US2986637A - High speed far infra-red detector and heat seeking control for guided missiles - Google Patents

Description

8 Sheets-Sheet 1 F. E. NULL SEEKING CONTROL FOR GUIDED MISSILES .xsxsgxssxxy May 30, 1961 HIGH SPEED EAR INERA-RED DETECTOR AND HEAT Filed Aug. 29, 1946 INVENTOR. FHY E ULL A mvo f HTTKNFYS May 30, 1961 F. E. NULL HIGH SPEED FAR INFRA- RED DETECTOR AND HEAT SEEKING CONTROL FOR GUIDED MISSILES 8 Sheets-Sheet 2 Filed Aug. 29, 1946 INVENTOR.

FAY E. /VUL HTTOE/VE'YS ummm..

May 30, 1961 F. E. NULL 2,986,537

HIGH SPEED FAR :NERA-RED DETECTOR AND HEAT SEEKTNG CONTROL FOR GUIDED MIssTLEs Filed Aug. 29, 1946 8 Sheets-Sheet 3 IN VEN TOR. F197 E. NULL firme/lfm l F. E. NULL A-RED FOR GUI May 30, 1961 2,986,637 DETECTOR AND HEAT DED MIssILEs HIGH SPEED FAR INF'R SEEKING CONTROL 8 Sheets-Sheet 4 Filed Aug. 29, 1946 EL?- E- INVENToR. Fay E NULL BY gua Hrm/HEY May 30, 1961 F. E. NULL 2,986,637

HIGH SPEED FAR INFRA-RED DETECTOR AND HEAT l SEEKING CONTROL FOR GUIDED MIssILEs Flled Aug. 29, 1946 8 Sheets-Sheet 5 May 30 1961 F. E. NULL 2,986,637

HIGH sPEED EAR :NERA-RED DETECTOR AND HEAT SEEKING CONTROL FOR GUIDED MrssILEs Filed Aug. 29, 194e 8 sheets-sheet e `FHY E /VULL 9 Trae/VEYS May 3o, 1961 F. E. NULL l 2,986,637

HIGH SPEED FAR INFRA-RED DETECTOR AND HEAT SEEKING CONTROL FOR GUIDED MISSILES 8 Sheets-Sheet '7 Filed Aug. 29, 1946 CYCL/ A fm CYCZE Z D N A May 30, 1961 F. E. NULL 2,986,637

HIGH SPEED EAR INTRA-RED DETECTOR AND HEAT SEEKING CONTROL FOR GUIDED MIssTLEs 8 Sheets-Sheet 8 Filed Aug. 29, 1946 INVENTOR. /CJ//V// du@ ZEE? NWETH tat HIGH SPEED FAR INFRA-RED DETECTOR AND SEEKINGv CONTROL FOR GUIDED MIS- The invention described herein may be manufactured and used by or for the Government for governmental purposes without paymentV to me of, any royalty thereon. This invention relates -to adetector for infra-red rays. Its. characteristic. ofl most value is its high speed action. Thismakes it suitable for use in explosives carrying guided missiles which are military weapons adapted to find and destroy enemy steel plants, refineries or the like. Such plants are suciently intense sources of infra-red raysto make it possible for a missile to direct itself into them, provided that the detector and control mechanisms of the missile are suiciently yfast and reliable to give suitable adjustments from a distance of say 4 to 6 miles.

This detector can also be applied lto heat mappers, target locators, etc. The invention resides not only in a detector itself, but also in the means employed for translatingA the detected signals into stronger impulsesl suitable for directing the movements of such servo systems with which missile may be provided for the adjustment of its aero-dynamic surfaces.

One object of the present invention is to provide a heat-seeking device which can detect infra-red radiation at a distance of several miles and which will provide proportional control signal in accordance with such detection in a form in which it can be used by a suitable guided missile to make a higher percentage of hits upon heat-detected targets than has been possible with previous heat-seeking missiles.

While an infra-red detector, or a heat seeker as it is more commonly called, needs to be sensitive, it is the conclusion of' the present inventor that sensitivity is not the controlling factor in insuring a hit upon the target. Sensitivity is of value so that the missile can pick out the hottest point in the iield of view, however differences in background can outweigh the heat given oi by a distant target in the previously employed systems in which one part of the eld of view was balanced against another part. Previous attempts to improve seekers have beenl in the direction of increasing their sensitivity.

Because of the very rapid exponential decay of transmitted radiation through the water vapor of the atmosphere, an increase in sensitivity alone does not give corresponding increase in performance. Thus, 14 cm., of condensable water in a path 1 cm. in cross section and 6 miles long (typical bombing conditions) reduce the transmitted radiation to the order of 6% of its original value by water vapor attenuation alone. Having a range of 12 miles and 28 cm. of condensable water, the transmitted radiation would be reduced to 0.36 of 1%, or for the same value of received energy, and multiplying by 4 to allow for the inverse square law, the device would have to be 6/0.36X4=67 times as sensitive to operate on a given target at 12 instead of 6 miles.

Increasing` the sensitivity of a device does not necessarily better its performance. For example, consider a device which balances one half of its field of view against the other, each side. of the half, field subtending an angle 236,637 Patented May 30, i961 of 8 from the seeker. Thus a target 200 feet square.

of one half of the eld of view. Assuming the targetto have an average temperature 55 degrees C. above the background, the seeker will receive from it approximately 2.28 times as much energy per sq. ft. as from the right half of the field of view. Then if the right half of the held of view exclusive of the target, radiated exactly as much as the left half, target on the right half of the eld of View would be equivalent to 1.28 times the target'V area (at the temperature of the background) added to the right half of the field of view. The elective radiant energy received from the right half of the iield of view would then be (4.85X106+5.12 104)/4.85 106=1.01 times the effective energy radiated by the left half of the field of view. If the left half of 4the held of view radiated one part in 100 more than the right side without the target, the result would be an exact heat balance between the two halves of the iield and no matter what its sensitivity, the device would be inoperative.

it is more eflicient therefore, to sac-rice supersensitivity by using a scanning system having a small scanning spot. Thus for example, a 1 square scanning spot is 277 feet on a side at a distance of 3 miles and covers an area of 7.68X104 sq. ft. The target is assumed to have an area of 4 1O4 sq. ft. and its equivalent area at the same temperature as the background is 5.12X104 sq. ft. which covers (5.12X104/7.68 104)=0.67 of the scanning spot. This unbalance must be 64 times larger to cause zero received signal in the case of the seeker having the scanning spot, than for a balanced field seeker.

In the drawings Fig. 1 is a longitudinal section, partly diagrammatic, of the head of a missile containing my detector.

Fig. 2 is a functional diagram of the detector and scanning circuit used therewith. The sensitive element is shown in vertical cross section.

Fig. 3 is a perspective view of the sensitive or mosaic element showing how the latter is scanned.

Fig. 4 is a vertical section of the mosaic taken on the line `4 4 of Fig. 5.

Fig. 5 is a rear View of the vertical section of the mosaic taken on the line 5-5 of Fig. 4.

Fig. 6 is a rear view of a vertical section of the mosaic taken on the line 6--6 of Fig. 4.

Fig. 7 is a vertical section of the condenser element of the mosaic.

Fig. 8 is a `diagram of an equivalent circuit for a mosaic element and its scanning device.

Fig. 9' is a diagram of the mosaic circuit in its first charging phase.

Fig. 10 is a diagram of an equivalent charging circuit for a mosaic condenser during the iirst phase.

Fig. 11 is a diagram of the mosaic element charging circuit in its second phase.

Fig. 12 is a diagram of the mosaic discharge circuit in its third or restoring phase.

Fig. 13 is a cross section of a cold generator and electric tempering circuit for use with said seeker.

Fig. 14 is a diagram of a master switch governing the sequence of operations for the phases of the mosaic circuit.

Fig. 15 is a voltage switch diagram showing the effect of thermal bias.

Fig. 16 is ay diagram of an electrical coupler circuit adapted to connect the mosaic circuit to the other circuits, i.e. missile control circuits for example.

Fig. 17v is a diagram of the cyclic switching circuit for producing the proper sequence of functions.

In order to be able to understand the drawings, it is first necessary to beinformed of the principles upon which the seeker operates. Briey stated, it is that an insulating iilm, say of certain types of Bakelite i.e. phenolformaldehyde synthetic resins in a thickness derived from the dried solutions or varnishes which approaches that of `a mono-molecular layer, Vhas electrical conductivity which is profoundly inuenced by the adsorption of water vapor upon it. lt has been shown experimentally that a dierence of 40% in the amount of adsorbed water vapor makes a dilerence of approximately 10,000 times in the electrical conductivity. In the device, the lhumidity of a circumambient atmosphere is 'kept as constant as possible, so that the only variable factoris the drying eect upon synthetic resin film which is exerted by the heatl signal itself. Since the amount of lheatY isfvery vsr'riall, insofar as it is received in signal form, it will be'understood that stringently controlled conditions must'prev'ailf General arrangement-Ambient temperature control Referring now to Fig. 1,720 is a cylindrical housing of shell which is intended to be attached to the Vforelpart of the missile (not shown). The shell has. a conical nose 19 which extends back to mirror 28. The shell 20 has a heavy coat 23 of heat insulating material which extends completely around it excepting the nose portion 19. The nose portion must transmit infra-red radiation, therefore it is made of silver chloride, suppported by metallic gridwork 21 which is an extension of the solid metallic lining 21'. The infra-red rays from the target pass through the silver chloride portion to impinge upon a heavy copper concave mirror 28 which is gold plated on its face and which has a central aperture 29. The mirror 28 reects the rays to a small concave mirror 26, also gold plated, which brings them to a focus at 59. The rays which diverge from the focus pass through the opening 29. The mirror 26 is mounted on the inner end of a cylinder which is preferably made of tough glass and which contains a water iilling (not shown). Extending from the'mirror 28 so as to be attached substantially to the rear of aperture 29, there is a conical wall 30 of a long cylindrical container 31, the function of which is to house a heat sensitive element 32 and the electronguns 33. Between the sensitive element 32 and the mirror aperture 29 there is a pair of rock salt plates 24, the function of which is to prevent local turbulences from producing hot spots which would affect the mosaic 32. So sensitive is the mosaic to such scattered radiation, that, the entire space enclosed by the silver chloride nose 27, the two copper mirrors 28 and 26, and the rock salt plates 24, the sensitive element 32 and the electron guns 33 is highly evacuated initially, watervapor being added later to the chamber between the rock salt plates 24 and mosaic 32.

In order to cut down thermal instabilityin the interior of shell 20, a second wall 34 of heat insulating material is provided within the shell at a substantial distance from the sidewalls thereof. The space between the two insulating walls 20 and 34 is used to house apparatus for the thermal stabilization of a cylindrical space within wall 34.

Such thermal stabilization apparatus comprises a cylinder 35 of compressed inert gas such as CO2, nitrogen or helium, which together with a remotely controlled expansion valve 36 constitutes a source of cold. A source of heat is a chamber 37 which contains water and an electrical heating coil 38. Heat and cold can therefore he used within the shell'as will be hereinafter described so that thel temperature and humidity conditions circumambient within the detector areV substantially constant. A third chamber is provided to have still a closer Yregulatory eechsaid chamber being the container 39, the exterior surfaces of which are provided with a multiplicity of radiating fins 40. Within this chamber is a coil 41 through which'gas may be circulated. Outside the coil but within the,chamber, there is a lling 42 of parain The lling 42 is intendedtobe maintainedwithin itvsws/oliYd-liquid phase, preferably at solid phase; while the changes in pressure produced on the inert gas within the chamber 39, which is brought about by change in volume of the paraffin due to changes in composition of the so1id-liquid phase are carried by a tube 43 which connects chamber 39 and a disc barometer 44. A small drop in pressure closes the contacts of a relay 45, to which the barometer is connected, thereby allowing electrical current'to ow in the water heating coil 38.l The water in chamber- 1 37 is prevented from pouring out by a porous wick 46 ,in which the heating element 38 is embedded. As water boils from the wick, its vapor passes forwardly inthe shell between the mirror 28 and forward end of the cylindrical wall 34 and then divides on each side of a partition formed by a large diameter copper tube 47 which houses the sensitive element compartment 30. The latter is provided with the conical tail piece 48 conforming to which the after end 49 of tube 47 is curved. By this arrangement a streamlined path for the water vapor stream encircling the sensitive element housing 30 is provided.y

This stream is also the source of heat tending to keep the paran 42 in a stable thermal condition. The purpose of the copper tube 47 is to equalize the water vapor stream from each Vof its sides to the other by means of heat transfer. Y

The container 40 with its filling 42 of paralln serves the purpose of a condenser and the heat carried to it by the water vapor must be equal to the heat carried away by means of the chilling coil 41. A pump 50 is provided outside the wall 34 and in circuit with coil 41 and a cold air chamber 51 to pump cold air through coil 41. As long as the temperature of chamber 51 is kept constant, the cooling elect of the coil 41 will also be constant and a nearly constant water vapor ow from the heat source 38 to the condenser 40 will result, regardless 0f the amount condensed due to changes in the ambient tem perature. `Chamber 51 is kept at. nearly constant temperature by a surrounding =bath formed by expanding CO2 passing from valve 36 through pipe 52 into the space 53 and out of the shell through a pipe 54. The release of CO2 is ,under the control of relay controlled valve 36. A second cylinder 55 of CO2, may also be controlled by an electrically operated valve 56 which allows CO2 to ow from cylinder 55 through a pipe 57, then around the nose 19 by means of a pipe 22, and on its way circulating through the water in the chamber 25 and then out through pipe 22a. A cold source 91 (see Fig. 2) is indicated through an opening 29 in mirror 28. Its function is to cool the mosaic of the sensitive element; its action thereon being described under Details of Sensitive Element." It is-to he understood that batteries vsuch as battery 58 for water evaporation wiring and other common circuit essentials are to be provided where required.

DETAILS OF SENSITIYE ELEMENT Referring to Figs. 2, 4, and 7, a vertical section of the heat sensitive elementor mosaic 32 is shown in each. The mosaic 32 is built upon an insulating lrn 75 which provides a base of very thin nitrocellulose, glass or synthetic resin. In the course of manufacturing the mosaic 32 and reading from left as front to right as back, there is deposited by sputtering in vacuo, in course of manufacture, through a mica stencil upon the film 7S, a metallic and metallic salt i'llm of the same order of thickness as the nitrocellulose 75, i.e., 5X 10-6 cm. On the front (left side in Fig. 2) the deposited material provides `a plurality of, plates 76 of black antimony trisulde, on the back of the lm 715 is deposited by sputtering in a vacuum a corresponding plurality of metallic gold plates 77. The deposition .is done in squares about 4 mm. on an edge. Deposition from the front is always on space left blank from' the rear and viceeversa. 'The lm is held in a suitable nonconducting frame 96 (Fig. 6) as. are the other parts of theY sensitive elements which are about Yto be decreed... f l,

The incident heat raysarc" rellected from the large mirror- 28 tothe smalllconcavemirror 26 which brings it to focus as the real image 59. Therays then diverge through the opening 29 in the large" mirror, through the rock salt windows 24 to strikefv the mosaic or sensitive element 32. A partition 60 separates the Water vapor around the thin tilm of theV mosaic from the evacuated space containing a cathode beam 61 which scans a condenser plate 92. The electron guns 33 supply the electron beam 61. The space enclosed by the conical AgCl nose or cone 27, the heavy copper mirror 28 which is plated with gold or other good infra-red reflector on the front face and the rock salt plates 24is evacuated. The after part of container 31, i.e., behind partition 60 is also evacuated. A porous wick 73 is a source of constant temperature, water in the space housing the sensitive element, which space is between the two evacuated spaces.

The condenser plate 92 just referred to is a part of the sensitive element on mosaic 32 and is mounted on the glass rear plate 78. There is an air space 79 between film 75 and' plate 92. Glass plate 7S is provided on its rear surface with a multiplicity of gold honeycomb cells 80 which are preferably square and areV so called because they are surrounded by low gold walls S1. The function of cells 80` is to act as a collector for electrons knocked out of condenser plates 92 by cathode beam 61. Glass plate 78 acts as the dielectric of the condenser plates 92.

The operation of the detector itself, is as follows: The insulating film 75 is very thin (of the order of 5 l06 cm.) and readily adsorbs radiant heat energy from the target and from the heat absorbing antirnony sulfide plates absorbing lm 76; this heat energy passes through the lrn 75 and heats rthe inner surface of the mosaic 32. To the right or back of the film 75 is a partial mono-molecular layer of water molecules 82V (Fig. 4). The incident heat causes a relatively enormous change in the electrical conductivity of this layer of Water molecules 82, antimony sulfide plates 76 acting as heat absorbing plates. The charging circuits of the electron collector gold cells S0 are scanned by cathode beam 61 to plate 92 and by induction through metal and glass plates 60 and 7S to negatively charged plates 77 on the rear of lm 75 and across the adsorbed water layer 82 on the rear of the front absorption plates 76 to the negative plates 77 to earth and electron gun 33. The incident energy falling on squares of cells 76 determines the completeness of the adsorbed water layer 82, hence its surface resistance and the charging current to the collector cells 80. A signal pickot resistor 83 in series with a battery 105, is placed in series with the plates 77' and 77, giving a voltage approximately proportional to the condenser charging current. Metal plate 60 is held gas tight, so that the best pressure of Water vapor for the adsorbed layer 82 of water molecules may be used for the film and a good vacuum used for the cathode ray beam 61.

The cathode ray beam 61 is operated in the way that is conventional in television transmission. It is formed by electron guns 33 and deflected by two pairs of opposed plates 84 (only one pair is shown). As the beam passes over one cell or plate 80 to another (Fig. 3) a corresponding signal is taken from the pick-offr resistor 104. ln the scanning process, condenser plates 92 become positive as secondary electrons are knocked out to the walls 81 of collectors 80.

The actual charging and discharging of the several condensers of the mosaic of the sensitive element occurs in several phases as will be later explained in detail under the heading entitled Detailed Description of Seeker Operation: The Mosaic.

In order to pick out the hottest part of the targeta negative thermal bias is used to cancel out all but the strongest signal in the field of view. As the magnitude of the pickoi voltage from resistor 104 increases, an increased output from the amplifier 85 passes over leads ss and 89 to a thermal biaisY regulator 90. The regulator electronically controls thetemperature of a cold source 91, decreasing its temperature for an increased signal. This increases the cooling eiecton the adsorbed layer of Water on the mosaic, acting in opposition to the energy from the target, thus acting as a negative thermal bias to cancel out all but the strongest signal in the field of view. When the scanning beam passes over the mosaic sectionfwhich is under the hottest part of the image of the target, a signal is picked up from resistor 104, amplified by an amplier and impressed on a delay network 86. Aiiter several cycles, the voltage on this network has built up suficently to trip an electronic coupler 87v which is in series with the resistor and ampli# fier whenV the-scanning beam 61 passes over the hottest part of the image.

The electronic coupler then generates a voltage equal in polarity and proportional in magnitude to that across the scanning beam deflection plates at that instant and impresses it across control leads 95. This gives proportional control to the output signal, since the voltage on the cathode ray detlecting plates has to be such as to produce av deflection of the beam equal to the position of the hottest mosaic receiver at that instant. The hottest mosaic cell has a position on the image mosaic corresponding to the position of the hottest target in the field of view.

General description.-The monomolecular layer The characteristics of a partial monomolecular layer of adsorbed water molecules on the surface of an insulator are utilized to produce a very great and rapid change of the surface resistance of the insulator with the temperature rise produced by incident radiation. As has been stated, the surface resistance of someV insulators changes by 10,000 fold for a 40% change in the completeness of a single layer of adsorbed Water molecules. The amount of adsorbed material on a solid surface de'- creases exponentially with rise in the absolute temperature of the adsorbed layer. Only a minute change in incident energy on the adsorbed layer is required'to produce a detectabie change in the surface resistance of the insulator on which the film is adsorbed. Since the Van der Waals type of adsorption occurs almost instantaneously, the speed of response to incident radiation is limited only by the thickness of the insulating film upon which the water molecules are adsorbed.

General descrpti0n.-St0rage principlev of the mosaic Small metallic condenser plates 92 in Figs. 4, 5, and 6 are about 1-1 mm. square and are stenciled on to the thin insulating film or glass plate 73 by evaporation. Each metallic wall 81 surrounds the glass surface 78. Incident radiant energy adsorbed on plates 76 and conducted to thin reverse sides varies the surface resistance of the insulatingy lm 75 surrounding the individual condenser plates 77. This varies the charging resistance in series with the condenser iilm squares Which form one set of plates 77 of the mosaic condensers, the set of condenser plates 912 of opposite polarity being on the rear of the mosaic, stenciled on the glass insulation plate 78. The pairs of plates forming the miniature condensers are charged and discharged by cathode ray beam 61, a resistor 104 in series with the collector 81, being used as signal pickoi. The image of the eld of view is impressed upon the mosaic at all times, and the incident energy is stored upon each square for a period equal to the time between successive transversals of the receiving elements by the scanning spot. In this respect it is very similar in action to the iconoscope and picko resistance circuits used in television. The two sets of condenser plates of the mosaic are separated by an `air tight partition 60 so that proper pressures may be used for the adsorption layer on one side of the mosaic and for the cathode ray scanner on the other side.

General description- Thermal bias control: (See Fig. 2) Without thermal bias control there would be no differentiationbetween signals big enough to completely remove the monomolecular layer. To prevent this and to dilerentiate against the weaker signals so that only the strongest comes through, a voltage is taken from the signal pickoff resistor 104 in series with the collector 81. This voltage is used to electronically control the energy received by the seeker from an artificial cold source 91, in such manner as to produce a negative thermal bias that cancels out the incident energy from all but the hottest target in the field of view. Thus, the only signal picked up bythe mosaic occurs when the scanning spot is passing over the hottest signal received in the image on the mosaic. (This corresponds to the target area emitting the most transmittable radiation.) .Y General description of the .weken-Proportional control from the position of the cathode ray scanning beam A high impedance pickoff from the deecting plates of the scanning beam is ideal for proportional control. For the polarity and magnitude of the voltage of the scanning beam deflection plates 84 accurately place the scanning spot on a corresponding portion of the image. The high impedance pick-olf is provided by connecting the control grid and cathode of tube 152 of electronic coupler 87 to the plates 84, as shown in Figure 16. As noted above for a given target position, an electric signal of suicient magnitude to trip the electronic coupler 87 is produced when the scanning beam passes over and only one of the mosaic elements representing the eld of view. It is therefore only necessary to have this electrical signal electronically connect a pickoif from the scanning beam deection plates 84 to the output signal leads 95, at the instant the scanning beam is passing over the mosaic element giving the signal. At that instant of time the voltage picked ofl` from the scanning beam deflection plates is of the correct polarity and magnitude to give almost perfect proportional control. Specific description of seeker details.-Change in resistance of monornolecnlar layer The surface resistivity of certain insulating lms changes by a factor of 10,000 fold for a change in relative humidity of 40%. Theory and experiment indicate that adsorbed lms on the surface of insulators do not exceed 1 molecule in thickness. An equilibrium exists at the surface of the film, the number of water molecules that evaporate from and condense on the insulating lm per sec. being equal. An increase in pressure of the water vapor surrounding the lilm causes an increase in the number of molecules condensing per sec., and the number of molecules in the unfilled monomolecular layer increases until the number evaporating equals the number condensing. An increase in temperature of the absorbed molecules-at constant-pressure ofthe surrounding water vapor-causes an exponential rise in their rate of evaporation, and the number of molecules in the layer decreases until the number evaporating is reduced to the number condensing on the surface. For example a change in :1 fold in resistance (which is ample for detection) corresponds (in the mean position between 50% and 90% relative humidity) to approximately,

@ein 2m kT DV T0 Y (i) where,

n:number of molecules in adsorbed layer 'n':number of molecules in the volume of gas -V=vo1ume of gas adjoining adsorbinglsolid i s:surface area of adsorbing solid T0:period of oscillation perpendicular to the surface of adsorbed molecules k=Boltzmann constant T:absolute temperature n0=potentia1 energy of adsorbed molecule (equals kinetic energy of adsorbed molecule necessary for escape) m:mass of adsorbed molecule In the range in which Van de Waals forces cause an almost instantaneous change in the adsorbed layer with temperature and pressure, the exponential term in Eq. l produces a muchgreater change than the square root term. The exponential term represents the decrease in the number of adsorbed molecules with increase in temperature because of the exponential increase in the rate of evaporation from the film when its temperature is raised.

Experimentally it is found that a fractional change of 0.09 in the adsorbed layer may occur for a change of 1 degree C. A fractional change of 4x106 in the adsorbed layer was required for a detectable signal in resistivity change, or

degrees C. change in temperature that is detectable by change in resistance of lm with adsorbed layer of water vapor.

The energy required to give this small rise of temperature of a thin lm is readily estimated: Assuming: Receiver-3 mm. square and 10-5 crn. thick; specific heat :0.3; density:2.0; and the heat required to raise the temperature of the receiver 4.45 l0*5 degrees C, 1s,

Heat:Volume x density x specific heat x temperature change Heat:9.0 l0'I X2.0 0.3 4.45 105 :2.41 X 10-11 calories: l .0l X l0*1 joules :1.01 X 10'3 ergs.

In the above estimation, dissipation losses were neglected. Since the dissipation loss is zero `for zero rise in temperature, and -for maxium rise in temperature, the average dissipation loss would be 1/2 the input for a linear temperature rise. If the signal is only stored for a time interval equal to t-he time constant of the receiver, the temperature rise would be roughly linear. Considering the dissipation loss, the minimum detectable signal on the above single receiver would be 2.02 10"3 ergs. Or the energy required per cm? of image mosaic area is (l/9.0X102) (2.02 10"):2.24 `102 ergs./cm.2/sig nal. If the target area is 3"X3", i.e., 7.6 cm. 7.6 cm. and the collecting mirror is 8" in dia. with an area of and the image area is approximately 7.6X7.6:57.7 cm.2, then the energy required per cm.2 at the mirror per signal is: 2.24 10"2(57.7/324):3.99 103 ergs./cm.2/signal. Since a scanning system is used whereby each receiving element stores energy in the interval between transits of the scanning spot across it, the energy required will be the energy per signal, 3.99)(103 ergs/cm.z into the number of times the eld of view is scanned per sec. Hence, if the field of view is scanned 50 times per sec., the energy required at the mirror surface is, 3.99 103 50:0.20 ergs/crn/sec. required at the mirror surface.

For 1 mm. square condenser plates, a unit mosaic 4 mm. square could be used, which in a 3" x 3" image area, gives 19 unit mosaic areas in a row and 19 rows, or 19 19:362 unit elements in the lield of view. At 50 scans per sec., this ygives `18,100 possible signals per sec. If the eld of view is 19 degrees square, each unit area of the mosaic would be about 1 degree square.

Thus, ,by sacricing some of the inherent sensitivity. of

across resistance 104.

adsense Q ai thel'device so that it ,is'aboutas sensitive a'stthefbe'stfheat seekers` at present developed, the effective scanning spot'` can`be`reduce`dto an areal degree lsquare in aeld of view 19`deg'ree's square, and the entire eld of view can be scanned 50 times per sec.

Specific description of seeker dermis-Mosaic and scanning circuit.-(See Figs. 3, 4, 5 and 6) The thin insulating lm 75 is secured to a frame 96. The center of each mosaic element Vis a condenser plate 77 which is stenciled by evaporation on the right side of the'thin synthetic resin film 75 facing the partition`60. The plate77V is surrounded by a square surface 209 of the insulating lm 75 with its adsorbed layer of Water molecules V82. On the'incident radiation side of the insulatingtilm is a thin film 76 of metal to adsorb the radiant energy. This heat energy is conducted through the insulating lm 7,5 to the innerv or right surface of the film where the adsorbed layer of water molecules is heated. Surrounding the exposed squares2tl9 on the inner surface of the lm is a common charging plate '77 which is" alsodeposited through a stencil by evaporation. An open space 79 behind the insulating nlm 75 allows rapid diiusion of' the water molecules in equilibrium with the adsorbed layer. The gas tight partition 69 allows the proper water vapor pressure to be used for the adsorbed layer, and a good vacuum to be obtained on the scanning beam side of the mosaic. The rear set of condenser plates* 92VY are stenciled upon the insulation 78. The outer and inner condenser plates 77 and 92 constitute a setuof` condensers (one at the center of each mosaic element) which' may be charged by the action of the scanning bleam 61` in knocking out secondary electrons to the honeycomb collector cells 80. The scanning beam isfcontrolledfin the same manner as in television, so as to sweep 'for instance from position liito 101 inthe upper row and to swing from one row to the next as, 1&1 f"1`02'. u A Y Fig'.N 8` shows' the equivalent chargingV circuit for one mosaic element. The upper electron beam is not used in Phasel".

The `following phases occur: Y

`Phase (1).-Incident energy from the lield of view isfalling upon the insulating lm resistor element 75 (with its absorbing metallic film in front, the insulating lnnand the layer of adsorbed moleculesv on the rear ofthe film), the element acting as a series resistance in the charging circuit. In this charging phase a steady divergentbeam of electrons 164 covers the rear of the mosaic uniformly. The number of secondary electrons emitted to the collector cells 81 by impact of the primary electrons of the beam 164 upon the composite layer surfaces of filme plate 92 is limited by the rise of potential on plate` 92 to slightly more than that of the honeycomb collector 81. The potential of plate' 92 is controlled by thercharging current allowed by heat conducted to film 82 by plate 76 as varied by theheat radiation from the eld of view.

Phase (2).-The steady shower of electrons 164 is stopped, and the beam 61 begins to scan over the mosiac. The artificial source 91 is now made colder than the average temperature of the mosaic and the mosaic 32 now radiates a relatively large amount of energy to it. This is equivalent toa partial electrical short as the heat from the'A target is completely cancelled and the mosaic 32 is momentarily cooled below its average temperature. This allows the formation of a more complete layer of adsorbed water molecules, and reduces the temperature of the mosaic suciently to allow the charging of plates 77 to be rapidly completed. The greater the amount of charge required to charge the rear condenser cells 92 to the potential of the collector cells 8l)I in Phase 2; the greater will be the time integral of the voltage p Thus, the greater the incident radiant energy and temperature of 75 and 82 in Phasev l;

the' greater will Ybe its resistance and*v the smaller. the final charge on condenser plate 92l in this-phase. The srn'aller the charge on the condenser in Phase l; the greaterv it will bein Phasev 2, with a corresponding larger signal. Thus, the greater the incident radiation in Phase l, the the greater will be the signal when the scanning'beam completes the charge of a given mosaic element, i.e., raises the potential of plate 92 Ato approximately that of the collector cells 81 in Phase 2. The coldsource 91 maintains an effective short circuit of water'lm resist'- ance 82 during Phase 3, so that the condenser cells 80 can quickly discharge to the initial value that theyl had in Phase l'.

Aj more detailed analyses of the mosaic circuits will now be made:

Phase (1).-(See Fig. 9) charging phase In this charging phase, a steady'diverging showerof electrons 164 covers the rear of the mosaic uniformly `so as to complete the charging circuit of the condenser element. Thefrear'oondenser plates`92 are covered with a composite layer suitable for copious secondary electron emission. At the start of Phase l, condensers 92`are charged to 50' volts above earth; the rear condenser plates 77 being negative. The electron gun 33, filament 160 and accelerating grid 161, gives the electron shower a kinetic energy of volts.` Since the collector is at a potential of 150 volts to earth, the electron shower will have zero potential on reaching the collector and hence does not reach it in appreciable amount. The electron shower is decelerated to reach plates 92 with an energy of 50 electron volts, but this is ample to emit secondary electrons from the composite layer on these plates, (e';g., caesium metal, caesium oxide and metallic silver in the order named). These secondary electrons are drawn to the collector which is 150 volts above ground, and the potentials of the rear condenser plates are raised until a limit is reached, e'.g., approximately volts, at which time the number of secondary electrons passing from the plate to the collector is just equal to the electrons receiving by the collector from the elec'- tron shower. All of the rear condenser plates thus have the saineV potential to'earth at the end of Phase l but the condenser charges may be quite different, depending' upon the relative resistances 82 during thisk period. (The volt'- age acrossl the positive and negative condenser plates varies with the` series resistance, but the potential of all condenser plates 92 with respect to earth is the same.) If upper filmv 82 of Fig. 9 receives more radiant energy than the lower 82 during this period, more adsorbed water molecules will be' driven from its surface andits resistance will be higher than that of the upper one. The equivalent'charging circuit for a mosaic condenser 80 during Phase 1 is given in Fig. 10. If the connecting lines representing the cathodeA ray shower, have a low resistance relative to 82, the condenser charge is controlledalmost entirely by the variable resistanceA 82 which represents the insulating' film and its adsorbed layer of molecules.

Phase' (2) (See Fig. 11.-Scannng. phase) The steady electron shower of Phase 1' is replaced by a focussed scanning beam withv electrons at the same potential above earth as in the steady electron shower. An articial cold source receives radiant energy 162, uniformly from all of the mosaic element resistors, 82 and 82', causing them to become' relatively good conductors and allowing the upper and lower condensers to charge up completely during Phase 2. The upper condenser with the smaller charge in Phase l (corresponding to a higher temperature) will receive the greater charge during Phase 2. The pickoi Voltage 163 from 104' is proportional tocharge Q added to a condenser mosaic element during Phase 2, i.e.,` proportional to Q=l--Q. where Q` is the charge added during Phase 1. Thus the larger the incident radiation, the higher the lm resist'- 11 ance will be, the smaller Q and the larger 1-Q and the voltage picked olf from `resistor 104 when the scanning beam passes `over ya given mosaic element in PhaseZ.

Phase (3) (See Fig. 12, Restoring phase) i The scanning beam is replaced by the electron shower 164 whose kinetic energy on leaving the electron gun 161 is 50 electron volts, and the radiation 162 to the cold source 91 from the lm resistors 82 effectively short circuits the resistors 82. VThe collector 81 also has a potential of 50 volts, and since at the start of Phase 3 the rear condenser plates 92 of the element condensers are more positive than 50 volts, no secondary emission can now occur and the rear condenser plates will drop in potential until only a few volts higher in potential than the collector 81, at plus 50 volts to ground. The conditions are now the same as at the start of Phase l and the cycle is repeated.

Specific description of seeker details-Secondary electron emission from bombarded insulation The insulation 78 in Fig. 4, used as a backing for the rear condenser plates of the mosaic elements, may also emit secondary electrons under the bombardment of the electron shower in Phase 1. These surfaces do not have any corresponding conducting surfaces with completed circuit on the front of the mosaic to act as opposite plates of a condenser. Fig. 4 shows that the metallic 'film heat absorbers 76 face it, but they are insulated on the'front surface of the insulator lm 75, and having no electrical connections, can not act as parts of a condenser. Hence, the exposed surface of insulation 78, having only'the capacity of an isolated surface, charges up quickly and completely during Phase l. Since the energy of the electrons in the beam of Phase 2 is the same as that of the electrons in the shower of Phase 1, the insulation surfaces will remain charged during Phase 2, and hence will contribute nothing to the pickoff voltage across resistor 104, in Fig. 2.

Specific description of seeker details-Heating ofl mosaic by the electron bombardment.' (See Fig. 4.)

The heat liberated by the electron showers and beam striking the rear of the mosaic is relatively large compared to the minute heat signals the device is capable of detecting from the target. The danger is that when the target image remains at one place on the receiving'mo'saic for a large number of signals, e.g., at the center of the mosaic, that this portion of the adjoining partition 60 and rear mosaic insulating plate 78 will become relatively hot to the other portions of the mosaic. Then if the target image changed its position on the mosaic, these previously heated regions of long vthermal time constant would give rise to a spurious signals. To preventthis, all mosaic condenser elements receive exactly the same amount of charge each cycle. Those condenser elements in the hotter part of the image receive less charge in Phase 1 but more in Phase 2. This charge is then removed from all condenser elements in the samemanner in Phase 3. Also the impinging electrons in -Phases 1 and 2 have the same voltage. Then, the number of electrons striking each condenser elementV is theV same, and the average energy of each colliding electron is the same for each element, so that the heating produced by electron impact is uniform over the entire mosaic.V VThe above description neglects second order defects in the electron optics of the beam. As av further safeguard (see Fig. 7) the partition 60 between the front and 'rear condenser plates of the mosaic can be made of heavy conducting metal as illustrated. As shown by the indicated distribution of charges,rorn the condenser plates 7,7 of the lm 75, those on the rear plates 92 and on the metal partition 60 and insulation 78, there need be no loss of capacity by introducing the thick metal sheet 60 between the condenser plates, Since thecopperplate 60 is insulated, its negative charges opposite the rear condenser plate 92 and insulation 78 must be equal and opposite to the induced positive charge opposite the front condenser plates. The heavy copper sheet shields the insulating film and its adsorbed water layer from an inequality from the heat produced by the impinging electrons on the rear side of the mosaic.

Specific description of seeker details-Cold source, audio frequency modulation:

Where rapid modulation of the cold source is required, it is constructed as in Fig. 13. Two small, practically point source hot and cold sources and 91, respectively, are placed close togetherV and sufficiently far from the mosaic to give uniform coverage. The variable cold source is made by superimposing the eect of the radiation received from the mosaic by a xed constant cold source, and the radiation to the mosaic from a variable hot source. The cold source 91 comprises small opening 126 in a black body enclosure 127, the heavy walls of which are cooled by the ice bath 128 in copper Waterice container 129. Container 129 has a heavy layer of insulation 130 and willmaintain an approximately constant temperature for several hours. The ice is frozen in the bath by remote control. A relay v133 opens a cock 132 from a small tank of CO2 131. The CO2 expands through a chiller coil 128 in the copper water-ice con- Irainer 129 around the outer wall of the black body source 127, producing a mixture of ice and water that will remain approximately constant. The rapidly controlled heat source consists of the light from neon bulb 1-25. The amount of light energy emitted can be made approximately proportional to the current. A filter 134 cuts out the infra-red which has too much lag to be used from the glass bulb. A small opening 135 in a screen 134 in front of the filter acts as a point source. The current through the neon lamp 12S can be controlled automatically by means later to be shown by varyingV the grid voltage of a radio tube 136 connected in series with the lamp.

Initial rough adjustment of the cold source can be made so that it approximately mergesV withthe background for the rst part of Phasel. A screen (not shown, and which is not used `during operation) at various temperatures of the optical background to be eni countered, covers half of the eldfof view, the other half being covered by a screen at the same temperature as the mosaic except for a small opening for radiation to the j The diverging and scanning cathode fray beams 164 and 61, respectively, can be started, stopped, and the accelerating voltage changed, by the use of the proper voltages as supplied by a master switching device MS. The proper sequence of voltages to produce the required characteristics for the electron guns and cold source in the Phases l, 2, and 3 may be obtained by arranging pickoff brushes 137 in sequence around a drum 138 carrying insulated segments 139 connected to slip rings 140, al cycle being completed every revolution and the drum making 50 revolutions per sec. Fig. 14 shows the master switching drum 138 with its surface described in a plane, and rotating in the direction of the arrow. The slip rings are connected to the metal plates` orinsulated segments 139 which are flush with the surface of the metal drum. The brushes 137 connect the'vgiven voltage sources Yat ythe. appropriate time *toV the devices 183v of the electron gun part 33.

. is they are to control.A The sequence of the connections is as follows:

At same time.

(10) Restore voltage on collector- (11) Cut on fun cold seme---" At same me' Switching connections Fig'.V 17 is a circuit diagram with the mosaic insertion of a heat detector and shows the over-all arrangement of the components therein, particularly the switching arrangement controlled bythe rotary drum 138 of the element designated MS (Master Switch) in this figure. In this circuit the electron beam 61 and the diiused electron shower 164 may be generated in the two parts 33 and 33' of the dual electron gun which may be housed in a single tube envelope (not shown). |Ifhe mosaic 32 is scanned normally by the electron beam 61. The beam passes through the anodes (not shown) of the gun and is deflected in the usual manner by the beam deflecting plates 874 after negative charge is applied to the mosaic condenser plates 92 by the electron shower 164. The arrangement of the elements of the circuit are shown in Fig. 17 and are described hereinafter together with their electrical functions during the various phases of their operation.

Phase Unef-Staring of the shower by the electron gun 33 Lead 172 between the master switch and the relay winding 181 is positively energized by a battery 210 through brushes 137 and slip rings 140 from one of the contacts 139 of the master switch. The application of positive potential to the lead 172 energizes the relay winding 181. The energization of the relay winding 181 closes a toggle switch 182 to the left contact thereof, connecting the positive terminal of a battery 183 to the beam accelerating grid 161 of the gun part 33. The cathode 160 is connected to the grounded negative terminal of battery A control grid 186 which is associated with both the acceleratinggrid 161' and with part 33 of the electron gun has the proper voltage impressed upon it to give the desired beam intensity when the grid 161' is made positive. The diiused elecf tron beam or shower 164 is thus started by impressing a positive potential upon the grid 161. The electron beam shower 164 is applied substantially uniformly over the mosaic and imparts negative charge to the condenser plates 92 thereof.

The start of Phase two-To stop the electron shower Lead 173 between the master switch and a relay winding 187 is energized positively by a battery 210 through brushes 137 and slip rings 140 `from one of the contacts 139 of the master switch. The application of positive potential to the relay winding 187 opens the toggle switch 182 and removes the positive voltage from the grid 184 thereby stopping the electron beam or shower 164.

To start scanning beam and to start full cold source A lead 175 between the master switch and a relay winding 188 makes contact with one of the contacts 139 of the master switch, thereby connecting battery 210 through brushes 137 and slip rings 140 to relay winding 188, and thereby energizes relay winding 188 and moves an arma ture 189 to the left. The positive pole of a battery 190 is thereby connected with the accelerating grid 161 which is associated with the cathode 160 to which the negative pole of battery 190 is connected. A control grid 186 14 associated with the accelerating grid 161 has such a rvoltage impressed upon it that the proper electron scanning beams intensity is produced when the accelerating grid 161 is made positive. At the same time a lead 174 makes contact with another one of the contacts 139 of the master switch, thereby energizing the relay winding 194 positively from the battery 210 inclosing the dual armature 195 to the left. This action connects a battery 196 across a resistor 197. The resistor 197 is connected with the grid 198 of the tube 136 and with the negative terminal of the battery 196. Such action drives the grid 198 of the tube 136 negative and stops the ilow of plate current of the tube 136. The tube 136 when conducting, supplies heat energy to the temperature regulator in the cold source 91 shown in Fig. 13. The conductivity of the tube 136 increases the temperature of the cold source 91 to the desired degree.

Start of Phase three- To slop the scanning beam 61 A lead 176 between the master switch and a relay winding 200 was energized from one of the contacts 139 of the master switch and the battery 210i to actuate the relay winding 200, thereby closing the right-hand contact of the relay armature 189. Such action connects a battery 201 in series with the electron gun 33 and the accelerating grid 161 thereof, thereby putting a negative voltage on the accelerating grid 161 and stopping the electron scanning beam 61.

To start the electron shower 164 at a reduced voltage A lead 177 between the master switch and a relay winding 202 makes contact with one of the contacts 139 of the master switch, thereby energizing from battery 211 the relay winding 202 and closing the switch 203 to the right. The battery 204 is thereby connected in series with the cathode of the electron gun 33' and the accelerating grid 161' thereof. Such action starts the electron shower 164 at a reduced voltage since the battery 204 produces less voltage than the battery 183.

To lower the voltage on the collector 81 A lead 178 between the master switch and a relay winding 205 makes contact with one of the contacts 139 on the master switch at the same time that lead 177 makes a contact 139. Relay winding 205 is thereby energized from battery 211 so that the switch 107 closes to the left. The positive potential battery 105 is then applied through the resistor 104 to the collector 81. The battery 105 is of lower voltage than battery 106. The negative terminal of battery 105 is connected to ground and the negative terminal of battery 106 is connected to the positive terminal of battery 201. The collector 81 is put on a lower voltage by switching in the battery 105 instead of battery 106.

To restore full voltage on the collector 81 The lead 179 between the master switch and the relay winding 207 is energized from one of the contacts 139 on the master switch and thereby energizes relay winding 207 from battery 2111. The energization of the relay winding 207 pulls the armature 107 to the right to connect the positive terminal of the battery 106 through the resistor 104 to the collector 81. Such action connects the Voltage of battery 106 with the collector 81. The latter is thereby enabled to collect electrons splashed from coudenser plates 92.

To cut of? the full cold source The lead 180 between the master switch and a relay winding 208 makes contact with one of the contacts 139 on the master switch at the same time as lead 179. The relay Winding 208 is thereby energized. The energization of relay winding 208 pulls dual armature to the right. Such action connects the leads 88 and 89 from the signal amplifier 85 across the resistor 197 which is associated with the control gridof the tube 136. A signal amplitude can then vary the electrical bias on the tube `136, which in turn can Vary the terminal bias produced lby-the cold source 91.

Specific details of seeker.-Thermal bias as rapidly as the electron beam can supply electrons.

Thus all of the spikesnwould start with the same slopev as the scanning spot moved onto the condenser plates, and the voltage would level ofiE at a constant value when the scanning spot was completely on the condenser plate.

is necessary to have a pickoi 88, 89 for thermal pips and a delay network 86 incorporated with amplifier 85 (see Fig. 2) that gives a response proportional to the voltage on a condenser, i.e., proportional to Iidt. Since In order to differentiate between the signals A and B it the output of the amplifier 85 is proportional to the energy only the spike A signal is received by the amplifier. The

hottest portion of the field of View has been selected. The delay network 86 of Fig. 2 stores up the energy of the spike for several cycles until it is sufficient to trip the electron coupler 87 and take a proportional signal off of the deflection plates 84.

Specific description of seeker details-The electron coupler A simple circuit for the electron coupler is shown in Fig. 16. The deflection plates 84 are connected by the electronic coupler to the signal leads 95. The output from the delay network 86 in Fig. 2 is impressed upon the leads 141 of the electronic coupler. Leads 141 are across battery 142 in series with rectifier 143 and resistance 144. The rectifier prevents current from flowing through the resistance until the input voltage exceeds that of ,battery 142. The voltage then picked up across the resistance is greatly amplified by tube 145 andV fed into thetranstiormer 146. The grid and cathode of tube 145Hare connected in parallel with the resistance 144. A battery 153 is connected in series with a transformer primary winding 146 and the cathode of tube 145. The secondary 146' of this transformer is connected to the grid of a tube 147. This tube is normally biased to cut'oi by battery 148. An all or nothing effect is obtained, for when the input leads 141 receive a voltage pulse only a small fraction larger than that of battery 142, the voltage from transformer 146 is suicient to overcomeY a biasing or cuto battery 148, and tube 147 is made conducting. Nearly all of the voltage of a battery 149 is then impressed across a resistance 150. Resistance 150 is made larger with respect to the resistance of tube 147 when activated by the transformer 146 and the tube grid. A tube 152 having a cathode, controlgrid, screen grid and plate is provided. The deflection plates 84 are connected in series with the control grid 155 of the tube 152.v A resistor 150 yand a battery 151 are connected in series with the screen grid of tube 152. Since the voltage across resistance 150 from battery 151 is the normal value for Vthe tube 152, the output to transformer 153 and signal leads 95 `from a battery 160 will be proportionall to the input voltage on control grid 155 and cathode 1560i tube4V 152 from the cathode ray beam deection plates 84 which isin turn responsive to mosaic output. It is necessary to use a thermal rather than an electrical -bias to eliminate all but the strongest signal, as large signals might drive .off all of the adsorbed water molecules from the thin insulating receiver films, and these mosaic elements would give the same response regardless of the relative signal size. Y Y Y Y. Y. Y

Specific description of seeker details.-Magnitude of A pickot voltage The order of magnitude of the capacity of one of the small condenser elements of the mosaic is,

C=KA/41rd A.

where,

K=die1ectric constant of insulation between condenser plates.

A :area of one condenser plate.

d=thickness of dielectric between plates.

This application of the above formula to the condensers of the mosaic is subject to a considerable error due to edge effect, but the order of magnitude is the only value considered. Most of the insulation between plates is l water vapor with a dielectric constant of approximately l 1.A=l mm'. l mm.=l02 cm?,

l/QX 10'11 3.v99 l03=4.43X 10-15 farads Ifthe change in condenser charge is that due to volts, Q=CV=4A3 X 10-15 1OZ=4.43 X10-13 coulombs. For 18,100 elements per sec. the time of passage of a scanning beam over a'condenser plate would be d=0.20 om. C:

=5.53 10*5 sec.

For `a pickoff resistance of 104 ohms, the pickof voltage Would be 8.01 l0 l04=8.01X10"s V. If the minimum detectable signal is 1/10 of this maximum value, minii mum signal 0.1 8.0l 105=8.01 106 volts, or 8 i micro-volts, which is readily detected. Since the frex quency range is much less than that in television, considerably higher values of pickoff resistance Vcould be; usedY with correspondingly higher pickoffY voltages, withi out thermal noise limitation. Y

Specific description of seeker details- Electrical time constant of mosaic element: (See 5) The square condenser plate 77, l mm. on aside, is separated from the outer charged plate 77d by the 3 mm. square of insulating film 75 with its partial layer of adsorbed molecules. A relative humidity may be chosen such that the specific resistance of the insulating lm is =5 107 ohms across the sides of a mm. square of surface. The resistance of the film surface between the l mm. square and the plate is, R=L/A, where L= 1` mm., and A as indicated by the dotted line 209 is approximately 4.5 mm. in effective length. Hence, R=L/A=approx1 mately 5 l07 (l/4.5)=1.1l 10'I ohms. The time constant T=l.l1 107x443 X 1()-15=4.92)1 10"8 `sec.

, AFunctional explanation of cycle of operations -Atthe start of Phase 1 the collector 81ispositive with respect to the discharged condenser plates 92 and 77. The electron shower 164 is started by contact 17V2`and relay 181, and the condenser plates 92Vare quickly brought up to a potential slightly exceeding that of the collector 81 so that no more secondary electrons leave condenser plates 92 than primary electrons arrive in shower164. Charging ofthe plates 92, however, does not charge up the .condenser elements consistingY of platesV 9.2A and 77.

ansehe? A condenser cannot be charged byraising :the potential vof :an isolated plate 92; to put appreciablefcharge on .the condenser consisting of plates 92 and 77, la complete charging circuit must be established. The charging circuit is completed through the surface resistance of lm 75, on the front surface of which is deposited infra-red absorbing lm 76. lf an infra-red image is focused on an absorbing plate 76, the lm 75 in that section ofthe mosaic 32 will have a higher resistance than in Vother sections and thecorresponding condenser element with plates 77 and 92 will have a relatively small charge on its plates at the end of Phase 1. The condenser plates 92 charge through the secondary electrons emitted'to collector 81, through resistor 104, and the battery 106 to earth, the toggle switch -107 having its'right contact closed'during Phasel l. The signal voltage that 4occurs across resistance .1104 during the charging of a'mosaic condenser `element is amplified by 85 whose output leads 88 and 89 are vconnected through switch 195 across the grid resistance 197 Aof'tube 196. When the heat signal on the mosaic is too Ylarge and would evaporate the water lilm from all .the mosaic elements alike, thus eliminating any diierence in signals from the section under the target image and'from therest of the mosaic, tube 196 is blocked byzthe extra vlarge negative signal on its grid. This stops the current through the lamp heater unit 125 of the cold source 9.1, fthus in effect providing an increased cold source 'which -acts as a thermal bias to limit the size ofthe heatsignal in the same way as automatic volume control limitszthe f size of the electrical signal. The. signal from amplifier-85 goes to the delay network '86 and can actuate` the coupling circuit 87, connecting the output leads 95.to.a volt- ;age'proportional to that across .the deectionfplates184. "No signals are obtained from 95 in therst phase, .howl ever, since scanning beam 61 has not ibeen; turned on .and no "deecting voltage is onplates 84, sincetheiconventional scan oscillator voltageis connected to 4.plates 84;by -fswitch 189 at the same time beam161 isstarted (starting f connection for standard scanning voltagernotV shown), at

:the end of Phase l, lead 173 is connectedandy relay 187- is energized, opening switch 182 andstopping the:elec .tron shower y164.

Phase II.-Scanning 'phase Y The charge on the mosaic condenserelements, `92and 77 in Phase I, diered from maximum by an amount V,proportional to the infra-red radiation absorbed by'the mosaic element. Hence, in Phase Il when the charge on the condenser elements is completed; the charge flowing t onto the condenser elements throughresistor104when [the charging circuit is completed by the scanning beam 561, is proportional to the infra-red energy -receivedby the given mosaic element. Lead 175 is` energized to close z relay 188 to start scanning beam 61. In order forthe mosaic condenser elements 92' and 77v to'be'complet'ely I. charged, it is necessary to `reduce the resistance of film 75 that is in series with the charging circuit. 'This lreduc- :tion in resistance is produced 'by starting the cold source 191. YTo start the cold source 91 at the beginningof Phase II, lead 174 is energized, operating relay 194, pulling Aswitch 195 to the left and placing battery 196 across resistor 197 driving the grid 198 of tube `196 sufficiently negative to completely stop the heating lamp 125 ofcold :source 91. This makes cold source 91 a maximum. `During APhase Il, signals are picked off yfrom resistor 104.as 4the mosaic condenser elements'are scanned. Th'esesig` vnal'voltages are ampliiied by 85, delayed by network 86 :and impressed on the coupling circuit 87Which4 plates a yvoltage across the output leads 95'proporti0nal tothat vacross the deflection plates 84 at theinstant the 'scanning `Ebeam 61 passes over the mosaic element that is made'the hottest bythe infra-red image. AmpliiierSS is equipped with an automatic bias `control to depress all signals so that only the strongest comes through. The delay circuit Icanf 'overcomethe effect of a1- decoy-'sgnaklastingffor only fone 'cr 'two cycles.

vless heating of mosaic.

`I8 ,Beam 61 is stopped at-the end of Phase II by the energization of lead 176, the activation of relay 200 and opening switch'189.

Phase IIL-Restoring phase To discharge the condenser elements 92 and 77 the full cold source is left on to reduce the resistance of lm in series with the discharge circuit. Lead 177 is energized to operate relay 202, pulling switch 203 to the right to put the accelerating grid 161 of electron gun 331 on the reduced voltage of battery 204. At the same time voltage is reduced on collector81 by lead 178 making contact with 139, relay 205l closing left contact of switch 107, `connecting the battery in series with collector 81. Condenser plates 92 will now be at a higher positive potential than collector 81 and no secondary electrons will be emitted until the potential of plates 92 is reduced to vapproximately that of collector 81. The voltage of electron shower 164 was lowered as plates 92 could be discharged by electron shower 164 without electron secondary electron emission, and the lower voltage produces At the end of Phase Ill just before the start of Phase I, again, lead 179 contacts 139,

relay 207 is energized and switch 1017 connects battery right-hand contacts of switch which removes the maximum cold source' by again connecting leads 88 and 89 -across Vresistor 197 so that tube 196 can now lbe reguvlated'bythe signaloutput'from amplifier 85 to provide thermal bias. Battery 204 for the lowered accelerating grid voltage onelectron gun 33 is automatically discon -nected'when switch 182 connects battery 183 in series ywith the grid 161 -atthe start-of Phase l. When relay 202 pulls switch 203 to close the right-hand side contact, it pulls itagains the tension of aspring (not shown) until a catch- (not shown) latches it in position. -When switch '182 closes to the left, it releases the latch on switch 203 allowing it to open.

The invention claimed is:

flfln an infra-redv radiation detector, a multiplicity of cells arranged as a` mosaic; in each cell thereof a substantially central condenser, a thin insulating iilm sur- 45' rounding said condenser, an adsorbed partially monomolecular aqueous tilm upon said insulating film whereby the resistance of the insulating film varies with the completeness of the adsorbed layer of water molecules as controlled by incident infra-redradiation.

`2. An infra-red radiation detector according to claim l in which the insulating film is synthetic resin ofthe phenol-formaldehyde varnish type.

3."In 'an infra-red detector, a mosaic element, a resist- Vance film bearing a multiplicity of individual cells, said lmbeing adapted to act as one plate of a condenser of said mosaic element a second plate bearing a multiplicity of mosaic condensing elements for assisting'the first plate to complete the functions of a condenser, the resistance of the first plate being in series with thel condenser so completed, a gas tight partition separating the two plates, means for maintaining a substantially constant humidity on that side of the partition containing the resistance film, so that the vapor pressure of the resistance side of the partition yis suitable for variation of the resistance by change kof the .number of adsorbed molecules ofthe vapor with incident infra-red radiation and means'for maintaining a partial vacuum around the con denser plate on the opposite side of the gas tight partition so that said condenser plate may be scanned over its individual condenser elements by a cathode ray beam. '4. In an infra-red radiation detector, the combination which comprises a heat-ray-sensitive electrical mosaic,

of the radiation of said cold source to said-mosaic, said means comprising an electrical resistance located between said cold source and said mosaic and an electronic coupling circuit adapted to use part of the electrical output of said mosaic to bias said cold source by means of the heat generated by said resistance so that the heat input to the mosaic from the field of view is cancelled except for the most intense infra-red local radiation derived from the field of view.

5. In an infra-red detector suitable for directing guided missiles for military purposes, a shell adapted to be attached to the front of a missile, heat insulating material substantially surrounding said shell except at the front portion thereof, an infra-red ray-transparent nose, an infra-red sensitive electrical element within said shell, means for maintaining stabilized conditions of humidity and temperature around said sensitive element and electrical means adapted to use a part of the electrical output of said sensitive element to thermally bias the infrared rays received by said element whereby to cancel out all infra-red signals except substantially the strongest ones.

6. In an infra-red detector, an infra-red sensitive electrical mosaic, said mosaic comprising a multiplicity of condenser plates, a resistance in film form in series with said plates, said resistance being variable with the incident infra-red radiation from the field of view, a regulatable artificial cold source disposed in the path of the infrared radiation on its way to the mosaic, an electrically resistant heat source between the cold source and the mosaic, said source being adapted to receive a part of the electrical output of said mosaic resistances, thereby causing a partial short circuit of said heat source whereby a rapid discharge of the condenser elements of the mosaic is brought about during the restoring phase of the operation of scanning said mosaic with a cathode ray beam.

7. In an infra-red ray detector, the combination which comprises a heat ray sensitive electrical mosaic, a multiplicity of condensers in said mosaic, a resistance in ihn form in series with said condensers, an electron gun, means connected with said gun for at one time showering all of said condensers and at another time for scanning all of said condensers serially with an electron beam having a small scanning spot and a rapid scanning action, said beam serving to connect serially, all of said condensers to earth.

8. In an infra-red sensitive electrical detector, the combination which comprises a mosaic element, a multiple condenser therefor, a s eries resistance in film form connected with said condenser and variable with the incident infra-red radiation, electron guns adapted to bombard condenser plates on one side of the mosaic, a multiplecelled electron collector of conducting mesh mounted in closely adjacent relation to the side of the mosaic which is to be scanned, said collector being adapted to receive the secondary emission from the condenser plates and to limit the charging potential of the condenser units.

9. In an infra-red radiation detector, the combination which comprises an approximately constant temperature boiler, an electrical heating coil therefor, a condenser, means for providing a streamline flow of vapor from said boiler to said condenser, a constant temperature bath Within the condenser, said bath containing a material having a liquid phase and a solid phase that can coexist at the temperature of condensing steam, a pressure-sensitive device under control of the pressure of gas or vapor at the top of said bath, a relay under control of said device, a cooling coil in the internal constant temperature bath thru which a cooling medium from another constant temperature bath is circulated whereby the vapor flow to the condenser will carry heat to it equal to the amount removed by the coolingcoil and the vapor ow is maintained constant regardless of varying condensation along its path as caused by changing ambient temperature. v

10. In combination, an artificialI cold source and means for tempering the cold from said source whereby to produce the overall effect of a cold source with high speed l modulation to create a constant temperature, said tempering means comprising a hot source emitting radiant light energy comprising a substantial proportion of infra-red rays, a modulating means for said hot source,

an electric heat detector placed in the zone of radiation of said hot source and of said cold source, said modulating means comprising an electronic tube of at least three electrodes, one of which is a grid, and an Velectronic cird cuit whereby a portion of the ouput of said heat detector is diverted to the grid of said tube.

11. The method of detecting a strong infra-red signal which comprises receiving saidV signal on an infra-red detector having a substantial extent of field, thermally biasing said detector to cancel out all background infrared so that only the strongest signals will come thru, ap-v plying said strongest signals to evaporate a partially monomolecular film of water from portions of a synthetic resin film in said detector, whereby to generate a static charge on said iilm, accumulating said charges on previously charged separate condenser cells of said detector, scan-` ning the cells after the addition or subtraction of charge due to the superposition of the film charge on the con;

.denser charge, amplifying the scanned image, then feeding said amplified image thru a delay network then thru an electronic coupler adapted to generate an output signal 'having a voltage and polarity sufficient to give said signal proportional control properties when it is supplied to an appropriate electro-mechanical system.

12. A missile head comprising, in combination, an' outer cylindrical shell, an inner shell concentric therewith, vheat insulating material substantially surrounding both shells, a conical nose for said outer shell, a light# lray-transparent'apex on said conical nose, a mirror sys tem for focussing the rays admitted through said trans- -parent apex and for 'projecting them rearward through the inner shell, a heat-ray-detecting mosaic mounted within said inner shell, means for scanning said heat-ray-detecting mosaic, a third shell surrounding said heat-ray-- detecting mosaic, said shell being partly Vcomposed of a material transparent to heat rays, said transparent material sealing the third shell near the forward end thereof, the space behind the transparent material and including the scanning means being evacuated of air and means for keeping constant the ambient temperature and humidity within the entire space within the shells except that space which is evacuated.

13. In an infra-red detector, a heat seeker comprising a vheat radiation-sensitive electrical mosaic capable of converting an impressed image into an electrical signal, an output circuit for said seeker, means for emitting a cathode-ray directed toward said Ymosaic, cathode ray deflecting means for causing the cathode ray to sweep said mosaic, a coupling circuit energized by signal from the mosaic caused by the image impressed thereon and transmitting a voltage signal to the seeker outputV circuit proportional to the voltage across the cathoderay deiiecting means, saidcoupling circuit having an output circuit and valso having an input circuit comprising, in series, a rectifier, a first battery and a first resistor with the coupling circuit input circuit voltage impressed thereacross so that no current iiows into the input circuit until the-input voltage exceeds that of the rst battery in the inputcircuit, a first input tube having a grid and a cathode across which the first resistor is paralleled and having a plate, a first transformer having primary and secondary windings, a second battery in series with said first transformer primary winding and with the plate and the cathode of said first tube, a second tube having a plate, a cathode and a grid, a grid-biasing third battery in series with the grid and cathode of said second tube and with the secondary winding of said first transformer, a ,fourth battery providing potential for the plate former primary winding, and said second transformer 10 secondary winding connected across said seeker output circuit .and providing seeker output signals whereby a voltage will be `furnished to the output circuit of the seeker as actuated by said mosaic.

References Cited in the tile of this patent UNITED STATES PATENTS 2,306,272 Levy Dec. 22, 1942

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