Real-time imaging spectrometer

• US 5,216,484 A
• Filed: 12/09/1991
• Issued: 06/01/1993
• Est. Priority Date: 12/09/1991
• Status: Expired due to Fees

• Current Assignee: The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Yu, Jeffrey W., Cheng, Li-Jen Cheng, Chao, Tien-Hsin, Lambert, James L.
• Primary Examiner(s): Evans, F. L.
• Application Number:
  US07/805,341
• Time in Patent Office: 540 Days
• Field of Search: 356/300, 356/308, 356/326, 356/328, 356/51, 359/308, 250/339
• US Class Current: 356/326
• CPC Class Codes: G01J 3/1256 using acousto-optic tunable filter; acousto-optic elements or systems G02F1/11, G02F1/33
• CPC Class Codes: G01J 3/2823 Imaging spectrometer
• CPC Class Codes: G01N 21/35 using infrared light G01N21/39 takes precedence
• CPC Class Codes: G01N 21/3563 for analysing solids; Preparation of samples therefor
• CPC Class Codes: G01N 21/359 using near infrared light

Abstract

An improved AOTF-based imaging spectrometer that has ability to electronically set the bandpass wavelength of an AOTF to any desired value in its wide tuning range, provides significant observational flexibility. This allows observations to be tailored in real-time and enables the spectrometer to address a wide range of objectives and permits real-time modification of the observational parameters, such as in flight or in other situations in which only remote control is possible. Various improvements in optical architecture provide simplified magnification variability, improved image resolution and light throughput efficiency and reduced sensitivity to ambient light. A preferred embodiment employs a camera zoom lens as the input lens. A TeO₂ AOTF and a relay lens are placed at selected distances behind the back focal plane of the input lens, respectively. A charge coupled device (CCD) camera, which comprises a camera, camera zoom lens and a CCD detector is placed at a selected distance behind the relay lens. An RF signal generator is used to drive the AOTF. Two embodiments of the invention are disclosed herein. One operates in the visible/near-infrared domain, in the wavelength range of 0.48 to 0.76 microns. The other operates in the infrared wavelength range of 1.2 to 2.5 microns.

Claims

1. An imaging spectrometer comprising an acousto-optic tunable filter of the type having an optical wavelength passband which is dependent on the selected frequency of an RF signal applied to said filter;

input optics forming an intermediate image plane for directing incident image light into said filter within a field of view of said filter;

a detector having an image plane for receiving image light passed by said filter;

output optics positioned between said filter and said detector for producing a focussed image on said detector image plane;

an RF signal source for applying said RF signal to said filter; and

means for controlling said source for generating an RF signal having said selected frequency;

wherein said filter optical wavelength passband is in the range of 0.4 to 5.0 microns.

2. An imaging spectrometer comprising an acousto-optic tunable filter of the type having an optical wavelength passband which is dependent on the selected frequency of an RF signal applied to said filter;

input optics forming an intermediate image plane for directing incident image light into said filter within a field of view of said filter;

a detector having an image plane for receiving image light passed by said filter;

output optics positioned between said filter and said detector for producing a focussed image on said detector image plane;

an RF signal source for applying said RF signal to said filter; and

means for controlling said source for generating an RF signal having said selected frequency;

wherein said selected frequency of said RF signal is in the range of 50 to 90 MHz.

3. An imaging spectrometer comprising an acousto-optic tunable filter of the type having an optical wavelength passband which is dependent on the selected frequency of an RF signal applied to said filter;

input optics forming an intermediate image plane for directing incident image light into said filter within a field of view of said filter;

a detector having an image plane for receiving image light passed by said filter;

output optics positioned between said filter and said detector for producing a focussed image on said detector image plane;

an RF signal source for applying said RF signal to said filter; and

means for controlling said source for generating an RF signal having said selected frequency;

wherein said controlling means comprises a computer.

4. An imaging spectrometer comprising an acousto-optic tunable filter of the type having an optical wavelength passband which is dependent on the selected frequency of an RF signal applied to said filter;

input optics forming an intermediate image plane for directing incident image light into said filter within a field of view of said filter;

a detector having an image plane for receiving image light passed by said filter;

output optics positioned between said filter and said detector for producing a focussed image on said detector image plane;

an RF signal source for applying said RF signal to said filter; and

means for controlling said source for generating an RF signal having said selected frequency;

wherein said detector comprises a charge-coupled device.

5. An imaging spectrometer comprising an acousto-optic tunable filter of the type having an optical wavelength passband which is dependent on the selected frequency of an RF signal applied to said filter;

input optics forming an intermediate image plane for directing incident image light into said filter within a field of view of said filter;

a detector having an image plane for receiving image light passed by said filter;

output optics positioned between said filter and said detector for producing a focussed image on said detector image plane;

an RF signal source for applying said RF signal to said filter; and

means for controlling said source for generating an RF signal having said selected frequency;

wherein said detector comprises a charge-coupled device detector within a CCD camera.

6. An imaging spectrometer comprising an acousto-optic tunable filter of the type having an optical wavelength passband which is dependent on the selected frequency of an RF signal applied to said filter;

input optics forming an intermediate image plane for directing incident image light into said filter within a field of view of said filter;

a detector having an image plane for receiving image light passed by said filter;

output optics positioned between said filter and said detector for producing a focussed image on said detector image plane;

an RF signal source for applying said RF signal to said filter; and

means for controlling said source for generating an RF signal having said selected frequency;

wherein said detector comprises a HgCdTe detector array.

7. An imaging spectrometer comprising an acousto-optic tunable filter of the type having an optical wavelength passband which is dependent on the selected frequency of an RF signal applied to said filter;

input optics forming an intermediate image plane for directing incident image light into said filter within a field of view of said filter;

a detector having an image plane for receiving image light passed by said filter;

output optics positioned between said filter and said detector for producing a focussed image on said detector image plane;

an RF signal source for applying said RF signal to said filter; and

means for controlling said source for generating an RF signal having said selected frequency;

wherein said output optics are positioned relative to said filter to receive only one polarization of light therefrom without the use of a spatial filter at said detector.

8. An imaging spectrometer comprising an acousto-optic tunable filter of the type having an optical wavelength passband which is dependent on the selected frequency of an RF signal applied to said filter;

input optics forming an intermediate image plane for directing incident image light into said filter within a field of view of said filter;

a detector having an image plane for receiving image light passed by said filter;

output optics positioned between said filter and said detector for producing a focussed image on said detector image plane;

an RF signal source for applying said RF signal to said filter; and

means for controlling said source for generating an RF signal having said selected frequency;

wherein said output optics comprises a zoom lens mounted directly on said detector.

9. An imaging spectrometer comprising foreoptics for collecting light incident on said spectrometer;

an AOTF positioned substantially at an intermediate imaging plane relative to said foreoptics for receiving said collected light;

imaging lenses for focussing an image passed by said AOTF;

a detector for detecting said focussed image;

an RF signal generator connected to said AOTF for altering the optical passband thereof, and

means for controlling the frequency of said RF signal generator for selecting the optical wavelength of said AOTF;

wherein said AOTF optical wavelength passband is in the range of 0.4 to 5.0 microns.

10. An imaging spectrometer comprising foreoptics for collecting light incident on said spectrometer;

an AOTF positioned substantially at an intermediate imaging plane relative to said foreoptics for receiving said collected light;

imaging lenses for focussing an image passed by said AOTF;

a detector for detecting said focussed image;

an RF signal generator connected to said AOTF for altering the optical passband thereof; and

means for controlling the frequency of said RF signal generator for selecting the optical wavelength of said AOTF;

wherein said frequency of said RF signal is in the range of 50 to 90 MHz.

11. An imaging spectrometer comprising foreoptics for collecting light incident on said spectrometer;

an AOTF positioned substantially at an intermediate imaging plane relative to said foreoptics for receiving said collected light;

imaging lenses for focussing an image passed by said AOTF;

a detector for detecting said focussed image;

an RF signal generator connected to said AOTF for altering the optical passband thereof; and

means for controlling the frequency of said RF signal generator for selecting the optical wavelength of said AOTF;

wherein said controlling means comprises a computer.

12. An imaging spectrometer comprising foreoptics for collecting light incident on said spectrometer;

an AOTF positioned substantially at an intermediate imaging plane relative to said foreoptics for receiving said collected light;

imaging lenses for focussing an image passed by said AOTF;

a detector for detecting said focussed image;

an RF signal generator connected to said AOTF for altering the optical passband thereof; and

means for controlling the frequency of said RF signal generator for selecting the optical wavelength of said AOTF;

wherein said detector comprises a charge-coupled device.

13. An imaging spectrometer comprising foreoptics for collecting light incident on said spectrometer;

an AOTF positioned substantially at an intermediate imaging plane relative to said foreoptics for receiving said collected light;

imaging lenses for focussing an image passed by said AOTF;

a detector for detecting said focussed image;

an RF signal generator connected to said AOTF for altering the optical passband thereof; and

means for controlling the frequency of said RF signal generator for selecting the optical wavelength of said AOTF;

wherein said detector comprises a charge-coupled device detector within a CCD camera.

14. An imaging spectrometer comprising foreoptics for collecting light incident on said spectrometer;

an AOTF positioned substantially at an intermediate imaging plane relative to said foreoptics for receiving said collected light;

imaging lenses for focussing an image passed by said AOTF;

a detector for detecting said focussed image;

an RF signal generator connected to said AOTF for altering the optical passband thereof; and

means for controlling the frequency of said RF signal generator for selecting the optical wavelength of said AOTF;

wherein said detector comprises a HgCdTe detector array.

15. An imaging spectrometer comprising foreoptics for collecting light incident on said spectrometer;

an AOTF positioned substantially at an intermediate imaging plane relative to said foreoptics for receiving said collected light;

imaging lenses for focussing an image passed by said AOTF;

a detector for detecting said focussed image;

an RF signal generator connected to said AOTF for altering the optical passband thereof; and

means for controlling the frequency of said RF signal generator for selecting the optical wavelength of said AOTF;

wherein said imaging lenses are positioned relative to said AOTF to receive only one polarization of light therefrom without the use of a spatial filter at said detector.

16. An imaging spectrometer comprising foreoptics for collecting light incident on said spectrometer;

an AOTF positioned substantially at an intermediate imaging plane relative to said foreoptics for receiving said collected light;

imaging lenses for focussing an image passed by said AOTF;

a detector for detecting said focussed image;

an RF signal generator connected to said AOTF for altering the optical passband thereof; and

means for controlling the frequency of said RF signal generator for selecting the optical wavelength of said AOTF;

wherein said imaging lenses comprise a zoom lens mounted directly to said detector.