High etendue imaging fourier transform spectrometer

• US 5,777,736 A
• Filed: 07/19/1996
• Issued: 07/07/1998
• Est. Priority Date: 07/19/1996
• Status: Expired due to Term

• Current Assignee: Leidos, Inc. (Leidos Holdings, Inc.)
• Original Assignee: Science Applications International Corporation
• Inventors: Horton, Richard F.
• Primary Examiner(s): Turner, Samuel A.
• Application Number:
  US08/684,925
• Time in Patent Office: 718 Days
• Field of Search: 356/346, 356/354, 250/339.07, 250/339.08
• US Class Current: 356/456
• CPC Class Codes: G01J 3/453 by correlation of the amplitudes

Abstract

A high etendue imaging Fourier transform spectrometer which utilizes a two beam interferometer is provided. The high etendue imaging Fourier transform spectrometer is adapted to create an image of an entire target area with the resolution and throughput comparable to that of a Michelson interferometer based spectrometer yet does not require moving parts. The disclosed imaging spectrometer is comprised of various image forming optical elements; an two beam interferometer, which provides two interfering images at the image plane, resulting in the spectral autocorrelation modulation of the scene image; a CCD array camera to detect this interference modulated image; a scanning mechanism to translate the image scene through the autocorrelation modulated image plane thereby allowing a sequence of the modulated scene images to be recorded; a control system for operatively controlling the image forming optical elements, the scanning mechanism and CCD array camera; and an image data analyzer adapted for recording and processing the intensity modulation signal including the step of Fourier transforming the intensity modulation signal for each of individual scene area of the target which yields the spectrum associated with each scene area.

Claims

1. A high etendue imaging Fourier transform spectrometer comprising:

an image forming optical element for converging radiation received from a defined target having a plurality of scene areas into a target wavefront;

an image plane interferometer adapted to receive said target wavefront and create a pair of wavefronts oriented at an angle with respect to one another, said angled wavefronts converging to an image plane where said angled wavefronts interfere to produce an interference wavefront such that an intensity of said radiation at each of said scene areas on said interference wavefront as a function of time as each scene area moves across said image plane corresponds to an autocorrelation function of each of said scene areas at an offset prescribed by said angle and a spatial position of said scene area;

an imaging detector disposed proximate said image plane and adapted for recording said intensity of said radiation at each of said scene areas on said interference wavefront as a function of time as each scene area moves across said image plane; and

an image data analyzer for processing said intensity data to produce an autocorrelation function data set for each of said scene areas and for determining a Fourier transform of said autocorrelation function data set for each of said scene areas, said image data analyzer comprising

means for collection of raw image data, said raw image data representing the time history of said intensity as measured by said imaging detector for each scene area,

means for calibrating said raw image data for each scene area using dark field and flat field calibration data,

means for array processing said calibrated image data to produce the autocorrelation function data set for each scene area,

means for Fourier transforming said autocorrelation function data set for each of said scene areas to produce an output of spectrum data for each scene area, and

means for displaying said output of spectrum data for each scene area.

2. The imaging Fourier transform spectrometer of claim 1 wherein said image forming optical element further comprises a lens for forming said target wavefront and an iris for controlling an amount of radiation received from said target.

3. The imaging Fourier transform spectrometer of claim 1 further comprising a scan mechanism for controlling the relative movement of said target across said image plane.

4. The imaging Fourier transform spectrometer of claim 3 wherein said scan mechanism includes a scan mirror disposed proximate said image forming optic element, said scan mirror adapted for rotation about an axis perpendicular to the relative motion of said target in said image plane.

5. The imaging Fourier transform spectrometer of claim 3 wherein said scan mechanism includes a moving platform wherein the motion of said platform provides said target with a relative velocity across said image plane.

6. The imaging Fourier transform spectrometer of claim 1 wherein said imaging detector for recording said intensity of said radiation at each scene area on said interference wavefront as a function of time as each scene area moves across said image plane is a charge coupled device (CCD) array camera.

7. The imaging Fourier transform spectrometer of claim 6 wherein said CCD array camera includes a CCD array of prescribed dimensions having a plurality of pixels such that each scene area moves across said image plane from a first pixel on said CCD array to an adjacent pixel in said CCD array in a given direction during each readout time interval of said CCD array.

8. The imaging Fourier transform spectrometer of claim 2 wherein said lens is adapted for focusing said radiation at an intermediate image plane located within said image plane interferometer and said imaging Fourier transform spectrometer further includes relay optics interposed between said image plane interferometer and said imaging detector, said relay optics adapted for re-focusing said angled wavefronts exiting said image plane interferometer to said image plane proximate said imaging detector.

9. The imaging Fourier transform spectrometer of claim 3 further comprising a control system operatively associated with said image forming optical element, said scan mechanism, and said imaging detector.

10. The imaging spectrometer of claim 1 wherein said image plane interferometer is a two-beam interferometer including a pair of beamsplitters and a plurality of mirrors adapted to create said pair of wavefronts oriented at said angle with respect to one another and wherein a difference in pathlengths traversed by each of said angled wavefronts is less than a coherent length of said radiation.

11. The imaging spectrometer of claim 10 wherein said beamsplitters include a pair of equal thickness glass substrates sandwiching a partially reflecting beamsplitting surface.

12. The imaging spectrometer of claim 10 wherein said beamsplitters include a glass substrate having a partially reflecting beamsplitting coating applied to one surface of said glass substrate.

13. The imaging spectrometer of claim 1 wherein said image plane interferometer includes a block prism having a beamsplitting surface which bisects said block prism.

14. The imaging spectrometer of claim 13 wherein said block is a segmented block prism including a plurality of prismatic blocks cemented together with optical cement.

15. The imaging spectrometer of claim 1 wherein said image plane interferometer further includes multiple inputs and multiple outputs.

16. Apparatus for determining a spectrum associated with radiation received from a target comprising:

an image plane; and

an interferometer that generates first and second wavefronts oriented at an angle with respect to one another, each wavefront having an image associated with the target with a broadband spectrum range and traveling a pathlength from the target to the image plane that is within the coherence length of the pathlength of the other wavefront, the interferometer including;

a first beamsplitter adapted to receive an input wavefront from the target radiation and produce the first and second wavefronts therefrom,

a first set of mirrors that creates a first optical path along which the first wavefront traverses,

a second set of mirrors that creates a second optical path along which the second wavefront traverses, and

a second beamsplitter adapted to recombine the first and second wavefronts after respectively traversing the first and second optical paths such that the first and second wavefronts are tilted at said angle with respect to one another while selected image points of the first and second wavefronts are co-located to ensure lateral coherence of the first and second wavefronts resulting in an interference pattern.

17. The apparatus for determining spectrum associated with radiation of claim 16 further comprising an optical detector having a pixel array, responsive to the interference pattern that is oriented in the plane of the image plane and that generates signals representing the intensity of radiation in the interference pattern collected by each array pixel.

18. The apparatus for determining spectrum associated with radiation of claim 17 further comprising an image data analyzer for processing the intensity data collected by each array pixel to produce an autocorrelation function data set for the radiation and for determining a Fourier transform of the autocorrelation function data set to yield the spectrum associated with the radiation.

19. The apparatus for determining spectrum associated with radiation of claim 17 further comprising:

a lens for imaging the radiation from the target at the image plane; and

an iris for controlling an amount of radiation received from the target.

20. The apparatus for determining spectrum associated with radiation of claim 17 wherein the optical detector is a charge coupled device (CCD) array camera.

21. An image plane interferometer comprising:

a first beamsplitter adapted to receive an input wavefront and produce a first wavefront and a second wavefront traveling separate optical paths;

a first set of mirrors adapted to reflect said first wavefront along a first optical path and a second set of mirrors adapted to reflect said second wavefront along a second optical path, wherein said first optical path and second optical path have substantially equal pathlengths; and

a second beamsplitter adapted to spatially recombine said first and second wavefronts such that said first and second wavefronts are tilted at a prescribed angle with respect to one another while selected image points of said first and second wavefronts are co-located to ensure lateral coherence of said first and second wavefronts resulting in said interference pattern.

22. The image plane interferometer of claim 21 wherein one or both of said beamsplitters include a pair of equal thickness glass substrates sandwiching a partially reflecting beamsplitting surface.

23. The image plane interferometer of claim 21 wherein one or both of said beamsplitters include a glass substrate having a partially reflecting beamsplitting coating applied to one surface of said glass substrate.

24. The image plane interferometer of claim 21 further comprising a prism block having a plurality of reflecting surfaces and a beamsplitting surface bisecting said prism block.

25. A method for determining a spectrum associated with radiation received from a target comprising:

receiving the radiation at a two-beam interferometer;

generating first and second wavefronts oriented at an angle with respect to one another, each wavefront having an image associated with the target with a broadband spectrum range and traveling a pathlength from the target to an image plane that is within the coherence length of the pathlength of the other wavefront;

superimposing the first and second wavefronts in the image plane to generate an interference pattern;

detecting the intensity of radiation of the interference pattern;

generating signals representing the intensity of radiation of the interference pattern;

collecting the signals representing the intensity of radiation of the interference pattern;

calibrating the signals using dark field and flat field calibration data to produce calibrated image data;

array processing the calibrated image data to produce an autocorrelation function data set; and

determining a Fourier transform of the autocorrelation function data set to yield the spectrum associated with the radiation.

26. The apparatus for determining spectrum associated with radiation of claim 25 further comprising the steps of:

imaging the radiation from the target at the image plane; and

controlling the amount of radiation received from the target.

27. A high etendue imaging Fourier transform spectrometer comprising:

an image forming optical element for converging radiation received from a defined target having a plurality of scene areas into a target wavefront;

an image plane interferometer comprising a pair of beamsplitters and a plurality of mirrors adapted to receive said target wavefront and create a pair of wavefronts oriented at an angle with respect to one another, said pair of beamsplitters including a pair of equal thickness glass substrates sandwiching a partially reflecting beamsplitting surface, wherein each wavefront of the pair of wavefronts traverses a different optical path, the different optical paths having a difference in pathlengths that is less than a coherent length of said radiation, said angled wavefronts converging to an image plane where said angled wavefronts interfere to produce an interference wavefront such that an intensity of said radiation at each scene area of said plurality of scene areas on said interference wavefront as a function of time as each scene area moves across said image plane corresponds to an autocorrelation function of each of said scene areas at an offset prescribed by said angle and a spatial position of said scene area;

an imaging detector disposed proximate said image plane and adapted for recording said intensity of said radiation at each of said scene areas on said interference wavefront as a function of time as each scene area moves across said image plane; and

an image data analyzer for processing said intensity data to produce an autocorrelation function data set for each of said scene areas and for determining a Fourier transform of said autocorrelation function data set for each of said scene areas;

wherein said Fourier Transform of said autocorrelation function data set for each of said scene areas yields a spectrum associated with said scene area.

28. A high etendue imaging Fourier transform spectrometer comprising:

an image forming optical element for converging radiation received from a defined target having a plurality of scene areas into a target wavefront;

an image plane interferometer comprising a pair of beamsplitters and a plurality of mirrors adapted to receive said target wavefront and create a pair of wavefronts oriented at an angle with respect to one another, said pair of beamsplitters including a glass substrate having a partially reflecting beamsplitting coating applied to one surface of said glass substrate, wherein each wavefront of the pair of wavefronts traverses a different optical path, the different optical paths having a difference in pathlengths that is less than a coherent length of said radiation, said angled wavefronts converging to an image plane where said angled wavefronts interfere to produce an interference wavefront such that an intensity of said radiation at each of said scene areas on said interference wavefront as a function of time as each scene area moves across said image plane corresponds to an autocorrelation function of each of said scene areas at an offset prescribed by said angle and a spatial position of said scene area;

an imaging detector disposed proximate said image plane and adapted for recording said intensity of said radiation at each of said scene areas on said interference wavefront as a function of time as each scene area moves across said image plane; and

an image data analyzer for processing said intensity data to produce an autocorrelation function data set for each of said scene areas and for determining a Fourier transform of said autocorrelation function data set for each of said scene areas;

wherein said Fourier Transform of said autocorrelation function data set for each of said scene areas yields a spectrum associated with said scene area.

29. A high etendue imaging Fourier transform spectrometer comprising:

an image forming optical element for converging radiation received from a defined target having a plurality of scene areas into a target wavefront;

an image plane interferometer adapted to receive said target wavefront and create a pair of wavefronts oriented at an angle with respect to one another, said angled wavefronts converging to an image plane where said angled wavefronts interfere to produce an interference wavefront such that an intensity of said radiation at each of said scene areas on said interference wavefront as a function of time as each scene area moves across said image plane corresponds to an autocorrelation function of each of said scene areas at an offset prescribed by said angle and a spatial position of said scene area, said image plane interferometer having multiple inputs and multiple outputs;

an imaging detector disposed proximate said image plane and adapted for recording said intensity of said radiation at each of said scene areas on said interference wavefront as a function of time as each scene area moves across said image plane; and

an image data analyzer for processing said intensity data to produce an autocorrelation function data set for each of said scene areas and for determining a Fourier transform of said autocorrelation function data set for each of said scene areas;

wherein said Fourier Transform of said autocorrelation function data set for each of said scene areas yields a spectrum associated with said scene area.

30. An image plane interferometer comprising:

a first beamsplitter adapted to produce first and second wavefronts from input radiation;

a first set of reflective surfaces adapted to reflect the first wavefront along a first optical path;

a second set of reflective surfaces adapted to reflect the second wavefront along a second optical path;

wherein the first and second optical paths have substantially equal pathlengths; and

a second beamsplitter adapted to recombine the first and second wavefronts such that the first and second wavefronts are tilted at a prescribed angle with respect to one another while selected image points of the first and second wavefronts are co-located to ensure lateral coherence of the first and second wavefronts, resulting in an interference pattern.