US3437752A - Apparatus for electron smoothing in image dissector tubes - Google Patents

Description

2 R o T C E L L o C INFRARED SOURCE HOLLAND C. FORD INVENTOR.

ATTORNEY April 8, 1969 H. c. FORD APPARATUS FOR ELECTRON SMOOTHING IN IMAGE DISSECTOR TUBES Filed Sept. 29. 1965 LIGHT $2 SOURCE ELECTRON BOMBARDMENT INDUCED cououcnow DIELECTRIC THIN METAL PHOTOEMITTING emu FIG. I

FIG. 2

IIra/nunnalrlrnunanunaznunnnu .Ifd IIllllllllll/f/l/ll/IIIIII COLLECTOR l COLLECTOR I (THIN CONDUCTING LAYER OF METAL) gwA w 3,437,752 APPARATUS FOR ELECTRON SMOOTHING IN IMAGE DISSECTOR TUBES Holland C. Ford, Inglewood, Calif., assignor to the United States of America as represented by the Secretary of the Navy Filed Sept. 29, 1965, Ser. No. 491,467 Int. C1. 1101 29/00, 29/89 U.S. Cl. 1787.87 4 Claims ABSTRACT OF THE DISCLOSURE The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to a dissector type lowlight-level imaging tube, and more particularly to an electron emitting target for use in imaging tubes for both improving the signal to noise ratio of the electron image (i.e., smoothing) and providing intensification of the image (i.e., multiplying).

To enable a low-light-level imaging tube with a multiple aperture dissector readout to be used for such applications as map matching, a smoothing target is required; for dissector readout, the target must emit electrons. There are no known thin film targets at the present time which simultaneously emit electrons and provide a smoothing action. At present the only way one can achieve smoothing and emission is through the use of a phosphor photocathode sandwich, the phosphor having a light decay time suitable to give smoothing. The disadvantages of this means of achieving smoothing are numerous, but primarily the process of converting electrons to light and then back to electrons will necessarily degrade resolution more than an arrangement using only a single thin target, and in addition to the high costs of a phosphor and photocathode, the achievement of a light tight coupling between the two can be both difiicult and expensive, especially if a fiber optics coupling is necessary. Also, there would be definite problems in processing more than one photocathode within the same tube. Additionally, there would be problems of the photocathodes poisoning one another and the phosphor, or vice versa.

The apparatus of the instant invention includes a thin film sandwich smoothing, emitting, multiplying target consisting of an electron bombardment induced conduction dielectric sandwiched between a thin metal conductor and a thin metal photoemitting grid. This target used nited States atent O in an imaging tube gives the tube the capability of storing an image indefinitely in the target or integrating a stationary scene for any desired length of time.

It is an object of the invention to provide a novel method and apparatus for electron smoothing for a dis sector type low-light-level imaging tube.

Another object of the invention is to provide a novel target for low-light-level dissector type imaging tubes.

A further object of the invention is to provide a thin film sandwich imaging tube target having the capability to emit, smooth, multiply, integrate indefinitely and store information.

Other objects and many of the attendant advantages of this invention will become readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:

FIG. 1 is a cross sectional diagrammatic illustration of a thin film sandwich target of the present invention.

FIG. 2 is a schematic diagram showing the target of FIG. 1 within an imaging tube.

Referring now to the drawing, like reference characters refer to like parts in each of the figures.

The thin film sandwich smoothing, emitting, multiplying target 10, shown in FIG. 1, consists of a thin conducting layer of metal, collector 1, on which is placed a thin layer of dielectric 12 and in turn on which is placed a thin grid of photoemitting metal 14. Photoemitting mosaic 14 i formed by evaporating the photoemitter onto dielectric 12 through a screen. The dielectric material 12 is the type that exhibits electron bombardment induced conduction. Target 10 is similar to an EBIC (electron bombardment induced conduction) target (F. Anabacher and W. Ehrenberg, Proc. Phys. Soc. (London), 64A, 362-379 (1951); W. E. Spear, Proc. Phys. Soc. (London), 68B, 991-1000 (1955)) with an added grid 14 of photoemissive material. Some examples of EBIC material are AS 8 A1 0 mica, Pyrex, antimonide sulphide and KCl. As will be seen, however, the new target, of the instant invention, operates much differently than an EBIC target.

The arrangement of the target 10 within an image tube is shown schematically in FIG. 2. To describe the operation of the target 10, assume that initially there is no image on the photocathode 24. In this initial state there will be no electrons hitting collector 1 and the dielectric 12 will not conduct. The photons from the light source 22 falling on the photoemissive surface of grid 14 will create photoelectrons which are emitted therefrom and then collected by collector 2, the phosphor dissector readout. However, since the dielectric 12 is nonconducting the positive charge resulting from the emission of electrons will accumulate on the photoemitting mosaic 14 and stop the photoemission process. Suppose now an image is imaged on the photocathode 24. Photoelectrons will result and these will be accelerated and focused on collector 1 of the smoothing target 10. The energetic electrons penetrate the thin conducting layer of collector 1 and dissipate their energy in the dielectric 12 with the formation of holes in the valence band and electrons in the conduction band. Due to the potential across the dielectric 12, the holes will drift to collector 1 and combine with electrons, whereas the electrons will drift to the photoemitting mosaic 14 and combine with the positive charges there. When the electrons combine with the positive charges, the potential from the photoemitting mosaic 14 to the phosphor dissector readout, collector 2, will be modified to the extent that the number of electrons which have combined can be emitted. One can consider that the number of electrons which have reached the photoemitting mosaic 14 have been made available for photoemission. It is apparent that the gain of the target will be the number of electrons reaching the photoemitting mosaic 14 per incident electron.

The smoothing time of target 10 will be determined by the length of time it takes the electrons in the conduction band to drift to the photoemitter mosaic 14. In other words, there will be an output representing one input electron for the length of time that the conduction electrons created by the input electron are arriving at the photoemitting grid. If this length of time is a frame time, the probability of seeing a signal for the input electron is greatly increased and hence the signal to noise ratio is im proved. Theoretical treatment shows that the maximum increase in signal to noise ratio is /G, where G is the total number of electrons resulting from one photocathode electron. Hence a smoother can use gain prior to the smoother to increase the signal to noise ratio.

Since the smoothing time depends on the drift time of the electrons in the conduction band, it can be varied. One way of doing this is by varying the light 22 intensity and the potential from photoemitting mosaic 14 to collector 2, since the greater the positive charge on the photoemitting grid the greater will be the field driving the conduction band electrons toward the grid. If the light intensity is such that only one photon/sec./cm. is incident on the photoemitting side 14 of target 10, then the maximum electron emission rate cannot exceed 1 electron/sec./cm. This rate would obviously take a long time to discharge the target. On the other hand, if the intensity is such that 10 photons/sec./cm. are incident on the photoemitting grid side of target 10, the target will be discharged very rapidly. The smoothing time can be controlled by varying the potential of the grid proximity focused on the photoemitting side of the smoothing target 10. Because the photoemitting mosaic 14 tends to charge to the potential of the proximity focused grid 15, the potential of the grid 15 determines the electric field strength within the target and hence the time it takes the electrons in the conduction band to drift to the photoemitter. Thus, by varying the potential of grid 15 we vary the persistence of the conduction and hence the smoothing time. Hence, variation of these two quantities can be used to vary the smoothing time. Another means of controlling the smoothing time is through the use of electron traps in the dielectric 12. Since it is possible to dump electrons from traps into the conduction band by using infrared radiation corresponding to trap depth (Ellickson and Parker, Physical Review, 70, 290-299 (1946)), one can control the time electrons spend in traps and hence the smoothing time by varying the amount of infrared applied to the target 10 from a variable intensity infrared source 23.

The smoothing target 10 may be illuminated by a variable intensity infrared light. When an energetic electron is incident on the smoothing target, it excites several electrons from the valence band to the conduction band, thereby inducing conduction in the smoothing target. Part of the electrons excited by the incident electron will be trapped in energy levels between the valence and conduction band. The gradual release of these electrons from the traps results in prolonged conductivity. If the target is now illuminated by infrared light of the appropriate wavelength, the electrons in the traps will be supplied with enough energy to be released from the traps into the conduction band. This emptying of the traps speeds up the decay time and thus shortens the smoothing time. Thus by use of the infrared smoothing can be controlled.

Target 10 may be operated in a storage mode by first exposing the photoemitting grid 14 to light from variable intensity light source 22 to charge it positively and then turn off the light. The target may now be used to integrate and store as long as one wishes, the readout being accomplished by turning light 22 back on. The length of time necessary for readout could then be varied by varying the intensity of the light.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A dissector type smoothing low-light-level imaging tube system comprising:

(a) an evacuated envelope having transparent opposite ends,

(b) a photocathode adjacent one transparent end,

(c) a phosphor dissector readout image forming surface adjacent the other transparent end,

(d) a thin film sandwich type smoothing, emitting, multiplying target means positioned between said photocathode and said dissector readout image forming surface,

(e) said target means sandwich comprising a thin conducting layer of metal forming a collector which faces said photocathode, a thin layer of dielectric material that exhibits electron bombardment induced conduction and a thin metal photoemitting grid, said dielectric material being sandwiched between said collector and said photoemitting grid,

(f) electrons from said photocathode penetrating the collector of said target means dissipating their energy in the layer of dielectric material with formation of holes in the valence band and electrons in the conduction band of the dielectric material, the gain of said target means being the number of electrons reaching said photoemitting grid per incident electron from said photocathode to the target means, and the smoothing time of said target means being determined by the length of time required for electrons in the conduction band of said layer of dielectric material to drift to said photoemitting grid,

(g) means for controlling the smoothing time of said target means by varying the drift time of electrons in the conduction band of said dielectric material; wherein said imaging tube is able to store an image indefinitely in said target means and integrate a stationary scene for any desired length of time, and

(h) said means for controlling the smoothing time of said target means comprising variable intensity light source for continuously illuminating the photoemitting grid which in turn operates to control the drift of electrons in the layer of dielectric material of said target means by varying the light intensity.

2. A system as in claim 1 wherein said means for controlling the drift of electrons in the layer of dielectric material and thus the smoothing time of said target means includes a means for varying the potential between said photoemitting grid side of said target and said dissector readout image forming surface together with said variable intensity light source for illuminating the photoemitting grid.

3. A system as in claim 1 wherein said variable intensive light source for controlling the drift of electrons in the layer of dielectric material and thus the smoothing time of said target means is a variable intensity infrared light source for illuminating the photoemitting grid of said target with infrared light of appropriate wavelength to release trapped electrons, causing a speeding up of decay time and thus shortening smoothing time.

4. A system as in claim 1 wherein said thin metal 5 6 photoemitting grid is a photoemitting mosaic of evapo- 3,073,989 1/ 1963 Amsterdam 250-213 rated photoemitter metal to said dielectric layer. 3,128,406 4/1964 Goetze 315-11 References Cited FOREIGN PATENTS UNITED STATES PATENTS 0 675,973 12/1963 Canada.

2 7 59 1/19 4 szegho 315 ROBERT L- GRIFFIN, Primary Examiner.

2,826,632 3/ 195 8 wflimer 315-11 1'. A. ORSINO, JR., Assistant Examiner.

2,875,371 2/1959 Perkins 313-95 U S cl X R 2,898,499 8/1959 Sternglass 313-103 2,992,346 7/1961 Farnsworth 315-11 178-72; 250-213; 315-11, '12; 313-65, 95, 103

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