Shutterless infrared imager algorithm with drift correction

• US 7,683,321 B1
• Filed: 12/19/2008
• Issued: 03/23/2010
• Est. Priority Date: 12/19/2008
• Status: Active Grant

• Current Assignee: Fluke Corporation (Fortive Corp.)
• Original Assignee: Fluke Corporation (Fortive Corp.)
• Inventors: King, Stephen R., Rekow, Mathew N., Carlson, Paul S.
• Primary Examiner(s): Porta; David P
• Assistant Examiner(s): Gaworecki; Mark R
• Application Number:
  US12/339,434
• Time in Patent Office: 459 Days
• Field of Search: 250/252.1, 250/330, 250/332, 250/338.1
• US Class Current: 250/332
• CPC Class Codes: G01J 2005/0077 Imaging
• CPC Class Codes: G01J 5/064 Ambient temperature sensor; Housing temperature sensor; Constructional details thereof
• CPC Class Codes: G01J 5/80 Calibration using comparison with reference sources G01J5/52
• CPC Class Codes: H04N 23/20 for generating image signals from infrared radiation only
• CPC Class Codes: H04N 25/671 for non-uniformity detection or correction
• CPC Class Codes: H04N 25/673 by using reference sources
• CPC Class Codes: H04N 25/76 Addressed sensors, e.g. MOS or CMOS sensors
• CPC Class Codes: H04N 5/33 Transforming infrared radiation cameras or camera modules for generating image signals from infrared radiation H04N23/20; circuitry of SSIS for transforming infrared radiation into image signals H04N25/20

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.

Claims

1. A method of maintaining image quality in presence of temperature drift for an infrared (IR) imaging system comprising:

a) taking base measurements from the IR imaging system while a shutter of the IR imaging system is closed, the base measurements comprising temperature with regard to the shutter and outputs of detector elements of a focal plane array (FPA) of the IR imaging system;

b) determining an offset output for the IR imaging system from the base temperature with regard to the shutter;

c) taking present measurements from the IR imaging system while the shutter is open and the IR imaging system is directed at a target scene, the present measurements comprising temperature with regard to the shutter and outputs of the FPA detector elements;

d) calculating thermal drift components with regard to the present outputs of the FPA detector elements in light of differences between the base and present measurements;

e) calculating outputs of the FPA detector elements attributable to the target scene in light of the present outputs of the FPA detector elements, the respective drift components of the FPA detector elements, and the offset output;

f) determining temperatures of the target scene from the output portions of the FPA detector elements attributable to the target scene; and

g) repeating steps (c) through (f) while the shutter is open during continued use of the IR imaging system.

2. The method of claim 1, wherein the base and present outputs of the FPA detector elements are converted into a digital format prior to being measured.

3. The method of claim 2, wherein the base and present outputs of the FPA detector elements are converted into digital counts, wherein quantity of digital counts measured corresponds to thermal intensity borne on the FPA detector elements.

4. The method of claim 1, wherein the offset output is derived from the base temperature measured with regard to the shutter using the calibration curve.

5. The method of claim 1, wherein a calibration curve is used with the IR imaging system in one or more of calculating the thermal drift components, calculating the output portions of the FPA detector elements attributable to the target scene, and determining the target scene temperatures, wherein the calibration curve depicts averaged output of one or more detector elements of the FPA for different temperatures of the target scene.

6. The method of claim 5, wherein the base and present temperatures measured with regard to the shutter correspond to outputs from the calibration curve, wherein the thermal drift component for each FPA detector element is based on a difference between such corresponding outputs, with such difference being multiplied by a respective drift coefficient for each FPA detector element.

7. The method of claim 5, wherein the output portions of the FPA detector elements attributable to the target scene are derived by respectively subtracting the base outputs from the present outputs of each of the FPA detector elements and multiplying result by a respective gain for each of the FPA detector elements, and further respectively subtracting the drift components of the FPA detector elements therefrom and adding the offset output thereto.

8. The method of claim 5, wherein the target scene temperatures are respectively derived from the output portions of the FPA detector elements using the calibration curve.

9. The method of claim 5, wherein the calibration curve is retrieved from memory of the IR imaging system, the memory being part of a processor in an electronic circuit of the IR imaging system.

10. The method of claim 1, wherein the shutter is in thermal communication with an enclosure of the IR imaging system, wherein the enclosure is disposed between the FPA and the target scene whereby IR energies from the target scene are directed there through and onto the FPA when the shutter is open and IR energies from the target scene are blocked from the FPA when the shutter is closed, wherein the base and present temperatures measured with regard to the shutter each comprises a temperature measurement of the enclosure.

11. The method of claim 10, wherein the enclosure is formed of one or more isothermal materials, wherein the enclosure temperature is measured via a single thermal sensor affixed to the enclosure.

12. An infrared (IR) imaging apparatus comprising:

a lens for focusing IR energy from a target scene;

a focal plane array (FPA) paired with the lens and adapted to receive the IR energy from the target scene via the lens, the FPA comprising a plurality of IR detector elements;

means for selectively exposing the FPA detector elements to the IR energy from the target scene, the means enabling maintenance of image quality in presence of temperature drift for the IR imaging apparatus following a single blocking of the FPA detector elements from the IR energy from the target scene; and

a processor operatively coupled to the FPA and configured for making initial measurements of the IR imaging apparatus during the single blocking of the FPA detector elements and for making continued measurements of the IR imaging apparatus during continued use of the IR imaging apparatus when the FPA detector elements are exposed to the IR energy from the target scene, the processor configured for storing the initial measurements and configured for using a calibration curve for the IR imaging apparatus with which the initial and continued measurements are used in maintaining image quality in presence of temperature drift during the continued use of the IR imaging apparatus.

13. The apparatus of claim 12, wherein the calibration curve is a graph depicting averaged output of one or more detector elements of the FPA for different temperatures of the target scene, wherein the calibration curve is stored in memory of the processor.

14. The apparatus of claim 13, wherein the initial measurements comprise a base temperature of the means and base outputs of the FPA detector elements, and wherein the continued measurements comprise a present temperature of the means and present outputs of the FPA detector elements.

15. The apparatus of claim 14, wherein the base and present temperatures of the means correspond to outputs on the calibration curve, wherein thermal drift components for each of the FPA detector elements are based on a difference between such corresponding outputs, with the thermal drift components comprising a multiplication of the difference by a respective drift coefficient for each FPA detector element.

16. The apparatus of claim 14, wherein output portions of the FPA detector elements attributable to the target scene derive respectively from a difference of the base outputs from the present outputs of each of the FPA detector elements and a multiplication of result by a respective gain for each of the FPA detector elements, and further respectively a subtraction of the drift components of the FPA detector elements therefrom and addition of an offset output thereto.

17. The apparatus of claim 16, wherein the offset output comprises an output on the calibration curve which corresponds to the base temperature of the means.

18. The apparatus of claim 16, wherein temperatures of the target scene correspond to outputs portions on the calibration curve.

19. The apparatus of claim 12, wherein the means comprise a shutter and an enclosure, wherein the enclosure is disposed between the FPA and the target scene, wherein the enclosure enables IR energies from the target scene to be directed there through and onto the FPA when the shutter is open, and wherein the infrared energies from the target scene are blocked from being directed onto the FPA when the shutter is closed.

20. The apparatus of claim 19, wherein the enclosure comprises apertures enabling the infrared energies to pass there through and onto the FPA, at least one of the apertures covered when the shutter is closed and both of the apertures uncovered when the shutter is open.

21. The apparatus of claim 19, wherein the shutter is in thermal communication with the enclosure, wherein the initial and continued measurements each comprises a temperature measurement of the enclosure.

22. A computer-readable medium programmed with instructions for maintaining image quality in presence of temperature drift for an infrared (IR) imaging system, the medium comprising instructions for causing a programmable processor to:

a) take base measurements from the IR imaging system while a shutter of the IR imaging system is closed, the base measurements comprising temperature with regard to the shutter and outputs of detector elements of a focal plane array (FPA) of the IR imaging system;

b) determine an offset output for the IR imaging system from the base temperature with regard to the shutter;

c) take present measurements from the IR imaging system while the shutter is open and the IR imaging system is directed at a target scene, the present measurements comprising temperature with regard to the shutter and outputs of the FPA detector elements;

d) calculate thermal drift components with regard to the present outputs of the FPA detector elements in light of differences between the base and present measurements;

e) calculating outputs of the FPA detector elements attributable to the target scene in light of the present outputs of the FPA detector elements, the respective drift components of the FPA detector elements, and the offset output;

f) determine temperatures of the target scene from the output portions of the FPA detector elements attributable to the target scene; and

g) repeat steps (c) through (f) while the shutter is open during continued use of the IR imaging system.

23. The computer-readable medium of claim 22, wherein the offset output is derived from the base temperature measured with regard to the shutter using the calibration curve.

24. The computer-readable medium of claim 22, wherein a calibration curve is used with the IR imaging system in one or more of calculating the thermal drift components, calculating the output portions of the FPA detector elements attributable to the target scene, and determining the target scene temperatures, wherein the calibration curve depicts averaged output of one or more detector elements of the FPA for different temperatures of the target scene.

25. The computer-readable medium of claim 24, wherein the base and present temperatures measured with regard to the shutter correspond to outputs from the calibration curve, wherein the thermal drift component for each FPA detector element is based on a difference between such corresponding outputs, with such difference being multiplied by a respective drift coefficient for each FPA detector element.

26. The computer-readable medium of claim 24, wherein the output portions of the FPA detector elements attributable to the target scene are derived by respectively subtracting the base outputs from the present outputs of each of the FPA detector elements and multiplying result by a respective gain for each of the FPA detector elements, and further respectively subtracting the drift components of the FPA detector elements therefrom and adding the offset output thereto.

27. The computer-readable medium of claim 24, wherein the target scene temperatures are respectively derived from the output portions of the FPA detector elements using the calibration curve.

28. The computer-readable medium of claim 24, wherein the shutter is in thermal communication with an enclosure of the IR imaging system, wherein the enclosure is disposed between the FPA and the target scene whereby IR energies from the target scene are directed there through and onto the FPA when the shutter is open and IR energies from the target scene are blocked from the FPA when the shutter is closed, wherein the base and present temperatures measured with regard to the shutter each comprises a temperature measurement of the enclosure.