Spectral imaging system

• US 7,369,229 B2
• Filed: 11/24/2004
• Issued: 05/06/2008
• Est. Priority Date: 11/26/2003
• Status: Expired due to Fees

• Current Assignee: Eathon Intelligence, LLC
• Original Assignee: Florida Environmental Research Institute Inc.
• Inventors: Kohler, David D. R., Bissett, William Paul III, Mobley, Curtis D., Steward, Robert G.
• Primary Examiner(s): EVANS, FANNIE L
• Application Number:
  US10/996,530
• Publication Number:
  US 20050151965A1
• Time in Patent Office: 1,259 Days
• Field of Search: 356/305, 356/326, 356/328
• US Class Current: 356/328
• CPC Class Codes: G01J 1/4204 with determination of ambient light solar light G01J2001/4266
• CPC Class Codes: G01J 2003/2866 Markers; Calibrating of scan
• CPC Class Codes: G01J 3/28 Investigating the spectrum using colour filters G01J3/51
• CPC Class Codes: G01J 3/2823 Imaging spectrometer
• CPC Class Codes: G01N 2021/1793 Remote sensing
• CPC Class Codes: G01N 2021/1797 in landscape, e.g. crops
• CPC Class Codes: G01N 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G01N 21/31 Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry G01N21/72 takes precedence
• CPC Class Codes: G01S 17/86 Combinations of lidar systems with systems other than lidar, radar or sonar, e.g. with direction finders

Abstract

An imaging system and methods for resolving elements of interest through and obscuring environment by removing undesired signals from the intervening, obscuring environment is disclosed. A passive hyperspectral imaging sensor is calibrated and integrated into a system having a positioning and attitude detection system and an instrument control and data acquisition system that records data from sensors in a three-dimensional data cube having two spatial dimensions and a spectral dimension. Either an active detection and ranging system or a look-up-table approach or both are used to remove the noise generated by the intervening, obscuring environment from the data relevant to the elements of interest.

Claims

1. A spectral imaging system for imaging elements of interest in an intervening, obscuring environment, the system comprising:

a passive spectral imaging sensor;

an active detection and ranging sensor, capable of emitting pulses of electromagnetic radiation having a narrow band of frequencies and sensing the return of the pulses;

at least one airborne platform;

at least one position and attitude detection system; and

an instrument control and data acquisition system, wherein the at least one position and attitude detection system, the passive spectral imaging sensor and the active detection and ranging sensor are mounted on the at least one airborne platform and signals from the at least one position and attitude detection system, the passive spectral imaging sensor and the active detection and ranging sensor are operatively received by the instrument control and data acquisition system, such that the signals are stored as a three-dimensional data cube recording two spatial dimensions and a spectral dimension.

2. The spectral imaging system of claim 1, wherein the passive spectral imaging system comprises a pushbroom sensor.

3. The spectral imaging system of claim 2, wherein the pushbroom sensor comprises a backthinned charged couple device.

4. The spectral imaging system of claim 2, wherein the pushbroom sensor comprises a diffraction grating, such that electromagnetic radiation having a range of frequency and entering the sensor is diffracted and the angle of diffraction depends on the frequency, wherein the electromagnetic radiation is spread across the backthinned charged couple device, providing a spectral dimension to the signals generated by the backthinned charged couple device.

5. The spectral imaging system of claim 4, wherein the passive spectral imaging sensor shielded from at least a portion of stray, undiffracted light.

6. The spectral imaging system of claim 5, wherein the passive spectral imaging sensor is calibrated using a multi-step process that includes spectral calibration and radiometric calibration, such that a calibration map is derived that is applied by the instrument control and data acquisition system to the signals received from the passive spectral imaging sensor, accounting for all viewing angles of the passive spectral imaging sensor.

7. The spectral imaging system of claim 6, wherein the calibration map further accounts for the combined residual stray light and frame transfer smear in the signals received by the instrument control and data acquisition system from the passive spectral imaging sensor.

8. The spectral imaging system of claim 1, further comprising:

a system for correcting signals from a passive sensor for the environment, the system for correcting signals comprising;

a look up table of known, tabulated remote-sensing reflectance spectra;

a processing system that is capable of accessing the look up table and comparing the known, tabulated remote-sensing reflectance spectra with actual remote-sensing reflectance signals received from the passive sensor; and

an algorithm that selects an appropriate known, tabulated remote-sensing reflectance spectra that is comparable to the actual, measured remote-sensing reflectance spectra, such that environmental information associated with the known, tabulated remote-sensing reflectance spectra are applied for correcting the signals from the passive sensor for the environment;

wherein the system for correcting signals from a passive sensor is operatively coupled to the passive spectral imaging system and the instrument control and data acquisition system, such that data recorded in a three-dimensional data cube is corrected to remove undesirable artifacts in the signals caused by the environment.

9. The spectral imaging system of claim 1, wherein the at least one airborne platform comprises a plurality of platforms, each having at least one position and attitude detection system, such that the instrument control and data acquisition system operatively integrates the signals from each of the plurality of platforms, such that the signals are stored as a three-dimensional data cube recording two spatial dimensions and a spectral dimension.

10. The spectral imaging system of claim 1, wherein the at least one airborne platform is a single satellite.

11. The spectral imaging system of claim 1, wherein the at least one airborne platform is a single aircraft.

12. The spectral imaging system of claim 11, wherein the aircraft is lighter than air at sea level.

13. The spectral imaging system of claim 1, wherein the at least one position and attitude detection system comprises at least one gyroscope.

14. The spectral imaging system of claim 13, wherein the at least one position and attitude detection system further comprises a differential global positioning satellite receiver.

15. The spectral imaging system of claim 13, wherein the at least one position and attitude detection system is capable of determining the yaw, pitch and roll of the airborne platform.

28. The spectral imaging system of claim 8, wherein the passive spectral imaging sensor is shielded from at least a portion of stray, undiffracted light, and the passive spectral imaging sensor is calibrated using a multi-step process that includes spectral calibration and radiometric calibration, such that a calibration map is derived that is applied by the instrument control and data acquisition system to the signals received from the passive spectral imaging sensor, accounting for all viewing angles of the passive spectral imaging sensor and the combined residual stray light and frame transfer smear.

16. A spectral imaging system for imaging elements of interest in an intervening, obscuring environment, the system comprising:

a passive spectral imaging sensor;

a system for correcting signals from the passive spectral imaging sensor the system comprising a look up table of known, tabulated remote-sensing reflectance spectra;

a processing system that is capable of accessing the look up table and comparing the known, tabulated remote-sensing reflectance spectra with actual remote-sensing reflectance signals received from the passive spectral imaging sensor; and

an algorithm that selects an appropriate known, tabulated remote-sensing reflectance spectra that is comparable to the actual, measured remote-sensing reflectance spectra, wherein environmental information associated with the known, tabulated remote-sensing reflectance spectra are applied, such that the system for correcting signals from the passive spectral imaging sensor is capable of operatively removing from the signals at least a portion of undesirable artifacts caused by an intervening, obscuring environment;

at least one airborne platform;

at least one position and attitude detection system; and

an instrument control and data acquisition system, wherein the at least one position and attitude detection system and the passive spectral imaging sensor are mounted on the at least one airborne platform and signals from the at least one position and attitude detection system and the passive spectral imaging sensor are operatively received by the instrument control and data acquisition system and the system for correcting signals from a passive sensor for correcting the signals, such that the signals are stored as data, the data being corrected to remove undesirable artifacts in the signals caused by the environment and being stored in a three-dimensional data cube, recording two spatial dimensions and a spectral dimension.

17. The spectral image system of claim 16, wherein the environmental information is selected from the group consisting of water inherent optical properties, shallow- water bottom depth, solar angles, viewing directions, and spectral reflectance.

18. The spectral image system of claim 16, wherein at least a portion of the look up table of known, tabulated remote-sensing reflectance spectra is generated using a radiative transfer numerical model for computing radiance distributions and related quantities, having a range of water inherent optical properties, bottom depths, bottom reflectances, solar angles, and viewing directions that are substantially similar to the range of water inherent optical properties, bottom depths, bottom reflectances, solar angles and viewing directions expected for elements of interest that are to be imaged by the passive sensor.

19. A method of calibration of a passive spectral imaging system, comprising:

conducting a spectral calibration of the passive spectral imaging system;

conducting a radiometric calibration of the passive spectral imaging system;

deriving, from the preceding steps of conducting, a correction factor; and

applying the correction factor to an instrument control and data acquisition system that receives signals from the passive spectral imaging system, accounting for diffraction of electromagnetic radiation from all viewing angles of the passive spectral imaging sensor.

20. The method of claim 19, wherein the step of conducting a spectral calibration comprises:

(a) placing a first spectral element lamp in front of a sensor in the passive spectral imaging system;

(b) positioning, between the lamp and the sensor, a piece of ground glass, such that the entire aperture of a detector is illuminated by the spectral element lamp;

(c) collecting data from the sensor;

(d) replacing the first spectral clement lamp with another spectral element lamp having a different range of emission frequencies;

(e) repeating steps (d) and (c) until data is collected for a full spectral range of the sensor; and

(f) calculating a regression of the known spectral wavelengths that correlates with the position of the pixels within the sensor in which a response was recorded.

21. The method of claim 20, wherein the step of conducting a spectral calibration further comprises fitting a Gaussian curve to the element lamp data for each element lamp used in the calibration and equating the position of the peak of the Gaussian curve to a true spectral position within the sensor.

22. The method of claim 21, wherein the step of conducting a spectral calibration further comprises generating a smile-tilt map of the sensor and using the smile-tilt map to warp all future signals produced by the sensor to a set wavelength vector, during both subsequent calibration steps and normal data collection.

23. The method of claim 19, further comprising a step of conducting an angular response characterization.

24. The method of claim 23, wherein the step of conducting an angular response characterization comprises generating a keystone for the sensor.

25. The method of claim 24, wherein the step of generating a keystone for the sensor comprises:

positioning a physically thin spectral light source a distance from the sensor, such that light source illuminates only one pixel width;

positioning the light source at the distance and inline with the center of the sensor'"'"'s focal plane;

repeatedly repositioning the light source at predetermined intervals, while keeping the light source in a plane perpendicular to the line between the sensor and the light source;

recording the response from the sensor before and during the step of repeatedly repositioning;

deriving the apparent angular displacement of the source, as determined from a regression against an actual measured relative position between the sensor and the light source.

26. The method of claim 19, wherein the step of conducting a radiometric calibration comprises:

selecting a known radiometric calibration standard having a spectra;

employing a series of filters selected to correct the spectra, such that the spectra is substantially representative of the spectra encountered during normal use of the sensor;

adjusting lamp intensities to provide a calibration series that substantially covers the spectral range of elements of interest during normal use;

calculating a stray light probability correction;

determining a radiometric calibration map; and

applying the radiometric calibration map to all of the signals generated by the sensor.

27. The method of claim 26, wherein the radiometric calibration standard is a ten lamp, 40-inch diameter, integrating sphere, utilizing tungsten-halogen lamps that are calibrated to known NIST-traceable standards, and the series of filters are three long-pass cutoff filters and three blue balancing filters.