THERMAL IMAGER INCORPORATING SENSOR WITHIN ELECTRONICS MODULE

Abstract: A thermal imaging system in which infrared radiation (56) from the viewed scene is transmitted to a two-dimensional detector array carried on the focal plane (52) of an optical/electronics module (54) which has embedded in it amplifying, filtering and multiplexing circuitry utilizing MOSFET transistors. The module is located inside the cooling device (62). Cooling requirements depend on the alternatives (a) of using detectors responsive to wavelengths in the 3.0 to 5.0 micron range, which require less cooling, or (b) of using detectors responsive to wavelengths in the 8.0 to 12.0 micron range, which require liquid nitrogen cooling. The two-dimensional detector array may be combined with a limited scanning, called ''nutation'', which causes each detector to view a plurality of pixels in the incoming infrared radiation.

Description: _ "Thermal Imager Incorporating Sensor within Electronics Module" Background of the Invention This invention relates to the field of thermal imaging, 5 and to the systems commonly known as forward looking infrared (FLIR) systems. The primary purpose of FLIR systems is night vision. Prior art systems, referred to as. Common Module FLIRs (CMFLIR), are designed to use infrared detection to permit 0 an operator to, in effect, "see" and identify objects which would otherwise be invisible. Such a system employs mechanically scanned line array of photo-conductive mercury- cadmium-telluride (MCT) detectors cooled to approximately 77°K. The term "common module" refers to the requirement 15 that each major component of the system be replaceable by an equivalent module. In the "Preliminary Common Module Design Handbook" published Jay Magnavox, the following common module FLIR elements are listed: (1) Detector, comprising an array of 20 180 vertically oriented elements of Mercury Cadmium Telluride (HgCdTe); 12) • IR (infrared) Imager. which directs infrared inputs toward the detector; (3) Scanner, which is a mechanically oscillating mirror sequentially directing infrared inputs onto the linear detector array; (4) £can -U- . 25. Interlace Electronics,, which provides mirror motion drive; (5) Cooler , which maintains the detectors at approximately 77°K; (6) Preamplifiers^ which provide a 70 to 1 amplifica¬ tion of the electronic output signals of the detectors; c (7) Post Amplifiers, which further amplify the detector output signals; (8) £fi Display, which converts the ampli¬ fied detector output signals into visible light, which is collimated by a Visual Collimator module and returned to the reflecting back surface of the scan mirror; and (9) Ima&s. Intertsifier. which receives the scanned LED display output, intensifies it, and directs it to a biocular eyepiece. The complexity and costliness of the present FLIR systems is apparent from the extensive number of major components. Also, they have significant performance limita¬ tions. Some of the needs of the prior art Common Module FLIRs are: (a) more producible detector arrays, presently a manu¬ facturing bottleneck; (b) TV display compatibility without the complex electronic interface units currently being used; (c) elimination of the logistic and maintenance requirements of liquid nitrogen cooling; and (d) reduced number and size of the system components. The purposes of the present invention are to meet those needs, and, perhaps more importantly, to provide a new FLIR system which will offer the option of either maintaining or improving performance at a greatly reduced cost, or providing much better performance without a cost increase. Summary of the Invention The present invention provides a basically altered FLIR system which incorporates a detector module having in a single miniature structure both a focal plane on which are 3 located a multipl icity of staring detectors and a three- dimensional electronics package which pre-amplifies, filters and multiplexes the electronic signals received from the detectors. Performance is vastly improved by prov iding a much greater number of detectors, reducing or el iminating scann ing requi rements , and l ocat ing l ow no ise , h igh performance electronic processing at the focal plane (inside the detector cooling unit) . The performance improvements permit the al ternative choices of (a) using relatively high efficiency detectors to provide a much more sensitive system than prior art systems, o r (b) us ing l owe r ef f ic iency detecto rs to prov ide sensitivity at least equal to that of prior art systems, but at a much lower cost.' Another choice of alternatives is to (a) el iminate entirely the scanning function, thereby removing a complex part of the system, or (b) use l imited (and simpl ified) scanning means to provide a plurality of resolution pixels for each detector . Brief Description of the Drawings Figure 1 is a diagrammatic showing of a prior art Common Module FLIR System; Figure 2 is a diagrammatic showing of the present invention, in which appl icants' detector module replaces several of the components in the prior art system; Figure 3 is a partly-sectional showing of a much simplified radiation input portion of the system of Figure 2; Figure 4 is an isometric close-up of the detector module and its cooling structure used in Figs. 2 and 3; Figure 5 is a partly exploded isometric view of the stacked circuit-carrying layers which constitute the detector module; c Figure 6 is an electronic block diagram of the layer- carried circuitry associated with each detector mounted on the module; Figure 7 is a schematic of a preamplifier circuit suitable for the circuitry of Figure 6; Figure 8 is a schematic illustration of the switched capacitance network used as a resistance-equivalent in the adaptive bandpass filter of Figure 6; Figure 9 is a schematic of a multiplexer circuit suitable for the circuitry of Figure 6; Figure 10 shows an optical intake system fordirecting radiation to the detector array, which system includes means (referred to as a "nutator") for causing limited scanning motion of the radiation directed to each detector; Figure 11 is an isometric view showing the nutating mirror element of Figure 10; Figures 12A and 12B are cross-sections taken through the structure of Figure 11 along the two pivot axes of the nutating mirror; Figure 13 shows a different embodiment of an optical 5 intake system in which the nutation is accomplished by a refracting element rather than a reflecting element; and Figures 14 and 15 show two possible scanning patterns which might be used in nutator scanning, including a pattern in which the same radiation is directed to more than one detector, for comparison and calibration purposes. Detailed Description of Specific Embodiments Figure 1 shows a typical Common Module FLIR system. Thermal (IR) radiation from the viewed scene is received by an optical module 12, having an afocal magnifying lens. A colli ated beam 14 from module 12 impinges on a mirror 16 incorporated in a scanner module 18. Mirror 16 reflects IR energy to an imaging module 20, which focuses it on a detec¬ tor module 22. The detector comprises a vertically-oriented line array of detector elements, which are formed of mercury cadmium telluride (HgCdTe, hereafter "MCT"). Because a line array detector module is used, it is necessary to scan across the viewed scene, in order to cause energy from successive segments of the scene to impinge seriatim on the detectors. This is accomplished by causing oscillation of mirror 16 about its axis. The mirror is o driven, by a suitable motor, through a scan angle up to 10 (5° to each side of center). The electronic signals output by the scanned detector array 22 are directed first to a pre-amplifier module 24, and then to a post-amplifier module 26. The amplified signals are then conducted to an LED module 28, which has an LED element corresponding to each detector element. Visible light from the LED module is collimated by a visual collimator module 30; and the collimated beam 32 is passed to the back of scan mirror 16 (which also is a reflecting surface) . The scanning movement of mirror 16 causes the visible light to image a visual representation of the infrared scene on the face of a light intensifier module 34, which permits viewing by an observer through a biocular eye-piece 36. An important and expensive module is the cooler, which is indicated at 38. The MCT detectors need to be cooled to approximately 77°κ. Only the detectors are cooled; the electronics are all in a relatively "warm" environment. The invention of the present application is a thermal imaging system (FLIR) in which: (a) a two-dimensional array of detectors is used instead of a line array; and (b) the 'detector array is on one surface of a module which contains electronics for pre-amplifying, filtering and multiplexing the detector output signals, all located in the cooler. Other FLIR systems are being developed having two- dimensional detector arrays, rather than a scanned linear array. As pointed out in a descriptive paper: "Staring arrays are being considered for many of the next generation IR systems because of the potential of increased performance with less mechanical complexity. Longer integration time of staring arrays offers a sensitivity increase when compared to scanned systems. Elimination of mechanical scanning gives size, cost and reliability advantages to staring ray,. τhls is especlally true . n ^^ ^^ ^ size constraints. τhese advantages aiso appiy fco thceat auction applications in whi ch many arrays are retired t. continuous!,, cover the total 4 steradians field of regard." E cept for the present invention, the proposed two- ■ nensional """ ° f St " in * **«*< .. for P LIR systems are Planar arrays, either hybrid or monolithic, which rely on non-fo=al- planβ electronics, i.e., the detector output "Snals are conducted to electronic processing circuitry located away from the d etector plane. Th e advantages of the Present invention over pi anar array Bysteιns w d i scussed in detail below. Figure 2 shows a p ia „ view _ iagtaa _ f ___ . .^^ Qf a FL IR system incorporating the present invention. An optical module 4, directs a colli d ed » beara to a module .42, which includes a chopper assembly 44 driven by motor 4 6 , and a f ixed (no „- soanning) mirroc ^ ^ ....^^ ^ " reflected by m irror 48 to an imaging module 50. The incoming « energy is directed by imaging module 5β to a two-dimensional detector array on the focal plane 52 of a aeteotor/electronics module 54. The IK beam 56, which -Pinges on focal pi ane 5 2, covers the entire area of the focal pi ane ( instead of being focused on a single detector, s xn the systeιn o£ Pigure ,,_ τhe ^ ^ ^ ^ -elude any desired number of detectors, an array of 128 X 128 being an exemplary choice. The detector/electronics module 54 includes electronics 8 for independen ly pre-amplifying and filtering the electronic information from each detector, and then multi¬ plexing the detector signals. This multiplexed output is conducted to a video signal generator 58, whose output is conducted to a video display 60. The detector/electronics module 54 may be mounted in a cooler 62, if efficient detector operation requires cooling. A major advantage of the present invention is its capability of providing substantial electronic processing at the focal plane (inside the cooler). As will be explained in detail below, this is possible because of the unusually low power dissipation of the electronic circuitry in module 54. It is apparent that the system of Figure 2 has substantially simplified the system of Figure 1. However, the present invention permits much further simplification, as is evidenced in Figure 3. The system of Figure 2 was ' designed for maximum compatibility with the Common Module FLIRs which are currently in widespread use. In the system of Figure 3, IR energy from the viewed scene is received by lens 64, and directed through suitable optics 66 and a chopper 68 to a two-dimensional focal plane 70 on a detector/electronics module 72. The module 72 is supported in a multi-stage thermoelectric cooler 74, and has its electronic output signals conducted to a video signal generator via connectors 76. The purpose of chopper 44 in Figure 2 and of chopper 68 in Figure 3 is to provide a DC reference value against which the detector signals are compared; i.e., the choppers are neede d to briefly cut off the radiation from the viewe d scene a „d to looκ, i„ st e ad , at . surface w ch ^ ^ formation for calibration purposes. This calibration '-notion in the prior art scanning „„ 0 f F igure 1 is accomplished by continuing the scanning sweep past the edges o f the viewed scene. F igure 4 shows an en.arged isometric view of the cooler 74 and the detector/electronics module 72. T he focal plane ". wh i ch may, f or example, contain an array o f l 28 X 128 d etectors, is one surface of module 72, w h ich comprises a -Itip.icity of l.yers 78 extending in p ι an es perpen di cu l ar to focal P ι ane „. τhese layers ^ __ _^ ^^^ c i rcuitry, w hi ch win be detailed below, A p l ura l ity of lea d s 8 0 are provided, which are e l ectrica l ly connecte d to the bottom, or bac k plan e, of module 72, for the purpose of .-putting control si g nals to the mo d u l e, an d outputting d etector-init t d s i gnals from the mo d u l e. T he mo d ule is mounted atop the coding pi at£θ rm o f mu l ti-s t a g e t he rmo ¬ e l ectric cooler 74, and is further protecte d f rom h ea t by ra d iat i on shie ld s 8 2, which are coole d b y t h e coo l er 74, an d wh.ch prevent spurious energy f rom reaching mo d ule 72. Th e t hree- d imensional detector/electronics modules 54 « f igure 2 , and 72 in rigure 3, may b e simi l ar to th ose aisclosed in „. S . S . N . „,,,„, f .^ ^^ ^ ^ ^ continuation u.s. s . N . 572#802 , fUed ^.^ ^ ^.^ to the assignee of this application. A s shown in Figure 5, -ch a mo d u l e comprises a stack of thin layers 84, each carrying circuitry which p rforms pre-amplifyi ng , ll t . rlnα and multiplexing f unctions on the individual photo-detector output signa l s. P referably the layers 84 are semicon d uctor ch i , having integrated circuits , Ϊ C ) formed thereon. The ch i ps are secured together to form the module, and each has a large number of C 1OR_ U B m~. J or c l osely spaced electrical leads at the focal plane, which has an array of d etectors 8 6 . E ach aetector is in d ependently i„ electrical contact with one of the c l ose l y space d electrical leads provi d e d b y t h e cir ¬ cuitry on the chips. B ackplane wiring 88 is used to connect the circuitry on the stacke d chips with the externa l e l ectrica l l ea d s 80 ( Figure 4) . F igure 6 exempl i f ies the general layout of circuitry connecte d to each detector, if . two- d imensiona l array o f 128 x 12 8 d etectors is used, each c h ip ( or layer ) 8 4 wi l l carry 12 8 such circuits, and 128 chips ( or l ayers ) 8 4 wi l l e stacked and bon d ed together. As shown, each d etector 9 0 has its output connected to a preamp l ifier 92. Each pre ¬ mp l if i er f ee d s its output signal to an in d ivi d ua l a d ap ti ve b an d pass f i l t β r 9 4, which selects the d esired b an d o f f luencies to be processed. The post-filter signal is i nput to a mu l tiplexer 96, which comprises para l l e l input branches from each of the separate detector circuits, an d a control circuit which causes sequencing of the mu.tiplexed output s i gnals conducted o f f t h e chip. Th e preamp l i f i er 92 may be similar to that d isc l ose d and claime d i„ common assignee app l ication O . S . S . N . S58 , 099 , 1 1 f iled - December 5, 1983; and the multiplexer 96 may be similar to that disclosed and claimed in common assignee U.S. Patent No. 4,490, 626, issued December 25, 1984 on Appl ication No. 403, 004, filed July 29, 1982. The circuit of Figure 6 is designed for the use of photo-voltaic detectors. if photo-conductive detectors are used, an exterior current source 98 (shown in phantom) is included. Al l of the "focal plane" circuitry, i.e., comprising the separate preampl ifiers, separate filters and multi- plexing circuitry, is located inside the cooled container, the reduced temperature of which is required for detector effectiveness. The transistors used in the focal plane circuitry located on the stacked chips are preferably MOSFETS, because of their ability to operate under cryogenic conditions, and because of major advantages which they bring to the problems addressed in developing suitable "on-focal-plane" circuitry. Λ fundamental problem is the requirement for low power operation, because power dissipation must be minimal. As a means of minimizing power dissipation, the pre¬ amplifier 92 is operated in the "weak inversion" mode (also referred to as "sub-threshold" or "current-starved") . The importance of this functional aspect is discussed in detail in ϋ.S.S.N. 558, 099. In that application, it is explained that the disclosed preampl ifier — an operational amplifier held in the weak inversion range — meets the severe require¬ ments, including space limitations, low power limitations. high gain requirements, and variable gain adjustability. The disclosure of that application is incorporated by reference in the present description. Figure 7 corresponds to Figure 2 of ϋ.S.S.N. 558, 009, but its description omits much of the specific description included in the prior application. In Figure 7, the pre¬ amplifier has a differential amplifier portion comprising six MOSFET transistors: a differential pair 104 and 106; a cascode pair 108 and 110; and a current mirror pair 112 and 114. Transistor 104 has its gate connected to the detector signa l ; and its matched trans istor 106 has its gate connected to ground. The sources of transistors 104 and 106 .are both connected to a constant current source 116. Another constant current source 118 supplies current to a source fol lower transistor 120, which provides the output signal from the preamplifier to the filter 94. The source follower transistor 120 has its source-to-drain current flowing between the constant current source 118 and the negative bias voltage 122. The positive bias voltage 124 is applied at one side of the two constant current sources 116 and 118. The feedback resistance of the operational amplifier, which is shown diagrammatical ly at 126 in Figure 6, is preferably provided by a switched capacitance network (as shown in Figure 7) , which comprises MOSFET switching transistors 128 and 130, capacitor 132, and clock-controlled inputs at gates 134 and 136. By changing the f requency of the switching network, the equivalent resistance value can if 13 be varied for optimal results. In other words, the pre¬ amplifier gain is variable on a real time basis. The amplified signal from each detector is fed into the adaptive bandpass filter 94, where the output signal is limited to the desired range of frequencies. The filter 94 couples the detector to the multiplexer and performs the function of real time clutter rejection, which is very important in preventing unwanted extraneous signals from entering the multiplexer, and thus from entering subsequent post-processing where they must be removed in a more compli¬ cated process. The bandpass filter is implemented using switched capacitors in place of resistive elements. Figure 8 shows an individual resistance-equivalent switched capaci¬ tance network comprising two MOSFET transistors 138 and 140, which act as frequency-controlled switches, and a capacitor 142. use in the filter of CMOS switching elements, which are compatible with the other on-chip transistors, simpli¬ fies filter fabrication and provides a high degree of uni¬ formity from one filter to another, since the equivalent resistance value is dependent on the size of the capacitors. MOSFET capacitors will vary due to geometry differences, but these can be held to a very tight tolerance. A second advantage of switched-capacitor implementation is that, by adjusting the clock frequency, the equivalent resistor can be altered in value. This principle allows the bandpass filter to be varied in frequency by adjusting the clocking rate of the switches. The thir d , an d f inal , on-chip segment of el ectronic circuitry is the multiplexer. Figure 9, which shows a d esira bl e multiplexer circuit, is substantially identical to F i g ure 3 of U . S . Patent No. 4, 490, 62 6 . B ecause the multip ¬ l exer circuit is di scussed in detail in that patent, that d i scl osure is incorporated herein by reference, an d the d escription of the circuit here will be relatively brief. F igu re 9 shows three pa ra l l e l b ranches of the multip l exer, each of which contains two MOSFET transistors. A MOSFET 150 in each branch is an ampl ifier of the analog signal f rom its respective detector. A MOS FET 15 2 in each b ranch acts as a switch which, when ena b le d , connects the MO S F E T 150 in the same branch to a multiplexer output l ine 154. Each MOSFET 150 receives at its gate the analog voltage signal originatin g from its respective d etector. S ignifi- cant l y, its out p ut analog signal is the source-to- d rain current f low, which varies proportional ly to the input vol ta g e, t h e source of each MOSFET 1 5 0 b eing connecte d to t h e common source voltage, and the d rain o f each MOSFET 15 0 b eing connecte d to the drain of the respective switch MOSFET 152. E ach switch MOSFET 152 has its gate connecte d to its respective line 156 from the logic circuit ; and its source connected to the common multiplexer output line 154. Under the control of the logic circuit, the switch MOS F E Ts 152 are norma l l y d isa b led, allowing no current f l ow through t h e respective ampl if ier MOSFETs 150. W hen a "trigger in" s gnal is received by the logic circuit, it first sends a voltage signal on the line 156 designated V ( SX- ) , causing the switch MO S FET 152 designated Q(B-O ) to turn on. A current analog output signal is thereupon permitted to flow in the amplifier 150 designated Q(A-O), which signal is proportional to, but amplified to a much higher power than, the input signal designated V(IN-O); and the amplified current output signal from Q(A-O) is transmitted through switch Q( B- O) to the multiplexer output line. The logic circuit then turns off the signal on the line V(S X-O ) , and sends a voltage signal on the next line designated v (S X-l ) to turn on the next switch 152 designated Q( B-l ) . This permits an amplified analog current signal to flow in the amplifier 150 designated Q( A -l ) , in the same branch as switch Q(B-1) ; and permits that signal to pass to the multiplexer output line. This process is continued until all detector-amplifier combinations have sent their output signals in the desired order to the output line. Each switch 152 may be controlled by a typical CMOS switch in the logic system, one of which is shown at the right side of Figure 9 connected to V ( S X- N) . Each CMO S logic switch comprises a P-channel MOSFET 160 having its source connected to V(DD), and an N -channel MOSFET 162 having its source connected to V (SS) . [ V alues of the source voltages may be approximately 5V at V(DD) and approximately 0 V at V ( SS ) ] . The gates of the respective MOSFETs 160 and 162 are both connected to the logic circuitry ; and the are oth connected to the gate of one of the switch MOSFETs 152. As explained in ϋ.S.S.N. 403,004, the multiplexer circuitry has important benefits, including (a) minimal power dissipation because no current flows in either transistor of a disabled branch, and (b) minimal cross-talk between branches because analog current signals are output instead of analog voltage signals. The use in an FLIR system of the stacked chips which have substantial electronic circuitry located at the focal Plane, and which are located inside the cooling element, provides important synergistic effects. it makes detector Performance much less critical, because electronics gain reduces the "work" required from the detectors. The focal plane pre-a plifier allows each detector to have the desired DC bias voltage, which is both "low" and very stable. Filtering and multiplexing at the focal plane complete the electronic processing which is important to accomplish before the signals are routed to external circuitry, in other words, the number of leads exiting the module 72 is minimized ; but all processing needed on each separate detector signal has been accomplished prior to multiplexing. Another advantage is the availability of random access readout, which is not possible with a line array. This permits the present system to provide a TV-display output with much simpler electronics. The common module FLIR system requires very complicated re- ormatting if a TV-display output is required. s ems are deslgned ^ ^ the spectral band from ... to .... micron „ using Mςτ sectors in the photo-conductive mode. However, attractive feoffs may be offered by PLIR systems operating in the t0 " miθr0n SPe ° t " 1 *>*"' ■ The present invention may P»vide, i„ CO m patison wifch prior ^ ^^ ^ ^ ^ costly system operating i„ the mid χ R range (3.. to 5.0,, or a higher performance system operating in the far IR range (8.0 to 12.0). If the detectors used i„ the present system are photo¬ voltaic „cτ detectors, operating in the 8.0 to 12.0 micron wavelength range, the system performance will gre atly exceed that of the existing common module FLIR systems. !t will h ve much higher sensitivity, better image quality, reduced number of components, lower WP i πh i ' - LOwer we i ght, lower power require ¬ ments, etc. 18 having limited operating time between replacements. As previously stated, the use of a two-dimensional array offers a major sensitivity improvement over a scanned line array, because of the longer integration time. The quantitative benefit is approximately the square root of the ratio of the numbers of detectors in the respective arrays. For example, in comparing a two-dimensional array of 128 x 128 detectors to a line array of 128 detectors, the ratio of the number of detectors in the former to the number of detectors in the latter is 128 to 1. The performance gain of the two-dimensional array is approximately represented by the square root of 128. So the improvement is greater than an order of magnitude. One potential limitation of the present invention relates to the fineness of resolution attainable. In modules of the type identified by numerals 54 and 72, the present center-to-center distance between detectors is .004 in. In the line array systems, a center spacing of .002 in. has been used. The modules 54 and 72 can be considered as having an X- axis which is parallel to the planes of the stacked layers 78 (Figure 4), and a Y-axis which is perpendicular to the planes of the layers. Because the Y-axis spacing of detector leads is controlled by the thickness of the respective layers, it requires extreme manufacturing accuracy and care to reduce the Y-axis center spacing of the leads. limited ° Cder t0 enhan ° e CeS ° 1Utl0n in thβ ""«««-. a ; βd ™ ° f ~ - - — uced. For example, 128 detectors on .00 in . oenter . c ,„ wM- ._ ^ « - «. Pixels on . aβ2 in. centers. This compromise -rxfxc. some of the performance advantage d ue to the i ntegration effect of st »r <„„ staring sensors, and also some of the advantage of eliminating mechanical scanning. However, e e remains a substantial performance benefit, and the 1-it.a scanning can be accomplished by a much simpler, less expensive and more re l i_ h i « . Cellable me chanism than that required in l i ne array systems. »9ure 10 illustrates a limited scanning feature, which i s referred to as -nutation-, incorporated in the system of the present invention , mcoming radiation from the viewed scene <at left in the figure, passes through an optical - ale 17f . and ce£lected towara the detectot/eiectroni rotate across the path of radiation focused on the detector rocal plane 70 of mo d u le τ> that it h " 0r " 2 1S SO arran s d " has a limited scanning motion. F or this purpose it «y be pivotally movable (for very small distances, about - Pivot axes, which are at right angles to one another. " i desired to have the mirror move in sequence to a Pl rality of fi χ e d posi t iθ n s ,. and spend as _„ ^ __ Possrble in a "staring- mode in each of those positions. « i ntegrating positions, the portion of time available for data-integrat ion is about seven-eights of the time cyc l e, in other words, if the time cycle of nutation from starting Point back to sta rting point is assumed to be 33 ms., the tota l motion t ime requi red (which is not available for detector sensing input ) is approximately 4 ms., leaving 29 ms. for detector sensing input. The mi rror should be stationa ry during each detector input period, in order to avoid image blurring. Figures 11, 12A and 12B show a structu re suitable for causing nutating motion of mirror 172. The mirror is pivot¬ al ly movable both on axis X-x and on axis ϊ-ϊ. it is car¬ ried by an inner gimbal ring 176, which is pivoted on the y- axis ( as shown in Figure 12A) , and which in turn is car ried by an outer gimba l ring 178. The outer g imbal r ing 178 is Pivoted on the X-axis ' (as shown in Figure 12B) . The outer gimbal ring ι 78 has atms „, . - oppoεlte __ d _ of ^ __^^ each of which is connected to a supporting element 180 by a Pa i r of l ex pivots 181. The inner gimbal ring 176 has arms "2 at opposite ends of the ϊ-axis, each of which is con¬ nected to the outer g imba l ring 178 by a pair of f l ex pivots 183. The supporting elements 180 are carried by a mounting Plate 184 which is retained in a fixed position in the structure of Figure 10, thereby locating the entire nutator sub-assembly in the desired position. As shown in both Figures 12A and 12B, a preferred means of causing pivotal motion of mirror 172 is a pie.o-electric elemen t, which acts as a vol tage-to-pressure transduce r. Such a piezo-electric element is shown at 190 in Figure 12A and at 192 in Figure 12B. Applying a direct electric field across either of the piezo-electric elements 190 or 192 causes it to flex in one direction or the other, depending on the polarity of the electric field. When voltage of one polarity is applied to piezo-electric element 190 (Figure 12A) , it moves the inner gimbal ring 176 into engagement with a stop member 194, which has a tapered engaging portion 196 at the end of a threaded portion 198, thus permitting adjustment of the stop-engaging position of the mirror. A similar stop member 194 is located at the opposite end of the inner gimbal ring 176, and is engaged by ring 176 when the polarity of the voltage across piezo- electric element 190 is reversed, causing element 190 to flex in the opposite direction. The total motion caused by flexure of the element 190 is only about 1 mil. One end of element 190 is secured to the inner gimbal 176 by a bracket 200, and its other end is secured to a pivoted arm 202, the position of which may be adjusted by a screw 204, thereby equalizing the flexing motion required in opposite directions. As shown in Figure 12B, the same arrangement may be used in controlling the pivotal movement of the outer gimbal ring 178, moving it either into engagement with the stop member 194 at the right side of the gimbal ring, or into engagement with a stop member (not shown) at the left side of the gimbal ring. Since . each of the gimbal rings 176 and 178 has two alternating stop-engaging positions, the mirror 172 has f our fixed signal-viewing positions. By using a sing l e d etector to sense ra d iation f rom four positions, t h e avai l a b le number o f pixe l s per d etector has been quadrup l ed. T his has b een accompl ished wi th a very slight amount o f accurate l y- controllable scanning motion of the mirror. '-$-. * 13 shows another mean of causing the nutation scanning ef f ect , whereas Figures 10-12 „.. . Electing " rW aS ,e SCanni "9 •!—«*. the structure shown in F gure 13 uses a refractive element, w h ich is a prism 21 0 . I ts motion may be caused by an annular electric motor 212 "ovin g a cam to cause sequential re l ocation o f t h e e l ement 21 0 at each o f the d esired positions. T he l ength o f motion regu.re d b y a re f ractive nutating prism is greater than that require d by the mirror of Figures 10-12; therefore, the i ezo-electric elements are not considered suitable f or the "•echanical nutator of Figure 13. Figure 1 3 s h ows an optica l nodule 17 0 a, . chopper 174a, and the detector foca l plane 7 0 on the surface o f optical/electronic mo d ule 72, which is mounte d inside a cooling unit. "sing t h e l i mited scanning motion, which has b een re f erred to as "nutation-, provides innumera bl e possibili ¬ ties f or performance enhancement o f the t h ermal imager system. F igures 14 and 15 illustrate diagrammatically j ust two of the possibilities. i n F igure 14, it is assumed that each of a plural ity of d etectors 22 0 ex t en d s 2 mils in each direction, and that the ec or center - _,-« enters are space d 4 mils apart in b oth the X-axis detector mo illustrated by plotting imaginary el ec t or movement. pi αιlrβ ,, .,, erΛ Λ 9 14 lllu strates the use of four space d samples t. < _r f fa ssttaarrtt bbyv vi • ewing at point A f < β ,. P k iXSi ""'« iε "«— to Point B th m0Vβ °" X"aXiS t0 View at P ' B ' thβn -*• on Y-axis t0 view at on - ay j e . . P" i n t : , then move n X axis to view at point n an _ f back . . P °' and fl ^ ll y move on Y-axis center rβSUlt *' ""' dete ° t0rS ta ' 1 " ir 1 " 98 " 4 ""' ~ P — — enter pixel o l ut.on o f 2 m i ls. Λssu ing the square root o f 12a as ens.tivity (p e r f ormance) . , m . g< _. , . . " _ ■ a n. I " 0r m ° SaiC "" "' e »»• •»«. ". nutation ta e 7ec '^ " '" ^r °~ °* «-* advan¬ ce use the frequency bandwidth must be increase d b y to Perf ° rmanCe ^ °" —" «"* « -ur compared to the compietely starin g case. has sen 15 UlUStrateS a " intβCβSti - ™ — -eh POtβntial adVanta9βS - «»'' " — nutat- I 0" r 0nβ dire0ti0n ° nly — — -- «ou„d „ ' • SeC ° nϋ ' " ■ "--• ««• -—r of required stop- pmg points. Thir d < . „ h lt d, lt causes viewing of a given portion o f scene by more than one detector, t h ereby enhancing -— t , i.e., re l ative detector outputs, or gains, can be electronically adjusted to provide uniform detector response. Fourth, it is particularly compatible with the stacked layer construction of module 72, because it provides increased resolution in one axis. The latter advantage refers to the fact that it is much easier to reduce center- to-center detector spacing along the X-axis, where the electrical leads lie along a single layer surface, than to reduce center-to-center detector spacing along the Y-axis, where the electrical leads are separated by the thicknesses of the stacked layers. Figure 15 illustrates the use of three samples per frame, one of which involves a second detector. The pixel center is assumed to start by viewing at point A on detector 220a, first move to a second viewing point B, and then move to a third viewing point C on detector 220b. Thereafter, the scanner can be returned to point A for repetition of the sequence. Because each detector 220a and 220b has viewed a common pixel, their outputs can be calibrated and adjusted to improve uniformity. This feature can be expanded by combining more than two detectors in the same frame, without eliminating the advantages of (a) simplified mechanical scanning structure, and (b) increased sensitivity because of available detector "staring" time. From the foregoing description, it will be apparent that the apparatus embodiments disclosed in this application will provide the significant functional benefits summarized in the introductory portion of the specification. The following claims are intended not only to cover the specific embodiments disclosed, but also to cover the inven¬ tive concepts explained herejn with the maximum breadth and comprehensiveness permitted by the prior art.

Claims: 26 What Is Claimed τ q? 1. A thermal imager, for detection of non-visible infrared radiation from objects, including: an optical input structure for receiving and focusing infrared radiation; a detector module having (a) a focal plane surface comprising a two-dimensional array of detectors which convert radiation signals from the optical input structure into electronic information, and (b) signal-processing elec¬ tronics embedded in the body of the module which ampl ify, filter and multiplex the electronic signals supplied by the detectors; and a video signal generator actuated by the multiplexed output signals from the detector module to provide a visible optical display. 2. The thermal imager of claim 1 which also comprises: a cooling element, within which the detector module is l oca ted, hav ing means for maintaining the cryogenic temperature needed for optimum detector sensitivity. 3. The thermal imager of claim 1 wherein the signal- processing electronics in the detector module comprise transistors all of which are MOSFET devices. 4. The therma l imager of claim 3 wherein the amplification portion of the signal-processing electronics comprises an operationa l ampl ifier operated in the weak- invers ion mode , the reby minimi z ing power dissipation requirements and increasing the signal amplification ratio. 5. The thermal imager of claim 3 wherein the filtering Portion of the signal-processing electronics comprises switched capacitance resistance-equivalents controlled by MOSFET switches, thereby permitting adjustment of the output ban d width by frequency variations, in order to accommodate the bandwith responsivity of the detectors. 6- The thermal imager of claim 3 wherein the multiplexer portion of the signal-processing electronics comprises branches each combining a f c OSF T amplifier transistor which has a variable-current output signal with a M OSFE T switch which prevents current flow in the branch except when enabled. 7 - The thermal imager of claim 5 wherein: the detectors are operative to detect signals in the approximate wavelength range of 3.0 to 5.0 microns. 8. T he thermal imager of claim 5 wherein: the detectors are operative to detect signals in the approximate wavelength range of 8.0 to 12.0 microns. 9- The thermal imager of claim 7 wherein: the detectors are formed of lead selenide material and operate in the photo-conductive mode. 10. The thermal imager of claim 8 wherein: the detectors are formed of mercury cadmium telluride material and operate in the photo-voltaic mode. 11. A thermal imager, for detection of non-visible infrared radiation from objects, including: an optical input structure for receiving and focusing incoming infrared radiation; a two-dimensional detector array which converts the incoming infrared radiation into electronic signals; a limited mechanical scanning device which causes each detector to view a plurality of pixels in the incoming infrared radiation; and a video signal generator actuated by the output electronic signals from the detector array. 12. The thermal imager of claim 11 which also comprises: a detector module having (a) a focal plane surface carrying the two-dimensional detector array, and (b) signal- Processing electronics embedded in the body of the module which amplify, filter and multiplex the electronic signals supplied by the detectors. 1 3 . T he thermal imager of claim 11 wherein the mec h anical scanning device has limited pivota l motion aroun d two axes, which are at right angles to one another. 14. The thermal imager of claim 13 wherein the mechanica l scanning device causes each detector to view at l east four pixels in the incoming infrare d ra d iation. 15. The thermal imager of claim 11 wherein each detector views at least two pixels in the incoming radia ¬ tion, and at least two detectors view the same pixel.

Inventors: CARSON JOHN C, CLARK STEWART A, CARSON, JOHN, C., CLARK, STEWART, A.
Applicants: IRVINE SENSORS CORP, IRVINE SENSORS CORPORATION
Application year: 1986
Main classification: H01L27
IPC codes: H01L27, H04N3, H04N5