Abstract:
A thermal imaging apparatus comprises a thermal image camera having a lens and a display. The camera further includes a focal plane array located behind the lens for converting imaging radiation to produce an image signal for further processing. A shutter mechanism is operative to selectively inhibit exposure of the focal plane array to the imaging radiation such that the focal plane array produces a reference signal. Processing circuitry is operative to receive the image signal and produce a corresponding thermal image on the display. The processing circuitry is further operative to utilize the image signal and the reference signal to derive temperature information.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to thermal imagers. More particularly, the invention relates to a thermal imager that utilizes an improved technique to calibrate a focal plane array so as to provide accurate radiometric information. 
   Thermal imaging cameras are used in a wide variety of applications, such as predictive maintenance in industrial facilities. While past imagers often utilized a scanning technique, modern imagers generally incorporate an infrared (IR) focal plane array (FPA) for producing the thermal image. FPAs have many advantages, such as the speed at which successive images can be produced for display. 
   Although FPAs provide good imagery of the scene, it is much more difficult to use an FPA imager to accurately measure the temperature of objects in the scene. It is believed that others have provided radiometers employing FPAs, but these devices have required complicated cooling or temperature stabilization mechanisms in order to operate properly. Thus, there is a need in the art for an FPA thermal imager that does not require a complicated cooling mechanism for radiometric operation. 
   SUMMARY OF THE INVENTION 
   According to one aspect, the present invention provides a thermal imaging apparatus comprising a thermal image camera having a lens and a display. The camera further includes a focal plane array located behind the lens for converting imaging radiation to produce an image signal for further processing. A shutter mechanism is operative to selectively inhibit exposure of the focal plane array to the imaging radiation such that the focal plane array produces a reference signal. Processing circuitry is operative to receive the image signal and produce a corresponding thermal image on the display. The processing circuitry is further operative to utilize the image signal and the reference signal to derive temperature information. 
   In presently preferred embodiments, the processing circuitry operates to produce the temperature information based on a difference between the image signal and the reference signal. A temperature sensor may also be provided to provide a temperature measure indicative of an ambient temperature of the focal plane array. For example, the temperature sensor may be associated with the shutter. In some cases, the shutter may comprise a flag element selectively rotatable into a closed position between the focal plane array and the scene. 
   Preferably, the apparatus may include a memory containing calibration information for the focal plane array. In such embodiments, the processing circuitry can utilize the calibration information during production of the temperature information. For example, the calibration information may indicate target temperature as a function of signal strength of the image signal. 
   Often, the memory may also contain adjustment information for each pixel of the focal plane array. The adjustment information, which may comprise gain and offset information, may be used by the processing circuitry to adjust the difference. 
   According to other aspects, the present invention provides a method of deriving temperature information in a focal plane array imager. One step of the method involves obtaining a reference signal from the focal plane array representing a uniform temperature reference scene. In another step, an image signal is obtained from the focal plane array representing an image scene. A difference between the image signal and the reference signal is also determined. In addition, a temperature measure indicative of an ambient temperature of the focal plane array is provided. The difference and the temperature measure is then utilized to derive temperature information for the image signal. 
   In accordance with preferred methodology, the reference scene may be provided by a shutter mechanism that selectively inhibits exposure of the focal plane array to imaging radiation. In such cases, the temperature measure may be provided utilizing a temperature sensor associated with the shutter mechanism. For example, the shutter mechanism may be operable to close on a periodic basis. 
   Often, the temperature information may be derived by applying adjustment information for each pixel of the focal plane array to the difference so as to yield an adjusted difference. Calibration information for the focal plane array can be utilized to produce the temperature information based on the adjusted difference and the temperature measure. For example, the adjustment information may comprise gain and offset information. 
   Still further aspects of the present invention are provided by an apparatus comprising a focal plane array for converting imaging radiation to produce an image signal for further processing. A shutter mechanism is operative to provide a uniform temperature reference scene such that the focal plane array produces a reference signal. A temperature sensor is also operative to provide a temperature measure indicative of an ambient temperature of the focal plane array. The apparatus also includes processing circuitry operative to utilize the image signal, the reference signal and the temperature measure to derive target temperature information. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying drawings, in which: 
       FIG. 1  is a rear perspective view of a thermal imager constructed in accordance with an embodiment of the present invention; 
       FIG. 2  is a front perspective view of the thermal imager of  FIG. 1 ; 
       FIG. 3  is an enlarged elevation of the thermal imager&#39;s display as depicted in  FIG. 1 ; 
       FIG. 4  is a diagrammatic representation showing internal components of the thermal imager of  FIG. 1 ; 
       FIGS. 5 and 6  are graphs representing signal strength versus target temperature of a particular FPA during calibration and use, respectively; and 
       FIG. 7  is a flow diagram showing steps performed in accordance with a preferred calibration technique of the present invention. 
   

   Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions. 
     FIGS. 1 and 2  illustrate a thermal imager  10  constructed in accordance with the present invention. Imager  10  includes a housing  12  in which the components of a thermal image camera are located. Preferably, housing  12  is formed by complementary left and right housing portions  12   a  and  12   b  which are joined together during assembly. While any suitable material can be utilized, housing portions  12   a  and  12   b  are preferably formed of a rigid high impact plastic material. Selected regions of housing  12  may be desirably overmolded with a softer polymeric material. 
   As shown in  FIG. 2 , housing  12  includes a front portion enclosing a hood  14  behind which the device&#39;s lens  16  is located. One skilled in the art will recognize that the target energy enters the device through lens  16 . A lens cover  18  is provided to cover lens  16  when imager  10  is not in use. Preferably, lens cover  18  is opaque to passage of infrared radiation so as to protect the imager&#39;s internal components when the unit is not being used. 
   In this case, lens cover  18  slides up and down in a channel provided in front shroud  20 . Shroud  20  extends to a location under handle  22  to facilitate placement of the entire unit in a computer docking station for recharging and specialized programming. A trigger  24  is located on handle  22 , as shown. Trigger  24  permits the user to store selected images in the device&#39;s internal memory. In the illustrated embodiment, laser diode  26  projects a dot of light forward of the imager to facilitate aiming. 
   Referring now to  FIG. 3 , a display  28  is preferably located at the rear of imager  10 . In this case, the display is configured as a color display of the LCD type. For example, the display may be a typical LCD touch panel as are often used in personal digital assistants (PDAs) and other types of common electronic devices. A plurality of function buttons  30 ,  32  and  34  are also located on the rear portion of imager  10 . 
   In this depiction, display  28  shows a variety of information, including a thermal image  36  of the equipment being inspected. A temperature gradient scale  38  and other information may also be provided in different regions of the display. For example, the display  28  indicates at  40  that the machine being inspected is a “compressor” based on stored information. The target temperature at the “crosshairs” of the image is indicated at  42 . 
   Referring now to  FIG. 4 , certain internal components of imager  10  will be described. When lens cover  18  is moved to the down position, incident radiation is allowed to enter hood  14 . The radiation passes through lens  16  and impinges focal plane array (FPA)  44 . In presently preferred embodiments, FPA  44  is an infrared (IR) FPA of any suitable type, such as alpha-silicon or vanadium oxide. (Such devices are available, for example, from Raytheon Company.) 
   FPA  44  converts the incident radiation into electrical signals S S  which are then provided to signal processing circuitry  46 . Circuitry  46  processes the raw signals to produce IR video/temperature data that can be shown on display  28 . In this regard, display  28  will typically depict a thermal image of the target equipment  48 , along with an accurate indication of the temperature at various locations on the image. For example, various temperatures on the image may be indicated by color, which can be correlated with the colors in gradient  38 . As noted above, a numerical indication of the actual temperature at the image crosshairs can also be depicted at  42 . 
   While FPA imagers are known to provide good imagery of the scene, it is much more difficult to utilize them for accurate temperature measurements. In the past, complicated cooling or temperature stabilization mechanisms have been provided to maintain the FPA at a predetermined temperature during use. Such cooling and temperature stabilization mechanisms are expensive, greatly add to the complexity of the imager, and increase the imager&#39;s power consumption. 
   Thus, the present invention provides a technique to ascertain temperature information from an FPA signal without internal cooling. Toward this end, imager  10  includes a shutter which can be used to provide a reference scene to processing circuitry  46 . In this embodiment, the shutter is configured as a “flag”  50  rotated by a small motor  52  (as indicated by arrow  54 ). Motor  52  is operated by shutter controller circuitry  56 , which is itself controlled by signal processing circuitry  46 . 
   Typically, flag  50  will be moved to a position in front of FPA  44  when imager  10  is initially powered on. Thereafter, motor  52  will move flag  50  into position in front of FPA  44  periodically during operation (such as every 2–3 minutes). When flag  50  is moved to this position, a reference signal S F  is produced by FPA  44 . This reference signal is correlated with an ambient temperature signal T F . In this regard, a suitable temperature sensor is typically mounted on flag  50  (or temperature well connected to flag  50 ) to provide a measure of the ambient temperature at this location. Signal processing circuitry utilizes signals S F , S S  and T F  along with stored calibration information in order to generate the desired temperature data. 
     FIG. 5  illustrates preferred calibration information that can be stored in memory  58 . At the time unit  10  is manufactured, the specific FPA  44  for that unit can be calibrated. In this case, calibration occurs at multiple scene temperatures throughout a range. Signal strength at each of the scene temperatures is obtained to generate a target temperature versus signal strength curve. 
   This is illustrated in the example of  FIG. 5 , where signal strength measurements have been taken at a total of eight calibration temperatures (a–h). In this exemplary case, these target temperatures may be temperature points that are equally spaced in a range extending from 0° C. through 250° C. The resulting measurements can be used to interpolate a curve  60 . It will be appreciated that each other pixel in FPA  44  will also exhibit this same shape of curve. Information representing curve  60  is preferably stored in memory  58  in the form of an electronic lookup table. 
     FIG. 6  illustrates a technique whereby curve  60  can be utilized to calculate actual temperature at a particular pixel, to wit: 
   a) Knowing the flag temperature use the lookup table to find the corresponding reference signal strength S R . This value S R  is then used for all pixels. 
   b) Subtract the reference frame from the signal frame thus generating a difference “DIFF” signal for each pixel. DIFF can be positive or negative, as one skilled in the art will appreciate. 
   c) For each pixel calculate the pixel scene signal S P , by adding the DIFF signal for the pixel to S R . 
   d) Knowing S P  for each pixel use the lookup table to find the scene temperature T S  for each pixel. 
   There are slight variations in gain and offset between adjacent pixels, but these can be corrected by individual adjustments. Thus, the raw difference signal may be adjusted by the pixel gain and offset before the actual temperature is obtained. 
   There are also variations in the responsitivity of each pixel with changes in ambient temperature. Because these variations are approximately linear, however, a relatively simple linear correction can often be applied. In accordance with a preferred embodiment, this linear correction can be accomplished using the following formula:
 
CORRECTION= DIFF ×[(1+α×( T−T   CAL )]
 
where,
 
   α is the value required to make the device read the same temperature at ambient and 50° C.; and 
   T CAL  is the ambient temperature of the instrument at calibration. 
   In many cases, it will also be desirable to provide a global gain adjustment depending on target temperature. For example, many preferred embodiments of the present invention provide a low gain or a high gain for temperatures above and below a predetermined threshold, respectively. In one particularly preferred embodiment, the threshold temperature may be 125° C. In other words, if something in the scene has a temperature of greater than 125° C., the instrument automatically chooses low gain. If nothing in the scene is greater than 125° C., high gain is automatically selected. 
     FIG. 7  provides an overview of the preferred methodology for determining temperature as described above. As indicated at  62 , flag  50  rotates to a position in front of FPA  44  in order to obtain a reference scene. Next, as indicated at  64 , a target image is obtained to produce an image signal. The “DIFF” for each pixel can then be determined (as indicated at  66 ). 
   As indicated at  68 , DIFF is then corrected for gain and offset on a pixel by pixel basis. Next, as indicated at  70 , the DIFF for each pixel can be corrected for variations in ambient temperature. Because the flag temperature T F  is known, calibration information can be utilized to determine T S  at each pixel (as indicated at  72 ). Finally, the signal processing circuitry can utilize the temperature information thus obtained in order to display a thermal image along with associated temperature data (as indicated at  74 ). 
   It can thus be seen that the present invention provides a thermal imager utilizing an improved radiometric calibration technique. According to the invention, accurate temperature information corresponding to points in a thermal image can be obtained using a focal plane array. This eliminates the need for complicated cooling or temperature stabilization mechanisms and the like in order to maintain the focal plane array at a reference temperature. 
   While preferred embodiments of the invention have been shown and described, modifications and variations may be made thereto by those of ordinary skill in the art without departing from the spirit and scope of the present invention. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to be limitative of the invention as further described in the appended claims.