US3056062A - Thermal image converter - Google Patents

Description

Sept. 25, 1962 M. GARBUNY ETAL THERMAL IMAGE CONVERTER Filed Aug. 15, 1952 Semi Tronsporeni 2 Mirror INVENTORS Mox Gorbuny and John S. Tolbot Scanning Control 5 iqnol Generator Emissive Amplifier 4 WITNESSES: 94 K [mm United States PatentD 3,056,062 THERMAL IMAGE CONVERTER Max Garbuny and John S. Talbot, Pittsburgh, Pa., us-

siguors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 15, 1952, Ser. No. 304,502

14 Claims. (Cl. 315-11) Our invention relates to radiation detectors and, more particularly, to thermal image converters.

In accordance with prior art of which we are aware, thermal imageconverters have been suggested comprising a screen of low heat capacity capable of emitting more thermal electrons when heated by radiation, such as infrared, impinging thereon than when not so heated. Such a device is disclosed in application of E. D. Wilson, Serial No. 293,522, filed June 14, 1952, and assigned to Westinghouse Electric Corporation, which application is now abandoned. While such a device is valuable for some purposes, in other situations a more sensitive device will be desirable. i

It is, accordingly, an object of our invention to produce a highly sensitive thermal image converter.

Another object of our invention'is to provide a thermal image converter employing a combination of thermionic and photoemissive effects.

Another object of our invention is sensitive thermal image converter.

An ancillary object of our invention is to provide a thermal image converter capable of reproducing an infrared heat image.

' Still another object of our invention is to provide a thermal image converter which is capable of supplying its energydirect to a television transmitter.

The invention with respect to both the organization and the operation thereof, together with other objects and advantages may be best understood from the following description of specific embodiments when read in connection with the accompanying drawing, in which:

FIGURE 1 is a schematic showing of a thermal image converter built in accordancewith one embodiment of our invention;

FIG. 2 is a schematic showing of a thermal image converter wherein the light scanning is produced by means of a fluorescent screen and electron gun; and

FIG. 3 is a showing in perspective, partly in cross section and greatly enlarged of the spot defining grid employed in the apparatus shown in FIG. 2.

In accordance with our invention, we provide a vacuumtight envelope 4 having a transparent wall 6. Inside the envelope 4, there is a photoelectric screen 8 comprising a supporting sheet 10 of low heat capacity which is transparent to certain wave lengths of light but capable of absorbing thermal radiation. Coated on the supporting sheet 10.is a layer of photoemissive material 12, such as cesium-antimony. The supporting sheet 10' forms the means for converting the thermal radiation into a corresponding temperature distribution on the photoemissive layer 12. It is also possible inaccordance with another embodiment of our invention to employ an isolated sheet of photoemissive material covered with a film of infrared absorbing material such as gold black without the necessity for the supporting sheet 10, the chief requirements of the screen being that it have low heat capacity, that it be photoemissively responsive and that it be capable of absorbing thermal radiation. Also, inside of the envelope, there is provided a collector electrode 14 for receiving electrons eitherdirectly or indirectly from the photoemissive surface 12.

'In accordance with one embodiment of our invention, it may be desirable to employ secondary electron amplification, as by the use of electron multiplier electo' produce a highly I from the photoelectric screen z trodes 16, two of which electrodes are shown in the drawing. These two multiplier electrodes 16 are, of course, representative of a larger number of such electrodes which would probably be employed in practice. A source of potential 18 is connected between the photm electric screen 8 and the dynodes 16 of the electron multiplier so as to cause electrons to be accelerated from the photoelectric screen 8 toward the dynode electrodes 16.

Connected to the collector electrode 14 through an amplifier 20 is the grid 22 of a viewing kinescope 24. Electrons impinging on the collector electrode 14 thus produce a change in the potential of the grid 22 of the viewing kinescope 24, thereby producing a change in theintensity of the electron beam produced within the viewing kinescope 24. Background currents maybe eliminated by capacitor coupling or suitable biasing.

Means are provided for focusingan infrared or thermal image onto the photoelectric screen 8. This means may comprise any of several known types of focusing apparatus such as a Cassegrainian telescope collecting mirror 26 and a semi-transparent mirror 28. The semitransparent mirror 28 is located with respect to the Cassegrainian telescope mirror 26 so as to reflect the 'infrared radiation, which is focused by the Cassegrainian telescope reflector 26, onto thephotoelectric screen 8.

On the opposite side of the semi-transparent mirror 28 8, there is provided an auxiliary kinescope 30 adapted to produce alight scan on the screen thereof. The auxiliary kinescope 30 is directed toward the semi-transparent mirror 28, and. a lens 32 is provided between the screen of the auxiliary kinescope 30 and the semi-transparent mirror 28 for focusing an image of the kinescope screen onto the photoelectric screen 8. In accordance with'one embodiment of our invention, a filter 34 may also be provided in front of the screen of the auxiliary kinescope 30 for filtering out light radiation above a predetermined frequency. This predetermined frequency lies below or near the socalled absolute threshold of the photoemissive material and in general corresponds to radiation which has not quite sutficient energy to emit electrons from the photoemissive surface 12 when the photoemissive surface 12 is not thermally excited.

A scanning control signal generator 35 is provided to supply energy to the deflection coils 36, 38 (or, alternatively, deflection electrodes) of the auxiliary kinescope 30 and the viewing kinescope 24, respectively.

The operation of the apparatus shown in FIG. 1 is substantially as follows: An infrared image of terrain, for example, is focused onto the photoelectric screen 8 by the Cassegrainian telescope reflector 26 and thesemi-transparent mirror 28. The infrared image impinging on the photoelectric screen 8 heats certain elemental areas of the photoemissive surface 12, thereby forming a temperature pattern corresponding to the temperature pattern of the observed scene. The elemental areas which have been heated now have electrons with higher average kinetic energies than the average energies of the electrons in the unheated areas of the photoemissive surface 12. Therefore, light photons with frequencies lower than the threshold frequency of the photoemissive material will cause the ejection of more electrons from the areas of the photoemissive surface 12 which have been heated by the thermal radiation than from the unheated areas.'

The auxiliary kinescope 30 has been provided with a filter 34 so that the light produced by the auxiliary kinescope 30, which is allowed to impinge on the photoemissive surface 12, has a photon energy near or below the threshold at which thermally unexcited electrons can be emitted.' Therefore, light from the auxiliary kinescope 30 causes electrons to be emitted with a density that is a function of the temperature of the photoemissive surface Patented Sept. 25, 19 62 Z, i.e. the higher the temperature of the area due to the ifrared radiation, the greater the density of emitted elecons. By causing the cathode-ray beam of the auxiliary inescope 30 to scan the kinescope screen, a scanning light lOt is provided which may be imaged onto the photonissive surface 12. Thus, the photoemissive surface 12 scanned with alight spot capable of causing emission of ectrons from the elemental areas of the photoemissive lrface 12 in numbers .which correspond to the tempera- 1'6 of these elemental areas so that the resulting photolrrent, element for element, is modulated by the tem- :rature distribution of the surface 12.

When the scanning light spot impinges on an area of the iotoemissive surface 12 which has been heated by the frared, electrons are emitted thereby in numbers correronding to the temperature, which electrons are collected ther directly or indirectly by the collector electrode 14. he electrons impinging on the collector electrode 14 'oduce a current pulse which changes the potential of the id 22 of the viewing kinescope 24. In a preferred em- )diment of my invention, the current pulses are fed to e grid either over a suitable bias or capacity coupling that only the current fluctuations appear as signals. herefore, the intensity of the electron beam of the viewg kinescope 24 is controlled by the density of electrons nitted at any particular time by the photoemissive surce 12. Since a scanning control signal generator 35 is mnected to the deflection coils of both the auxiliary nescope 30 and the viewing kinescope 24, the scanning :tion of both of the kineseopes 30 and 24 is coordinated that the location of the electron-beam-produced spot 1 the screen of the viewing kinescope 24 will correspond the location of the scanning spot on the photoemissive irface 12. An image is, therefore, produced on the reen of the viewing kinescope 24 which corresponds the infrared image which is being focused onto the iotoemissive surface 12.

In accordance with a preferred embodiment of our in- :ntion, the heat capacity of the photoelectric screen 8 is [05611 so that most of the heat from an elemental area F the screen is dissipated in a period approximately equal the time required for the scanning of the screen 8 by e light spot. Thus, when infrared radiation impinges 1 an elemental area of the screen, it continues to heat at elemental area during the entire scan or frame time the light spot. There is thus produced a storing feet whereby the energy from the infrared is stored up er a complete scanning cycle but is emitted in the form electron kinetic energy only during that short interval time when the scanning light spot impinges on that :mental area. In FIGURE 2, there is shown another embodiment of Ir invention wherein the light scanning spot is produced means of an electron gun 40 and a fluorescent screen substantially adjacent the photoelectric screen 44. Acrdingly, an envelope 46 is provided having an electron n 40 therein near an end thereof and a fluorescent reen 42 is provided which is capable of emitting light response to electrons impinging thereon. Near the iorescent screen 42 and substantially parallel thereto is photoelectric screen 44 of low heat capacity. Between e fluorescent screen 42 and the photoelectric screen 44 ere may be provided a threshold filter 48 for filtering .t light photons having energies near or above the photo reshold of the emissive material of the photoelectric teen 44. Also, between the fluorescent screen 42 and the photo- :ctric screen 44 there may be provided a spot-defining id 50. The spot-defining grid, as shown in FIG. 3, comises a sheet 52 of material opaque to the light emitted the fluorescent screen 42 and having holes 54 extendtherethrough perpendicular to the surface thereof. lese holes 54 should have a diameter about equal to the ickness of the grid 50. The spot-defining grid 50 prevents light emitted from one elemental area of the fluorescent screen 42 from striking elemental areas of the photoemissive surface 44 other than that elemental area of the photoemissive surface 44 which is directly opposite its source on the fluorescent screen 42.

A window 56 is provided in the end of the envelope 46 opposite the photoelectric layer 44, which is transparent to infrared radiation, and a Cassegrainian telescope reflector 58 is provided for focusing infrared radiation through that window onto the photoelectric screen 44. A collector electrode 60 is provided to collect electrons emitted by the photoelectric screen 44 and transmit pulses in response thereto to a viewing kinescope in the manner shown in FIGURE 1. If the collector electrode 60 comprises a large sheet of conducting material as shown in FIG. 2, it is desirable that it be constructed of a material which is transparent to infrared radiation.

Although we have shown and described specific embodiments of our invention, we are aware that other modifications thereof are possible. Our invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and the spirit of the invention.

We claim as our invention: I

1. In combination: a vacuum-tight envelope, a fluorescent screen inside said envelope, an electron gun inside said envelope directed toward said screen, scanning means associated with said gun for causing the electron beam produced by said gun to scan said screen, a low heat capacity photoelectric screen comprising a photoemissive layer and means for converting a thermal radiation image into a corresponding temperature distribution on said photoemissive layer, said photoelectric screen being substantially parallel to said fluorescent screen and located near said fluorescent screen so that light emitted by said fluorescent screen impinges on said photoelectric screen, a light filter between said fluorescent screen and said photoelectric screen to limit transmitted light to that having a frequency below the threshold of said photoemissive layer, a spot defining grid between said fluorescent screen, and said photoelectric screen comprising a sheet of opaque material having a plurality of holes therein extending substantially perpendicular to the surface of said sheet, and a collector electrode inside said envelope for collecting electrons emitted by said photoelectric screen, said electrons being emitted in accordance with said temperature distribution on said photoemissive layer.

2. An infrared detector comprising a photoelectric screen on which a thermal radiation image is focused, said screen comprising a layer of photoemissive material and means for converting said radiation image into a corresponding temperature distribution image on said photoemissive layer, and means for scanning said photoemissive layer with light of such wave length for which said photoemissive layer shows a variation of photoemission with temperature variation of said photoemissive layer.

3. An infrared detector comprising a thin photoemissive layer on which a thermal radiation image is focused and is converted into a corresponding temperature distribution image on said photoemissive layer, means for scanning sequentially point by point said photoemissive layer with a light beam below a selected frequency and said photoemissive layer being of a heat capacity so that most of the heat of an elemental area is dissipated in a time required to scan said photoemissive layer with said light beam, said photoemissive layer being operable to emit electrons in accordance with said temperature distribution image.

4. An infrared detector comprising a thin layer of 0 photoemissive material, means for focusing a thermal radiation image on said photoemissive layer so as to set up a corresponding temperature distribution image on said photoemissive layer, and means for scanning said photoemissive layer with a light beam of such wave length 5 for which said photoemissive layer shows a strong variation of photoemission with temperature variation.

5. An infrared detecting system, comprising evacuum-tight envelope having a window therein transparent to infrared and visible radiations, a photoelectric screen on which a thermal image is focused positioned within said envelope and parallel to said window, said screen comprising a'layer of. photoemissive material and means for converting said thermal image into a corresponding temperature distribution image on said photoemissive layer, and means positioned in 'front of saidwindow for focusing said thermal image through said window onto said photoelectric screen and light scanning means positioned in front of said window for scanning a raster on said photoelectric screen, said photoemissive layer being operable to emit electrons in accordance with said temperature distribution image.

6. An infrared image system comprising an envelope having a window therein transparent to infrared and visible radiations, a photoemissive layer positioned within said envelope and parallel to said window, focusing means positioned exterior to said envelope and in front of said window for focusing an infrared image on said photoemissive layer, and light scanning means also positioned exterior to said envelope and in front of said window for scanning said photoemissive layer with a light beam of such wave length for which said photoemissive layer shows a strong variation of photoemission with temperature variation.

7. An infrared imaging system comprising an envelope having a window therein transparent to infrared and visible radiations, a photoemissive layer positioned within said envelope and parallel to said window, a collector positioned within said envelope on the opposite side of said photoemissive layer with respect to said window, focusing means positioned in front of said window for focusing an infrared image onto said photoemissive layer so as to form a corresponding temperature distribution image on said photoemissive layer, means for scanning said photoemissive layer with a light beam from a cathode ray tube positioned in front of said window, and filtering means positioned in front of said cathode ray tube limiting the wave length of said scanning light beam to that which said photoemissive layer shows a variation of photoemission with temperature variation.

8. In combination, a photoelectric screen of low heat capacity, said photoelectric screen comprising a photoemissive layer and means for converting a thermal radiation image into a coresponding temperature distribution on said photoemissive layer, light scanning means having a control means, said light scanning means being directed toward the surface of said photoemissive layer, and an electron collector electrode near said photoelectric screen for collection of electrons emitted therefrom, said electrons being emitted in accordance with said temperature distribution on said photoemissive layer.

9. In combination, a screen of low heat capacity, said screen comprising a photoemissive layer and means for converting a thermal radiation image into a corresponding temperature distribution on said photoemissive layer, light scanning means directed toward the surface of said photoemissive layer, and an electron collector electrode near said photoemissive layer for collection of electrons emitted therefrom, said electrons being emitted in accordance with said temperature distribution on said photoemissive layer.

10. In combination a thin supporting infrared-absorbing support layer of low heat capacity, a layer of photoemissive material coated on said support layer, said support layer comprising means for converting a thermal radiation image into a corresponding temperature distribution on said photoemissive layer, light scanning means having a scanning control means located so as to scan the surface of said photoemissive layer, an electron collector electrode near said layer of photoemissive material for collection of electrons emitted therefrom, said electrons being emitted in accordance with said temperature distribution on said photoemissive layer, electronir picture-reproducing means having an intensity control and a picture-reproducing scanning control, electrical connections between said collector electrode and said in tensity control, and meaning signal generator connected to'said scanning control of said light scanning mean: and to said picture-reproduceing scanning control.

11. In combination, a screen comprising a thin con tinuous infrared-absorbing support layer of low heat capacity and a layer of photoemissive material coated on said support layer, said support layer comprising means for converting a thermal radiation image into a corresponding temperature distribution on said photoe missive layer, light scanning means located so as to scar the surface of said photoemissive layer, an electron col lector electrode near said layer of photoemissive material for collection of electrons emitted therefrom, said elec' trons being emitted in accordance with said temperature distribution on said photoemissive layer, and infrared foscusing apparatus directed at said screen.

12. In combination: a vacuum-tight envelope, a fluorescent screen inside said envelope, an electron gun inside said envelope directed toward said screen, scanning means associated with said gun for causing the electror beam produced by said gun to scan said screen, a lov heat capacity photoelectric screen comprising a photoemissive layer' and means for converting a thermaf radiation image into a corersponding temperature distribution on said photoemissive layer, said photoelectrir screen being substantially parallel to said fluorescen screen and located near said fluorescent screen so tha light of a frequency below the absolute threshold of sair photoemissive layer emitted by said fluorescent screer impinges on said photoemissive layer, and a collectoi electrode inside said'envelope for collecting electron: emitted by said photoemissive layer, said electrons being emitted in accordance with said temperature distribution on said photoemissive layer.

13. In combination: a vacuum-tight envelope having a window therein transparent to infrared radiation, fluorescent screen inside said envelope near said window an electron gun inside said envelope directed toward saic screen, scanning means associated with said gun for caus ing the electron beam produced by said gun to scan sait screen, a low heat capacity photoelectric screen com prising a photoemissive layer and means for converting a thermal radiation image into a corresponding tempera ture distribution on said photoemissive layer, said photo electric screen being substantially parallel to said fiuo rescent screen and located between said fluorescen screen and said window so that light of a frequency nea the absolute threshold of said photoemissive layer emitter by said fluorescent screen impinges on said photoemissivi layer, and a collector electrode inside said envelope ii the region between said photoemisive layer and saic window for collecting electrons emitted by said pho toemissive layer.

14. An infrared detector comprising: a photoelectril screen having a low heat capacity, said photoelectri screen comprising a photoemissive layer and means fOl converting a thermal radiation image into a correspond ing temperature distribution on said photoemissive layer infrared focusing means directed toward said screen light scanning means directed toward said photoemissivi layer, said light scanning means emitting light of slight]: lower frequency than the photo threshold of said pho toemissive layer at the ambient temperature so that elec trons are emitted from said photoemissive layer in ac cordance with said temperature distribution.

References Cited in the file of this patent UNITED STATES PATENTS 7 8 UNITED STATES PATENTS 661,162 Great Britain Nov. 21, 1951 150,159 Gray Mar. 14, 1939 OTHER REFERENCES Cage 1946 Goerljch- Measurenae fits on Com poslte Photo-Cath- 4901o52 Harms 1949 5 odes, Zeit fur Physik, Vol. 109, pages 374-386 (1938). 518,761 F 1952 Morton et a].: An Infrared Image Tube and Its Military 519,531 Wflghmn 1952 Applications, R.C.A. Review (September 1946), vol. 7,

pp. 385-413. FOREIGN PATENTS Golay: A Pneumatic Infra Red Detector-Review of 541,959 Great Britain Dec. 19, 1941 Scientific Instruments (May 1947), Vol.18, pp. 357-662.

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