On-board non-uniformity correction calibration methods for microbolometer focal plane arrays

• US 9,167,179 B2
• Filed: 02/11/2012
• Issued: 10/20/2015
• Est. Priority Date: 02/21/2011
• Status: Active Grant

• Current Assignee: Safran Defense and Space Incorporated
• Original Assignee: Vectronix Inc. (Safran SA)
• Inventors: Lavik, Nils A., Kwan, Greg, Linsacum, Deron L.
• Primary Examiner(s): Porta, David
• Assistant Examiner(s): Igyarto, Carolyn
• Application Number:
  US13/371,414
• Publication Number:
  US 20120211648A1
• Time in Patent Office: 1,347 Days
• Field of Search: 250/252.1, 250/232
• US Class Current: 1/1
• CPC Class Codes: G01J 2005/0077 Imaging
• CPC Class Codes: G01J 2005/106 Arrays
• CPC Class Codes: G01J 2005/202 Arrays
• CPC Class Codes: G01J 5/53 Reference sources, e.g. standard lamps; Black bodies
• CPC Class Codes: G01J 5/80 Calibration using comparison with reference sources G01J5/52
• CPC Class Codes: G01N 2201/1211 for temperature
• CPC Class Codes: G01N 2201/12707 Pre-test of apparatus, e.g. dark test, sensor test
• CPC Class Codes: G01N 2201/12715 Zero adjustment, i.e. to verify calibration
• CPC Class Codes: G01N 2201/12746 Calibration values determination
• CPC Class Codes: G01N 2201/12753 and storage
• CPC Class Codes: H04N 23/23 from thermal infrared radiation
• CPC Class Codes: H04N 25/63 applied to dark current
• 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 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

On-board non-uniformity correction calibration methods for a microbolometer focal plane array in a thermal camera are disclosed. The methods include performing first calculations in the processor unit of the thermal camera to generate and apply a set of coarse correction bias voltages to the detector elements. The method also includes performing calculations in the external computer based on image data collected by the thermal camera with the coarse correction bias voltages applied to the detector elements to generate a set of fine correction bias voltages. The method also includes downloading the fine correction bias voltages to the thermal camera and applying the fine correction voltages to the detector elements to establish a fine calibration of the microbolometer focal plane array.

Claims

1. A method of calibrating a microbolometer focal plane array (MFPA) having detector elements and that reside in a thermal camera that has a processing unit that is operably coupled to an external computer and a shutter, comprising:

determining, in the processing unit, detector non-uniformity corrections (NUCs) for corresponding ones of the detector elements based on reference voltages from the MFPA;

applying the detector NUCs to the detector elements as coarse corrections that establish a coarse calibration;

sending the detector NUCs to the external computer;

in a first measurement step;

collecting image data DL1 at a first ambient temperature and image data DH1 at a first calibration source temperature;

collecting image data DL2 at a second ambient temperature and image data DH2 at a second calibration source temperature; and

sending image data DL1, DH1, DL2 and DH2 to the external computer;

calculating, in the external computer, fine corrections based on the collected image data DL1, DH1, DL2 and DH2 and the detector NUCs; and

sending the fine corrections from the external computer to the processor unit and applying the fine corrections to the detector elements to establish a fine calibration.

2. The method of claim 1, wherein calculating the fine corrections in the external computer further comprises calculating from collected image data DL1, DH1, DL2 and DH2 gain values based on a gain slope and a gain intercept, and calculating offset values based on an offset slope and an offset intercept.

3. The method of claim 1, wherein the first and second ambient temperatures differ by a temperature increment δ

T, and wherein the first and second calibration source temperatures differ by the temperature increment δ

T.

4. The method of claim 3, wherein δ

T =4°

C.

5. The method of claim 1, wherein the determining the detector NUCs, applying the detector NUCs, sending the detector NUCs, and the first measurement step are repeated for additional measurement steps that sequentially increment the first and second ambient temperatures and the first and second calibration source temperatures by a temperature increment δ

T.

6. The method of claim 5, wherein the additional measurement steps are completed before the calculating fine corrections and sending the fine corrections are performed.

7. The method of claim 1, wherein the detector NUCs are defined by a six-bit numbers for each detector element, wherein the first bit is a sign bit and the remaining five bits define a 0 to 32 bias voltage increment.

8. The method of claim 1, wherein the first and second calibration source temperatures are maintained about 20 C.°

higher than the first and second ambient temperatures, respectively.

9. The method of claim 1, wherein the image data DL1 is obtained at the first ambient temperature with the shutter closed, image data DH1 is obtained at the first ambient temperature with the shutter open so that the MFPA is exposed to the first calibration source temperature, image data DL2 is obtained at the second ambient temperature with the shutter closed and image data DH2 is obtained at the second ambient temperature with the shutter open so that the MFPA is exposed to the second calibration source temperature.

10. A method of calibrating a microbolometer focal plane array (MFPA) having detector elements and that reside in a thermal camera that has a processing unit that is operably coupled to an external computer and a shutter, comprising:

performing first calculations in the processing unit to generate a set of coarse correction bias voltages;

applying the coarse correction bias voltages to the detector elements;

collecting image data with the coarse correction bias voltages applied to the detector elements;

performing second calculations in the external computer based on the image data collected by the thermal camera to generate a set of fine gain and fine offset correction values;

downloading the fine correction values; and

applying the fine correction values to the detector elements to establish a fine calibration of the MFPA;

wherein performing first calculations is repeated at a plurality of ambient temperatures to calculate a plurality of coarse correction tables corresponding to various ones of the ambient temperatures, and wherein performing second calculations includes calculating fine correction values to create a plurality of fine correction tables corresponding to the various ones of the ambient temperatures.

11. The method according to claim 10 wherein performing the first calculations includes determining detector non-uniformity corrections (NUCs) for corresponding ones of the detector elements.

12. The method according to claim 10, wherein performing the second calculations includes calculating gain and offset values based on image data collected at four different temperatures.

13. The method of claim 10, wherein:

performing first calculations includes calculating detector non-uniformity corrections (NUCs) and;

collecting image data includes collecting image data DL1 at a first ambient temperature and image data DH1 at a first calibration source temperature, and collecting image data DL2 at a second ambient temperature and image data DH2 at a second calibration source temperature, wherein the NUCs and image data DL1 are sent to the external computer before image data DH1, DL2, and DH2 are obtained.

14. A method of calibrating a microbolometer focal plane array (MFPA) having detector elements and that reside in a thermal camera that has a processing unit that is operably coupled to an external computer and a shutter, comprising:

performing first calculations in the processing unit to generate a set of coarse correction bias voltages;

applying the coarse correction bias voltages to the detector elements;

collecting image data with the coarse correction bias voltages applied to the detector elements;

performing second calculations in the external computer based on the image data collected by the thermal camera to generate a set of fine gain and fine offset correction values;

downloading the fine correction values; and

applying the fine correction values to the detector elements'"'"' output signals to establish a fine calibration of the MFPA;

wherein;

performing first calculations includes calculating detector non-uniformity corrections (NUCs); and

collecting image data includes;

collecting image data DL1 at a first ambient temperature and image data DH1 at a first calibration source temperature; and

collecting image data DL2 at a second ambient temperature and image data DH2 at a second calibration source temperature, wherein the NUCs and image data DL1 are sent to the external computer before image data DH1, DL2, and DH2 are obtained.

15. The method according to claim 14, wherein performing the second calculations includes calculating gain and offset values based on image data DL1, DL2, DH1, and DH2.

16. The method of claim 14, wherein DL1 and DL2 are obtained with the shutter closed and DH1 and DH2 are obtained with the shutter opened.

17. The method of claim 14, wherein performing first calculations is repeated at a plurality of ambient temperatures to calculate a plurality of coarse correction tables corresponding to various ones of the ambient temperatures, and wherein performing second calculations includes calculating fine correction values to create a plurality of fine correction tables corresponding to the various ones of the ambient temperatures.