Progress on characterization of a dualband IR imaging spectrometer

1. Progress on characterization of a dualband IR imaging spectrometer 18 March 2008 Orlando, Florida SPIE Conference 6940 Infrared Technology and Applications XXXIV Brian Beecken, Cory Lindh, and Randall Johnson Physics Department, Bethel University, St. Paul, MN Paul LeVan Air Force Research Lab, Kirtland AFB

2. Overview • The Goal: Hyperspectral IR Imaging from a space-based sensor • Why? - More Info with • Our Method: • Using a dualband FPA gives improvements over traditional 2 channel approach • Precise wavelength calibration • Demonstrated recovery of BB spectral content • One Application: • When scanning for targets, only a few pixels may be available for each target. Can you still determine what it is? • Our instrument is a resource that can be used to test a method of determining T of “small targets” in large FOV "COLOR"

3. Broadband Hyperspectral ImagingClassic “2 channel” Spectrometer • Efficiencies change with λ • Gratings • FPA detectors • Classic Solution: 2 channels • Common aperture & FOV • Beamsplitter • 2 Dispersive elements and 2 FPAs • Each channel optimized for roughly 1 octave of λ • Issues • Size • Mass • Power consumption • λRegistration • Complex FPA Dispersive Elements

4. Spectral Image, but only 1 spatial dimension Dualband FPA Diffraction Concept Dualband FPA Dispersive Element Multispectral IR Spatial Dimension • Improvements: • No beam splitter • One dispersive element • One FPA Spectral Dimension

5. Using Dual-band FPA • Gratings • nλ = d sin θ • Peak efficiencies at λB, λB/2, λB/3,… • Designed Bands: 3.75 – 6.05 µm (MWIR) 7.5 – 12.1 µm (LWIR) • λGap chosen to prevent spectral crosstalk • Advantages: • Reduced Complexity • Smaller mass & size • Less cooling required • Perfect λ registration 2nd order is MWIR 1st order is LWIR 320 cols x 240 rows

6. Schematic of Dewar Optics Dualband FPA grating Image formed on slit • Only 4 optical components • Near-collimation (2 mirrors) • Grating • Refocusing (“camera” mirror)

7. Dualband Focal Plane Array “Stacked” detection sites • Shorter waveband material absorbs shorter wavelength photons, • transmitting longer wavelength photons to the (deeper) longer waveband • “Simultaneous”operation • both photocurrents integrated during the same frame time with overlapping integration times • alternative is switched with shared duty cycle, t1 + t2 < 100% MWIR Layer Courtesy DRS IR Technologies LWIR Layer

8. No FPA is Perfect LWIR MWIR Decreasing Wavelength

9. Wavelength Calibration 0.0078 μm/col 0.0157 μm/col

10. Dualband BB Calibration • Two Point Gain and Offset Calibration at 498 K and 373 K • Data shown is average down each full column of the array • Intermediate BB spectrums recovered • Efficacy of recovered spectrum is limited by a compromised bias voltage

11. BB Calibration at MWIRonly

12. Calibration with 2nd and 3rd Order! 2nd order 3rd order • Columns 331 to 433 • Small MCT response in 2nd order • Poor grating efficiency in 3rd order • Competition between these two effects • May be able to “tease out” proper calibration

13. Derived space object temperatures: 50% visible reflection 50% infrared emissivity 394 K equilibrium Modeling Determination of Space Object Temperatures Longer band fixed @ 12 μm Variation of shorter bands Uncertainties decrease at shorter wavelengths, but still some increase in dilution by reflected solar 5 & 12 μm seem to provide good tradeoff in this case

14. Two Wavebands to determineBB Temperature 423 K

15. Recovered BB Spectrum Actual 423 K Recovered 407-424 K

16. Greater Separationof the Two Wavebandsused to determine BB Temperature 423 K

17. Better Results Actual 423 K Recovered 422-427K

18. Using Dualband Capability • Two Point Gain and Offset Calibration at 498 K and 373 K • Data shown is average down each column of the array, but only 5 pixels • Intermediate BB spectrums recovered, but look poor due to limited average • Quality of recovered spectrum is also limited by a compromised bias voltage

19. Two widely separated wavebandsto determine BB Temperature 423 K

20. Results compromised bynoisy LWIR band Actual 423 K Recovered 397-449 K

21. Two more widely separated wavebandsto determine BB Temperature 423 K

22. Good results despite noisy LWIR Actual 423 K Recovered 413-423 K

23. Summary • Novel Dualband IR Imaging Spectrometer • Several advantages for space-based applications • Precisely wavelength calibrated over two octaves • Successfully recovered BB spectrum between offset and gain calibration temperatures • Demonstration of Determination of Space Object T’s • Use only two very narrow wavebands • Low noise within wavebands helps • Greater separation of wavebands helps • Determination of T’s to within 1 % demonstrated