substances, and in particular certain crystalline substances such as the diamond, which normally behave as electric insulators, have the property of becoming conducting when bombarded by gamma rays or electrons. For example, if a diamond is mounted in vacuo between two electrodes maintained at a sufficient potential difference and an electron beam is directed on to the crystal, a current will flow between the electrodes through the crystal according to the value of the potential difierence and the intensity and velocity of the incident electrons, and this current may be many times greater than the current in the beam. The present invention makes practical application and utilization of this phenomenon to increase the sensitivity of photo-electric tubes of the character described.

According to the present invention, a tube is provided with a photo-cathode arranged so that an optical image may be projected thereon, and with a target comprising a normally insulating substance having the property above described arranged so that an electron image formed by photo-electrons emitted from the photo-cathode under the influence of the incident optical image may be projected on to one side of the target, the said substance being provided with a transversely non-conducting electron-emissive surface at the opposite side of the target, and the tube also being provided with a second target arranged so that there may be projected thereon an electron image formed by electrons emitted from the emissive surface of the first-mentioned target. The tube is also provided with electrodes associated with the first-mentioned target (which may be termed the image amplifying, intensifying or multiplying target) for establishing a potential difierence necessary for obtaining current conduction through the said substance of that target under the influence of electron bombardment.

In operation, the emissive surface of the first target is caused to emit electrons. An optical image is projected upon the photo-cathode and causes an intensity of emission of photo-electrons from each point of the photo-cathode correresponding to the light intensity of the corresponding point of the incident image, thus developing an electron image which is projected, under the influence of a suitable field, on to the aforesaid substance of the first target. The electrons leaving each point of the photo-cathode and impinging on a corresponding point or" the said substance cause a conduction current to flow through said substance from that point thereof to a corresponding point of the emissive surface of the target, the conduction current corresponding in degree to the incident electron current intensity but being greatly amplified in relation thereto. The conduction current flowing to any given point of the electron emitting surface of the target causes the electron current emitted from that point to assume the same value. In this way, incidence of an electron image on one side of the target develops a greatly intensified electron image at the opposite, emitting side of the target and, under the influence of a suitable field, this electron image is projected upon the second target, which may be a fluorescent screen if the tube is constructed as an image converter, or may be a storage plate or mosaic if the tube is constructed as an image analyzer.

The invention is susceptible of embodiment in various constructions of tube according to the purpose for which the tube is intended to be used, and according to the scanning process employed in the case of image analyzer tubes, but in order that the invention, both as to its organisation and its method of operation, may be more clearly understood, it will be exemplified by the embodiments illustrated in the accompanying drawing, in which Fig. l diagrammatically depicts one example of image converter tube embodying the invention.

Fig. 2 diagrammatically depicts one example of image analyzer tube embodying the invention, and

Fig. 3 is a diagrammatic fragmentary view depicting a modification.

The image converter tube depicted in Fig. 1 comprises a generally cylindrical envelope l2 containing a transparent photo-cathode li, a first (image intensifying) target it and a second target comprising a fluorescent screen 53, all disposed in parallel relation spaced along the envelope axis and perpendicular thereto, with the target 52 between the photo-cathode ii and fluorescent screen it which are at opposite ends of the envelope and may be formed on its respective end walls.

The target 22 comprises a substance having the property hereinbefore described, in the form of a layer or plate M which is coated on the side facing the photo-cathode l l with a thin electronpermeable conducting metallic film l5, and is coated on the opposite side with an electronemissive film 56 which is nonconductive in a direction parallel to its surface. The latter may be photo sensitive and be caused to emit photoelectrons by illumination with a suitable light source l'i which may be arranged outside or inside the envelope ll). A shield l8 may be provided to prevent background illumination of the fluorescent scr en if by the source ll, but if a metallised fluorescent screen is employed, the shield is is unnecessary. The source I? is also prc vented from illuminating and thus exciting the photo-cathode ll. Accordingly, it is proposed to arrange the source ii, for instance, in the form of a filamentary ring within the envelope Hi as shown, so that the. photo-cathode H is shielded from the source i'i by the intervening target 52. The combination of photo-emissive film it light source ii is preferred, but as an alternative, the surface HE could be made sec ondary emissive and the tube provided with a subsidiary electron source suitably arranged for bombarding the surface it with electrons in order to cause secondary emission therefrom.

The envelope Ill also contains a fine mesh screen electrode 59 facing the emissive surface IE5 of the target l2 and spaced closely thereto. The envelope I0 is surrounded by a coaxial focus coil 29 which extends from the photo-cathode Ii the fluorescent screen l3 and produces strong uniform axial magnetic field for forming an electron image of the emission from the photo-cathode i l on the target i2 and for forr'ing an elec tron image of the emission from emitting side [6 of the target l on the fluorescent screen l3.

In the operation of the tube, the emissive side or film N5 of the target i2 is caused to emit electrons by illumination thereof from the light source ll. In consequence, the emitting surface it assumes a potential at or near that of the mesh electrode is which is made positive with respect to the electrode film it: at the other side of the target. It is proposed to connect the electrode E5 to ground (zero) potential and to impress upon electrode l9 apotential of approximately 30 volts positive. An optical ige of the subject for reproduction is projected on to the photo-cathode ll through the adjacent end wall of the envelope l9 by means of a lens or optical system 2 I. It is proposed to iin recs upon the photo-cathode a potential of approximately 1000 volts negative. The electron obtained by emission from the photo-cathode l i and corresponding to the optical image projected thereon, is projected from the photo-cathode on to the target l2 so that the electrons of the electron image have sufiicient velocity to penetrate the electrode film l5 and bombard the ayer 45, to render the aforesaid substance thereof conducting to an extent at different points thereof depending on the incident electron intensity at those points. Conduction currents will thus fiow from the various points of the electrode it through the intervening substance i l to corresponding points of the emitting surface it of the target 52, and the potential of any given elementary part of the said substance comprising layer it through which current flows will dec ease by reason of the current flow therethrough until an equilibrium is established such that the emission current from the corresponding point of the emitting surface 56 is equal to the conduction current through the substance to that point of the surface it. Since, by virtue of the currentamplifying property of the su stance comp sing layer M, the conduction current therethrough is much greater than the incident electron image current, the emission current leaving the emitting surface lfi, which will correspond in tude to the electron image current incident upon layer i l, will also be much greater than that current. The fluorescent screen 33 posi tive with respect to the mesh electrode l2. It is proposed to impress upon screen 53 a potential in the range of approximately 2C4 940060 volts positive. The electrons emitted from the surface 55 are drawn through the mesh electrode 59 to form an electron image on the fluorescent screen l3 which converts the impinging electron image into a visible optical image.

As will be appreciated by those skilled in the art, in comparison with image converter t cos of conventional type in which the electron image developed from a photo-cathode (similar to ll) is directly transformed into an optical image by direct projection upon the fluorescent screen (corresponding to 3) the present invention introduces an intermediate stage of image gain at the target l2 prior to the transformation process at screen l3.

The same principle may be applied in like manner prior to the scanning process in image analyzer tubes of the kind hereinbefore mentioned. That is to say, a stage of image gain is introduced at a target similar to i 2 inserted between the photo-cathode (similar to ii and the scanned charge storage target upon which, in the heretofore known tubes, the electron image developed from the photo-cathode was directly projected. Thus, in image analyzer tubes according to the present invention, the scanned storage target becomes what is hereinbeiore termed the second target, and, in comparison with the image converter of Fig. 1, takes the place of the fluorescent screen 13. To illustrate the foregoing, Fig. 2 depicts an image analyzer tube embodying the invention, the tube in other respects being based on the known principles of construction and operation of the Image Orthicon.

In the tube in Fig. 2, the envelope lit, the photo-cathode iii, the first target H2 comprising current-amplifying layer lid, film electrode H5 and emitting film H5, the light source ill and the mesh electrode 5 59 are respectively similar to, and similarly arranged as, the parts iii. ll, i2, i i5, 16, El and it in Fig. 1, except that the envelope He is sufiiciently longer to accommodate means for scanning the storage target 22 which takes the place of the fluorescent screen iii in Fig. 1. In the tube in Fig. 2, a shield corresponding to shield is in Fig. l is not required, it being only necessary to ensure (similarly as in Fig. 1) that the light source i i? does not illuminate the photo-cathode iii.

The described operation of the tube in Fig. 1 applies equally to the corresponding parts of the tube in Fig. 2, with the difference that, in this case, the intensified electron image developed by the emission from surface iii; of the target H2 is projected upon one side of two-sided secondary-e .itting target 22 such as used in the Image Orthicon. Ihe projected electron image forms a charge image on that side of the target 22 by secondary emission from the latter. A fine-mesh metallic target screen 23 spaced closely to the image side of the target 22 collects the secondary emission from the latter, and also serves as a common capacity member for all the elements of the charge image on the target 22. With potentials on the photo-cathode ill and the el ctrodes M5 and lid of the order men tioned in connection with 1, it is proposed to impress upon the target screen 28 a potential of approximately 600 volts positive.

The target 22 is scanned from the opposite side by an electron beam 26 of the low-velocity type generated by an electron gun 25. From the gun to the target 22, the beam M is in a strong uniform axial magnetic field, the same as focuses the electron image from the photo-cathode ill upon the target H2 and the intensified electron image from target H2 upon target 22. Accordingly, for producing this axial field the envelope Hill is surrounded by a coaxial focus coil 2923 which, in part, corresponds to and serves the same purpose as the coil 29 in Fig. 1, but in the present case extends from the photo-cathode iii beyond the target 22 substantially up to the electron gun 25 in order also to focus the scanning beam. The beam 24 is shot from the gun 25 into the axial magnetic field parallel to its lines of force and is deflected by suitable means, of which a deflecting coil 26 is shown, so that the beam emerges from the deflecting means cathode and comprising a normally insulati g parallel to its original direction under the influence of the axial magnetic field and arrives with perpendicular incidence and substantially zero velocity at all points of the target 22, in accordance with the known principle of lowvelocity scanning. In operation, the cathode 2? f the gun 25 is given the same potential as the target screen 23. As the arriving beam 24 neutralizes the charges on the target 22, the return beam 23 is modulated by the charge pattern. Accordingly, the signal is derived from the return beam current arriving at a collector 29. If desired, electron multiplication of the signal current may be accomplished by making the collector 22s the first stage of an electron multiplier.

It will be understood that application of the invention to an image analyzer tube utilizing constructional and operating characteristics of the Image Orthicon is purely illustrative. It will be apparent to those skilled in the art that the invention may equally readily be embodied in an image analyzer tube which in other respects is based on the constructional and operating characteristics of the Image Iconoscope.

In the modification depicted in Fig. 3, the target 212 corresponds to and serves the same purpose as the target #2 in Fig. l and target 5 ii in Fig. 2. The target 2E2 has a core consisting of a fine metal mesh 215 corresponding to and serving the same purpose as the metallic electrode film 15 in Fig. 1 and H5 in Fig. 2. In the interstices of the mesh 215 are secured particles 2M of a substance having the property hereinbefore described, such as diamond dust, corresponding to and serving the same purpose as the layer i i in Fig. 1 and H4 in Fig. 2. On one side of the target structure 2E5, 2H3 is formed a photo-emissive film 2H6 corresponding to the film it in Fig. 1 and H6 in Fig. 2. Although the film 21B is essentially required on the surface of the particles 25% at the one side of the target 212, the film Zlt may, in practice, he formed over the whole surface (including that of the mesh 2E5) at that side of the target, since it is an essential characteristic of the film 2H6 that it be non-conducting transversely. The formation of film ZiS may proceed in accordance with well-known technique for producing thin-film transparent photo-cathode structures (such as i! in Fig. 1 and ill in Fig. 2) which conduct from one side of the film to the opposite but are non-conducting transversely. For instance, a very thin, transparent film of such as silver or antimony may be deposited, on

.one side of the target structure 2i5, 2M and the film subsequently photo-sensitized with, e, g. caesium.

The operating action of target M2 is the same as that of target [2 in Fig, 1 or H2 in Fig. 2.

I claim:

1. Photo-electric apparatus comprising a photo-emissive cathode adapted for projection of an optical image thereupon to release photoelectrons therefrom, a target facing said photcsubstance capable, under bombardment by a on rent of electrons, of conducting a current which is amplified with respect to the incident current, electrode means associated with said target for establishing a potential dilTerence between opposite sides of said substance, means for accel ating and focussing photo-electrons reicas I from said photo-cathode to form an electron image on the side of said target facing said photo-cathode, whereby under impact upon said substance of the target by said photo-electrons conduction currents of varying degrees proportional to the intensity of said electron image are caused to flow through said substance from said one side to the opposite side thereof, said target having an electron emissive surface at said opposits side, means for causing said emissive surface continuously to emit electrons, whereby under the influence of the conduction currents flowing to said surface the emission of electrons therefrom varies correspondingly to the intensity of the arriving conduction currents, a second target spaced from said first-mentioned target and on the said opposite side thereof, and means for forming upon said second target an electron image of the electron emission from said emissive surface.

2. Apparatus as defined in claim 1, said emissive surface being photo-emissive, and means comprising the first target, adapted to screen said light source from said photo-cathode.

3. Photo-electric apparatus comprising a photo-emissive cathode adapted for projection of an optical image thereupon to release photo-electrons therefrom, a target facing said photo-cathode and comprising a normally insulating substance capable under bombardment by a current of electrons, of conducting a current which is ampliwith respect to the incident current, electrode means associated with said target for establishing a potential difference between oppos sides of said substance, means for accelerating and focussing photo-electrons released from said photo-cathode to form an electron image on the side of said target facing said photo-cathode, whereby under impact upon said substance of the target by said photo-electrons conduction currents of varying degrees proportional to the intensity of said electron image are caused to flow through said substance from said one side to the opposite side thereof, said target having an electron emissive surface at said opposite side, means for causing said emissive surface continuously to Clllli] electrons, whereby under the influence of the conduction currents flowing to said surface the emission of electrons therefrom varies corrcspondingly to the intensity of the arriving conion currents, a second target spaced from said -mentioned target and on the said opposite side thereof, means for projecting upon said :econd target an electron image of the electron emission from said emissive surface, whereby sec ondary electrons are released from said second to store charges thereon, means for collecting the released secondary electrons, and means scanning said second target with an electron n to discharge the stored charges from said second target.

4 Apparatus as defined in claim 3, said emissive ace being photo-emissive, and means comprising the first target, adapted to screen said light source from said photo-cathode.

5. Photo-electric apparatus comprising a phoio-emissive cathode adapted for projection of an ical image thereupon to release photoelecons therefrom, a target, means for accelerating and jocussing photo-electrons released from said "photomathode to form an electron image on one of said target, said target comprising a noruv insulating substance capable of becoming ducting under electron bombardment, an electrode constituting a cathode in contact with said substance, and an electron emissive surface on said substance at the opposite side of said. target and separated from said cathode by inter- 8 vening portions of said substance, means for causing, said emissive surface continuously to emit electrons, a fine mesh screen electrode constituting an anode at said opposite side of said target and closely spaced thereto for causing said emissive surface of the target to assume a potential in the neighborhood of that of said anode, whereby to establish a potential difference through said portions of said substance between said cathode and said emissive surface so that under impact of said photo-electrons upon said substance of said target conduction currents of varying de grees proportional to the intensity of said electron image are caused to flow through said portions of said substance from said one side to said emissive surface at said opposite side to vary electron emission from said surface correspondingly to the intensity of the arriving conuction currents, at second target spaced from first-mentioned target and on the said oppo- -itc side thereof, and means for forming upon id second target an electron image of the electron emission from said emissive surface.

6. Apparatus as defined in claim 5, said emissive surface being photo-emissive, and means comprising the first target, adapted to screen said light source from said photo-cathode.

'7. Photo-electric apparatus comprising a photo-emissive cathode adapted for projection of an optical image thereupon to release photo-electrons therefrom, a target, means for accelerating and focussing photoelectrons released from photo-cathode to form an electron image on one side of said target, said target comprising a normally insulating substance capable of becoming conducting under electron bombardment, an electrode constituting a cathode in contact with said substance, and an electron emissive surface on said substance at the opposite side of said target and separated from said cathode by intervening portions of said substance, means for causing said emissive surface continuously to omit electrons, a fine mesh screen electrode constituting an anode at said opposite side of said target and closely spaced thereto for causing said emissive surface of the target to assume a potential in the neighborhood of that of said anode, whereby to establish a potential difference through said portions of said substance between said cathode and said emissive surface so that under impact of said substance of said photoelectrons upon said target conduction currents of varying degrees proportional to the intensity of said electron image are caused to flow through said portions of said substance from said one side to said emissive surface at said opposite side to vary the electron emission from said surface correspondingly to the intensity of the arriving conduction currents, a second target spaced from said first-mentioned target and on the said opposite side thereof, means for projecting upon said second target an electron image of the electron emission from said emissive surface, whereby secondary electrons are released from said second target to store charges thereon, means for coliecting the released secondary electrons, and means for scanning said second target with an electron beam to discharge the stored charges from second target.

8. Apparatus as defined in claim 7, said emissive surface being photo-emissive, and means comprising the first target, adapted to screen said light source from said photo-cathode.

9. Photo-electric apparatus comprising a photo-emissive cathode adapted for projection of an optical image thereupon to release photo-electrons therefrom, a target said photo-cathode and comprisin a plate of a normally insulating substance capable of becoming conducting under electron bombardment, electrode constituting a cathode applied to the side of d plate facii said ph to-cathode and. an ele ion ve layer a lied to the opposite side of v 01 plate for uing electrons from said opposite side, means for causing said e-missive layer continuously to emit electrons, means for accelerating ant iocussing photo-electrons released from said photo-cathode to form an electron image on said side of said plate facing said photo-cathode, a fine mesh screen electrode constituting anode at said opposite side of said plate and closely spaced thereto for causing said emissive surface to assume a potential in the neighborhood t of said anode, whereby to a potential difference through said substance between said cathode and emissive surface so that under impact of said photo-electrons upon said substance of said conduction currents of varying degreeproportional to the intensity of said electron are caused to flow through said substance from said one side to said emissi've surface at said opposite side to vary the electron emission from said surface correspondingly to the intensity of the arriving conduction currents, a second ta" spaced from said first-inentioned target and on opposite side thereof, and means for forming upon said second target an electron image of the electron emission from said einissive surface.

10. Photo-electric apparatus comprising a photo-emissive cathode adapted for projection of an optical image thereupon to release photoelectrons therefrom, a target facing said photocathode and comprising a plate of a normally insulating substance capable of becoming conducting under electron bombardment, an electrode constituting a cathode applied to said side of said plate facing said photo-cathode and an electron emissive layer applied to the opposite side or" said plate for emitting electrons from said opposite side, means for causing said emission layer continuously to emit electrons, means for accelerating and iocussing photo-electrons releasedfrom said photo ca'thode to form an e1ectron image on said one side of said plate, a fine mesh screen electrode constituting an anode at said opposite side of said plate and closely spaced thereto for causing emissive surface to assome a potential in the neighborhood of that or" said anode, wher y to establish a potential difi'erence through said substance between sai cathode and substance of said em ssive surface so that under impact of said photo-electrons upon target conduction currents of v rying degrees proportional to the intensity said electron image caused to flow through suhstance from said one side to said emissive surface at said opposite side to vary the electron emission from said surface correspondingl the intensity of the arriving conduction currents, a sec ond target spaced from said first-mentioned target and on said opposite side thereof, means for projecting upon said second target an electron image of the electron emission from said emissive surface, whereby secondary electrons are released from said second target to store charges thereon, means for collecting the released secondelectrons, and means for scanning id second target with an electron beam to discharge the stored charges from said second target.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2.177360 Busse Got. 24, 1939 2,258,294 Lubszynski et a1 Oct. '7, 1941 2,324,505 Iams et a1 July 20, 1943 2,537,388 Wooldridge Jan. 9, 1951 2,544,753 Graham Mar. 13, 1951 2,544,754 Townes Mar. 13, 1951 2,544,755 Johnson et a1 Mar. 13, 1951 2,550,316 Wilder Apr. 24, 1951

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