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PHYSICS
Progress on characterization of a dualband IR imaging spectrometer
Brian Beecken, Cory Lindh, and Randall JohnsonPhysics Department, Bethel University, St. Paul, MN
Paul LeVanAir Force Research Lab, Kirtland AFB
18 March 2008Orlando, Florida
SPIE Conference 6940Infrared Technology and Applications XXXIV
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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 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
– λ Registration
– Complex
Dispersive Elements
FPA
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PHYSICS
Spectral Image, but only 1 spatial dimension
Sp
atia
l Dim
ensi
on
Dualband FPA Diffraction Concept
Improvements:•No beam splitter•One dispersive element•One FPA
DispersiveElement
SpectralDimension
DualbandFPA
Multispectral IR
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PHYSICS Using Dual-band FPA
• Gratings
– nλ = d sin θ
– Peak efficiencies atλB, λB/2, λB/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
2nd orderis MWIR
1st order is LWIR
320 cols x 240 rows
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PHYSICS Schematic of Dewar Optics
DualbandFPA
gratingImage formed on slit
Only 4 optical components
• Near-collimation (2 mirrors)
• Grating
• Refocusing (“camera” mirror)
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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, t1 + t2 < 100%
Dualband Focal Plane Array “Stacked” detection sites
p-type
IR
p-Type
n-Type
HgCdTe-2
Insulatedvia
ROIC
n-type
diode-1current
diode-2current
HgCdTe-1 LWIR Layer
MWIR Layer
Courtesy DRS IR Technologies
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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
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PHYSICS Calibration with 2nd and 3rd Order!
3rd order 2nd 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
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PHYSICS
Tsol
404.405
398.186
392.031
Tsolve 393.561
0 5 10 15 200
5 10 9
1 10 8
1.510 8
2 10 8
Wavelength, microns
lam
da*S
-lam
bda,
W/c
m2
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
Shorter waveband, microns (SNR)
Derived temperature +/- uncertainty
(Kelvin)
12 (50) ∞
11 (53) 394 +76 / -53
9 (56) 394 +17 / -15
7 (47) 395 +8 / -8
5 (23) 398 +6 / -6
3 (6) 470 +12 / -13
Derived space object temperatures: 50% visible reflection 50% infrared emissivity 394 K equilibrium
Modeling Determination of Space Object Temperatures
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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
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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