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

2 PHYSICS 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

3 PHYSICS Broadband Hyperspectral Imaging Classic “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 – λ Registration – Complex Dispersive Elements FPA

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

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

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

7 PHYSICS  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, t 1 + t 2 < 100% Dualband Focal Plane Array “Stacked” detection sites LWIR Layer MWIR Layer Courtesy DRS IR Technologies

8 PHYSICS No FPA is Perfect MWIRLWIR Decreasing Wavelength

9 PHYSICS Wavelength Calibration μm/col μm/col

10 PHYSICS 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 PHYSICS BB Calibration at MWIR only

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

13 PHYSICS Longer band 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 Shorter waveband, microns (SNR) Derived temperature +/- uncertainty (Kelvin) 12 (50)∞ 11 (53) / (56) / (47) / -8 5 (23) / -6 3 (6) / -13 Derived space object temperatures: 50% visible reflection 50% infrared emissivity 394 K equilibrium Modeling Determination of Space Object Temperatures

14 PHYSICS Two Wavebands to determine BB Temperature 423 K

15 PHYSICS Recovered BB Spectrum Actual 423 K Recovered K

16 PHYSICS Greater Separation of the Two Wavebands used to determine BB Temperature 423 K

17 PHYSICS Better Results Actual 423 K Recovered K

18 PHYSICS 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 PHYSICS Two widely separated wavebands to determine BB Temperature 423 K

20 PHYSICS Results compromised by noisy LWIR band Actual 423 K Recovered K

21 PHYSICS Two more widely separated wavebands to determine BB Temperature 423 K

22 PHYSICS Good results despite noisy LWIR Actual 423 K Recovered K

23 PHYSICS 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