US2999177A - Infra-red-sensitive mosaic - Google Patents

Description

Sept. 5, 1961 F. E. NULL ETAL INFRA-REn-SENSITIVE MosAIc Filed Aug. 19, 1946 IN V EN TORS w l 6 MMMM/ m f z ya. f NM N E 5w o YM. W MMV. H B

United States Patent D F 2,999,177 INFRA-RED-SENSITIVE MOSAIC Fay E. Null, 2008 Oakridge Drive, Dayton, Ohio, and William D. Adams, 4737 Drexel Blvd., lChicago', Ill. Filed Aug. '19, 1946, Ser. No. 691,648 Y y 14 Claims. (Cl. 313-66) (Granted under Title '35, U.S. `Code (1952), sec. 26

The invention described herein may be manufactured and used by or for the Government for governmental pur- 1946, for Infra-Red Television Detector and Controller,V

now U.S. Patent No. 2,955,777, granted October 11,1960, in which the mosaic as here described and claimed is shown in an illustrative circuit installation.

In the past, mosaics embodying thenmocouples, bolometers, photo-electric cells and the like, have been used to guide military missiles toward heat emitting targets with varying degrees of success. Previously developed heat seeking devices that depended upon the use of photo; electric cells, had the limitation of not distinguishing with a sufficiently high degree of precision, military targets from other sources of radiation. Infra-red sensitive cells, such as form parts of the present mosaic, have the characteristic of selectively distinguishing sources of maximum heat radiation, such as that given off by an operating steel works, oil cracking plants and the like, as distinguished from lesser sources of heat in the background. Previously produced infra-red sensitive cells have very largely lacked suicient sensitivity when mounted in the nose of a missile, to direct the flight course of the missile with satisfactory accuracy toward the target that is farther away than five miles from the missile and have lacked a desirable degree of accuracy and clarityv when used for the purpose of presenting on a television lscreen the observations gathered by the cells.

The mosaic type infra-red heat detector that forms thel subject matter of the present invention, when subjected to the type of scanning that is indicated `in the descript-ion of the mosaic that is presented hereby has the important advantage over scanning systems using asingle cell in that energy from aninfra-red target image is stored throughout the period between electron scans of adjacent condenser plates. In the present types of' scanning, the current path is only completed during the time in which the scanning beam is passing over a mosaic element. 'I'he sensitivity of a mosaic made of photo-conductive material is limited by the smallest change in pickup4 currents that can be detected above the noise level as the electron beam passes over a mosaic element. The

mosaic that is described herein is not dependent upon the' electron scanning beam to produce current flow through the photo-conductive materiaL'but has an auxiliary circuit which discharges condenser elements by amounts depending upon the infra-red radiation that is received from the target image. This provides, as an object of the present invention, a storage principle that is analogous to that in an inconoscope as used in television. For example, if the time of scanning a mosaic element by the cathode ray beam is lr4 seconds and the time intervalbetween scans is .10 of a second, the mosaic that is disclosed herein would be of the order of possibly 1000 fold more sensitive than pre-existing types of mosaics, assuming that the target image stays in one spot onkthe mosaic for .10 of.

Patented Sept. 5, 1961 lCe tivity by the storage principle is not strictly accurate 'and' is intended merelyto give an order ofvmagnitudecom- Y parisien. Some photo-conductivemater-ials have accumulative action in that they pass more current thanother materials after a longer exposure to light energy ofr the same intensity. If this effect is not linear with time for the material considered, there might not be strict proportionality between the charge accumulated and the period of charge.

Another object of the present invention is to provide an improved mosaic wherein the mosaic elements are s0. arranged thatV the` scanning electron beam passes ovei only condenser plates and not over photo-conductive ma- Vterial elements of the mosaic' that might b'e injured thereby. lThis provision permits the use of more advantageous photo-conductive materials as elementsA of the present mosaic than materials used heretofore, the sensitivity peaks of the present material being higher than the sensitivity peaks of corresponding materials used -previously in that region of the infra-red where good atmospheric windows exist, Vas for example, from 2 to 4 mu, and where water vapor absorption is relatively less than for the windows Y at 8.5 .to 13.5 mu. The presentlmosaic substantially prevents electron scanning of the photo-conductive material, the electronv bombardment of which destroys some of thequantum activated centers of lconduction and raises the temperature of the photo-conductive material, which are detrimental to they sensitivity of the photo-conductive material. For this reason, provision ismade'for maintaining eilcient operation of the present mosaic by holding it at` substantially the temperature of solid carbon dioxide or Dry Ice. In the older forms'of mosaic, Vthe electron bombardment of the photo-conductive material is prevented in some mosaics by providing the mosaic with a double wall, the photo-conductive material being exposed to the infra-red on the front wall and being connected by wires to electrode plates Yon the rear wall which are scanned by the cathode ray beams. The manufacture of older mosaics of the described type involves a prohibitive amount of hand work where the number of elements' involved becomes appreciable. .L Y I Y An infra-reddetector should'have a relatively large iield of view, Vsuch as, when measured lfrom theV detector, an

mosaic elements or cells. l t

ment thereof that 4is shown inthe angle of 20 degrees fora square mosaic and should `be able to resolve two point targets that are but one degreeA Y apart. Such` a mosaic would require Ia *minimum of 400v Another object of the present invention is toV provide a mosaic having improved functional advantages,"and that may be manufactured rapidly'and accurately with a mini-4 mum expenditure of time and effort. Y 'Y Other objects of the present invention will-beapparent from the following description Yof .anillustrative embodiaccompanying drawing;-

wherein:

FIG. 1 is a front elevationalfragmentaryiview of-aI mosaic that forms the subject matter of the present inven-v tion; 1 i" FIG. Z -is an enlarged sectional vview takenalongthe2 line2-2ofFIG.l; 'I' FIG. 3 is a further enlarged fragmentary sectional viewof aportion of FIG: 2, showing the electrical chargeson elements thereof; andv FIG. 4 is a schematic electrical circuit diagramvofY illusfl trative means Yfor scanningrthe mosaic. 1 Inthedrawings: -g Y y 'f l The back plate 24 serves as'a Ymount 'forl the mosaic assembly and is preferably of copper'or'otherUelect/rical. conducting material. An insulationV sheet 27 is-disposeclI upon and secured to one sidel ofthe back plate 24, 'le'av" ing aV suitable margin between adjacent edges thereof'for making electrical connections tov some elements offthd mosaic. The-'insulationsheet 27 preferably is a thin sheet of mica suitably cemented to the metal back plate 24.

In the making of the mosaic, `there is disposed upon the insulation-plate 27 =through a mica stencil, notshowm by any suitableprocess of evaporation, a desired plurality of strip-layers of1photo-sensitive material 331Whichfpre'ferably is llead snlphide; which has-beentactivated by exposure'to oxygen gas.

This Aphoto-sensitive material hasthe property .ofA changingV its resistance upon exposure -to intermediate wave lengths in the-infra-red. The lead sulphide deposit of photo-*sensitive material 33 is disposed in strip form horizontally ofithefinsulation material 27 the individual strips terminating-inwardly-from the-margins of the insulation material *.27 soithat the lead-'sulphideis insulated from the back plate24. Above each strprof photo-sensitive material233gispositioned a positive grid strip 28- of gold or platinum deposited upon the insulating material 27 by evaporation, sputtering o1- the like. Below each strip of photo-sensitive material 33, there is a horizontally disposed row of` spaced small substantially square condenser plates.3t1.-that also are applied to the insulation material 27 by 'the vevaporation or sputtering of gold or platinum. Air,- gaps.separate` the condenser plates 30 from each other. Electrical contact ismade between 4the positive grid strips 281and the condenser plates 30 by the highly resistant photo-sensitive material 33 throughout the face of the mosaic'that is.exposed to electron bombardment.

Below the row of condenser plates 30 there is another depositor strip of lead sulfide material 33 and below that another positive grid lstrip .28 of gold. This structure is repeated until there are preferably not less than four hundred combinations.of; elements 28, 33 and 3i)l carried by a backplate 24 with the Vmica insulation 27 therebe tween. As a ,practical matter, it is equivalent to saying that'there .should be at least 400 condenser plates 30. For the proper guidance of a missile, the seeker should be large enough to cover 20 of the iield of view, both vertic ally andhorizontally.

The function of the three-element combination just described is as follows:

A condenser plate 30 upon receiving heat signal, loses electrns. (negative charges) thereby acquiring a positive proportionate charge. An electrical picture of a heat sig-V nal is therefore formed on some of the gold plates 30 which serve as condenser plates. Backplate v24 forms the- Other plate of the condenser'and the mica sheet 27 the dielectric.

Thefcondenser plates 30may be scanned by an electron beam 22 (see FIG. 4) without injury. T o scan the lead sullidematerial 33 directly would cause loss of sensitiveness. 'Ihe bombarding electrons can easily destroy the disturbance centers set up by the incident infra-red radiation conductivity of a semi-conductor due to incident radiation. These disturbance centers can exist only as longfas electrons are removed from filled levels to conductngflevels, which leaves aconducting positive hole in the formerly filled levels. These levels are either in the crystalvla'ttice or -impurities held in the lattice. The conduction-dueto the radiation can only continue until electrons recombine with the positive holes. Electron bombardmentscan free electrons `in the vicinity of the positive holes to recombne with them, or can redetach ejected electrons from impurity centers, allowing them to recombine-with the positive holes and `stop the conduction caused by the radiation. Thus, electron bombardment can readily destroythe lead sulphide sensitivity by causing the recombination of the electrons and positive holes Vproduced by the radiation. 'Ihe beam 22 in scanning the rows of condenser plates 30 brings the .potential of each one in turn 'equalto that of the cathode 23 from which the scanning been; originates.

,Electrons are reflected from the plates 3.0 andthese mnstbecau'g'htbefore they drift to as yet-undischarged,

condenser plates 3'0 and partially discharge them before the cathode ray (scanning beam 22) discharges them.

A collecting screen 19 bearing a small positive charge is provided for this purpose. The screen 19 is disposed over the face of the whole mosaic 20 and is penetrated by the scanning beam 22. It should not besoiine as to interfere with the action of the beam. The rdistance between the screen 19.and the-mosaic 2 0 is only-a fewlmillimeters. A second 4function of; the screenA19-is to assist in keeping the face of the mosaic cool. The lead sulfide photosensitive material33ris sensitive at thetemperature of Dry Ice. The slight decrease in the electron bombardment offered by the gridand the lesseningof-ele`ctron collisions between condenser platesM30-will `be! of value in helping. to keeplthe temperaturelow. Y

A preferredsource-ofcooling the mosaic A20 is 4filling of Dry Ice (not shown) contained in apan37=mounted upon the rearside of vthebackplatef24 (see FIG'. 2)'t The pan 37 is. attached by any suitable mechanical fastener to one or more bus bars 36 which extend from theibackplate 24 of the mosaic 20 through a glass envelope t 39 which houses the mosaic t and A which isy identical .fwith -the tube envelope. Any other provision for cooling th'c mosaic may be substituted if it -isreasonably compact and Vcankeep .the mosaic-at least as cold as -70C.

A scanning circuit is shown in FIG. 4; VltoV inclusive, shown asresistances, are successive-rowsof lead suldetphotosensitive material=33 on the mosaic. AA pair of deilector plates V38 deflect the electron beam -22 in a usual scanning operation. Reversal oftl polarity on plates* 38 accomplishesfthe scanning motion -of the beam in l the usual manner. The beam 221 is'shown passingfthrough the screen grid 19 Whichis` chargedpositively-byfbeing' connected to thepositive terminal VVcfa-battery 2'1. secondbattery 26 is Aprovided to render positive the Vgrid strips .28. The positive terminal of the battery 26 is'connected by lead 34 to thetpositive gridstrips 28in parallel; its negative terminaltogether with the negativeterminal of battery 21 is led to ground 35. A third battery 40' provides focussing for the electron beam 22' from the.

cathode 31 within the tube 32. A lead 41 groundsthe negative side of battery40 at ground 35.

A pickoff resistor 25 is connected to the mosaic 20' by its backplate 24. A lead 42'grounds the resistor at-ground 35. A circuit is therefore completed from battery. 26' through the mosaic 20.and resistor 25v to ground'gandanother circuit from battery V21 to collector screen119' to beam 22 to ground 35. The current of battery `26'isfava`ilable to form signal takenfromthe ends of resisten 25 as indicated by a pair of arrows extending therefrom. The mosaic acts as a condenser to block the passageloftdirect current through it.

Upon incidence `of heat on the photosensitivematerial 33, its conductivity. is increased sufliciently toallow positive potential from battery 26 to How` to the condenserplates 30.

The electrical action may be described as follows: When the most eiective infra-red energy strikes oneofthe photo-sensitive material resistances such as' V1, then that condenser plate 30 to which resistance V1 is connected, is charged more than the othersduningthe interval between scans. When the scanningbeam 22 passes over the most highly charged plate 30, the vlargest-pickotfy voltage is obtained across resistor 25. At eachpassage of the electron beam 22 over a condenser plater30, that' plate is discharged and brought essentially to .the potential of the cathode 31. It is impor-tant thatasmall, sharply focussed scanning spot bey applied to the con. denser plates 30 so that Vbombardment of the photo-sensitive material 33 is minimized. Aligningl tabsv for-this purpose are shown in operation with the present mosaic- 20 in our above-mentioned co-pending application.

As a matter of dimensions, it has beenvfound that the condenser plates 30 'm ay be about one millimeter sqpare. The other elementsmay be of ,proportionate as lildicated on the drawings. The various elements of the mosaic may be provided with electrical connections as will be seen from FIG. 4. It will be noted in FIG. 1 that the positive grid strips 28 are connected in parallel.

The invention claimed is:

1. A mosaic, comprising a backplate made of an electrically conductive metal, a sheet of insulating material substantially covering one side of said backplate, a strip of infra-red-sensitive material on said insulating material, a strip of thin gold bordering one side of said infra-redsensitive strip and in contact therewith, and a plurality of small plates of thin gold on the other side of said infra-red-sensitive strip, said plates being of substantially equal area and in contact with the infra-red-sensitive strip.

2. A mosaic according to claim 1 in which the infrared-sensitive strip is composed of lead sulfide which has been activated by exposure to oxygen.

3. A mosaic according to claim l in which the mosaic comprises not less than 400 cells comprising infra-redsensitive material bordered on one side by a gold strip and on the other by a separate area of thin gold.

4. A mosaic comprising a backplate made of a metal which is a good conductor of electricity, a thin sheet of insulating material covering one side of said backplate except the margins thereof, a plurality of strips of a material having the property of becoming positively charged on exposure )to infra-red light, on said sheet of insulation, a strip of thin gold bordering one side ofV each of said infra-red sensitive strips and in contact therewith, and a multiplicity of small deposits of thin gold on the other side of said infra-red-sensitive strips, said deposits being of substantially equal area and also in contact with the infra-red-sensitive strips.

5. A mosaic according tovclaim 4 in which the infrared-sensitive strips are composed of lead suliide which has been activated by exposure to oxygen.

6. A mosaic according to claim 4 in which the firstmentioned gold strip is provided with an electrical connection for maintaining a charge thereon.

7. A mosaic according to claim 4 which comprises not less than 400 small deposits of thin gold.

8. A mosaic intended to be scanned by an electron beam, comprising a backplate of a metal lwhich is a good electrical conductor, a sheet of insulating material covering one side of said plate except the margins thereof, a sensitive element comprising lead suliide activated by oxygen, said sensitive element being closely adherent to said sheet of insulating material, a positive grid of a noble metal, said grid being adherent to said sheet of insulation and in electrical contact with said sensitive element, and a multiplicity of condenser plates, not less than 400, said condenser plates being also made of noble metal and adhering to said sheet of insulating material and in electrical contact with said sensitive element, said condenser plates being arranged in rows adapted to be scanned by the electron beam having a small focal spot.

9. In combination, a mosaic comprising a backplate of a. metal which is a good electrical conductor, a sheet of insulating material covering one side of said plate lexcept the margins thereof, infra-red sensitive material disposed in strips on said insulating material, an electrically conductive grid on said insulating material and making electrical contact with said infra-red sensitive materialand to which grid a source of positive charge may be connected, and a multiplicity of small condenser plates attached to said sheet of insulating material for receiving positive electrical charges from said gridupon increasing the temperature of said sensitive material and said condenser plates being mounted in superposed rows to be scanned by an electron beam and thereby discharged to provide an electrical signal.

10. In combination, a mosaic, comprising a heat sensitive element which is most sensitive when extremely cold, said mosaic having a ybackplate of a metal which is a good .electricalconducton means attached to. said backplate for the conduction of heat therefrom, and a pan mechanically attached to said means, said pan being adapted to contain, a iilling of Dry Ice, -said mosaic including also a sheet of insulation between the heat sensitive element and the backplalte, a grid ou the insulation for supplying potential to the heat sensitive element and a multiplicity of condenser elements on the insulation for collecting potential fromsaid heat sensitive element.

ll. A combination according to claim l0 having in addition a glass envelope enclosing the mosaic and in which the heat-conducting means is a plurality of heavy bus bars, said bus bars extending through said envelope, and said pan being outside the envelope.

l2. In combination, a mosaic in part sensitive to infrared radiation, a back plate of conductive metal for said mosaic, a source of an electron beam, means attached to said beam source for focussing the beam issuing therefrom to a small scanning spot, means for oscillating said beam, a screen before said mosaic for capturing electrons reflected from said mosaic, means for keeping said screen charged positively, a circuit including a source of direct current and connecting the infra-red sensitive part of said mosaic through said source of current to said backplate, and a pickolf resistor in said circuit.

13. An infra-red sensitive mosaic comprising an electr'ically conductive backplate, insulation covering a substantial pant of said backplate, anv electrically conductive grid attached to said insulation, a strip of electrically resistive infrared-sensitive material of increasing electrical conductivity with increase in temperature and attached to said insulationand making electrical contact with said conductive grid, anda plurality of condenser plates attached to said insulation and making electrical contact with the infra-red-sensitive material by one of their edges, by which arrangement said condenser plates are disposed in parallel rows on the insulation.

14. In a device for detecting heat made of a highly electrically conductive metal of substantial thickness, a covering of electrical insulation over a substantial area of one side of said plate, strips of infrared-sensitive material laid on said insulation, said strips being of a material which has a high electrical resistance when extremely cold and a lower resistance when exposed to faint infra-red radiation, strip means for supplying positive electrical potential to said infra-red-sensitive material mounted adjacent thereto, and collecting means also mounted adjacent to the infra-red-sensitive material y for removing and holding discrete electrical charges from said` infra-red-sensitive material whenever incidence of infra-red radiation lowers the electrical resistance of the infra-red-sensitive material to an extent permitting leakage of potential across said infra-red-sensitive means to`said collecting means.

References Cited in the le of this patent UNITED STATES PATENTS 2,190,020 Goldsmith Feb'. 13, 1940 2,238,381 Batchelor Apr.l5, 1941 2,250,721 Moller et al. July 29,1941

signals, a backplate

Images