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Real-time radiation sensor calibration Number:7,030,378 from the United States Patent and Trademark Office (PTO) owispatent

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Title: Real-time radiation sensor calibration

Abstract: One embodiment of the invention is directed to methods and apparatus for determining a variation of a calibration parameter of a pixel of the thermal sensor during operation of the imaging apparatus, after an initial calibration procedure. Another embodiment of the invention is directed to methods and apparatus for calculating a gain calibration parameter using first and second ambient temperature values and respective first and second resistance values for a pixel of a sensor. A further embodiment of the invention is directed to calculating an offset calibration parameter for at least one pixel using a gain of the at least one pixel between first and second times and an ambient temperature at a third time, wherein the pixel is exposed to both scene and ambient radiation at the third time.

Patent Number: 7,030,378 Issued on 04/18/2006 to Allen,   et al.


Inventors: Allen; Thomas P. (Allston, MA); Butler; Neal R. (Acton, MA)
Assignee: BAE Systems Information and Electronic Systems Integration, Inc. (Nashua, NH)
Appl. No.: 634215
Filed: August 5, 2003


Current U.S. Class: 250/332 ; 250/252.1
Current International Class: G01J 5/00 (20060101); H01L 25/00 (20060101)
Field of Search: 250/252.1,330,332,338.1,338.3,338.4,339.02,339.03,339.04,339.09,341.5,351,353


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Primary Examiner: Gagliardi; Albert
Attorney, Agent or Firm: Wolf, Greenfield, & Sacks, P.C.

Claims



What is claimed is:

1. An imaging apparatus, comprising: a plurality of pixels to detect radiation and to output image signals based on the detected radiation; a temperature sensor to detect an ambient temperature; and means, coupled to the plurality of pixels and the temperature sensor, for determining a calibration parameter of a pixel during operation of the imaging apparatus, based on at least a first ambient temperature of the pixel and a second ambient temperature of the pixel, each measured after an initial calibration procedure.

2. The imaging apparatus of claim 1, wherein the means for determining a calibration parameter comprises means for determining an offset of the pixel.

3. The imaging apparatus of claim 1, wherein the means for determining a calibration parameter comprises means for determining a gain of the pixel.

4. The imaging apparatus of claim 1, wherein the means for determining a calibration parameter is actuated to determine the calibration parameter when a predetermined time period has elapsed.

5. The imaging apparatus of claim 1, wherein the means for determining a calibration parameter is actuated to determine the calibration parameter when a predetermined ambient temperature change has occurred.

6. The imaging apparatus of claim 1, wherein the means for determining a calibration parameter comprises at least one processor, and wherein the at least one processor is programmed to perform an act of: calculating an offset calibration parameter for the pixel based on a change in resistance of the pixel over a time period and a change in the ambient temperature of the pixel over the time period.

7. The imaging apparatus of claim 6, wherein the offset calibration parameter is a change in a resistance of the pixel caused by a change in an ambient temperature of the pixel.

8. The imaging apparatus of claim 1, wherein the plurality of pixels are sensitive to radiation in the infrared range.

9. The imaging apparatus of claim 1, wherein the plurality of pixels are sensitive to thermal radiation.

10. The imaging apparatus of claim 1, wherein the means for determining includes means for determining the calibration parameter after an initial calibration procedure during which calibration is performed at only one calibration temperature.

11. A method of calibrating an imaging system comprising a thermal sensor, comprising an act of: determining a calibration parameter of a pixel of the thermal sensor during operation of the imaging apparatus, based on at least a first ambient temperature of the pixel and a second ambient temperature of the pixel, each measured after an initial calibration procedure.

12. The method of claim 11, wherein the act of determining a calibration parameter includes comparing first and second output signals of the pixel.

13. The method of claim 12, wherein the act of determining a calibration parameter further includes comparing first and second temperature signals associated with the first and second output signals.

14. The method of claim 11, wherein the act of determining a calibration parameter includes determining an offset calibration parameter of the pixel.

15. The method of claim 14, wherein the act of determining an offset calibration parameter includes determining a change in resistance of the pixel over a time period and a change in the ambient temperature of the pixel over the time period.

16. The method of claim 11, wherein the act of determining a calibration parameter includes determining a gain calibration parameter of the pixel.

17. The method of claim 11, wherein the act of determining a calibration parameter occurs when a predetermined time period has elapsed.

18. The method of claim 11, wherein the act of determining a calibration parameter occurs when a predetermined ambient temperature change has occurred.

19. The method of claim 11, wherein the act of determining a calibration parameter includes determining a calibration parameter of a pixel sensitive to infrared radiation.

20. The method of claim 11, wherein the act of determining includes determining the calibration parameter after an initial calibration procedure during which calibration is performed at only one calibration temperature.

21. The method of claim 16, wherein the act of determining a gain calibration parameter of the pixel comprises acts of: shielding the pixel from scene radiation at a first time and measuring a resistance of the pixel and an ambient temperature at the first time; shielding the pixel from scene radiation at a second time and measuring a resistance of the pixel and an ambient temperature at the second time; calculating a first gain calibration parameter using the resistance of the pixel and the ambient temperature at the first time and the resistance of the pixel and the ambient temperature at the second time; and determining a second gain calibration parameter for the pixel.

22. The method of claim 21, wherein the act of calculating the first gain calibration parameter includes determining a change in the resistance of the pixel between the first and second times relative to a change in the ambient temperature between the first and second times.

23. The method of claim 22, wherein the act of calculating the first gain calibration parameter further comprises acts of: subtracting the ambient temperature at the first time from the ambient temperature at the second time to generate an ambient temperature difference; subtracting the resistance of the pixel at the first time from the resistance of the pixel at the second time to generate a resistance difference; and dividing the ambient temperature difference by the resistance difference.

24. The method of claim 21, wherein: the act of shielding the pixel from scene radiation at the first time comprises performing a shutter operation at the first time; and the act of shielding the pixel from scene radiation at the second time comprises performing a shutter operation at the second time.

25. The method of claim 21, wherein the act of determining a second gain calibration parameter for the pixel comprises acts of: shielding the pixel from scene radiation at a third time and measuring a resistance of the pixel and an ambient temperature at the third time; and calculating a second gain calibration parameter using the resistance of the pixel and the ambient temperature at the second time and the resistance of the pixel and the ambient temperature at the third time; wherein the method further comprises an act of updating the gain calibration parameter with the second gain calibration parameter.

26. The method of claim 21, wherein the pixel is a first pixel in an array of pixels, and wherein the method further comprises: measuring a resistance of a second pixel in the array of pixel at the first time; measuring a resistance of the second pixel at the second time; and calculating a gain calibration parameter for the second pixel using the resistance of the second pixel at the first and second times and the ambient temperature at the first and second times.

27. The method of claim 21, wherein: the act of measuring the ambient temperature at the first time comprises measuring a substrate temperature at the first time; the act of measuring the ambient temperature at the second time comprises measuring a substrate temperature at the second time; and the act of calculating the first gain calibration parameter comprises calculating the first gain calibration parameter using the resistance of the pixel at the first and second times and the substrate temperature at the first and second times.

28. A method of claim 21, further comprising acts of: receiving scene radiation via the pixel at a third time and measuring an ambient temperature at the third time; calculating a second gain calibration parameter using the ambient temperature at the third time and a predetermined function that relates an ambient temperature change to a gain calibration parameter change; and updating the gain calibration parameter with the second gain calibration parameter.

29. The method of claim 21, wherein the pixel is a first pixel in an array of pixels, and wherein the method further comprises acts of: shielding a second pixel of the array from scene radiation at a first time and measuring a resistance of the second pixel at the first time; shielding the second pixel from scene radiation at a second time and measuring a resistance of the second pixel at the second time; and calculating a gain calibration parameter for the second pixel using the resistance of the second pixel and the ambient temperature at the first time and the resistance of the second pixel and the ambient temperature at the second time.

30. The method of claim 21, further comprising an act of: applying the second gain calibration parameter to correct a gain error of the pixel.

31. The method of claim 30, wherein the act of applying includes applying the second gain calibration parameter to an output signal of the pixel to correct the gain error of the pixel.

32. The method of claim 30, wherein the act of applying includes applying the second gain calibration parameter to an operating parameter of the pixel to correct the gain error of the pixel.

33. The imaging apparatus of claim 1, further comprising: a data storage device to store first and second ambient temperature values and first and second resistance values for each pixel of the plurality of pixels; wherein the means for determining a calibration parameter comprises: means for calculating a first gain calibration parameter for each pixel of the plurality of pixels using the first and second ambient temperature values and first and second resistance values for each pixel of the plurality of pixels; and means for determining a second gain calibration parameter for each pixel of the plurality of pixels.

34. The imaging apparatus of claim 33, further comprising: a shutter mechanism to block scene radiation; wherein the first and second ambient temperature values and first and second resistance values for each pixel of the plurality of pixels are each detected during actuation of the shutter mechanism.

35. The imaging apparatus of claim 33, further comprising: a substrate coupled to the plurality of pixels; wherein the temperature sensor is thermally coupled to the substrate so as to detect a temperature of the substrate.

36. The imaging apparatus of claim 33, wherein the plurality of pixels are sensitive to radiation in the infrared range.

37. The imaging apparatus of claim 33, wherein the plurality of pixels are sensitive to thermal radiation.

38. The imaging apparatus of claim 33, wherein at least some of the plurality of pixels are bolometers.

39. A method of calculating an offset calibration parameter of a pixel of a camera, comprising acts of: determining a gain of the pixel during a period of operation of the camera between first and second times, after an initial calibration procedure; exposing the pixel to both scene and ambient radiation at a third time; measuring an ambient temperature of the pixel at the third time; and calculating the offset calibration parameter of the pixel using the gain of the pixel between the first and second times and the ambient temperature of the pixel at the third time.

40. The method of claim 39, further comprising an act of: determining a portion of a change in temperature of the pixel between the second and third times based solely on a change in scene radiation using the offset calibration parameter and a resistance of the pixel measured at the third time.

41. The method of claim 40, further comprising an act of: determining the portion of a change in temperature of the pixel between the second and third times based solely on a change in scene radiation by multiplying a gain calibration parameter by the resistance of the pixel measured at the third time to generate a product, and adding the offset calibration parameter to the product.

42. The method of claim 39, wherein the act of calculating the offset calibration parameter includes an act of determining the portion of the resistance of the pixel at the third time that is attributable to ambient radiation.

43. The method of claim 39, further comprising acts of: shielding the pixel from scene radiation at the first and second times; and exposing the pixel to ambient radiation and scene radiation at the third time.

44. A method of claim 43, wherein the act of shielding comprises performing a shutter operation at the first and second times.

45. The method of claim 39, wherein the act of determining a gain of the pixel comprises acts of: shielding the pixel from scene radiation at a first time and measuring a resistance of the pixel and an ambient temperature at the first time; shielding the pixel from scene radiation at a second time and measuring a resistance of the pixel and an ambient temperature at the second time; and calculating the gain of the pixel using the resistance of the pixel and the ambient temperature at the first time and the resistance of the pixel and the ambient temperature at the second time.

46. The method of claim 39, wherein the pixel is a first pixel in an array of pixels, and wherein the method further comprises: determining a gain of a second pixel in the array between the first and second times; measuring a resistance of the second pixel; and calculating a change in the resistance of the second pixel between the second time and the third time resulting from a change in the ambient temperature between the second time and the third time.

47. The method of claim 39, wherein: the act of measuring an ambient temperature of the pixel at a third time comprises measuring a substrate temperature at the third time.

48. The method of claim 39, further comprising an act of: applying the offset calibration parameter to an output signal of the pixel at the third time to correct an offset error of the pixel.

49. The method of claim 48, wherein the act of applying includes applying the offset calibration parameter to a resistance of the pixel at the third time to correct the offset error of the pixel.

50. The method of claim 48, wherein the act of applying includes applying the offset calibration parameter to an operating parameter of the pixel to correct the offset error of the pixel.

51. An imaging apparatus, comprising: at least one pixel to detect radiation and to output image signals based on the detected radiation; a temperature sensor to detect an ambient temperature; and means for calculating an offset calibration parameter for the at least one pixel using a gain of the at least one pixel during a period of operation of a camera between first and second times after an initial calibration procedure, and an ambient temperature at a third time, wherein the pixel is exposed to both scene and ambient radiation at the third time.

52. The imaging apparatus of claim 51, further comprising: a substrate coupled to the at least one pixel; wherein the temperature sensor thermally coupled to the substrate so as to detect a temperature of the substrate.

53. The imaging apparatus of claim 51, wherein the at least one pixel is sensitive to radiation in the infrared range.
Description



BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to radiation sensors, and in particular, to compensating operating parameters and/or output signals of radiation sensors for changes in temperature of the sensors.

2. Discussion of the Related Art

A radiation detector is a device that produces an output signal which is a function of an amount of radiation that is incident upon an active region of the radiation detector. Radiation detectors may be designed and constructed to be sensitive to particular regions of the electromagnetic spectrum. For example, infrared detectors are radiation detectors that are sensitive to radiation in the infrared region of the electromagnetic spectrum. One example of an infrared detector includes a thermal detector, which detects radiation based upon a change in the temperature of an active region of the detector due to absorption of radiation incident to the detector.

A variety of imaging sensors may be constructed using an array of radiation detectors. Such sensors may be used in an imaging system that produces an image (e.g., on a display) based on radiation impinging on the imaging sensor. Based on the type of detectors used, the imaging sensor may be responsive to a particular region of spectrum. For example, an infrared or thermal imaging sensor may include a number of thermal detectors that detect a representation of an object by the objects' thermal emissions. In particular, energy emitted by an object may depend on numerous quantities such as, for example, the emissivity and the temperature of the object. Infrared thermal sensors typically detect one or both of these quantities and use the detected information to produce an object image that may be viewed, for example, on a display.

One issue in connection with at least some types of radiation detectors is that often it may be challenging to separate those signals output by the detector that are actually due to radiation of interest incident to the detector from various undesirable components which may be present in the detector output signals. For example, detector output signals may include various undesirable components due to variations in temperature of the detector itself that are not necessarily due to the radiation of interest. In particular, temperature changes in the vicinity of the detector that may affect the temperature of the detector, sometimes referred to as ambient temperature variations (e.g., changes in temperature of a substrate on which the detector is fabricated, changes in temperature of a package in which the detector is housed, average temperature changes in a scene of interest itself), in turn may cause undesirable components to be present in the detector output signals. In some cases, these undesirable components may be hundreds of times larger than the instantaneous signals resulting from the radiation of interest, thereby detrimentally reducing the dynamic range of the detector and/or processing circuitry associated with the detector with respect to the radiation of interest.

In view of the foregoing, some conventional imaging systems employing imaging sensors comprising a number (e.g., array) of radiation detectors require some type of temperature stabilization of the detectors to reduce such undesirable components in the detector output signals. In particular, with respect to conventional thermal imaging systems, it is generally thought to be impractical to operate such systems without active stabilization of the temperature of the detectors. In some cases, thermal stabilization components may include a thermoelectric cooler (hereinafter, "TE cooler") that is thermally coupled to the detectors (e.g., the substrate on which the detectors are fabricated is mounted on the TE cooler) to hold the detectors at a predetermined temperature. Depending on the difference between the predetermined stabilization temperature and the actual ambient temperature in the vicinity of the detectors, the TE cooler may consume appreciable power resources of the imaging system.

SUMMARY OF THE INVENTION

One embodiment of the invention is directed to an imaging apparatus, comprising a plurality of pixels to detect radiation and to output image signals based on the detected radiation and a temperature sensor to detect an ambient temperature. The imaging apparatus further comprises means, coupled to the plurality of pixels and the temperature sensor, for determining a variation of a calibration parameter of a pixel during operation of the imaging apparatus after an initial calibration procedure.

Another embodiment of the invention is directed to a method of calibrating an imaging system comprising a thermal sensor. The method comprises an act of determining a variation of a calibration parameter of a pixel of the thermal sensor during operation of the imaging apparatus after an initial calibration procedure.

A further embodiment of the invention is directed to a method for generating a gain calibration parameter of a pixel. The method comprises acts of shielding the pixel from scene radiation at a first time and measuring a resistance of the pixel and an ambient temperature at the first time, shielding the pixel from scene radiation at a second time and measuring a resistance of the pixel and an ambient temperature at the second time, and calculating the gain calibration parameter using the resistance of the pixel and the ambient temperature at the first time and the resistance of the pixel and the ambient temperature at the second time.

Another embodiment of the invention is directed to an imaging apparatus, comprising a plurality of pixels to detect radiation and to output image signals based on the detected radiation, a temperature sensor to detect an ambient temperature, and a data storage device to store first and second ambient temperature values and first and second resistance values for each pixel of the plurality of pixels. The imaging apparatus further comprises means for calculating a gain calibration parameter for each pixel of the plurality of pixels using the first and second ambient temperature values and first and second resistance values for each pixel of the plurality of pixels.

A further embodiment of the invention is directed to a method comprising acts of determining a gain of the pixel between first and second times, exposing the pixel to both scene and ambient radiation at a third time, and measuring an ambient temperature of the pixel at the third time. The method further comprises calculating an offset calibration parameter of the pixel using the gain of the pixel between the first and second times and the ambient temperature of the pixel at the third time.

Another embodiment of the invention is directed to an imaging apparatus, comprising at least one pixel to detect radiation and to output image signals based on the detected radiation and a temperature sensor to detect an ambient temperature. The imaging apparatus further comprises means for calculating an offset calibration parameter for the at least one pixel using a gain of the at least one pixel between first and second times and an ambient temperature at a third time, wherein the pixel is exposed to both scene and ambient radiation at the third time.

A further embodiment of the invention is directed to a method of performing an offset and gain calibration procedure after an initial calibration procedure. The calibration procedure comprises acts of calculating a gain of a pixel between first and second times, measuring an ambient temperature of the pixel and a resistance of the pixel at a third time, wherein the pixel is exposed to both ambient and scene radiation at the third time, and determining a change in temperature of the pixel between the second and third time attributable to solely scene radiation using the gain of the pixel between the first and second times and the ambient temperature and resistance of the pixel at the third time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a portion of an imaging system according to one embodiment of the invention;

FIG. 2 is a diagram showing a portion of an integrated sensor array and some associated signal processing circuitry used in the imaging system of FIG. 1, according to one embodiment of the invention;

FIG. 3 is a diagram showing a more detailed view of a portion of the sensor array illustrated in FIG. 2, according to one embodiment of the invention;

FIG. 4 is a diagram showing one example of a digital-to-analog converter used in the processing circuitry shown in FIGS. 2 and 3, according to one embodiment of the invention;

FIG. 5 is a diagram showing one example of a preamplifier/integrator used in the processing circuitry shown in FIGS. 2 and 3, according to one embodiment of the invention;

FIG. 6 is a diagram showing a more detailed view of the portion of the imaging system shown in FIG. 3, illustrating one possible implementation of various temperature compensation features, according to one embodiment of the invention;

FIG. 7 is a diagram showing examples of a current sampler/DC global offset adjust circuit and a global bias adjust circuit as analog circuit implementations of the temperature compensation features shown in FIG. 6, according to one embodiment of the invention;

FIG. 8 is a diagram showing an example of a current sampler/AC global offset adjust circuit as an analog circuit implementation of a sensor self-heating temperature compensation feature shown in FIG. 6, according to one embodiment of the invention;

FIG. 9 is a diagram showing a more detailed view of the portion of the imaging system shown in FIG. 3, illustrating another implementation of various temperature compensation features, according to one embodiment of the invention;

FIG. 10 is a block-diagram showing one example of a portion of a compensation algorithm implemented by a controller of the imaging system of FIG. 1, according to one embodiment of the invention;

FIG. 11 is a diagram showing a portion of an imaging system according to another embodiment of the invention;

FIG. 12 is a flow diagram illustrating an exemplary method for calculating a gain calibration parameter according to one embodiment of the invention; and

FIG. 13 is a flow diagram illustrating an exemplary method for calculating an offset calibration parameter according to one embodiment of the invention.

DETAILED DESCRIPTION

As discussed above, a radiation sensor outputs signals based on radiation that impinges on the sensor. Such a sensor may be used in an imaging system that produces images (e.g., on a display) based on radiation incident to the sensor from a scene of interest. The sensor output signals, however, may contain significant undesirable components due in part to changes in temperature of the sensor itself that are not necessarily due to the radiation of interest. In some cases, these undesirable signal components may be hundreds of times larger than the instantaneous signals resulting from the radiation of interest in the scene being imaged, thereby detrimentally reducing the dynamic range of the sensor and/or processing circuitry associated with the sensor with respect to the radiation of interest.

With respect to undesirable signal components, changes in temperature of the sensor that are not related to the radiation of interest may have an average effect over time (i.e., a DC drift in the sensor output signals). Such changes in temperature may be related to ambient temperature variations, such as a change in temperature of a substrate on which the sensor is fabricated, a change in temperature of a package in which the sensor is housed, or a change in the average temperature of the scene of interest around the sensor. Additionally, essentially instantaneous (i.e., AC) undesirable signal components may be observed due to "self-heating" of the sensor, for example, when a bias voltage initially is applied to the sensor and current begins to flow through the sensor, producing heat. Furthermore, individual detectors of a detector array constituting an imaging sensor each may respond differently to temperature variations, creating undesirable signal components due to detector non-uniformities.

In view of the foregoing, the present invention is directed generally to methods and apparatus for compensating operating parameters and/or output signals of a radiation sensor for temperature variations of the sensor that are not due to radiation of interest. The compensation provided by various embodiments of methods and apparatus of the invention significantly reduces undesirable components in the instantaneous signals output by the sensor. In one aspect of the present invention, the radiation sensor is an infrared thermal imaging sensor including an array of thermal detectors, such as bolometers. It should be appreciated, however, that the invention is not limited in this respect, as various compensation methods, apparatus, and concepts discussed herein may be applied generally to a variety of sensors and detection devices.

In one embodiment, methods and apparatus of the invention provide compensation for temperature variations of a sensor without thermally stabilizing the sensor itself; in particular, the sensor is allowed to freely vary in temperature and is dynamically compensated for temperature variations. In one aspect of this embodiment, one or more operating and/or calibration parameters associated with the sensor are dynamically determined or updated based on temperature variations of the sensor, and used to dynamically compensate for changes in operational characteristics of the sensor due to the temperature variations of the sensor. Essentially, in one aspect, the methods and apparatus of this embodiment provide continuous thermal compensation feedback without thermal stabilization of the sensor through dynamic operating and/or calibration parameter adjustments. Accordingly, methods and apparatus of the invention according to one embodiment facilitate the design of a thermal imaging system that does not require thermal stabilization components (e.g., a thermoelectric cooler), thereby providing for reduced system power consumption and potential production cost savings.

Some examples of operating parameters associated with the


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