Pulsed laser linescanner for a backscatter absorption gas imaging system

• US 6,690,472 B2
• Filed: 09/26/2001
• Issued: 02/10/2004
• Est. Priority Date: 09/28/2000
• Status: Active Grant

• Current Assignee: National Technology & Engineering Solutions Of Sandia LLC (Honeywell International Inc.)
• Original Assignee: Sandia National Laboratories (Government of the United States of America)
• Inventors: Schmitt, Randal L., Kulp, Thomas J., Reichardt, Thomas A., Bambha, Ray P.
• Primary Examiner(s): Rosenberger, Richard A.
• Application Number:
  US09/965,527
• Publication Number:
  US 20020071122A1
• Time in Patent Office: 867 Days
• Field of Search: 356/437, 356/438, 250/334, 250/338.5, 250/330, 358/474, 358/482, 358/483, 348/166, 348/203
• US Class Current: 356/437
• CPC Class Codes: G01M 3/38 by using light G01M3/02 takes precedence
• CPC Class Codes: G01N 21/314 with comparison of measurements at specific and non-specific wavelengths dual wavelength spectrometry G01J3/427
• CPC Class Codes: G01N 21/53 within a flowing fluid, e.g. smoke

Abstract

An active (laser-illuminated) imaging system is described that is suitable for use in backscatter absorption gas imaging (BAGI). A BAGI imager operates by imaging a scene as it is illuminated with radiation that is absorbed by the gas to be detected. Gases become “visible” in the image when they attenuate the illumination creating a shadow in the image. This disclosure describes a BAGI imager that operates in a linescanned manner using a high repetition rate pulsed laser as its illumination source. The format of this system allows differential imaging, in which the scene is illuminated with light at least 2 wavelengths—one or more absorbed by the gas and one or more not absorbed. The system is designed to accomplish imaging in a manner that is insensitive to motion of the camera, so that it can be held in the hand of an operator or operated from a moving vehicle.

Claims

1. A method for imaging a gas plume, comprising:

(i) providing pulses of light including an on-wavelength that is absorbed by a gas to be detected and an off-wavelength that is not absorbed by said gas to be detected;

(ii) shaping said pulses into a sheet of light;

(iii) directing said sheet of light onto a line of a target area, wherein light will be reflected and backscattered to produce a return signal;

(iv) directing the instantaneous field-of-view (IFOV) of an imaging detector to intersect with said line of said target area that is illuminated by said sheet of light, wherein said IFOV intersects said line at an intersection zone;

(v) imaging said return signal onto said detector;

(vi) translating said intersection zone across said target area;

(vii) repeating steps (i)-(vi) at various positions of said target area to create an image containing imagery of said target area and said plume of said gas to be detected.

2. The method of claim 1, wherein said light comprises electromagnetic radiation of wavelengths within the range from the ultraviolet to the infrared.

3. The method of claim 1 wherein said sheet of light comprises an illumination pattern selected from the group consisting of a linear illumination pattern and a rectangular illumination pattern.

4. The method of claim 1, wherein said intersection zone is translated at a rate sufficient to ensure that pixels in the top row of said image are well-registered with pixels at the bottom row.

5. The method of claim 4, wherein said intersection zone is translated such that said target area is completely scanned at a rate of at least 10 Hz.

6. The method of claim 1, wherein said intersection zone is translated across said target area by the motion of a platform on which said camera is contained.

7. The method of claim 1, further comprising displaying said image in real-time on a video display monitor.

8. The method of claim 1, wherein said return signal includes background signal, the method further comprising producing an image that contains only an image of said gas to be detected by removing said background by a log-ratio process.

9. The method of claim 1, further comprising calibrating said imagery of said plume of said gas to be detected to produce a quantitative measurement of concentration of said gas to be detected integrated over a path of a light pulse of said light pulses to a backscattering surface.

10. The method of claim 1, wherein each light pulse of said light pulses comprises an energy, wherein said return signal is scaled for said energy of the light pulse transmitted.

11. The method of claim 1, wherein a time between imaging of said return signal that comprises said on-wavelength and imaging of said return signal that comprises said off-wavelength is set to avoid loss of spatial registration due to motion of said camera or of said gas plume.

12. The method of claim 1, wherein step (iv) includes sweeping a single scan mirror smoothly in time to steer said intersection zone.

13. The method of claim 1, comprising adjusting the launch angle of said pulses of light from a scan mirror so that two consecutive pulses illuminate the same said line of a target area as said scan mirror moves continuously in time.

14. The method of claim 1, wherein step (iii) includes directing said sheet of light onto two lines of a target area simultaneously.

15. The method of claim 1, wherein step (iv) includes sweeping a single scan mirror in angular steps in time to steer said intersection zone.

16. The method of claim 1, wherein a first mirror translates said IFOV and a second mirror translates said sheet of light.

17. The method of claim 1, wherein said imaging detector is operated in a windowed mode in which a subset of lines in said detector are used to image backscatter from a particular light pulse, wherein said subset of lines is translated through said detector to form a complete image.

18. The method of claim 1, further comprising simultaneously measuring backscattered radiation at said on-wavelength and at said off-wavelength.

19. The method of claim 1, wherein said detector comprises a plurality of lines of detectors having pixels, wherein both said on-wavelength and said off-wavelength are imaged onto the same line of detectors of said plurality of lines of detectors to avoid errors caused by pixel nonuniformity.

20. The method of claim 1, wherein said detector comprises a plurality of lines of detectors, wherein said on-wavelength and said off-wavelength are imaged onto a different line of detectors of said plurality of lines of detectors, the method further comprising performing nonuniformity correction.

21. The method of claim 1, further comprising suppressing ambient light collected by said detector.

22. The method of claim 21 wherein the step of suppressing ambient light is carried out by a suppression method selected from the group consisting of temporal suppression, spatial suppression, and spectral suppression.

23. The method of claim 21, wherein the step of suppressing ambient light is carried out by placing a filter in front of said detector, wherein said filter transmits over a narrow spectral bandwidth centered at a wavelength being detected.

24. The method of claim 21, wherein the step of suppressing ambient light is carried out by electronically gating said detector to integrate signal for a short time window that contains the arrival time of the pulse of backscatter photons.

25. The method of claim 21, wherein the step of suppressing ambient light is carried out by limiting the dwell time of said IFOV o n a particular region of said target area.

26. A pulsed linescanner (PLS), comprising:

a lightsource that is capable of providing (i) a pulsed light beam at a wavelength that is absorbed by a gas to be detected and (ii) a pulsed light beam at a wavelength that is not absorbed by a gas to be detected;

optics for shaping said pulsed light beam into a sheet of light;

a detector; and

imaging optics for imaging said sheet of light onto said detector.

27. The PLS of claim 26, wherein said detector comprises a focal plane array (FPA) having a field-of-view that overlaps said pulsed light sheet on a target.

28. The PLS of claim 26, wherein said imaging optics comprise at least one galvanometrically-driven optic and control electronics, wherein said at least one galvanometrically driven optic is positioned to simultaneously scan said pulsed light beam and said field of view.

29. The PLS of claim 26, wherein said at least one lightsource is configured to provide pulses that alternate between said pulsed light beam at a wavelength that is absorbed by a gas to be detected and said pulsed light beam at a wavelength that is not absorbed by a gas to be detected.

30. The PLS of claim 26, wherein said at least one lightsource is selected from a group consisting of a seeded optical parametric device and an unseeded optical parametric device.

31. The PLS of claim 27, wherein said FPA is selected from the group consisting of a linear array and a 2-dimensional array with windowing.

32. The PLS of claim 26, wherein said optics for shaping said pulsed light beam into a sheet of light comprises a pair of anamorphic cylindrical projection lenses that will spread the beam as a line onto a target.

33. The PLS of claim 26, wherein said optics for shaping said pulsed light beam into a sheet of light comprises a mixing rod and a pair of cylindrical optics, wherein said mixing rod is operatively positioned to homogenize said beam into a uniform rectangular illumination, present at the exit face of said rod, wherein said exit face is then projected onto a target with the appropriate anamorphic magnification by said pair of cylindrical optics.

34. The PLS of claim 26, wherein said optics for shaping said pulsed light beam into a sheet of light comprises a multifaceted projection optic and a pair of cylindrical optics, wherein said multifaceted projection optic is operatively positioned to homogenize said beam and project it as a uniform rectangle that is then imaged onto a target by said cylindrical optics.

35. The PLS of claim 26, wherein said optics for shaping said pulsed light beam into a sheet of light are selected from the group consisting of a quasi-phasematched frequency conversion device and an amplifier with laterally patterned periodic poling that produces a flat-toped output beam from a Gaussian input beam.

36. The PLS of claim 26, wherein said optics for shaping said pulsed light beam into a sheet of light comprise an aspherical optical system to convert a Gaussian input beam to a flat-top output beam.

37. The PLS of claim 26, wherein said optics for shaping said pulsed light beam into a sheet of light comprise a diffractive optic to convert a Gaussian input beam to a flat-top output beam.

38. An apparatus for imaging a gas plume, comprising:

a lightsource for providing pulses of light including an on-wavelength that is absorbed by a gas to be detected and an off-wavelength that is not absorbed by said gas to be detected;

optics for shaping said pulses into a sheet of light and for directing said sheet of light onto a line of a target area, wherein light will be reflected and backscattered to produce a return signal;

a detector having an instantaneous field-of-view (IFOV);

optics for directing said IFOV to intersect with said line of said target area that is illuminated by said sheet of light, wherein said IFOV intersects said line at an intersection zone;

imaging optics for imaging said return signal onto said detector; and

a linescanner for translating said sheet of light and said IFOV such that said intersection zone translates across said target area.

39. A method for imaging a gas plume, comprising:

(i) providing pulses of light at an on-wavelength that is absorbed by a gas to be detected;

(ii) shaping said pulses into a sheet of light;

(iii) directing said sheet of light onto a line of a target area, wherein light will be reflected and backscattered to produce a return signal;

(iv) directing the instantaneous field-of-view (IFOV) of an imaging detector to intersect with said line of said target area that is illuminated by said sheet of light, wherein said IFOV intersects said line at an intersection zone;

(v) imaging said return signal onto said detector;

(vi) translating said sheet of light and said IFOV such that said intersection zone translates across said target area;

(vii) repeating steps (i)-(vi) at various positions of said target area to create an image containing imagery of said target area and imagery of said plume of said gas to be detected.