Abstract:
A lightweight camera or detector that does not require a shutter, chopper or thermoelectric stabilizer. Lightweight materials and lightweight packaging techniques are used, and in some embodiments, some or all of the calibration, compensation and processing hardware are moved from the camera itself to a remote station. Such a shutterless, lightweight, IR camera, which may operated at ambient temperature, can be mounted on a micro air vehicle (MAV) or the like, with the raw IR sensor data received and processed by a ground station.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is related to U.S. patent application Ser. No. 09/748,756, filed Dec. 26, 2000, entitled “LENS FOR INFRARED CAMERA”, and U.S. patent application Ser. No. 09/748,784, filed Dec. 26, 2000, entitled “MICROBOLOMETER OPERATING SYSTEM.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to infrared (IR) cameras and detectors. More particularly, the present invention relates to lightweight infrared (IR) cameras and detectors. 
     BACKGROUND OF THE INVENTION 
     Infrared cameras and detectors in general, and microbolometer cameras in particular, are well known to those skilled in the art. See, for example, U.S. Pat. Nos. 5,688,699; 5,999,211; 5,420,419; and 6,026,337, all of which are incorporated herein by reference. Infrared microbolometer cameras typically include an array of infrared sensitive sensing detectors, each having a resistance that changes with temperature, with each detector having an infrared absorber that may be formed in several ways. See, for example, U.S. Pat. Nos. 5,939,971 and 5,729,019, herein incorporated by reference. 
     During operation, the incoming infrared radiation heats each sensing detector in proportion to the amount of infrared radiation received. The sensing detectors are then queried, typically one by one, to determine the resistance of the sensing detectors, and thus the amount of infrared radiation received. Typically, supporting electronics are provided with the camera to process the detector output signals, provide calibration and compensation, and provide a resulting image. 
     Because heat is used to measure the amount of incoming infrared energy, changes in the ambient temperature of the microbolometer array can significantly affect the detector signals. To compensate for this, many infrared cameras or detectors have a thermoelectric stabilizer to regulate the temperature of the array. In one example, thermoelectric stabilizers are used to maintain the array temperature at a known value. A limitation of using thermoelectric stabilizers is that they can draw significant power and can add significant weight to the system. 
     Because of manufacturing tolerances, each sensing detector in the camera may have a slightly different zero point than other detectors within the system. To compensate for these detector-to-detector differences, many infrared cameras or detectors have a means for providing a zero radiation baseline value, which is made available to interpret or calibrate the detector output signals. One method for providing the zero radiation baseline is to use a shutter or chopper to periodically block the incoming infrared energy. When the shutter or chopper is activated, a zero radiation baseline is read and stored. A limitation of this approach is that the shutter or chopper can add significant complexity and weight to the system, which for some applications, can be particularly problematic. Another approach for providing a zero radiation baseline is to periodically point the camera at a uniform infrared source such as the sky. This, however, can require significant control circuitry to periodically change the direction of the camera, again adding weight to the system. 
     For some applications, the weight of the infrared camera can be important. For example, in lightweight micro air vehicle (MAV) applications, the weight of the infrared camera can significantly impact the size, range and other critical performance parameters of the vehicle. For these and other applications, a lightweight infrared camera would be highly desirable. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes many disadvantages of the prior art by providing a lightweight infrared camera. This is accomplished primarily by eliminating the shutter or chopper, eliminating the thermoelectric stabilizer, using lightweight materials and lightweight packaging techniques, and/or moving some of the calibration, compensation, processing and display hardware from the camera to a remote station. 
     In one illustrative embodiment of the present invention, the infrared camera includes a microbolometer array as the radiation sensing device. The microbolometer array includes a plurality of addressable radiation sensing detectors each having an output that depends on the intensity of the infrared radiation that strikes the detector. 
     To reduce the weight of the infrared camera, the measured infrared signals may be transmitted to a remote station in analog or digital form. The signals may be transmitted by wireless or optical fibers or wires as best suits the application. The remote station receives the transmitted signals, and formats the signals into an array that corresponds to the original microbolometer detector array. The remote station may include the necessary processing hardware for compensating both inter-detector differences and variations in the ambient temperature of the transmitting detector array. A temperature sensor also may be provided near the microbolometer array, which can send a temperature signal that is transmitted to the remote station for use in signal calibration and compensation, if desired. By moving the calibrations compensation and/or processing hardware from the infrared camera to the remote station, significant weight savings can be realized in the infrared camera. 
     The calibrating and compensating values are typically dependent upon the temperature of the microbolometer array, typically being different for each bolometer array temperature and for each individual sensing detector on the array. The remote station may select the proper calibrating and compensating values to apply. The required numbers may be stored in the remote station, or generated for each individual sensing detector using algorithms which use the array temperature as a variable. 
     In camera applications in which the scene has known statistical properties, for example, when each sensing detector views a target which on average is identical to every other sensing detector, as is usually the case in MAV and many other moving-vehicle applications, the compensating and calibrating values may also be computed within the ground station by using multiple measured values of each sensing detector signal. 
     Another way to reduce the weight of the infrared camera is to provide the microbolometer in an integrated vacuum package (IVP). An integrated vacuum package may include an infrared transmitting cover that includes a cavity that fits over the microbolometer detector array. Silicon is a typical cover material. The silicon cover is bonded to the microbolometer substrate to collectively form a lightweight vacuum package. In a preferred embodiment, the silicon cover does not extend over the bonding pads of the microbolometer. Configured in this way, the IVP may be directly bonded to a motherboard, with wire bonds, bump bonds or other bonding mechanisms used to directly connecting the bonding pads of the microbolometer to bond pads on the motherboard. Motherboards are typically ceramic. This is known as “hybridizing” the IVP with the motherboard. This may eliminate the need for a conventional chip carrier, which may further reduce the weight of the camera. 
     It is also contemplated that any supporting electronics in the camera, such as A/D converters and/or transmitting circuitry, may be hybridized with the ceramic motherboard. That is, rather than including the supporting electronics in conventional packages, the integrated circuit dice of the supporting electronics may be directly bonded to the ceramic motherboard, with wire bonds, bump bonds or the like connecting the supporting electronics to the motherboard. This may also reduce the weight of the camera. 
     The infrared camera may also use a lens system. The lens system is used to focus the incoming infrared radiation on the microbolometer array of detectors. The lens is typically a germanium lens, and may be a singlet, doublet, or triplet. The lens is preferably spaced from the ceramic motherboard by lightweight supports made from a material such as titanium. The use of doublets or singlets can further reduce the weight of the infrared camera. If doublets or singlets are used, the resulting image blur may be removed by the ground station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a shutterless radiation detector device having a plurality of radiation sensing detector, a multiplexer, and a transmitter; 
     FIG. 2 is a receiving system for receiving transmitted signal values including a temperature compensator for compensating raw, received signal values from a system as in FIG. 1, the compensator using values which vary with array temperature, and which are typically different for each sensing detector; 
     FIG. 3 is a schematic view of a receiving system for receiving transmitted radiation signal values from a system as in FIG. 1, including a compensator using multiple signal values to compensate for inter-detector differences; 
     FIG. 4 is a schematic view of a row of infrared detectors and sampling circuitry which is used in one embodiment of the radiation detector device of FIG. 1; 
     FIG. 5 is a cutaway, perspective view of an integral vacuum package including an infrared transparent silicon top cap providing a vacuum environment for a bolometer array; and 
     FIG. 6 is a perspective view of a camera system including a triplet lens spaced from an integral vacuum package array as in FIG. 5 by multiple legs upon a hybrid circuit board. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a first illustrative infrared camera in accordance with the present invention. In this embodiment, the infrared camera only contains an array of detectors for receiving the infrared energy, measuring array temperature, and transmitting the raw data to a remote station. An illustrative remote station is shown in, for example, FIG.  2 . 
     The infrared camera of FIG. 1 is generally shown at  20 , and includes an array of radiation detectors  22 , a column selector  24 , a row selector  26 , a controller  28 , and an optional temperature sensor  52 . Radiation detector array  22  includes numerous radiation sensing detectors  50  which, in a preferred embodiment, are infrared sensing microbolometers. Radiation detectors  50 , in the embodiment illustrated, are deployed in a series of columns  48 , and rows  46 . A controller  28  is provided for controlling column selector  24  and row selector  26 . Controller  28  can include a counter for stepping selectors  24  and  26  through a series of nested row and column addressing sequences. In one embodiment, a single radiation detector is addressed or selected for a reading at any instant in time by the selection of a single column selector line  40  and a single row selector line  42 . In another embodiment, all detectors in a column are addressed or selected for reading simultaneously, by the selection of a single column and accepting all row signals into row selector  26 , after which individual detector signals are passed out sequentially to the amplifier  30 . In one embodiment, the array of radiation detectors  22 , the column multiplexer or selector  24 , the row multiplexer or selector  26 , the controller  28 , and the optional temperature sensor  52  are all part of the microbolometer device and are all formed on the same substrate. In a preferred embodiment, a 160 by 120 (160×120) array of 35- micrometer sized microbolometers is used to form the microbolometer array. 
     In another embodiment, detector offset signals may be applied directly to the array of radiation detectors, so as to partially compensate the individual radiation detectors for zero offsets. Such non-uniformity correction is disclosed in U.S. Pat. No. 4,752,694. With particular readout circuits described in this invention, offset correction circuits similar to those in U.S. Pat. No. 4,752,694 may be used, or other offset correction circuits well known to persons skilled in the electronic arts may be employed, as in U.S. Pat. No. 5,811,808. 
     When a particular detector signal is passed to readout line  44 , that signal corresponds to the radiation intensity striking the selected radiation detector, which can be amplified by an amplifier.  30 . The amplified signal is then provided to an analog-to-digital converter  32 , which can, in turn, provide an output signal to a transmitter  34 . In the embodiment illustrated, transmitter  34  is coupled to an antenna  36  for emitting wireless signals  38 . In one embodiment, signals  38  are radio frequency signals transmitted without wires, while another embodiment uses optical wireless transmissions, which can include infrared signals. Signals  38  can also be transmitted over electrical wires or optical fibers. 
     It is contemplated that temperature sensor  52  can be selected or addressed by a first temperature sensor selector line  56  with the temperature sensor value being read out by a temperature sensor readout line  54  which may be coupled to radiation detector readout line  44 . The temperature sensor value may be added as an additional value at the beginning or the end of a series of transmitted radiation detector values. In another embodiment, the temperature sensor  52  is coupled independently to transmitter  34 , and the temperature sensor value is periodically transmitted to the remote station. 
     Referring now to FIG. 2, a remote station  100  is illustrated for receiving a series of transmitted radiation detector values and array temperature signals from the camera  20  of FIG.  1 . Remote station  100  can include an antenna  102  coupled to a receiver  104  through line  110 , with receiver  104  being coupled to a controller  106  though a line  108 . Controller  106  can provide a raw data structure, array or device  114  with a plurality of raw radiation detector data values through a line  112 . Storage device array  114  can be any device capable of storing the plurality of radiation detector values obtained from the transmitting system. The raw values can be fed to a compensator  118  through a connecting line  116 . In the embodiment illustrated in FIG. 2, a series of constants, such as zeroing or amplification constants, may be stored in a data structure, array or device  132  and provided to comparator  118  through a connecting line  130 . 
     In some embodiments, a temperature value is received from the transmitting system and stored in a temperature storage location  124 , either as a temperature or a raw detector output value. Temperature storage location  124  can be supplied by controller  106  which can retrieve the temperature value from the stream of received radiation detector values and supplied to temperature storage location  124  through line  126 . The temperature value can be retrieved by compensator  118  through line  128 . In one embodiment, compensator  118  takes the raw radiation detector signal values  114 , the temperature values stored in device  124  and the constants stored in device  132 , and calibrates or compensates the raw radiation detector signal values both for inter-detector differences and for the array ambient temperature. The result may be a series of compensated values that can be stored in a compensated storage data structure, device, or array  122  through line  120 . 
     In one embodiment, detector array  22  of FIG. 1 is tested at the factory to measure differences from detector to detector. In particular, in one embodiment, the output values of each detector at a baseline of zero (0) received radiation is stored in a table such as constant array  132  of FIG. 2, plus a number of array temperatures. The constants stored in array  132  can be used to adjust the values received from each detector  50  of FIG. 1 to effectively zero the values received by receiver  104  and stored in raw value array  114 . 
     A single temperature compensation model may be stored and used by compensator  118  to adjust the received values of all detectors in raw array  114  according to the onboard temperature of detector array  22 . Alternatively, a separate model may be provided for each detector across a range of array temperatures. In this embodiment, the temperature dependence of each detector  50  of FIG. 1 can be measured at the factory or at some time prior to deployment of the infrared camera  20 . The temperature dependence of each detector  50  may be independently stored, for example, by storing a different set of temperature coefficients in constant storage array  132 . Alternatively, optional temperature storage location  124  is not provided, and the compensation performed by compensator  118  eliminates only the inter-detector differences. 
     It is contemplated that the controller  106  may retrieve the temperature data by identifying a particular location in the received data stream, thus isolating the onboard temperature sensor value. Alternatively, the temperature sensor value may be effectively marked by a bit or a particular series of bits to identify the temperature sensor value as a temperature value, rather than a received radiation intensity value. 
     Referring now to FIG. 3, another remote station  200  is illustrated which can be used to receive transmitted radiation detector signal values as transmitted by infrared camera  20  of FIG.  1 . Remote station  200  includes many of the same components previously described and identically numbered in FIG. 2, which are not discussed further. Receiving system  200  includes a controller  206  for processing received signals through input line  108 , outputting raw detector data values into raw detector storage device  114 . In this embodiment, the raw detector values can be stored over time on a detector-by-detector basis, providing multiple signal values for each detector through input line  233 . The signal values, averaged over time, should be substantially identical for all detectors  50  if the target is similarly identical. However, due to the inter-detector differences, detectors that received essentially identical cumulative radiation, will nonetheless output slightly different radiation signal values due to the inter-detector differences. These differences, or the cumulative values, may be stored in time average data structure or array  232 . 
     In one embodiment, the average signal values over time are used to effectively normalize the detector values. In another embodiment, time average array  232  stores the positive or negative number required to bring the detector value to the average over time. In this embodiment, a compensator  218  can obtain the raw data values through input line  116  and the time average values through line  234 , producing a compensated array of values stored in compensated data array  122 . Compensation can be applied to the raw detector values to reduce or eliminate inter-detector differences using time average values and a received onboard temperature measurement, such as illustrated in FIG.  2 . Further, a temperature correction model may be performed by compensator  218 , as discussed with respect to FIG.  2 . In one embodiment, such as shown in FIGS. 2 and 3, if a camera lens is employed which produces a known image blurring, the compensators  118  and  218  may be used to remove the image blur to an acceptable degree. 
     Referring again to FIGS. 2 and 3, in one embodiment, the temperature compensation and inter-detector compensation and image de-blurring can be provided by a general purpose computer executing software operating upon the received detector data. In one embodiment, the data is received through input line  108 , separated into raw data and, optionally, temperature data, and stored into arrays or other data structures within a general purpose computer operating a computer program. In this embodiment, the general purpose computer running the program can retrieve the needed constants and raw data, from data stores such as arrays. The retrieved data can be compensated within a compensating portion of the program, and output to a data storage area containing the compensated detector values. In a similar manner, with respect to FIG. 3, the time averaged values stored in array  232  of FIG. 3 can be averaged within a general purpose computer executing a computer program. 
     As can be seen from inspection of FIGS. 1-3, the bulk of the processing can be performed in the remote station, rather than in the infrared camera. In particular, the temperature compensation and inter-detector or inter-pixel compensation or normalization, and image de-blurring can be performed at the receiving end, for example, at a ground station. In this way, a lightweight, sometimes airborne, infrared camera can have the temperature compensation and image processing offloaded to the ground station. 
     Referring now to FIG. 4, a single row of detectors  300  in a microbolometer array with readout is illustrated, for an embodiment having infrared detectors as the radiation detectors discussed with respect to FIG.  1 . Detector row  302  can include infrared sensitive variable resistance elements  304  which are supplied with a reference voltage at  305  providing a variable current therethrough at  307 . Each resistance element  304  can be selected or addressed by a row selector circuit  306  and a column selector line  310 . When the corresponding row and column are selected, the selected detector element  304  may be read. 
     In the embodiment illustrated in FIG. 4, an n-type transistor  308  is switched by the proper selection of a row line  306  and a column line  310 . Row selector circuit  306  includes a transmission gate  318  which includes a p-type transistor  316  and an n-type transistor  314 . When the reading of a row is desired, only one column addressing line  310  and one row readout line  306  are typically selected. Current then flows from the power supply  305 , through the selected variable resistance element  304 , through the selected transistor  308  to the selected row readout line  312 , and out a common readout line  320 . In another embodiment, in which all detectors in a column are addressed or selected for reading simultaneously by the selection of a single column, an optional integrator  321  is provided for each row within the row selector, and means  314  for passing individual detector signals out sequentially to the amplifier  30 . 
     Referring now to FIG. 5, an integral vacuum package (IVP)  500  suitable for use with the current invention is illustrated. IVP  500  preferably includes an infrared transparent silicon top cap  502 . The silicon top cap  502  is preferably micromachined to include a cavity that is in registration with the microbolometer detector elements  508 . The silicon top cap  502  is bonded to the substrate of the microbolometer array  508  to provide a vacuum environment for a microbolometer detector elements  506 . The bonding is typically performed on a wafer scale. 
     In a preferred embodiment, the silicon top cap  502  is provided with vias so that it does not extend over the bonding pads  510  of the microbolometer. Configured in this way, the IVP  500  may be directly bonded to a ceramic motherboard  528 , with wire bonds, bump bonds or other bonding mechanisms used to directly connecting the bonding pads  510  of the microbolometer to bond pads on the ceramic motherboard  528 . This is known as “hybridizing” the IVP  500  with the ceramic motherboard  520 , as better shown in FIG.  6 . This may eliminate the need for a conventional chip carrier, which may reduce the weight of the camera. 
     It is also contemplated that any supporting electronics in the camera, such as A/D converters and/or transmitting circuitry, may be hybridized with the ceramic motherboard  528 . That is, rather than including the supporting electronics in conventional packages, the integrated circuit dice  530  of the supporting electronics may be directly bonded to the ceramic motherboard  528 , with wire bonds, bump bonds or the like connecting the supporting electronics to the ceramic motherboard  528 . This may also reduce the weight of the camera. 
     FIG. 6 shows a perspective view of an infrared camera  520  in accordance with a preferred embodiment of the present invention. IVP  500  is shown directly bonded to a ceramic motherboard  528 . The infrared camera  520  also includes a germanium triplet lens  522  including three individual germanium lens elements  524  which can be spaced apart by multiple titanium spacer legs  526  mounted on the ceramic motherboard  528 . The lens system is used to focus the incoming infrared radiation on the microbolometer array in the IVP  500 . Radiation shields may be added around the array (not shown) to reduce the stray radiation. The ceramic motherboard is preferably of multi-layer construction and is about one inch (1″) on each side. Ceramic is preferably used to further reduce the weight of the camera  520 . In one embodiment, infrared camera  520  accepts DC supply voltages and supplies 12-bit digital data corresponding to the data to the transmitter provided by microbolometer array  506  in IVP  500 . 
     As can be seen from inspection of FIG. 6, infrared camera  520 , in the embodiment illustrated, requires no shutter and requires no temperature stabilization, and may use a light lens that produces a blurred image. As indicated above, onboard temperature regulation can add significant weight to the camera. The preferred lack of shutter or chopper, temperature stabilizer, high quality lens and complex onboard processing can combine to provide an extremely lightweight infrared camera or detector. In one embodiment, camera system  520  weighs less than 25 grams, or more preferably less than 10 grams. 
     A lightweight infrared camera, as can be provided by the present invention, is ideal for several applications. In one application, a lightweight micro air vehicle (MAV) can be used with downloading capabilities to relay the raw infrared image of a selected area over an RF link to a ground station. In another application, an expendable, single use, lightweight infrared camera can be deployed through use of a downwardly drifting projectile slowed by a small chute to provide maximum time over the recognizance target. In another application, the present invention may be incorporated into a helmet mounted sensing device. Numerous other applications are contemplated. 
     Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.