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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

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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 No FPA is Perfect

MWIRLWIR

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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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

Two Wavebands to determineBB Temperature

423 K

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PHYSICS Recovered BB Spectrum

Actual 423 KRecovered 407-424 K

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PHYSICS

Greater Separation of the Two Wavebandsused to determine BB Temperature

423 K

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PHYSICS Better Results

Actual 423 KRecovered 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 wavebandsto determine BB Temperature

423 K

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PHYSICS

Results compromised bynoisy LWIR band

Actual 423 KRecovered 397-449 K

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PHYSICS

Two more widely separated wavebandsto determine BB Temperature

423 K

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PHYSICS Good results despite noisy LWIR

Actual 423 KRecovered 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