Quantitative imaging of gas emissions utilizing optical techniques

• US 20030025081A1
• Filed: 06/27/2002
• Published: 02/06/2003
• Est. Priority Date: 12/28/1999
• Status: Active Grant

• Current Assignee: Gas Optics AS
• Original Assignee: Gas Optics AS
• Inventors: Svanberg, Sune, Sandsten, Jonas, Edner, Hans
• Granted Patent: US 6,803,577 B2
• Time in Patent Office: Days
• US Class Current: 250/339.09
• CPC Class Codes: G01M 3/38 by using light G01M3/02 takes precedence
• CPC Class Codes: G01N 2021/3509 Correlation method, e.g. one beam alternating in correlator/sample field
• CPC Class Codes: G01N 21/3518 Devices using gas filter correlation techniques; Devices using gas pressure modulation techniques

Abstract

A method for quantitative imaging of gas emissions utilizing optical techniques combining gas correlation techniques with thermal background radiation or as self-emission radiation is presented. A simultaneous recording of images with and without filtering through a gas-filled cell is utilized for the identification of a selected gas A new calibration method provides the display of the integrated gas concentration spatially resolved in the generated final image. The procedure includes methods for a correct subtraction of the zero level, consisting of self-radiation from the dual-image camera device including the as correlation cell,, and electronic offset, and for the calculation of the specific absorption as a function of the difference temperature between the background and the gas emission.

Claims

1. A method for imaging of gas distributions utilizing optical techniques, comprising:

the use of gas correlation techniques for spectral identification of substances and cancellation of spatially varying background temperatures and emissivities;

the utilization of absorption of natural thermal background radiation or selfemission spectrum due to a selected gas (passive recording technique); and

wherein two images, A and B are stored using a dual-image infrared camera device adapted to a selected wavelength region where the gas absorption or emission spectrum is present;

A—

is the infrared scene recorded in one of the images (direct image);

B—

is the same scene recorded with the infrared light passing a gas correlation cell;

characterized by a calibration procedure as follows;

the background temperature is recorded using the information contained in image A;

the relevant zero images A₀ and B₀, consisting of self-radiation from the dual-image camera device including the gas correlation cell and electronic offset, are subtracted from A and B, respectively, wherein the individual zero level in each pixel of the images has been determined before the gas measurement by recording a black body radiator at different temperatures and plotting the pixel intensity obtained versus a theoretically calculated intensity, and the axis intercept of a straight line, which is fitted to the data, provides the zero level;

the images are digitally overlapped within a field of interest containing the gas release, and the continuing image processing is constrained to this field;

a gas correlation image, G=(A-A₀)/(B-B₀), is calculated;

the concentration level in each pixel of image G is calculated using a diagram showing the integrated transmission within the chosen spectral profile as a function of the integrated concentration of the gas expressed in ppm×

meter for the particular gas, temperature difference between the background temperature and the gas emission temperature, and absolute temperatures; and

finally, the resulting gas concentration image is superimposed on a visible image C of the scene and the result is displayed.

2. A method as claimed in claim 1, characterized in that the resulting gas concentration image is colour-coded.

3. A method as claimed in claim 1 or 2, characterized by the adaption of the infrared camera detector integration time, spatial/temporal filtering, and concentration threshold to the measured gas concentration level and dynamics at a rate allowing concentration calibrated images to be shown as movies.

4. A method as claimed in claim 1 or 2, characterized by a determination of the gas flux by combining the gas. concentration image with the gas flow velocity, calculated through correlation of the displacement of the gas cloud in temporally separated images.

7. A method as claimed in claim 1 or 2, characterized by the use of two infrared cameras mounted together for simultaneous capturing of the images A and B. The visible image C is simultaneously captured with a camera mounted in close proximity to the infrared cameras.

8. A method as claimed in claim 1 or 2, characterized by the use of an infrared camera with two detectors capturing images A and B with the aid of a beamsplitter and gas correlation cell inside the camera. The visible image C is simultaneously captured with a camera mounted in close proximity to the infrared camera.

9. A method as claimed in claim 1 or 2, characterized by the use of an infrared camera with one detector sequentially capturing images A and B by the use of a chopper switching the gas correlation cell in and out in front of the detector. The visible image C is simultaneously captured with a camera mounted in close proximity to the infrared camera.

5. A method as claimed in claim I or 2, characterized of the utilization of a reflector double telescope with off-axis parabolas for simultaneous capturing of the images A and B. The visible image C is simultaneously captured with a camera mounted in close proximity to the telescope.

6. A method as claimed in claim 5, characterized by the use of plane and angled secondary telescope mirrors designed to avoid the self-radiation of the camera to be reflected back into the camera detector.

10. A device for imaging of gas distributions utilizing optical techniques, comprising:

a dual-images infrared camera device for storing two( images A and B and adapted to a selected wavelength region where the gas absorption or emission spectrum is present wherein;

A—

is the infrared scene recorded in one of the images (direct image);

B—

is the same scene recorded with the infrared light passing a gas correlation cell;

characterized in that the camera device includes means for calibration comprising;

means for recording the background temperature using the information contained in image A;

means for determining and storing the relevant zero images A₀ and B₀ including

means for recording a black body radiator at different temperatures and plotting the pixel intensity obtained versus a theoretically calculated intensity, and the axis intercept of a straight line, which is fitted to the data, for providing the individual zero level in each pixel of the images, consisting of self-radiation from the dual-image camera device;

means for calculating a gas correlation image, G=(A-A₀)/(B-B₀);

means for calculating the concentration level in each pixel of image G arranged to use a diagram showing the integrated transmission within the chosen spectral profile as a function of the integrated concentration of the gas expressed in ppm×

meter for the particular gas, temperature difference between the background temperature and the gas emission temperature, and absolute temperatures; and

means for displaying the result by superimposing the resulting gas concentration image on a visible image C of the scene.

11. A device according to claim 10, characterised in that the display means is arranged to colour-code the gas concentration image.

12. A device according to claim 10 or 11, consisting of a reflector double telescope with two off-axis parabolas for simultaneous capturing of the images A and B, two plane and angled secondary mirrors designed to avoid the self-radiation of the infrared camera to be reflected back into the camera detector, and a camera, which is used to simultaneously capture the visible image C, mounted in close proximity to the telescope.

13. A device according to claim 10 or 11, consisting of two infrared micro-bolometer cameras mounted together for simultaneous capturing of the images A and B. The sensitivities of the infrared cameras are tuned to the selected wavelength region with the use of antireflection coatings and interference filters, and a camera, which is used to simultaneously capture the visible image C, mounted in close proximity to the infrared cameras.

14. A device according to claim 10 or 11, consisting of two infrared QWIP (Quantum Well Infrared Photodetector) cameras mounted together for simultaneous capturing of the images A and B with sensitivities of the detectors optimized for the selected wavelength region, and a camera, which is used to simultaneously capture the visible image C, mounted in close proximity to the infrared cameras.

15. A device according to claim 10 or 11,consisting of an infrared camera with two detectors capturing images A and B with the aid of a beam-splitter and gas correlation cell inside the camera, and a camera, which is used to simultaneously capture the visible image C, mounted in close proximity to the infrared camera.

16. A device according to claim 10 or 11, consisting of an infrared camera with one detector sequentially capturing images A and B by the use of a chopper switching the gas correlation cell in and out in front of the detector, and a camera, which is used to simultaneously capture the visible image C, mounted in close proximity to the infrared camera.