Multi-slit hyperspectral imager

Abstract

A system and method for a multi-slit hyperspectral imager. The imager is configured with multiple slits parallel to each other. Each slit produces its own hyperspectral cube and is limited to a specific wavelength range. The multiple slits produce multiple data sets for the same section of the area to be imaged, the multiple data sets being acquired in rapid succession. In optical spectrometry applications such as trace gas detection and quantification, this allows for increased measurement accuracy. The imager can be used for any gas of interest by tuning the wavelength range to one that includes the absorption characteristics of the gas of interest.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to U.S. Provisional Application No. 63 / 336,655, filed April 29, 2022, the entirety of which is incorporated herein by reference.

[0002] [Technical field] The present invention relates to remote sensing, and more particularly to systems and methods for hyperspectral imagers suitable for collecting data relating to specific gases. [Background technology]

[0003] Recent attention and concern about climate change has highlighted the need for better and more accurate tracking and assessment of atmospheric gases produced by human activities. While remote sensing of gases such as methane to this end is well known and has been practiced for many years, the methods and instruments used in such endeavors are often left on the back burner.

[0004] Typical tracking and measurement of gas leaks from man-made facilities has been accomplished using instruments designed to detect multiple items / artifacts. Such hyperspectral imagers can be used for vegetation mapping, archaeological sensing, underground mineral detection, and of course gas detection. Such a wide range of applications, while convenient for the equipment manufacturers, means that the equipment is not typically optimized for gas detection and characterization. Thus, the error and accuracy levels for gas detection and characterization may be less than ideal.

[0005] In light of the above, there is a need for systems and methods that reduce the amount of error and increase the accuracy of instruments used in hyperspectral imaging. Preferably, such systems and methods can be optimized for a particular application, such as gas characterization and detection. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention provides a system and method for a multi-slit hyperspectral imager. The imager is configured with multiple slits parallel to each other. Each slit produces its own hyperspectral cube and is limited to a specific wavelength range. The multiple slits produce multiple data sets for the same section of the area to be imaged, the multiple data sets being acquired in rapid succession. In optical spectrometry applications such as trace gas detection and quantification, this allows for increased measurement accuracy. The imager can be used for any gas of interest by tuning the wavelength range to one that includes the absorption characteristic of the gas of interest. [Means for solving the problem]

[0007] In a first aspect, the present specification discloses a hyperspectral imager comprising: a plurality of slit apertures through which a particular portion of a scene is imaged multiple times as a platform on which the imager is mounted traverses the scene; at least one collimating lens through which an image of a particular portion of a scene passes after being received through at least one of the plurality of slit openings; and at least one spectrally dispersive element, through which an image of a particular portion of a scene passes after passing through at least one collimating lens; and at least one imaging system for refocusing an image of a particular portion of the scene onto the focal plane pixel array; a plurality of pixels on a focal plane pixel array for receiving an image of a particular portion of a scene; A hyperspectral imager comprising Each of the slit openings generates a hyperspectral dataset corresponding to a limited number of specific wavelengths of light received through the slit opening. Each of the slit openings corresponds to a determined number n of pixels out of a plurality of pixels such that n < m, where m is the maximum number of available pixels. A hyperspectral imager.

[0008] In a second aspect, the present invention provides a hyperspectral imager as follows. A plurality of parallel slit openings, wherein when a platform to which the above imager is attached traverses a scene, a specific portion of the scene is imaged multiple times through the plurality of slit openings. A plurality of pixels for receiving split light on a focal plane, the split light being derived from light received through at least one of the plurality of slit openings. A hyperspectral imager comprising Each of the slit openings generates a hyperspectral dataset corresponding to a limited number of specific wavelengths of light received through the slit opening. A hyperspectral imager.

[0009] In one embodiment, the spectral dispersion element is a grating. In another embodiment, the spectral dispersion element is a prism.

[0010] In a further embodiment, the slit openings are evenly spaced from each other. The divided openings may be parallel to each other. Similarly, the divided openings may be perpendicular to the direction of movement of the platform.

[0011] In another embodiment, the above specific wavelengths of light include the absorption wavelengths of a specific gas. The specific gas may be methane.

[0012] In another aspect, this specification discloses a method of hyperspectral imaging, the method comprising: (a) receiving data from an imaging platform, the imaging platform flying over an area, the imaging platform comprising a hyperspectral imager having a plurality of slit apertures, each of the plurality of slit apertures providing an individual data stream; (b) sorting the data into a plurality of data sets, each of the plurality of data sets corresponding to a respective one of the plurality of slit apertures, the number of the plurality of data sets being equal to the number of the plurality of slit apertures, and each of the plurality of data sets corresponding to a single data stream; (c) synchronizing the plurality of data sets to thereby generate synchronized data for the area; (d) combining the synchronized data to thereby generate a single reading for the area. In another embodiment, this specification discloses a method in which each slit aperture corresponds to a limited number of specific wavelengths of light received through the slit aperture.

[0013] In another embodiment, this specification discloses a method in which the hyperspectral imager further comprises a plurality of pixels for receiving split light in a focal plane, the split light being derived from light received through at least one of the plurality of slit apertures.

[0014] In another embodiment, this specification discloses a method in which each slit aperture corresponds to a determined number n of pixels out of a plurality of pixels such that n < m, where m is the maximum number of available pixels.

[0015]

[0016] ​In another embodiment, the present specification discloses a method in which the slit openings are parallel to each other in the slit plane.

[0017] In another embodiment, the present specification discloses a method in which a plurality of slit openings are evenly spaced from each other.

[0018] In another embodiment, the present specification discloses a method in which the multiple slit openings are perpendicular to the direction of movement of the imaging platform.

[0019] In another embodiment, the present specification discloses a method, wherein said particular wavelengths of light include absorption wavelengths of a particular gas.

[0020] In another embodiment, the present specification discloses a method, wherein said particular gas is methane.

[0021] In another embodiment, the present specification discloses a method, wherein the hyperspectral imager further comprises at least one spectrally dispersive element, the at least one spectrally dispersive element comprising at least one of a diffraction grating and a prism.

[0022] Embodiments of the present invention will now be described with reference to the following figures, in which like reference numbers in different figures indicate like elements, and in which: [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a schematic diagram showing general components of a hyperspectral imager. [Diagram 2] 1 is a detailed image of an exemplary configuration of one embodiment of the present invention using 10 slits, showing the sample area to be imaged. [Diagram 3] FIG. 1 is a conceptual diagram showing components for implementing the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] A conventional hyperspectral imaging system 10 is shown in Figure 1. As can be seen, a scene to be imaged is scanned and light from a portion of the scene to be imaged is received at a lens 20. The lens 20 focuses the light into a slit 30 which is then received at a collimating optic 40. A dispersive element / grating 50 then receives the light from the collimating optic 40. The light is dispersed and then imaged using imaging optics 60 for eventual receipt by a sensor 70. The sensor 70 generates a hyperspectral cube from its output, where the xy axes correspond to the 2D axes of the imaged scene and the z axis corresponds to the different frequencies of light collected from the imaged scene.

[0025] In many gas remote sensing applications, the desired performance index is the detection threshold, or the minimum detectable emission from a point source. For a fixed wind speed, this index is determined primarily by the column precision and the spatial resolution (i.e., ground sampling distance). It should be clear that the term "column" refers to "column density" (a measure of the gas abundance along the optical path). It should also be clear that "column precision" refers to the precision with which the column density is measured.

[0026] As shown below, the column accuracy can be improved by using a multi-slit hyperspectral imager to reduce the average noise. It will be apparent that a multi-slit hyperspectral imager is a hyperspectral imager with multiple slit apertures for receiving images of the scene to be imaged.

[0027] TIFF2025514666000002.tif37164

[0028] TIFF2025514666000003.tif24164

[0029] TIFF2025514666000004.tif78164

[0030] TIFF2025514666000005.tif63164

[0031] TIFF2025514666000006.tif45164

[0032] TIFF2025514666000007.tif63164

[0033] TIFF2025514666000008.tif35164

[0034] TIFF2025514666000009.tif23164

[0035] For a given set of circumstances (i.e., a particular instrument, spectral range, and gas of interest), it is therefore possible for the accuracy to remain nearly invariant for n much less than the total number N of available spectral pixels.

[0036] Due to the above, only a subset of the spectral pixels need to be used. The remaining spectral pixels can then be used to repeat the measurement using multiple / additional slits. By making further repeated measurements of the same signal, the noise can be averaged out and reduced, and the column accuracy can be increased accordingly.

[0037] TIFF2025514666000010.tif52164

[0038] TIFF2025514666000011.tif43164

[0039] TIFF2025514666000012.tif16164

[0040] Thus, in one aspect, the invention provides a hyperspectral imager configured with multiple parallel slits. The imager may be placed on a platform flying over the area to be imaged. Conventionally, each slit is configured to be perpendicular to the direction of platform movement. As can be imagined, each slit images a portion of the scene to be imaged. As the platform flies over the area, each section of the scene is repeatedly imaged as each successive slit passes over it. This allows each section to be repeatedly imaged over a period of time. The data collected for each image may be correlated / aligned such that multiple data sets covering the same section may be overlaid, for example for averaging purposes. Ideally, the period over which a section is repeatedly imaged should be short enough that the emission plume being imaged remains relatively stable. Preferably, the local wind speed and direction do not change during this period over which the section is repeatedly imaged.

[0041] Referring to FIG. 2, the area imaged by a 10-slit hyperspectral imager is illustrated. The portion imaged on each slit is represented by the cyan lines, while the emission plume is represented by the red portion of the area. As can be seen from FIG. 2, all of the slits are perpendicular to the direction of motion of the platform on which the imager is mounted / placed. One advantage of this configuration and its multiple data sets is that when the multiple views / data sets are aligned, the noise for all measurements can be averaged out and reduced without compromising the average plume density (assuming the emission plume does not vary significantly).

[0042] For gas / plume applications, as mentioned above, ideally the section of the scene to be imaged is imaged repeatedly over a period during which the plume remains relatively stable, so implementations in which the scene or plume is imaged repeatedly over a period of, for example, about 15 seconds, have been found to provide useful results.

[0043] It will be apparent that multiple slits enable the hyperspectral imager to operate as multiple independent spectrometers, with each of the slit-based subsystems producing a time-delayed data set for the same section of the scene.

[0044] Referring to FIG. 3, a schematic configuration of one embodiment of the present invention is illustrated. As can be seen, in this configuration there are four slits 30A-30D, all four sharing the same collimating optics 40, dispersive element 50, and imaging optics 60. Additionally, between the four slits 30A-30D and the scene to be imaged are fore-optics 25 and band-pass filters 27. In FIG. 3, light from a section of the area to be imaged for slit 30B is shown passing from the area through the fore-optics and band-pass filters, passing through slit 30B, and being received by collimating optics 40. From collimating optics 40, said light is dispersed by dispersive element 50 (diffraction grating or prism, if necessary or desired). As a result, it is imaged by imaging optics 60 before being received by sensor 70 (pixel) on the focal plane. As each slit flies over the section of the area to be imaged, a corresponding section of a hyperspectral data cube is generated for that section of the area. Thus, for this embodiment, four sections of four hyperspectral data cubes are generated, with all four sections corresponding to a single section of the area being imaged. These four sections of the data cube can be correlated or synchronized, and four readings of the sections are generated. From these four sections of the data cube, a suitable single reading can be synthesized, which has better column accuracy than that obtained by a single slit configuration imager.

[0045] It will also be clear that Figure 3 shows a "transmissive" dispersive element as an example. Although transmissive gratings exist, other dispersive elements such as diffraction gratings are generally reflective. If a reflective grating is used as the dispersive element, the system needs to be folded. The reflective grating can be curved or flat. Other mirrors can also be used (e.g. the collimating optics and imaging optics can be curved mirrors), resulting in even more folding in the system arrangement.

[0046] To account for the delay between readings of different slits in an imager with a multi-slit configuration, suitable parameters for implementation for an airborne multi-slit imager with 10 evenly spaced slits may be as follows: Platform ground speed: 60m / s, The focal length is 44mm to generate a 698m wide swath on a 3000m high platform.

[0047] The above parameters result in a speed along the trajectory of 88 pixels per second, traversing the entire spectral dimension of the focal plane array in 14.5 seconds. Assuming 10 uniformly / evenly spaced slits, this results in a delay of about 1.45 seconds between the search fields of successive slits. Thus, 10 simultaneous readings of the swath are performed in 14.5 seconds, resulting in more accurate spectroscopic measurements. It will be clear that the parameters given above merely provide an implementation example, and of course other parameters are possible.

[0048] While the above implementation discusses targeting methane plumes and methane emissions, it will be apparent that other gases can also be targeted. Such targeting can be achieved by simply adjusting the target frequency range for the hyperspectral data cube.

[0049] Similarly, hyperspectral imagers in a multi-slit configuration can be used for applications other than trace gas detection and quantification. Multi-slit hyperspectral imagers can be used in any implementation or field that can benefit from a better effective SNR. The concept of using multiple slit apertures simultaneously is useful in LIBS spectroscopy (laser-induced breakdown spectroscopy) as well as Raman spectroscopy.

[0050] It will be apparent that unless otherwise specified, any reference herein to an "image" or "images" refers to a digital image or digital images comprising pixels or picture cells.

[0051] As used herein, the interpretation of "at least one of [x] and [y]" should be interpreted as meaning "[x], [y], or both [x] and [y]."

[0052] Those understanding the present invention may now conceive of all of the above alternative structures and embodiments or variations which are intended to fall within the scope of the present invention as defined in the claims which follow.

Claims

1. 1. A hyperspectral imager, comprising: The hyperspectral imager includes: a plurality of slit apertures through which a particular portion of the scene is imaged multiple times as a platform on which the imager is mounted traverses a scene; and at least one collimating lens through which an image of the particular portion of the scene passes after being received through at least one of the plurality of slit openings; and at least one spectrally dispersive element, wherein an image of the particular portion of the scene passes through the at least one spectrally dispersive element after passing through the at least one collimating lens; and at least one imaging system for refocusing the image of the particular portion of the scene onto a focal plane pixel array; a plurality of pixels on the focal plane pixel array for receiving the image of the particular portion of the scene; Equipped with each of the slit apertures generating a hyperspectral data set corresponding to a limited number of specific wavelengths of light received through said slit apertures; each of the slit apertures corresponds to a fixed number n of the plurality of pixels, such that n<m, where m is the maximum number of pixels available; Hyperspectral imager.

2. The imager of claim 1 , wherein the spectrally dispersive element is a diffraction grating.

3. The imager of claim 1 , wherein the spectrally dispersive element is a prism.

4. The imager of claim 1 , wherein the slit openings are evenly spaced from each other.

5. The imager of claim 1 , wherein the slit openings are parallel to each other in the slit plane.

6. The imager of claim 1 , wherein the plurality of slit openings are perpendicular to a direction of movement of the platform.

7. The imager of claim 1 , wherein the particular wavelengths of light include absorption wavelengths of particular gases.

8. The imager of claim 7, wherein the particular gas is methane.

9. 1. A hyperspectral imager, comprising: a plurality of parallel slit apertures through which a particular portion of the scene is imaged multiple times as a platform on which the imager is mounted traverses a scene; and a plurality of pixels for receiving split light on a focal plane, the split light being derived from light received through at least one of the plurality of slit openings; Equipped with each of the slit apertures generating a hyperspectral data set corresponding to a limited number of specific wavelengths of light received through said slit apertures; Hyperspectral imager.

10. 10. The imager of claim 9, wherein each slit opening corresponds to a fixed number, n, of the plurality of pixels, where n is less than a maximum number of available pixels.

11. 1. A method of hyperspectral imaging, comprising: The method comprises: (a) receiving data from an imaging platform, the imaging platform flying over an area, the imaging platform comprising a hyperspectral imager having a plurality of slit apertures, each of the plurality of slit apertures providing a separate data stream; (b) organizing the data into a plurality of data sets, each of the plurality of data sets corresponding to a separate one of the plurality of slit apertures, a number of the plurality of data sets equal to a number of the plurality of slit apertures, and each of the plurality of data sets corresponding to a single data stream; (c) synchronizing a plurality of said data sets, thereby generating synchronized data for said region; (d) combining said synchronization data, thereby generating a single read for said region; Including, method.

12. The method of claim 11 , wherein each slit aperture corresponds to a limited number of specific wavelengths of light to be received through said slit aperture.

13. 12. The method of claim 11, wherein the hyperspectral imager further comprises a plurality of pixels for receiving split light on a focal plane, the split light being derived from light received through at least one of a plurality of the slit apertures.

14. 14. The method of claim 13, wherein each slit opening corresponds to a fixed number, n, of the plurality of pixels, such that n<m, where m is the maximum number of pixels available.

15. The method of claim 11 , wherein the slit openings are parallel to one another in a slit plane.

16. The method of claim 15 , wherein the plurality of slit openings are evenly spaced from one another.

17. The method of claim 11 , wherein the plurality of slit openings are perpendicular to a direction of movement of the imaging platform.

18. The method of claim 12 , wherein the particular wavelengths of light include absorption wavelengths of particular gases.

19. 20. The method of claim 18, wherein the particular gas is methane.

20. 12. The method of claim 11, wherein the hyperspectral imager further comprises at least one spectrally dispersive element, the at least one spectrally dispersive element comprising at least one of a diffraction grating and a prism.

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