Transcript Extending hyperspectral capabilities with

PHYSICS
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
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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
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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
FPA
Dispersive Elements
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PHYSICS
Dualband FPA Diffraction Concept
Spectral Image, but only 1 spatial dimension
Dualband
FPA
Spatial Dimension
Dispersive
Element
Multispectral IR
Improvements:
Spectral
Dimension
•No beam splitter
•One dispersive element
•One FPA
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Using Dual-band FPA
PHYSICS
•
Gratings
–
–
•
2nd order
is MWIR
nλ = d sin θ
1st order is LWIR
Peak efficiencies at
λB, 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
320 cols x 240 rows
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Schematic of Dewar Optics
PHYSICS
Dualband
FPA
grating
Image formed on slit
Only 4 optical components
• Near-collimation (2 mirrors)
• Grating
• Refocusing (“camera” mirror)
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PHYSICS
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%
IR
MWIR Layer
HgCdTe-2
p-type
Courtesy DRS IR
Technologies
n-Type
HgCdTe-1
n-type
Insulated
via
LWIR Layer
p-Type
diode-1
current
ROIC
diode-2
current
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PHYSICS
No FPA is Perfect
LWIR
MWIR
Decreasing Wavelength
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PHYSICS
Wavelength Calibration
0.0078 μm/col
0.0157 μm/col
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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
BB Calibration at MWIR only
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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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Modeling Determination of Space Object Temperatures
PHYSICS
Derived space object temperatures:
404.405
50% visible reflection
Tsol  398.186
50% infrared emissivity
Tsolve  393.561
394 K equilibrium 392.031
lamda*S-lambda, W/cm2
2 10
1.5 10
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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
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8
1 10
5 10
Longer band fixed @ 12 μm
Variation of shorter bands
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0
0
5
10
15
20
Wavelength, microns
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
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PHYSICS
Two Wavebands to determine
BB Temperature
423 K
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PHYSICS
Recovered BB Spectrum
Actual 423 K
Recovered 407-424 K
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PHYSICS
Greater Separation of the Two Wavebands
used to determine BB Temperature
423 K
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PHYSICS
Better Results
Actual 423 K
Recovered 422-427K
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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
Two widely separated wavebands
to determine BB Temperature
423 K
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PHYSICS
Results compromised by
noisy LWIR band
Actual 423 K
Recovered 397-449 K
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PHYSICS
Two more widely separated wavebands
to determine BB Temperature
423 K
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PHYSICS
Good results despite noisy LWIR
Actual 423 K
Recovered 413-423 K
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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
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