Abstract:
An infrared imaging system having functionality for maintaining image quality in the presence of temperature drift of the system. Such functionality is applied repetitively to maintain image quality of a target scene, yet without continuous actuation of a shutter of the system. The functionality of the imaging system results from implementing an imager algorithm. In use, the imager algorithm functions with a calibration curve created for the imaging system, with the curve comprising a plot of system output versus target scene temperature.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments of the present invention pertain to infrared imaging systems and, more particularly, to such systems using an imager algorithm to limit shutter actuations of the systems without sacrificing image quality. 
       BACKGROUND 
       [0002]    As is known, infrared cameras generally employ a lens working with a corresponding infrared focal plane array (FPA) to provide an image of a view in a particular axis. The operation of such cameras is generally as follows. Infrared energy is accepted via infrared optics, including the lens, and directed onto the FPA of microbolometer infrared detector elements. Each detector element responds to the heat energy received by changing its resistance value. An infrared (or thermal) image can be formed by measuring the detector elements&#39; resistances—via applying a voltage to the detector elements and measuring the resulting currents or applying current to the detector elements and measuring the resulting voltages. A frame of image data may, for example, be generated by scanning all the rows and columns of the FPA. A dynamic thermal image (i.e., a video representation) can be generated by repeatedly scanning the FPA to form successive frames of data, with such frames being produced at a rate sufficient to generate a video representation of the thermal image data. 
         [0003]    Individual detector elements have unique response characteristics. These response characteristics are found to produce non-uniformities, which often result in fixed pattern noise. Many infrared cameras have functionality to correct for such noise. For example, some infrared cameras can automatically or manually perform offset compensation, which corrects for variations in the individual detector element responses by observing a uniform thermal scene (e.g., by placing a shutter between the optics and the array) and measuring offset correction data for each detector element which provides the desired uniform output response. These measured offset corrections are stored, then later applied in subsequent infrared measurements (e.g., with the shutter open) to correct for fixed pattern noise. Other compensations can also be applied, such as 2-point correction. 
         [0004]    As is known, offset compensation functionality is found in most conventional infrared cameras because it leads to improved imaging capabilities. However, offset compensation can be an inconvenience to the user as it necessitates activation of the camera shutter, thereby “freezing” the camera image for a short period of time when the shutter is closed. Therefore, it is desirable to keep the period between offset compensations lengthy so as to limit the general inconvenience to the user of the camera, while still maintaining good image quality. 
         [0005]    Temperature changes within or surrounding an infrared camera can be further found to result in the individual detector elements exhibiting their unique response characteristics. In particular, the change in temperature of the camera&#39;s internal components, e.g., due to self-heating or as the result of changes to the surrounding ambient temperature, leads to the individual detector elements exhibiting fixed pattern noise over extended lengths of time. For example, during initial powering of an infrared camera, the internal components can be found to continue to rise in temperature for a period of time before the camera becomes thermally stable. Because of this, offset compensation is often performed at an increased frequency during such period so as to maintain good image quality from the camera. Such increased frequency of offset compensation correspondingly results in an increased frequency of shutter actuation. Consequently, there is further inconvenience for the user as the shutter is closed more often during such period. 
         [0006]    What are needed are apparatus and systematic methods to address or overcome one or more of the limitations briefly described above with respect to offset compensation functionality in infrared imaging systems. 
       SUMMARY 
       [0007]    Embodiments of the invention involve an infrared imaging system having functionality for maintaining image quality in the presence of temperature drift of the system. The functionality is applied repetitively to maintain image quality of a target scene, yet without continuous actuation of a shutter of the system. The functionality of the imaging system results from implementing an imager algorithm. In use, the imager algorithm functions with a calibration curve created for the imaging system, with the curve comprising a plot of system output versus target scene temperature. 
         [0008]    In certain embodiments, the imaging system includes a focal plane array (FPA); an enclosure disposed over the FPA, yet allowing for infrared energy from a target scene to be directed onto the FPA; and a shutter that can be selectively actuated so as to block infrared energy emanating from the target scene from reaching the FPA. When the imaging system is used in the field to view a target scene, the shutter of the system is initially closed to ascertain a base output for the detector elements of the FPA. Used in conjunction with the calibration curve and real-time measurements from the imaging system, the base output serves as a reference measurement in calculating the output of the FPA detector elements attributable to the target scene on an ongoing basis without necessitating further actuation of the shutter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The following drawings are illustrative of particular embodiments of the present invention and therefore do not limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements. 
           [0010]      FIG. 1  is a representative cross-sectional top view of part of an exemplary infrared imaging system in accordance with certain embodiments of the invention. 
           [0011]      FIG. 2  is an exemplary plot of a calibration curve showing output of a reference detector element of the focal plane array (FPA) of the infrared imaging system of  FIG. 1  for different temperatures of a target scene in accordance with certain embodiments of the invention. 
           [0012]      FIG. 3  is the calibration curve of  FIG. 2  demonstrating how target scene temperature is calculated with reference to output of a reference detector element immediately after a shutter of the imaging system has been opened, in accordance with certain embodiments of the invention. 
           [0013]      FIG. 4  is the calibration curve of  FIG. 2  demonstrating how target scene temperature is calculated with reference to output of each detector element of the FPA, immediately after a shutter of the imaging system has been opened, in accordance with certain embodiments of the invention. 
           [0014]      FIG. 5  is the calibration curve of  FIG. 2  demonstrating how target scene temperature is calculated with reference to output of each detector element of the FPA with thermal drift accounted for in accordance with certain embodiments of the invention. 
           [0015]      FIG. 6  is an exemplary plot depicting temperature calculation accuracy of the imaging system of  FIG. 1  over time without shutter closure in accordance with certain embodiments of the invention. 
           [0016]      FIG. 7  are further exemplary plots depicting image quality of the imaging system of  FIG. 1  over time without shutter closure in accordance with certain embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides practical illustrations for implementing exemplary embodiments of the present invention. In the foregoing detailed description, the invention has been described with reference to specific embodiments. However, it may be appreciated that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims. 
         [0018]    As described above, infrared imaging systems are typically configured to perform offset compensation by actuating or closing a shutter thereof to reset offsets for detector elements of a focal plan array (FPA) based on the current temperature of the shutter. In turn, such offsets are used to correct non-uniformities from the individual detector elements of the FPA. However, one limitation to this compensation technique is the high frequency of shutter actuation that is warranted, particularly when ambient temperature surrounding the imaging systems varies or the internal components of the imaging systems are found to exhibit thermal change. As described above, high frequency of shutter actuation can inconvenience the user as the image provided by the infrared imaging system is often frozen for a short period of time when the shutter is closed. As alluded to above, embodiments of the invention are provided to eliminate the need to actuate a shutter during periods of continued use of the imaging system. 
         [0019]      FIG. 1  shows a representative cross-sectional top view of part of an exemplary infrared imaging system  10  in accordance with certain embodiments of the invention. As illustrated, such system  10  includes optics  12  (e.g., one or more lenses) and an infrared focal plane array (FPA)  14 . As described above, in operation, the system  10  receives image information in the form of infrared energy  16  through the optics  12  from a source  18  (e.g., a target scene). In turn, the optics  12  direct the infrared energy  16  through a FPA window  20 , and onto the FPA  14 . 
         [0020]    As is generally understood, the FPA  14  can include a plurality of infrared detector elements (not shown), e.g., including bolometers, photon detectors, or other suitable infrared detectors well known in the art, arranged in a grid pattern (e.g., an array of detector elements arranged in horizontal rows and vertical columns). The size of the array can be provided as desired. For example, an array of 160×120 detector elements can be employed, but the invention should not be limited to such. Further detailed description of an FPA and its functioning can be found in patent application Ser. No. 11/553,373, entitled “Multiple View Infrared Imaging System”, the disclosure of which is incorporated by reference herein in relevant part. 
         [0021]    While not shown, one or more electrical circuits (not shown) are provided downstream from the FPA  14  to create an image based on the image view captured by the optics  12  and the corresponding infrared energy  16  projected onto the detector elements of the FPA  14 . Further detailed description of such electrical circuits and their functioning can be found in patent application Ser. No. 12/054,818, entitled “Offset Compensation Scheduling Algorithm for Infrared Cameras,” the disclosure of which is incorporated by reference herein in relevant part. 
         [0022]    Further included in the system  10 , and generally disposed between the optics  12  and the FPA  14 , are an enclosure  22  as well as a shutter  24 . The enclosure  22 , as shown in  FIG. 1 , is disposed over the FPA  14 . In certain embodiments, the enclosure  22  and shutter  24  are separate components of the imaging system  10 ; however, the invention should not be limited to such. For example, while not shown as such, the shutter  24  may be formed as a portion of the enclosure  22 , with the shutter  24  extending from the enclosure  22  and being movable between an open or closed position, as described below. 
         [0023]    The enclosure  22  defines opposing apertures, a first or outer aperture  26  and a second or inner aperture  28 . Each of the apertures  26 ,  28  are in alignment with the FPA  14  such that the infrared energy  16  from the source or target scene  18  can be directed through the apertures  26 ,  28  and onto the FPA  14 . In certain embodiments, the infrared imaging system  10  can be a camera, and as shown, the enclosure  22  can be operatively coupled to an outer surface  30  of such camera so that the inner aperture  28  is located near the FPA  14 , while the outer aperture  26  is located away from the FPA  14 . In certain embodiments, the enclosure  22  can be formed of one or more isothermal materials, e.g., aluminum, so that its temperature is uniformly provided across its surfaces; however, the invention should not be limited to such. As further described below, the enclosure  22  can just as well be formed of one or more non-isothermal materials. 
         [0024]    In certain embodiments, as shown in  FIG. 1 , the shutter  24  is located within the enclosure  22 . As described above, the shutter  24  can be shifted in position. In a first or “closed” position, as illustrated in  FIG. 1 , the shutter  24  is situated so that it covers the outer aperture  26 . Consequently, when in its closed position, the shutter  24  sufficiently blocks the infrared energy  16  emanating from the source  18  from reaching the FPA  14 . Alternately, in a second or “open” position, the shutter  24  is situated so that it does not cover the outer aperture  26 . For example, when in such open position, the shutter  24  may be disposed underneath or covered by an inner surface  32  of the enclosure  22 . Accordingly, the infrared energy  16  emanating from the source  18  is allowed to pass through the outer aperture  26  and reach the FPA  14  (shown via dashed lines in  FIG. 1 ). In certain embodiments, whether being in its closed or open position, the shutter  24  is in close proximity to the enclosure  22 . 
         [0025]    As should be appreciated, the detector elements of the FPA  14  are exposed to varied sources of infrared energy depending on whether the shutter  24  is in its closed position or its open position. In particular, when the shutter  24  is in its closed position, the detector elements of the FPA  14  are only exposed to infrared energy  34  emanating from the shutter  24  and the enclosure  22 . Conversely, when the shutter  24  is in its open position, the detector elements of the FPA  14  are exposed to the infrared energy  16  emanating from the target scene  18  (via the outer and inner apertures  26 ,  28  of the enclosure  22 ), as well as the infrared energy  34  emanating from the enclosure  22  and the shutter  24 . It should be appreciated that the thermal properties of the enclosure  22  and the shutter  24  are substantially similar in light of their close proximity and their similar compositions (e.g., both are generally formed of metal). When in its open position, the shutter  24  is found to lie adjacent to a corresponding surface portion of the enclosure  22 . Accordingly, infrared energy  34  emanating from the enclosure  22  and the shutter  24  in its open position can be generally thought of as the energy emanating from the enclosure  22 . 
         [0026]    In general, by taking the output of the detector elements of the FPA  14  when the shutter  24  is in its closed position and subtracting such from the output of the detector elements of the FPA  14  when the shutter  24  is in its open position, one can selectively parse the output of the FPA  14  that is specifically associated with the target scene  18 . As should be appreciated, such technique is well known and used in many conventional infrared cameras. However, as described above, such technique necessitates frequent actuating of the shutter or flag when camera temperature, e.g., either within or ambient thereto, fluctuates. To address this limitation, the infrared imaging system  10  of  FIG. 1  functions with an imager algorithm in order to negate the need for frequent actuation of the shutter  24 , as described herein. 
         [0027]      FIG. 2  shows an exemplary plot of a calibration curve  40  showing output of a reference detector element of the FPA  14  of the infrared imaging system  10  of  FIG. 1  for different temperatures of a target scene in accordance with certain embodiments of the invention. Such curve  40  is generally created following manufacture of the system  10 , and is used in conjunction with the imager algorithm embodied herein. In certain embodiments, the reference detector element represents a plurality of detector elements of the FPA  14 . 
         [0028]    As alluded to above, the curve  40  is generated by calculating output of the reference detector element (e.g., 6-Counts) on the y-axis for different temperatures of a target scene (e.g., ° C.) on the x-axis. As shown, in certain embodiments, output of the reference detector element can be quantified as magnitude of counts. Such counts, in certain embodiments, can be provided from an output of an analog-to-digital converter (ADC). As briefly alluded to above, the outputs of the FPA detector elements are transmitted through one or more electrical circuits of the system  10 , with the circuits configured to generate an image based on infrared energy from the view passing through the optics  12  and there from directed onto the FPA  14 . In certain embodiments, the signals from the detector elements would be amplified and converted from an analog form to a digital form in the electrical circuits (e.g., using an ADC) so the intensities of the signals can be quantified in terms of counts. Generally, the quantity of digital counts measured corresponds to thermal intensity borne on the detector elements of the FPA  14 . 
         [0029]    In one example, the target scene temperature for T 1  (referenced as  42 ) may be 20° C. At this value of T 1 , a corresponding output of the reference detector element can be measured from the system  10 . In certain embodiments, when the reference detector element represents a plurality of detector elements of the FPA  14 , the measured outputs of the plurality of elements are averaged together to arrive at a single output for the reference detector element. In certain embodiments, the corresponding output may be derived from more than one measurement. For example, at such T 1  value, measurements can be taken for the reference detector element both (i) when the shutter  24  is in its closed position and (ii) when the shutter  24  is in its open position. In turn, the corresponding reference detector element output, C 1  (referenced as  44 ), is calculated by taking the measured output in counts from (i) above and subtracting it from the measured output from (ii) above. Accordingly, the output for the reference detector element attributable to the target scene (C 1 ) is calculated relative to the target temperature at 20° C. (T 1 ), using the following equation: 
         [0000]        C   1 =δ−Counts (T1=20° C.) =Counts (shutter open) −Counts (shutter closed) .  (i)
 
         [0030]    Using the T 1  and C 1  values, a corresponding point  46  can be plotted in generating the calibration curve  40 . The same process can then be repeated for other temperature values of the target scene in generating the curve  40 . That is, for other target scene temperature values, e.g., T 2  (referenced as  48 ) and T 3  (referenced as  50 ), corresponding outputs for the reference detector element can be calculated, e.g., C 2  (referenced as  52 ) and C 3  (referenced as  54 ), from which corresponding points,  56  and  58 , can be further plotted in generating the curve  40 . 
         [0031]    With further reference to the curve  40  of  FIG. 2 , while the outputs of the FPA reference detector element (on the y-axis of the curve  40 ) are exemplified herein as being derived from more than one output measurement of the system  10 , the invention should not be limited to such. Instead, the output could just as well be derived from a single output measurement, e.g., output of the reference detector element with the shutter  24  in its open position. As should be appreciated, while such change would likely uniformly impact the magnitudes of the output points (on the y-axis of the curve  40 ), the slope of the curve  40  would largely remain unchanged. 
         [0032]    During generation of the calibration curve  40  of  FIG. 2 , the temperatures of the FPA  14  as well as of the enclosure  22  and shutter  24  are generally maintained at or near constant values. As a result, the calculated output of the reference detector element of the FPA  14  is controlled to be primarily reflective of the changes made to the target scene temperature. In certain embodiments, the temperature of the FPA  14  can be maintained through use of a TE stabilizer (not shown). Such TE stabilizer is well known in the art, and is often coupled to an FPA so as to monitor and maintain the temperature of the FPA at a certain desirable value. 
         [0033]    As described above, the temperatures of the enclosure  22  and the shutter  24  are substantially similar in light of their close proximity and their similar compositions. Accordingly, in certain embodiments, as shown in  FIG. 1 , the temperature of the enclosure  22  and shutter  24  can be collectively monitored through a temperature sensor  36  operatively coupled to the enclosure  22 . In certain embodiments, if the enclosure  22  is formed of one or more isothermal materials, a single temperature sensor is generally sufficient for monitoring the temperature of the enclosure  22 . However, if the enclosure  22  is not formed of isothermal material(s), multiple temperature sensors can be alternatively used, e.g., with such sensors positioned around the enclosure  22 . The temperatures of the enclosure  22  (and shutter  24 ) can be maintained by controlling the periods of use of the system  10  during the calibration process and by controlling the ambient temperature surrounding the system  10 . 
         [0034]    Generating the calibration curve  40  of  FIG. 2  in the above manner or other like fashion enables one to obtain the response of the reference detector element of the FPA  14  for different target scene temperatures. In certain embodiments, the curve  40  can be stored in memory of the imaging system  10 , e.g., memory of a processor of the electrical circuits (not shown) located downstream of the FPA  14 . In turn, when the system  10  is subsequently used in the field, such curve  40  can be retrieved from the memory and used by the imager algorithm in calculating real-time temperatures of the target scene  18 , as described below. 
         [0035]      FIG. 3  shows the calibration curve  40  of  FIG. 2  demonstrating how temperature of a target scene is calculated with reference to output of the reference detector element of the FPA  14  immediately after the shutter  24  has been opened, in accordance with certain embodiments of the invention. While  FIG. 3  demonstrates such process for the FPA reference detector element, it should be appreciated that such process is contemporaneously performed for each individual detector element of the FPA  14  by the imager algorithm of the system  10 , as is further described herein with respect to  FIG. 4 . 
         [0036]    As described above, the calibration curve  40  of  FIG. 2  enables one to obtain the response of the reference detector element of the FPA  14  for different target scene temperatures. However, one variable that needs to be accounted for in calculating the target scene temperature is how the enclosure  22 /shutter  24  compare to their temperature when the curve  40  of  FIG. 2  was generated. As described above, during calibration, the temperatures of such structures of the system  10  are generally maintained. However, when the system  10  is used in the field, the temperatures of these structures may likely vary. For any such variance, the outputs of the FPA detector elements need to be correspondingly compensated, in terms of their response, so that the curve  40  can be used in accurately calculating real-time target scene temperature. In certain embodiments, the temperature of such system structures is based on the temperature of the shutter  24 . 
         [0037]    As described above, the temperature of the shutter  24  (T Shutter , referenced as  60 ) is generally reflective of the temperature of the enclosure  22 , which in certain embodiments, as described above, can be retrieved from the temperature sensor  36  located on the enclosure  22 . With regard to this example, such shutter temperature (T Shutter ) provides a reference temperature on the x-axis of the calibration curve  40 , which is reflective of the temperatures of the system structures proximate to the FPA  14 . In turn, the imager algorithm of the system  10  uses the curve  40  in determining a corresponding reference output of the reference detector element (Ct Shutter , referenced as  62 ) for such shutter temperature (T Shutter ). As described above, such reference output of the reference detector element (Ct Shutter ) represents an offset in output of the reference detector element in light of the variance in temperature proximate to the FPA  14  from its generally maintained temperature during generation of the curve  40 . In turn, as further described below, the reference output of the reference detector element (Ct Shutter ) is the point on the y-axis from which output of the reference detector element attributable to the target scene is projected. Accordingly, such reference output represents a reference offset for the system  10 . 
         [0038]    For example, the imager algorithm derives the output of the reference detector element attributable to the target scene  18  relative to its reference output (Ct Shutter ). With reference to  FIG. 1 , such derivation involves gathering the output of the reference detector element (e.g., in counts) with the shutter  24  in its closed position. As such, in certain embodiments, the reference detector element output with the shutter  24  in its closed position is initially measured (e.g., in counts), stored, and further retrieved from memory until the shutter  24  is closed again (as further detailed below). Accordingly, such stored output can be thought of as a base output of the reference detector element. The derivation further involves measuring the output of the reference detector element (e.g., in counts) immediately after the shutter  24  is moved to its open position. In turn, the output of the reference detector element attributable to the target scene is calculated by subtracting the stored output of the reference detector element with the shutter  24  closed from the output of the reference detector element with the shutter  24  open. In turn, this difference in output of the reference detector element (δc Ref , referenced as  64 ) represents an output of the reference detector element attributable to the target scene. 
         [0039]    Once the difference in output of the reference detector element (δc Ref ) is determined, the output of the reference detector element attributable to the target scene (Ct Target , referenced as  66 ) can be obtained by adding such output of the reference detector element (δc Ref ) to its reference output (Ct Shutter ) and locating such combined output value on the y-axis. Subsequently, in using the calibration curve  40  with respect to the output of the reference detector element attributable to the target scene (Ct Target ), a corresponding target scene temperature (T Target , referenced as  68 ) can be located on the x-axis. 
         [0040]    The following equation represents the output relationship described above, with reference to  FIG. 3 : 
         [0000]        Ct   Target   =Ct   Shutter   +δc   Ref .  (ii)
 
         [0041]    From the above equation, it should be understood that Ct Shutter  is used to compensate or offset the output of the reference detector element for a real-time temperature of the enclosure  22 /shutter  24  as opposed to the temperature of such elements maintained during generation of the calibration curve  40 . Accordingly, a reference or offset point is provided on the y-axis of the curve  40 , from which output of the reference detector element attributable to the target scene (δc Ref ) is projected, with such output being derived relative to output measured during the most recent closure of the shutter  24 . 
         [0042]    As described above, while  FIG. 3  demonstrates how temperature of a target scene is calculated using the calibration curve  40  of  FIG. 2  with respect to the reference detector element of the FPA  14 , such process must be contemporaneously performed by the imager algorithm for every individual detector element of the FPA  14 .  FIG. 4  illustrates such expanded functionality with respect to the imager algorithm of the system  10 . In particular,  FIG. 4  illustrates the calibration curve of  FIG. 2  demonstrating how temperature of a target scene is calculated, for example, with reference to output of each detector element of the FPA  14 , immediately after the shutter  24  has been opened, in accordance with certain embodiments of the invention. 
         [0043]    With reference to  FIG. 1 , and similar in manner to that exemplified above with respect to  FIG. 3 , the imager algorithm of the system  10  derives the output of the detector elements attributable to the target scene  18  relative to the reference output (Ct Shutter ). However, while  FIG. 3  was exemplary with respect to the reference detector element, in the case of  FIG. 4 , all of the outputs of the detector elements of the FPA  14  are contemporaneously measured with the shutter  24  closed. Accordingly, a distinct output is measured for each detector element of the FPA  14 . As described above, the detector elements are often arranged on the FPA  14  in a matrix format. As such, each detector element is herein referenced via “i” and “j” variables, e.g., with “i” representing the row in the FPA matrix in which a detector element is located and “j” representing the column in the FPA matrix in which the same detector element is located. Thus, with the imaging system being in the vicinity of a target scene  18 , the shutter  24  is closed and an output of each of the detector elements of the FPA  14  is measured and stored. 
         [0044]    Following measurement of the outputs of the detector elements when the shutter  24  is closed, the shutter  24  is opened. As should be appreciated, the system  10 , or rather, the optics  12  of the system  10 , would be directed at the target scene  18  soon after the shutter  24  is opened. In turn, a distinct output is measured for each detector element of the FPA  14 . Following such measurements, the imager algorithm of the system  10  derives portions of the just-measured detector element outputs attributable to the target scene  18 . Such derivation, similar to what was exemplified for the reference detector element with respect to  FIG. 3 , involves subtracting the stored outputs of the detector elements (measured when the shutter  24  was closed) from their respective outputs just measured with the shutter  24  open. Such output portions of the detector elements, collectively shown as δc i,j  (referenced as  70 ), are the collective output of the FPA  14  attributable to the target scene  18 . 
         [0045]    As alluded to above, the detector elements of the FPA  14  have unique response characteristics. Put another way, each detector element of the FPA  14  has a unique gain component. The gain component for the reference detector element is generally considered to have a fundamental value of 1, while the gain components of the other detector elements of the FPA  14  may vary from such fundamental value. Gain components for each of the detector elements, collectively shown as G i,j  (referenced as  72 ), can be calculated by the manufacturer for the imaging system  10  and stored in memory (e.g., of a processor) of the system  10  during the system&#39;s calibration. The calculation of such gain components (G i,j ) would be known to those skilled in the art. In turn, the imager algorithm retrieves these gain components (G i,j ) from memory and respectively multiplies them by the derived output portions of the detector elements (δc i,j ), resulting in products collectively shown as δc i,j ·G i,j  (referenced as  74 ). 
         [0046]    Similar to that described above with reference to  FIG. 3 , in order to determine real-time outputs of the FPA detector elements attributable to the target scene  18 , a reference output or offset is determined by the imager algorithm. As described above, in using a calibration curve (such as the curve of  FIG. 2 ) in determining real-time temperatures of the target scene  18 , a reference output is necessary to predict how the current temperature of the enclosure  22 /shutter  24  compare to their temperature when such calibration curve was generated. As described above, in certain embodiments, the temperature of such system structures is based on the previously-measured temperature of the shutter  24 . 
         [0047]    With reference to  FIG. 4 , such shutter temperature (T Shutter , again referenced as  60 ), taken when the shutter  24  was most recently closed, is used by the imager algorithm of the system  10  with the curve  40  in determining a corresponding reference output of the reference detector element (Ct Shutter , again referenced as  62 ). As described above, such reference output of the reference detector element (Ct Shutter ) represents the offset in output for the FPA reference detector element in light of the variance in temperature proximate to the FPA  14  from its generally maintained temperature during generation of the curve  40 . In turn, the reference output of the reference detector element (Ct Shutter ) is the point on the y-axis from which each of the output portions of the detector elements attributable to the target scene, collectively shown as δc i,j ·G i,j , are projected. As described above, in certain embodiments, this output projection involves calculating the output portions of the FPA detector elements attributable to the target scene  18  (each being a distinct Ct Target  value, referenced as  76 ) immediately after the shutter  24  is opened. 
         [0048]    The following equation represents the output relationship described above, with reference to  FIG. 4 : 
         [0000]        Ct   Target   =Ct   Shutter +(δ c   i,j   ·G   i,j ).  (iii)
 
         [0049]    From the above equation, it should be understood that Ct Shutter  is used to compensate or offset each of the output portions of the detector elements for real-time temperatures of the enclosure  22 /shutter  24 . Accordingly, a reference or offset point is provided on the curve  40  from which outputs of the detector elements attributable to the target scene are projected (resulting in a distinct Ct Target  value for each detector element). As described above, such output portions are derived relative to output measured immediately after a last (or most recent) activation of the shutter  24 . In turn, the temperatures of the target scene  18  (each being a distinct T Target  value, referenced as  78 ) with respect to each of the projected outputs (each Ct Target  value) can be found on the x-axis by the imager algorithm of the system  10  via use of the curve  40 . 
         [0050]    It should be appreciated that the shutter  24  may be manually closed for certain events. For example, the infrared imaging system  10  may be moved to another area so as to view another target scene and/or the system  10  may be turned off. Accordingly, following such events, in certain embodiments, the imager algorithm is configured to keep the shutter  24  initially closed upon further use of the system  10 . As such, when the shutter is initially closed, new outputs of the detector elements are measured and stored, thereby replacing the prior stored outputs. In turn, such newly measured and stored outputs are used by the imager algorithm in maintaining the image quality of the system  10  during further use of the imaging system  10 . 
         [0051]    As alluded to above, the embodied infrared imaging system  10  is configured to maintain image quality in presence of temperature drift for the system  10  without frequent actuation of the shutter  24 . With reference to  FIG. 1 , such temperature drift of the system  10  is generally a reflection of how the temperature of the structures surrounding the FPA  14  varies or shifts over time, with such temperature variance known to adversely affect the output of the detector elements of the FPA  14 . In particular, it has been found that a 1° C. change in temperature proximate to the FPA  14  can be seen by the FPA detector elements as almost a 10° C. change in scene temperature. As such, it is essential to account for such temperature variance proximate to the FPA  14  to ensure the outputs calculated from the FPA  14  are valid, particularly in the instant embodiments, when the shutter  24  is kept open for extended periods of time during use of the imaging system  10 . 
         [0052]    Accounting for such temperature variance, and its impact on the outputs of the FPA detector elements, is buoyed by the inclusion of the enclosure  22  within the imaging system  10 . In particular, the temperature variance proximate to the FPA  14  is generally confined within the enclosure  22 . Such confinement enables accurate accounting, and subsequent offsetting, of a drift component (attributable from the temperature variance) from the calculated outputs of the FPA detector elements over time. To that end,  FIG. 5  shows the calibration curve  40  of  FIG. 2  demonstrating how target scene temperature is calculated with reference to output of each detector element of the FPA with thermal drift accounted for in accordance with certain embodiments of the invention. 
         [0053]    As described above, the detector elements of the FPA  14  are exposed to varied sources of infrared energy depending on whether the shutter  24  is in its closed position or its open position. In particular, when the shutter  24  is in its closed position, the detector elements of the FPA  14  are only exposed to infrared energy  34  emanating from the shutter  24  and the enclosure  22 . Conversely, when the shutter  24  is in its open position, the detector elements of the FPA  14  are exposed to the infrared energy  16  emanating from the source  18  (via the outer and inner apertures  26 ,  28  of the enclosure  22 ), as well as the infrared energy  34  emanating from the enclosure  22  and the shutter  24 . 
         [0054]    Because the detector elements of the FPA  14  have unique response characteristics, each of the elements are uniquely affected by the infrared energy  34  emanating from the shutter  24  and the enclosure  22 . Accordingly, each detector element of the FPA  14  has its own distinct drift component, collectively represented as Δ i,j  (and referenced as  80 ). As described above with respect to  FIG. 4 , the reference detector element output (Ct Shutter ) is derived from the shutter temperature (T Shutter ), and provides the offset for the detector element outputs (δc i,j ·G i,j ). These two outputs ((Ct Shutter ) and (δc i,j ·G i,j )) are then summed to determine the target temperatures (T Target  values) using the curve  40 . However, as further described above, this model only applies when determining target temperatures (T Target  values) immediately after the shutter  24  is opened, where there is no drift component Δ i,j  for the FPA detector elements. During continued use of the imaging system  10  (where the shutter  24  remains open for extended periods of time), the detector elements outputs (δc i,j ·G i,j ) are generally found to be impacted by variances in temperature proximate to the FPA  14 . As such, the imager algorithm uses a further model in determining the drift component Δ i,j  of each of the detector element outputs (δc i,j ·G i,j ), as detailed below with reference to  FIG. 5 . 
         [0055]    As can be gathered from  FIG. 5 , the imager algorithm performs certain steps which are similar to the steps described above with respect to  FIG. 4  (i.e., performed immediately after the shutter  24  is opened). For example, with reference to curve  40  of  FIG. 5 , the imager algorithm initially uses (and continues to use for subsequent derivations while the shutter  24  is open) the temperature of the shutter  24  (T Shutter , again referenced as  60 ). As described above, T Shutter  is the temperature taken during the last (or most recent) closure of the shutter  24 , and in certain embodiments, is attributed to the temperature of the enclosure  22 , e.g., measured using the temperature sensor  36  operatively coupled to the enclosure  22 . In turn, the shutter temperature (T Shutter ) is used with the curve  40  to determine the corresponding output offset for the FPA detector elements (Ct Shutter , again referenced as  62 ). 
         [0056]    The imager algorithm, in turn, measures the outputs of the detector elements (δc i,j , again referenced as  70 ); retrieves the gain components for each of the detector elements (G i,j , again referenced as  72 ) from memory and respectively multiplies the detector element outputs by the gain components, leading to (δc i,j ·G i,j ) values (again referenced as  74 ); and adds these gain-corrected outputs (δc i,j ·G i,j ) to the output offset for the FPA detector elements (Ct Shutter ), with these sums representing the corrected output for the detector elements (referenced as  82 ). 
         [0057]    However, as described above, such corrected outputs  82  contain components not only of the scene  18 , but also from any increase in infrared energy  34  (depicted in  FIG. 1 ) emanating from the shutter  24  and enclosure  22 . This additional energy  34  from the shutter  24  and enclosure  22  (collectively represented as Δ i,j ) must be removed before using the curve  40  in determining the temperatures of the target scene  18  (each being a distinct T Target  value, referenced as  78 ). The following equation represents the output relationship described above, with reference to  FIG. 5 : 
         [0000]        Ct   Target   =Ct   Shutter +(δ c   i,j   ·G   i,j )−Δ i,j   (iv)
 
         [0058]    It should be appreciated that during an initial image maintenance iteration by the imager algorithm (immediately after the most recent closure of the shutter  24 ), as depicted in  FIG. 4 , there is no need to account for drift because the temperature of the shutter  24  has not changed. However, with every subsequent image maintenance iteration (as exemplarily depicted in  FIG. 5 ), drift components (Δ i,j ) need to be accounted for because the temperature of the shutter  24  may have changed. Thus, using a calibration curve (such as the curve of  FIG. 5 ) in determining target scene temperatures (each T Target  value) for each FPA detector element, one is able to further determine and subsequently remove the corresponding drift component (Δ i,j ) from each determined output of the elements (δc i,j ·G i,j ). 
         [0059]    The drift components (Δ i,j ) are largely based on a difference in measurements taken by the imager algorithm between when the shutter  24  was last closed and a current state with the shutter  24  open. In certain embodiments, the derivation of the drift components (Δ i,j ) involves use of the temperatures of the enclosure  22  when the shutter  24  was last closed (T Shutter (last taken (with shutter closed) ) and in the present when the shutter  24  is open (T Shutter (present (with shutter open)) ). In turn, the respective outputs corresponding to these enclosure temperatures (each obtained by using the curve  40 ) are used in calculating the drift components (Δ i,j ). In certain embodiments, the calculation of the drift components (Δ i,j ) further involves use of a drift coefficient, D Shutter i,j , with the following equation being used: 
         [0000]      Δ i,j   =D   Shutter i,j   ·[Ct   Shutter (present (with shutter open))   −Ct   Shutter (last taken (with shutter closed)) ],  (v)
 
         [0000]    where D Shutter i,j  is an array of calibration constraints (one for detector element) which correct for drift. 
         [0060]    The drift coefficients (D Shutter i,j ) are generally calculated at time of calibration and stored in memory of the imaging system  10 . Calculating for the drift coefficients (D Shutter i,j ) involves taking measurements from the imaging system  10  at a time “ 0 ”, immediately after the shutter  24  is opened (i.e., so there is no drift component (Δ i,j )) and a later time (i.e., time “ 1 ”) with the shutter  24  still open but with variance to the temperature of the enclosure  22 /shutter  24  from time “ 0 ” (i.e., so there is now a drift component (Δ i,j )). As should be appreciated, such variance to the enclosure/shutter temperature impacts the outputs from the FPA detector elements taken at time “ 1 ”. As should be appreciated, such points in time (“ 0 ” and “ 1 ” times) have already been described above with respect to  FIGS. 4 and 5  above, and further, with respect to equations (iii) and (iv), respectively. Keeping the target scene temperatures (each T Target  value) for each FPA detector element the same during these points in time (time “ 0 ” and the later time), the corresponding Ct Target  values would be kept the same. Accordingly, equations (iii) and (iv) can be provided so as to be equal to one another, as shown below: 
         [0000]        Ct   Shutter     —     0 +((δ c   i,j ) 0   ·G   i,j )= Ct   Shutter     —     0 +((δ c   i,j ) 1   ·G   i,j )−Δ i,j .  (vi)
 
         [0061]    In turn, using equation (v) above, the drift component (Δ i,j ) can be replaced in equation (vi), resulting in: 
         [0000]        Ct   Shutter     —     0 +((δ c   i,j ) 0   ·G   i,j )= Ct   Shutter     —     0 +((δ c   i,j ) 1   ·G   i,j )− D   Shutter ij   ·[Ct   Shutter     —     1   −Ct   Shutter     —     0 ],  (vii)
 
         [0000]    which can be reconfigured to solve for the drift coefficients (D Shutter i,j ), resulting in the following equation: 
         [0000]        D   Shutter i,j   =[G   i,j ·[(δ c   i,j ) 1 −(δ c   i,j ) 0 ]]/[ Ct   Shutter     —     1   −Ct   Shutter     —     0 ].  (viii)
 
         [0062]    With the drift coefficients (D Shutter i,j ) previously calculated and stored in memory of the imager system  10 , the imager algorithm can calculate the drift components (Δ i,j ) using equation (v). Such drift component can be used by the imager algorithm in deriving output portions for each of the FPA detector elements attributable to the target scene (Ct Target  values), using equation (iv) above. In turn, the imager algorithm uses these outputs (Ct Target  values) with the curve  40  in determining the corresponding target scene temperatures (T Target  values). It should be appreciated that the above model provides good image quality for the imaging system  10  without the need to flag, or close the shutter  24 , even in the presence of significant swings in temperature with respect to the enclosure  22 /shutter  24 . 
         [0063]      FIGS. 6 and 7  help illustrate the above-noted excellent image quality that can be maintained without closing the shutter  24  of the imaging system  10  by using the imager algorithm.  FIG. 6  is a plot of three different lines, where line I represents the calculated target temperature over time, line II represents the measured temperature of the enclosure  22  over time, and line III represents the Noise Equivalent Temperature Difference (NETD) over time. As illustrated, the time line on the x-axis shows the test duration being almost three days over which the imager system  10  was used without closing the shutter  24 . The target temperature was kept at a constant temperature of 55° C. over the test duration. As can be seen, the calculated target temperature (line I) exhibited no significant change in temperature (from 55° C.) over the test duration, while the measured temperature of the enclosure  22  (line II) showed dramatic change in temperature (due to the imaging system cooling down, going from about 49° C. to about 41° C.) over the same period. In addition, the calculated target temperature (line I) showed negligible noise equivalent temperature difference over the same period. Accordingly, accuracy and noise quality remained excellent, even with the shutter  24  not being closed over the test duration. 
         [0064]      FIG. 7  further illustrates the above superior performance of the imager algorithm when used with the imaging system  10 . As shown, two graphs depicting image uniformity for the FPA  14  are shown side by side, the graph on the left showing the uniformity immediately after opening the shutter  24  and the graph on the right showing the uniformity after 90 minutes without further closing the shutter  24 . Similar to that described above with respect to  FIG. 6 , the target temperature was kept at a constant temperature of 55° C. over the test duration. In comparing the two plots, there is no noticeable difference between the two even though the unit started cooling down from a 50° C. shortly after the shutter  24  was opened. Accordingly, image quality was maintained through use of the imager algorithm. 
         [0065]    It will be appreciated the embodiments of the present invention can take many forms, and it is not intended that the embodiments of the invention presented herein should limit the scope thereof.