Methods and apparatus for performing scene based uniformity correction in imaging systems

• US 6,184,529 B1
• Filed: 01/28/1998
• Issued: 02/06/2001
• Est. Priority Date: 01/28/1998
• Status: Expired due to Fees

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Inventors: Contini, Casey L.
• Primary Examiner(s): Hannaher, Constantine
• Assistant Examiner(s): GAGLIARDI, ALBERT J
• Application Number:
  US09/014,910
• Time in Patent Office: 1,105 Days
• Field of Search: 250/252.1, 250/339.02, 250/339.09, 250/339.14, 250/341.5, 250/342, 250/347, 250/350, 250/351, 250/332, 359/350, 359/353, 359/355, 359/356, 359/359
• US Class Current: 250/347
• CPC Class Codes: G01J 1/04 Optical or mechanical part supplementary adjustable parts
• CPC Class Codes: G01J 1/0411 using focussing or collimating elements, i.e. lenses or mirrors; Aberration correction
• CPC Class Codes: G01J 1/0474 Diffusers cavities G01J2001/0481
• CPC Class Codes: G01J 1/08 Arrangements of light sources specially adapted for photometry standard sources, also using luminescent or radioactive material
• CPC Class Codes: G01J 2005/0077 Imaging
• CPC Class Codes: G01J 5/08 Optical arrangements
• CPC Class Codes: G01J 5/0806 Focusing or collimating elements, e.g. lenses or concave mirrors
• CPC Class Codes: G01J 5/0878 Diffusers
• CPC Class Codes: G01J 5/53 Reference sources, e.g. standard lamps; Black bodies
• CPC Class Codes: G02B 26/0875 by means of one or more refracting elements
• CPC Class Codes: H04N 25/683 by defect estimation performed on the scene signal, e.g. real time or on the fly detection

Abstract

Methods and apparatus for generating uniform reference sources for use in calibrating multiple-detector imaging devices. In exemplary embodiments, a plano-plano reference lens is inserted at a non-focusing position in the optics path of a multiple-detector imaging device. The reference lens includes a highly polished side coated with a semi-reflective film and a rough polished side coated with an anti-reflective coating. The smooth polished side causes the detector array to see a reflected image of itself and a portion of the incoming scene energy simultaneously. The energy emitted by the detector array thus mixes with the scene energy to create a detected photon flux level which is dependent upon, but deviates in a controlled manner from, that of the scene energy alone. At the same time, the rough polished side and the thickness of the reference lens blur the detected image to eliminate scene structure and to provide a uniform photon flux level at each detector in the array. Since the detected flux is uniform and dependent upon the scene energy, the inserted reference lens effectively provides a reference source suitable for detector calibration. Additional reference lenses of varying reflectivity can be successively inserted to provide additional calibration points. Advantageously, the reference source levels vary automatically and passively with the viewed scene flux, and the present invention eliminates many of the drawbacks associated with conventional calibration techniques.

Claims

1. An imaging device, comprising:

a plurality of radiation detectors positioned in an optical path of said imaging device;

a plano-plano reference lens; and

a lens carrier supporting said reference lens and configured to selectively position said reference lens within and without the optical path.

2. An imaging device according to claim 1, wherein said radiation detectors are cryogenically cooled photoconductors.

3. An imaging device according to claim 1, wherein said radiation detectors are cryogenically cooled photovoltaic devices.

4. An imaging device according to claim 1, further comprising optics configured to focus scene radiation received at said imaging device at a focal plane in the optical path.

5. An imaging device according to claim 4, wherein said plurality of radiation detectors are positioned at the focal plane and form a staring-type two-dimensional focal plane array.

6. An imaging device according to claim 4, wherein said plurality of radiation detectors are positioned in the focal plane and form a scanning-type linear focal plane array.

7. An imaging device according to claim 4, wherein said lens carrier is configured to selectively position said reference lens within the optical path and between said optics and the focal plane.

8. An imaging device according to claim 1, wherein a material used to construct said reference lens is transparent to radiation in a waveband used by said plurality of radiation detectors for imaging.

9. An imaging device according to claim 1, wherein a first side of said reference lens is rough polished and a second side of said reference lens is smooth polished.

10. An imaging device according to claim 9, wherein the first rough polished side and the second smooth polished side of said reference lens are positioned, respectively, away from and toward said plurality of radiation detectors when said reference lens is positioned within the optical path.

11. An imaging device according to claim 9, wherein the first rough polished side of said reference lens is coated with a non-reflective material and wherein the second smooth polished side of said reference lens is coated with a partially reflective material.

12. An imaging device according to claim 11, wherein the partially reflective material coating the second smooth polished side of said reference lens reduces a level of scene radiation received by said plurality of radiation detectors when said reference lens is positioned within the optical path.

13. An imaging device according to claim 11, wherein the partially reflective material coating the second smooth polished side of said reference lens reflects radiation emitted by said plurality of radiation detectors back toward said plurality of radiation detectors when said reference lens is positioned within the optical path.

14. An imaging device according to claim 11, wherein said reference lens provides said plurality of radiation detectors with a uniform level of radiation when said reference lens is positioned within the optical path, a value of the uniform level of radiation being within an instantaneous dynamic flux range of said plurality of radiation detectors.

15. An imaging device according to claim 14, wherein the value of the uniform level of radiation provided by said reference lens is approximately equal to a value corresponding to a center of the instantaneous dynamic flux range of said plurality of radiation detectors.

16. An imaging device according to claim 14, further comprising a processor configured to compute an offset value for each of said plurality of radiation detectors, wherein the offset value for a particular radiation detector is computed as a difference between an output value provided by the particular radiation detector when said reference lens is positioned within the optical path and a reference value which is based on output values provided by each of said plurality of radiation detectors when said reference lens is positioned within the optical path.

17. An imaging device according to claim 16, wherein the reference value is computed as one of a mean, median and mode of output values provided by said plurality of radiation detectors when said reference lens is positioned within the optical path.

18. An imaging device according to claim 16, wherein said processor is configured to compute a corrected output value for each of said plurality of radiation detectors when said reference lens is positioned without the optical path, wherein the corrected output value for a particular radiation detector is computed by subtracting an offset value previously computed for the particular radiation detector from an output value provided by the particular radiation detector when said reference lens is without the optical path.

19. An imaging device according to claim 9, wherein the first rough polished side of said reference lens blurs a scene image detected by said plurality of radiation detectors when said reference lens is positioned within the optical path.

20. An imaging device according to claim 1, wherein a thickness of said reference lens shifts a position of a focal plane in the optical path when said reference lens is positioned within the optical path.

21. An imaging device according to claim 20, wherein the thickness of said reference lens and a rough polished side of said reference lens are effective, in combination, to provide said plurality of radiation detectors with a uniform level of radiation when said reference lens is positioned within the optical path.

22. An imaging device according to claim 20, wherein the thickness of said reference lens is at least 0.03 inches.

23. An imaging device according to claim 1, wherein said lens carrier is configured as a wheel including an aperture into which said reference lens is inserted.

24. An imaging device according to claim 23, wherein said wheel is configured to rotate and to thereby alternately position said reference lens within and without the optical path.

25. An imaging device according to claim 1, further comprising a second plano-plano reference lens, wherein said lens carrier is configured to selectively position each of said first and second reference lenses within and without the optical path.

26. An imaging device according to claim 25, wherein said first reference lens provides said plurality of radiation detectors with a first uniform level of radiation when said first reference lens is positioned within the optical path, and wherein said second reference lens provides said plurality of radiation detectors with a second uniform level of radiation when said second reference lens is positioned within the optical path, a value of each of the first and second uniform levels of radiation being within an instantaneous dynamic flux range of said plurality of radiation detectors.

27. An imaging device according to claim 26, wherein the value of the first uniform level of radiation provided by said first reference lens corresponds to an upper end of the instantaneous dynamic flux range of said plurality of radiation detectors, and wherein the value of the second uniform level of radiation provided by said second reference lens corresponds to a lower end of the instantaneous dynamic flux range of said plurality of radiation detectors.

28. An imaging device according to claim 26, further comprising a processor configured to compute a scale factor and an offset value for each of said plurality of radiation detectors, wherein the scale factor and the offset value for a particular radiation detector are computed based on first and second output values provided by the particular radiation detector when said first and second reference lenses are positioned within the optical path, respectively, and on first and second reference values which are based on output values provided by each of said plurality of radiation detectors when said first and second reference lenses are positioned within the optical path, respectively.

29. An imaging device according to claim 28, wherein the first and second reference values are computed as one of a mean, median and mode of output values provided by said plurality of radiation detectors when said first and second reference lenses are positioned within the optical path, respectively.

30. An imaging device according to claim 28, wherein said processor is configured to compute a corrected output value for each of said plurality of radiation detectors when neither of said first and second reference lenses is positioned within the optical path, wherein the corrected output value for a particular radiation detector is computed based on a scale factor and an offset value previously computed for the particular radiation detector and on an output value provided by the particular radiation detector when neither of said first and second reference lenses is positioned within the optical path.

31. An imaging device according to claim 25, wherein said lens carrier is configured as a wheel including first and second apertures into which said first and second reference lenses are inserted, respectively.

32. An imaging device according to claim 31, wherein said wheel is configured to rotate and to thereby alternately position said first and second reference lenses within and without the optical path.

33. An imaging device according to claim 1, wherein a waveband used by said plurality of radiation detectors for imaging is an infrared waveband.

34. An imaging device according to claim 1, further comprising a plurality of plano-plano reference lenses, wherein said lens carrier is configured to selectively position each of said plurality of reference lenses within and without the optical path, and wherein each of said plurality of reference lenses provides said plurality of radiation detectors with a uniform level of radiation when positioned within the optical path, a value of the uniform level of radiation provided by each reference lens being within an instantaneous dynamic flux range of said plurality of radiation detectors.

35. An imaging device according to claim 34, further comprising a processor for computing piece-wise linear correction factors for each of said plurality of radiation detectors, wherein the piece-wise linear correction factors are computed based on output values provided by said plurality of radiation detectors as said reference lenses are successively positioned within the optical path.

36. An imaging device, comprising:

optics configured to focus scene radiation received at an input of said imaging device at a focal plane in an optical path of said imaging device;

a plurality of radiation detectors positioned in the focal plane;

a plano-plano reference lens having first and second sides, said first side being rough polished and coated with a non-reflective material and said second side being smooth polished and coated with a partially reflective material; and

a lens carrier supporting said reference lens and configured to selectively position said reference lens within and without the optical path,

wherein said reference lens is configured to provide said plurality of radiation detectors with a uniform level of radiation when said reference lens is positioned within the optical path, the uniform level of radiation having a value within an instantaneous dynamic flux range of said plurality of radiation detectors.

37. An imaging device according to claim 36, wherein said radiation detectors are uncooled thermal detectors.

38. An imaging device according to claim 36, wherein said radiation detectors are cryogenically cooled photoconductors.

39. An imaging device according to claim 36, wherein said radiation detectors are cryogenically cooled photovoltaic devices.

40. An imaging device according to claim 36, wherein said plurality of radiation detectors are positioned at the focal plane and form a staring-type two-dimensional focal plane array.

41. An imaging device according to claim 36, wherein said plurality of radiation detectors are positioned in the focal plane and form a scanning-type linear focal plane array.

42. An imaging device according to claim 36 wherein, when said reference lens is positioned within the optical path, said reference lens is situated between said optics and said plurality of radiation detectors such that said first and second sides of said reference lens are facing, respectively, away from and toward said plurality of radiation detectors.

43. An imaging device according to claim 36, wherein the first rough polished side of said reference lens blurs a scene image detected by said plurality of radiation detectors and a thickness of said reference lens shifts a position of the focal plane when said reference lens is positioned within the optical path.

44. An imaging device according to claim 36, wherein the partially reflective material coating the second smooth polished side of said reference lens reduces a level of scene radiation received by said plurality of radiation detectors and reflects radiation emitted by said plurality of radiation detectors back toward said plurality of radiation detectors when said reference lens is positioned within the optical path.

45. An imaging device according to claim 36, further comprising a processor configured to compute an offset value for each of said plurality of radiation detectors, wherein the offset value for a particular radiation detector is computed based on an output value provided by the particular radiation detector when said reference lens is positioned within the optical path and on a reference value which is based on output values provided by each of said plurality of radiation detectors when said reference lens is positioned within the optical path.

46. An imaging device according to claim 45, wherein said processor is configured to compute a corrected output value for each of said plurality of radiation detectors when said reference lens is positioned without the optical path, wherein the corrected output value for a particular radiation detector is computed based on an offset value previously computed for the particular radiation detector and on an output value provided by the particular radiation detector when said reference lens is without the optical path.

47. An imaging device according to claim 36, wherein said lens carrier is configured as a wheel including an aperture into which said reference lens is inserted.

48. An imaging device according to claim 47, wherein said wheel is configured to rotate and to thereby alternately position said reference lens within and without the optical path.

49. An imaging device according to claim 36, further comprising a second plano-plano reference lens, wherein said lens carrier is configured to selectively position each of said first and second reference lenses within and without the optical path.

50. An imaging device according to claim 49, wherein said first and second reference lenses provide said plurality of radiation detectors with first and second uniform levels of radiation when said first and second reference lenses are positioned within the optical path, respectively, a value of each of the first and second uniform levels of radiation lying within an instantaneous dynamic flux range of said plurality of radiation detectors.

51. An imaging device according to claim 49, further comprising a processor configured to compute a scale factor and an offset value for each of said plurality of radiation detectors, wherein the scale factor and the offset value for a particular radiation detector are computed based on first and second output values provided by the particular radiation detector when said first and second reference lenses are positioned within the optical path, respectively, and on first and second reference values, wherein the first and second reference values are based on output values provided by each of said plurality of radiation detectors when said first and second reference lenses are positioned within the optical path, respectively.

52. An imaging device according to claim 51, wherein said processor is configured to compute a corrected output value for each of said plurality of radiation detectors when neither of said first and second reference lenses is positioned within the optical path, wherein the corrected output value for a particular radiation detector is computed based on a scale factor and an offset value previously computed for the particular radiation detector and on an output value provided by the particular radiation detector when neither of said first and second reference lenses is positioned within the optical path.

53. An imaging device according to claim 49, wherein said lens carrier is configured as a wheel including first and second apertures into which said first and second reference lenses are inserted, respectively.

54. An imaging device according to claim 53, wherein said wheel is configured to rotate and to thereby alternately position said first and second reference lenses within and without the optical path.

55. An imaging device according to claim 36, wherein a waveband used by said plurality of radiation detectors for imaging is an infrared waveband.

56. An imaging device according to claim 36, further comprising a plurality of plano-plano reference lenses, wherein said lens carrier is configured to selectively position each of said plurality of reference lenses within and without the optical path, and wherein each of said plurality of reference lenses provides said plurality of radiation detectors with a uniform level of radiation when positioned within the optical path, a value of the uniform level of radiation provided by each reference lens being within an instantaneous dynamic flux range of said plurality of radiation detectors.

57. An imaging device according to claim 56, further comprising a processor for computing piece-wise linear correction factors for each of said plurality of radiation detectors, wherein the piece-wise linear correction factors are computed based on output values provided by said plurality of radiation detectors as said reference lenses are successively positioned within the optical path.

58. A method for correcting non-uniformities among a plurality of radiation detectors in an imaging device, said plurality of radiation detectors being positioned in a focal plane of an optical path of said imaging device, comprising the steps of:

positioning a piano-plano reference lens within the optical path to provide the plurality of radiation detectors with a uniform level of radiation;

computing an offset value for each of the plurality of radiation detectors based on values output by the plurality of radiation detectors when the reference lens is positioned within the optical path;

removing the reference lens from the optical path; and

computing a corrected output value for each of the plurality of radiation detectors based on the computed offset values and on values output by the plurality of radiation detectors when the reference lens is removed from the optical path.

59. A method according to claim 58, wherein said step of computing an offset value for each of the plurality of radiation detectors comprises the step of:

computing one of a mean, median and mode output value provided by the plurality of radiation detectors when the reference lens is positioned with the optical path.

60. A method according to claim 58, further comprising the steps of:

positioning a second plano-plano reference lens within the optical path to provide the plurality of radiation detectors with a second uniform level of radiation;

computing a scale factor and an offset value for each of the plurality of radiation detectors based on values output by the plurality of radiation detectors when the first and second reference lenses are positioned within the optical path; and

computing a corrected output value for each of the plurality of radiation detectors based on the computed scale factors and offset values and on values output by the plurality of radiation detectors when neither of the reference lenses is positioned within the optical path.

61. A method according to claim 58, further comprising the steps of:

successively positioning each one of a plurality of additional plano-plano reference lenses within the optical path;

computing piece-wise linear scale factors and offset values for each of the plurality of radiation detectors based on values output by the plurality of radiation detectors when the first and additional reference lenses are positioned within the optical path; and

computing piece-wise linear corrected output values for each of the plurality of radiation detectors based on the computed piece-wise linear scale factors and offset values and on values output by the plurality of radiation detectors when none of the reference lenses is positioned within the optical path.

62. A method according to claim 58, wherein one side of the reference lens is rough polished and blurs an image detected by the plurality of radiation detectors when the reference lens is positioned within the optical path.

63. A method according to claim 58, wherein one side of the reference lens is coated with a partially reflective material and reduces a level of scene radiation received by the plurality of radiation detectors when the reference lens is positioned within the optical path.

64. A method according to claim 63, wherein the partially reflective material reflects radiation emitted by the plurality of radiation detectors back toward the plurality of radiation detectors.

65. A method according to claim 58, wherein a waveband used by the plurality of radiation detectors for imaging is an infrared waveband.