Hyperspectral imager development at Army ... - Physics-Control

Hyperspectral imager development at Army Research Laboratory Neelam Gupta U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783-1197 ABSTRACT Development of robust compact optical imagers that can acquire both spectral and spatial features from a scene of interest is of utmost importance for standoff detection of chemical and biological agents as well as targets and backgrounds. Spectral features arise due to the material properties of objects as a result of the emission, reflection, and absorption of light. Using hyperspectral imaging one can acquire images with narrow spectral bands and take advantage of the characteristic spectral signatures of different materials making up the scene in detection of objects. Traditional hyperspectral imaging systems use gratings and prisms that acquire one-dimensional spectral images and require relative motion of sensor and scene in addition to data processing to form a two-dimensional image cube. There is much interest in developing hyperspectral imagers using tunable filters that acquire a two-dimensional spectral image and build up an image cube as a function of time. At the Army Research Laboratory (ARL), we are developing hyperspectral imagers using a number of novel tunable filter technologies. These include acousto-optic tunable filters (AOTFs) that can provide adaptive no-moving-parts imagers from the UV to the long wave infrared, diffractive optics technology that can provide image cubes either in a single spectral region or simultaneously in different spectral regions using a single moving lens or by using a lenslet array, and micro-electromechanical systems (MEMS)-based Fabry-Perot (FP) tunable etalons to develop miniature sensors that take advantage of the advances in microfabrication and packaging technologies. New materials are being developed to design AOTFs and a full Stokes polarization imager has been developed, diffractive optics lenslet arrays are being explored, and novel FP tunable filters are under fabrication for the development of novel miniature hyperspectral imagers. Here we will brief on all the technologies being developed and present highlights of our research and development efforts. Keywords: Acousto-optic tunable filter, AOTF, two-transducer, UV, VNIR, SWIR, MWIR, LWIR, diffractive optics lens, dual-band, MWIR/LWIR, lenslet array, MEMS Fabry -Perot etalon, hyperspectral, imaging, mercurous bromide

1. INTRODUCTION The Sensors and Electron Devices Directorate at the U.S. Army Research Laboratory (ARL) has been actively carrying on research in the development of hyperspectral imagers based on using a number of different tunable dispersive elements for standoff detection of chemical and biological agents as well as targets and backgrounds. The main emphasis is on the development of robust imagers that are compact in size and are field-portable. Traditional hyperspectral imagers evolved from spectrometer technology and use either wiskbroom or pushbroom approach with a grating(s) or prism(s). Typically a two-dimensional focal plane array (FPA) that is sensitive in the required range of operation is used. To obtain a hyperspectral image cube of a scene of interest, a fair amount of data processing is involved because such an imager obtains one-dimensional images of the scene at all wavelength intervals at a given instant of time and the second spatial dimension is filled by the relative motion of the scene and the imager. Use of a tunable dispersive element in front of an FPA in a camera instead of a grating or prism results in obtaining a hyperspectral image cube in a much simpler manner without any relative motion of the scene and the imager and a two spatial dimensional image in one color is obtained at a given instant of time and the image cube is filled by tuning the output wavelength of the dispersive element. Such dispersive elements can be obtained by using an electronically tunable filter, a Fabry-Perot (FP) etalon, a diffractive optics lens, or a Michelson interferometer. There are two types of tunable filters that are being used in the development of hyperspectral imagers—(i) liquid crystal tunable filters (LCTFs) and (ii) acousto-optic tunable filters (AOTFs). In general LCTFs have much slower response time (50 to 500 ms) as compared to the AOTFs (tens of µs).

Infrared Technology and Applications XXXIV, edited by Bjørn F. Andresen, Gabor F. Fulop, Paul R. Norton, Proc. of SPIE Vol. 6940, 69401P, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.777110

Proc. of SPIE Vol. 6940 69401P-1

LCTF-based hyperspectral imagers operating in the visible and near infrared (IR) regions are now commercially available.1 AOTFs operating in the visible to midwave IR (MWIR) are also commercially available and some work has been done in the development of imagers using commercial components.2 Much progress has been made in FP etalonbased imagers especially in the longwave IR region.3 Similarly, diffractive optics lens based imagers operating in a single wavelength region (visible, MWIR, or LWIR) are also commercially available.4 Michelson interferometer based imaging FTIR spectrometers is also commercially available.5 At ARL, we are using three of the above technologies to develop novel hyperspectral imagers and also to reduce their size and increase performance. We have developed prototype hyperspectral imagers operating in the ultraviolet (UV), visible-near IR (VNIR), shortwave IR (SWIR), MWIR, and LWIR.6–17 In general, an AOTF-based imager operates over one octave in wavelength but we have demonstrated imagers that operate over greater than two octaves in range.18-–20 The most commonly used AOTF crystal is tellurium dioxide (TeO2). We have developed new AOTFs operating in the UV using existing nonlinear optical materials (KDP, MgF2) that were previously not used for this purpose.6–9 We are also developing new materials like Hg2Cl2, Hg2Br2 and Te that can be used over wide spectral range to fabricate an AOTF.21–24 Some of our AOTF-based imagers can also acquire spectropolarimetric images.10,13 We also developed a prototype simultaneous dual-band (MWIR and LWIR) hyperspectral imager that acquires image cubes in MWIR and LWIR regions simultaneously by using a diffractive optics lens with a dual-band FPA and demonstrated its performance in a field test in acquiring bioagent signatures.25–28 Recently, a diffractive lens based LWIR hyperspectral imager using a single band FPA is operating in our laboratory. In these imagers the wavelength is changed by moving a diffractive lens in front of the FPA over a short distance. We are also working on miniaturizing the size of a hyperspectral imager by reducing the size of the dispersive element and optics. Our first approach is the development of an imager that uses a micro-electromechanical systems (MEMS) based diffractive optics lenslet array in front of a dual-band FPA where each lens is for a different wavelength and no motion is required to change the wavelength. Our second approach is based on using a MEMS-based FP etalon with one fixed mirror and the other mirror with a voltage-controlled motion operating in front of an FPA. Recently, we have also used two liquid crystal tunable retarders in front of an AOTF to demonstrate a full Stokes vector spectropolarimeter.29 In this paper we will give a brief overview of our research and present some results that illustrate utility of our imagers in diverse range of applications: (i) detection of a concealed camouflaged target and (ii) standoff detection of a bioagent simulant.

2. APPROACH In figure 1 we show our generic optical design for a hyperspectral imager that uses a tunable dispersive element. As shown in this figure, in this concept of a hyperspectral imager the block representing the optical elements used for light dispersion and imaging on the camera can be (i) a noncollinear AOTF that tunes the transmitted wavelength by changing the applied radio frequency (RF) signal with an imaging lens or (ii) a diffractive optics lens that changes the transmitted wavelength by its motion along its optical axis and forms the image or (iii) a tunable FP etalon with an imaging lens where the transmitted wavelength is changed by changing the etalon spacing by moving one of the mirrors. We are using all these approaches to develop field-portable hyperspectral imagers operating in different regions of the optical spectrum from the UV to the LWIR and we have demonstrated hyperspectral imaging from the UV to the LWIR using some of these imagers. We have also demonstrated spectropolarimetric imaging with our VNIR and SWIR imagers where we collect two orthogonally polarized images at each wavelength. 2.1 AOTF-based hyperspectral imagers We have demonstrated AOTF-based imagers from the UV to the LWIR and a summary of our imagers is listed in table 1.6–17 Each of these imagers uses a different camera as well as a different AOTF cell fabricated in a different birefringent crystal and each has a different spectral resolution. The main consideration in designing such a hyperspectral imager is the signal to noise ratio (S/N) arriving at each pixel of the FPA. To improve the S/N we have tried to use an AOTF with high transmission efficiency as well as with both large linear and angular apertures. We also use a high sensitivity FPA. We used a KDP AOTF with an extended range response silicon (Si) charge coupled device (CCD) camera to cover the UV to the visible region from 220 to 480 nm,9 a TeO2 AOTF with an off-the-shelf Si CCD camera to cover the VNIR


region from 400 to 800 nm,10 a TeO2 AOTF to cover the SWIR region from 900 to 1700 nm with a room temperature indium gallium arsenide (InGaAs) camera,13 another TeO2 AOTF with a liquid nitrogen-cooled indium antimonide

Optics White Light λ(t1)

Hyperspectral Image Cube

λ1

Camera

λ2 Scene

λ3

t1

Light Dispersion and focusing—AOTF+lens or Diffractive lens or Tunable etalon+lens

λ4

t2 y x

t3 t4

t or λ

time

Figure 1: A schematic drawing of a hyperspectral imager concept that uses a tunable dispersive element to obtain the image cube as a function of time.

(InSb) camera to cover the MWIR region from 2 to 4.5 µm, and a thallium arsenic selenide (Tl3AsSe3 or TAS) AOTF with a liquid nitrogen-cooled mercury cadmium telluride (HgCdTe or MCT) camera to cover the LWIR region from 7.8 to 10.5 µm.15,16 Each imager has a suitable optical train for its spectral region of operation. Since an AOTF is a polarization sensitive device, we can obtain polarization signature by using another polarization sensitive device with it. To utilize the polarization property of an AOTF, we use a nematic liquid crystal variable retarder (LCVR) in front of the tunable filter to obtain two orthogonally polarized images at each wavelength (to cover wavelength regions from 400 nm to 4.5 µm) by changing the applied voltage of the LCVR. The operation of each imager and its image acquisition is computer controlled. TABLE 1. ARL AOTF-based hyperspectral imagers

Imager

AOTF crystal

Cell FOV

Spectral Range (µm)

Spectral Resolution (nm)

FPA

UV

1.2

0.22–0.48

1.4 at 0.3 µm

Si CCD room temperature

VNIR

KDP, MgF2, TeO2 TeO2

4.2

0.4–0.9

10 at 0.6 µm

SWIR

TeO2

8.4

0.9–1.7

10.4 at 1.3 µm

MWIR

TeO2

3

2.0–4.5

77 at 3 µm

LWIR

TAS

7.75

7.8.0–10.5

80 at 10 µm

Si CCD room temperature InGaAs room temperature InSb 77 K MCT 77 K

Recently, we have also used two LCVRs in front of an AOTF to demonstrate a full Stokes vector spectropolarimeter.29 We have developed a TeO2 AOTF to obtain high quality imaging down to 360 nm in the UV.6 We have developed MgF2


AOTF to obtain spectral images in the deep UV down to 200 nm.7 We have developed TeO2 AOTFs (0.43–2.12 µm and 0.69–4.0 µm) with two transducers to obtain greater than two octaves coverage and used these AOTFs to obtain spectral images with CCD, InGaAs and InSb cameras.18–20 We have worked with Northrop Grumman Corporation, Brimrose Corporation and Moscow State University, Russia to develop new birefringent materials that cover wide spectral regions and have high birefringence—Hg2Cl2 (0.35–20 µm), Hg2Br2 (0.40–30 µm)21–23 and Te (4.0–20 µm)24. Currently, we are working with Brimrose under an SBIR phase II contract to grow Hg2Br2 crystals to fabricate novel AOTFs operating in the 8–12 µm region.23 We are also working with Physical Sciences Inc. (PSI) under another SBIR phase II to develop a prototype hyperspectral imager using one of these Hg2Br2 AOTFs with an MCT FPA. 2.2 Diffractive optics lens based hyperspectral imager We have two different hyperspectral imagers based on this technology both using a single diffractive optic lens. The first hyperspectral imaging system is a dual-band simultaneous MWIR/LWIR with a dualband MCT FPA and the second one uses a single LWIR MCT FPA and is much more compact. Using a custom design diffractive optics lens we demonstrated a dual-band (MWIR/LWIR) hyperspectral imager that obtains hyperspectral image cubes in both MWIR and LWIR.25–28 An f/1.8 diffractive optics lens with a 75 mm focal lens was designed by Pacific Advance Technology (PAT) Inc. for the center wavelength of 9.0 µm. This Ge lens has special anti-reflection coatings to cover both 4–5.25 µm and 8–10.5 µm regions and was placed in front of a 320×240 dual-band HgCdTe FPA cooled with a Sterling cooler. The dual-band 320×240 MCT FPA was developed by DRS Infrared Technologies, under ARL’s Federated Laboratory program.30 The FPA consists of two layers of MCT with different compositions. The top layer is for 3–5.4 µm radiation and the bottom layer is for 8–10.5 µm radiation. The combined imaging system is f/3. As the lens moves by the motion of a screw in front of the dual-band FPA, it focuses different wavelengths at different points along the optical axis as shown in figure 2 and its technical specifications are listed in table 2. Due to the lens design the depth of spectral focus is quite shallow and the spectral defocus that takes place due to the lens motion makes the image slightly blurred due to some contribution from wavelengths other than the focused one. This blur is removed in the post processing of the image data. Also, since the focal length of the lens changes with wavelength and the lens moves in front of the FPA to form spectral images there is a change in the magnification of the imaged object. This is also corrected in post processing.31 Data collection is quite straightforward and fast. Data for both an MWIR and an LWIR image cube are collected for each scene simultaneously, taking a total time of approximately 5 seconds. Each cube contains 128 different wavelengths for a total of 256 wavelengths. When the data collection for each scene is completed, the cubes from the MWIR and LWIR are combined into a single file and stored on the computer. A photograph of this imager is shown in figure 3. Since this is a hybrid system it is quite cumbersome (it requires one desk top computer, one laptop computer, two monitors, a power supply, an electronic controller to run the FPA, camera with lens, and a lens controller) and can be made much more compact if packaged in a manner similar to other imagers from PAT.32 We used this prototype imager at the Dugway Proving Ground (DPG), UT for a biological standoff detection field test held in June 2005 to evaluate the capability of this imager for biological standoff detection. A single LWIR spectral region diffractive optics lens based hyperspectral imager called Warlock was developed under an STTR program by PAT and uses a 320×256 MCT FPA by DRS Infrared Technologies with a Ge diffractive optics lens. The wavelength range of operation is 8–10.5 µm and the f/# is 2.38. The FPA is cooled by a Sterling cooler. The lens operates similar to what is shown in figure 2 except now we only use the first order diffraction from the lens. Detailed specifications of this imager are listed in table 3 and a photograph of the imager is shown in figure 4. The entire imaging system is packed very compactly including a Power PC. It is run by a notebook computer. The image acquisition from this imager is very fast and the image processing is done in similar manner as described above. 2.3 Fabry-Perot etalon-based hyperspectral imager A hyperspectral imager called AIRIS operating in LWIR has been developed by PSI using a large aperture FP etalon with bulk optical components and it has been used to collect field test data over past few years.33 We are working on miniaturizing such an imager using MEMS technology. Under a congressional interest program we are working with the Infotonics Technology Center, NY to develop MEMS-based FP etalons using a number of different materials including quartz, Si, etc. A VNIR etalon will be tested in near future and after that SWIR, MWIR and LWIR etalons will be


developed. These will be quite small in size and each can be combined with an FPA and packaged compactly to develop low power, lightweight handheld hyperspectral imagers which will be useful in micro-UAVs/UGVs, unattended microsensor networks, individual soldiers, etc.

10 µm and 5 µm

8 µm and 4 µm

9 µm and 4.5 µm Dual-band FPA

MWIR ROIC LWIR

Figure 2. Diffractive-optics lens acts both as an imaging and a dispersive element and its operation for three wavelengths in the LWIR and the MWIR range is shown.

TABLE 2. Diffractive optics-based dual-band hyperspectral imager specifications

Parameter

Value

Focal Length

75 mm @ 9 µm

Diameter

41.7 mm

f/#

3

FPA

MCT

LWIR coverage

8 – 10.5 µm

MWIR coverage

4 – 5.25 µm

Spectral resolution

0.1 µm @ 9 µm

Number of bands in each cube

128

Spatial resolution

320×240

Cooling

Closed cycle Sterling cooler

Voltage requirements

120 V

Weight

~ 50 lbs (camera only)

Collection time/scene

~ 5 seconds


\

Figure 3. A photograph of the simultaneous dual-band hyperspectral imaging system showing the camera, electronic controller, one of the two computers, one of the two monitors and power supply. Another monitor, laptop and the lens controller are inside the trailer.

Figure 4. A photograph of the Warlock hyperspectral imager. It contains a Power PC inside and requires a laptop to run it.

2.4 Diffractive optics lenslet array-based hyperspectral imager As discussed in section 2.2 we have demonstrated a simultaneously dual-band hyperspectral imager using a specially designed bulk diffractive lens in Ge and used it with a dual-band MCT FPA. We are now working with PAT under an SBIR phase II program to develop an imager using a MEMS based 3×3 lenslet array with a dual-band FPA. Each of the nine lenses will operate at a specific wavelength. In such an imager relay optics is used to image the whole scene at nine different wavelengths simultaneously. There is no lens motion required and no magnification correction needs to be carried out during data processing. In other words, this is a no-moving-parts hyperspectral imager that can be small and


light-weight and the imager sensitivity can be significantly enhanced by cooling the lenslet array along with the FPA. The major disadvantage is that the spatially resolution is reduced by a factor equal to the number of lenses in the array. Table 3. Specifications of Warlock imager

Parameters

Value

Focal length

70 mm @ 8 µm

FOV

10.48°×10.48°

f/#

2.38

Spectral range

8.0–10.5 µm

Spectral resolution

0.034 µm at 9 µm

Spatial resolution

320×256

FPA

MCT

Weight

15 lbs

Size

12”×7”×8”

Image acquisition rate

120 frames/s

Cooling

Closed cycle Sterling cooler

Voltage requirement

90–265 VAC Can also run with a 12 V battery

3. IMAGING EXPERIMENTS AND RESULTS We have characterized and demonstrated the performance of the above discussed hyperspectral imagers. Here we discuss an outdoor imaging experiment carried out with the AOTF-based VNIR imager as well as an experiment to detect bioagents using the simultaneous dual-band imager in a field test. 3.1 Imaging with VNIR AOTF imager We used the VNIR AOTF imager10 to image a blue car concealed under a camouflage net on a bright summer day. In figure 5 we show two sample spectral images obtained at two different wavelengths—625 nm and 750 nm each with both horizontal and vertical polarizations. We selected to show these two wavelength results because we can see that the camouflage net works well for one of these wavelengths, i.e., 750 nm but is not effective at 625 nm but when we combine polarization detection with hyperspectral detection, the use of the net is not effective at either of these wavelengths. It is clear from these images that at 625 nm we can sort of see the car at both polarizations and it becomes more evident in the polarization difference image. On the other hand at 750 nm we cannot see the car with either polarization and it can be seen only in the polarization difference image. From these results it is clear that spectropolarimetric imaging is a better tool to detect manmade objects hidden under a camouflage net than just hyperspectral imaging.


Car revealed

Car+net625 nm-H

Car+net625 nm-V

Car+net625 nm-pol.diff.

Car revealed D

-

--

.4_,,-;_ - ,e

-

-L

Car+net750 nm-H

Car+net750 nm-V

— -r-. -

r..'- .T

Car+net750 nm-pol.diff.

Figure 5. Spectral and polarization images of a blue car hidden under a camouflage net obtained with our VNIR AOTF imager on a summer day. Car can be clearly seen in the polarization difference image in both sets of images, while it can be sort of seen in 625nm images but not in 750-nm images.

3.2 Field test measurements of bio aerosols We took measurements of absorption by bio aerosols in the Standoff Ambient Breeze Tunnel (sABT) at Dugway Proving Ground, UT during a field test in June 2005 using our simultaneous dual-band hyperspectral imager. Our imager was placed inside the tunnel. A blackbody at 40 C was located at the outside end of the tunnel and bioagents were dispersed inside the tunnel. The distance between the imager and blackbody was 99 m. All field tests were carried out at night. The ambient temperature of the bio aerosol was cooler than the blackbody and our imager detected absorption of light that propagated through the cold aerosol. Before the scenes were imaged, non-uniformity correction for the camera was done. As described in section 2.2, after each measurement the two spectral images corresponding to the MWIR (i.e., 4 µm) and the LWIR (i.e., 8 µm) were stored as a double frame forming a 640×240×128 image cube. During the image processing first the double frame image cube was broken into two cubes. The size of each spectral image was 320×240 in each of these two image cubes. The image size was cropped to 280×210 to eliminate the noisy edge pixels due to mismatch in the field of views of the lens and the cold stop in the camera. Next a spectral deconvolution was performed for each cropped image cube to remove the contributions of out of band radiation. After this a magnification correction was carried out for each cube to account for the motion of the diffractive lens along the optical axis.31 Spectral analyses of each cube was carried out by using PAT software to obtain the absorption spectrum of the bio agent. We show some representative spectra of BG obtained in the tunnel in figure 6. It is important to note that our instrument measured BG absorption spectrum both in the LWIR and MWIR regions. It should also be pointed out that the wavelength scales in these graphs are not correct due to some calibration problems with the lens. The problem with lens calibration has subsequently been fixed by modifying the lens holder.


0

0 1800

800 900 1000 1100 1200 1300 -50

2000

LWIR -100

2400

MWIR -200 Counts

Counts

2200

-100

-150

-300

-200

-400

-250

-500

-300

-600 Wavenumber (cm-1)

-350 Wavenumber (cm-1)

Figure 6. Absorption spectra of BG in LWIR and MWIR bands obtained from the tunnel experiment.

4. CONCLUSIONS ARL has developed a number of hyperspectral imagers using tunable dispersive elements, particularly AOTFs and diffractive optics lens with two-dimensional FPAs to demonstrate hyperspectral imaging from the UV to the LWIR. Such hyperspectral imagers can be used for the standoff detection of chemical and bio agents as well as detection of targets and backgrounds. We have found that spectropolarimetric imaging is a more powerful tool in finding manmade objects even when they are hidden under a camouflage net or foliage. By doing hyperspectral imaging simultaneously at two different spectral regions we can get more spectroscopic information than doing it only in one spectral region. We would like to package our dual band hyperspectral imager more compactly to make it more portable and easier to use. We are growing Hg2Br2 crystals that can be used to fabricate AOTF cells from 0.4 to 30 µm. Miniaturized imagers using MEMS based FP etalons and diffractive optics lenslet array technologies are being developed for application in microUAVs/UGVs, unattended microsensor networks, individual soldiers, etc.

REFERENCES [1] www.chemimage.com [2] www.brimrose.com [3] http://www.patinc.com/products/index.htm [4] http://www.psicorp.com/products/airis.shtml [5] http://www.telops.com/ [6] Gupta, N.and Voloshinov, V. B., “Hyperspectral imaging performance of a TeO2 imaging acousto-optic tunable filter in the ultraviolet region,” Opt. Lett. 30, 985–987 (2005). [7] Voloshinov, V. B. and Gupta, N., “Investigation of magnesium fluoride crystals for imaging acousto-optic tunable filter applications,” Appl. Opt. 45, 3127–3135 (2006). [8] Voloshinov, V.and Gupta, N., “Ultraviolet/Visible Imaging Acousto-Optic Tunable Filters in KDP,” Appl. Opt. 43, 3901–3909 ( 2004). [9] Gupta, N. and Voloshinov, V., “Hyperspectral Imager from Ultraviolet to Visible Using KDP AOTF,” Appl. Opt. 43, 2752-2759 (2004).


[10] Gupta, N., Dahmani, R. and Choy, S., “Acousto-optic tunable filter based visible-to-near-infrared spectropolarimetric imager,” Opt. Eng. 41, 1033–1038 (2002). [11] Suhre, D. R. and Gupta, N., “Acousto-optic tunable filter sidelobe analysis and reduction using telecentric confocal optics,” Appl. Opt. 44, 5797-5801 (2005). [12] Suhre, D. R., Denes, L. J., and Gupta, N. ,“Telecentric confocal optics foraberration correction of Acousto-optic tunable filters,” Appl. Opt. 43, 1255 (2004). [13] Gupta, N., Dahmani, R., Bennett, K., Simizu, S., Suhre, D. R., and Singh, N. B., “Progress in AOTF Hyperspectral Imagers,” Proc. SPIE 4054, 30–38 (2000). [14] Voloshinov V. B. and Gupta, N., “Acousto-optic imaging in the mid-infrared region of the spectrum,” Proc. SPIE 3900, 62–73 (1999). [15] Gupta, N., “Acousto-optic tunable filters for Infrared Imaging,” Proc SPIE 5953, 59530O 1–10 (2005). [16] Gupta, N., Suhre, D. R., and Gottlieb, M., “LWIR spectral imager with an 8-cm-1 passband acousto-optic tunable filter,” Opt. Eng. 44, 094601 1–7 (2005). [17] Singh, N. B., Suhre, D., Gupta, N., Rosch, W. and Gottlieb, M.,“Performance of TAS crystal for AOTF Imaging,” Jour. Crystal Growth 225, 124–128 (2001). [18] Gupta, N., “Development of Agile Wide Spectral Range Hyperspectral/Polarization Imagers,” Technical Digest CLEO/QELS/PhAST, PThA3 (2005). [19] Gupta, N., “Hyperspectral and Polarization Imaging with Double-Transducer AOTF for Wide Spectral Band Coverage,” IJHSES (International Journal of High Speed Electronics and Systems), vol. 17, no. 4, 845–855 (2007). [20] Gupta, N. and Voloshinov, V. B., “Development and Characterization of Two-Transducer Imaging Acousto-Optic Tunable Filters with Extended Tuning Range,” Appl. Opt. 46, 1081-1088 (2007). [21] Knuteson, D. J., Singh, N. B. Gupta, N., Gottlieb, M., Suhre, D., Berghmans, A., Thomson, D., Kahler, D., Wagner, B., Hawkins, J., and Fitelson, M., “Design and fabrication of mercurous bromide acousto-optic tunable filters,” Proc. SPIE 5881, 58810E 1–8 (2005). [22] Knuteson, D. J., Singh, N. B. Gupta, N., Gottlieb, M., Suhre, D., Berghmans, A., Kahler, D., Wagner, B., Lears, C., and Hawkins, J. J., “Performance of crystals; operational characteristics of mercurous bromide crystals for acousto-optic applications,” Proc. XIII International Workshop on Physics of Semiconductor Devices, Section H, Vol. II, 1184–1189 (2005). [23] Kim, J., Trivedi, S. B., Soos, J., Palosz, W., and Gupta, N.,”Development of Mercurous Halide Crystals for Acousto-Optic Devices,” Proc. SPIE 6661, 66610B 1–12, 2007. [24] Voloshinov, V. B., Balakshy, V. I., Kulakova, L. A. and Gupta, N., “Acousto-optic properties of tellurium useful in anisotropic diffraction,” paper under preparation (2008). [25] Hinnrichs, M., Gupta, N., and Goldberg, A., “Dual-band (MWIR/LWIR) Hyperspectral Imager,” Proc. of 32nd AIPR Workshop, 73–78 (2003). [26] Smith, D. J., and Gupta, N., “Data collection with a dual-band infrared hyperspectral imager,” Proc. SPIE 5881, 588106-1–11 (2005). [27] Gupta, N. and Smith, D., “A simultaneous dual-band infrared hyperspectral imager for standoff detection,” Proc. SPIE 5995, 59950L 1–10 (2006). [28] Gupta, N. and Smith, D., “A field-portable simultaneous dual-band infrared hyperspectral imager,” Proc. 2005 AIPR Workshop 87–92 (2006). [29] Gupta, N. and Suhre, D. R., “AOTF imaging spectrometer with full Stokes polarimetric capability,” Appl. Opt. 46, 2632–2037 (2007). [30] Goldberg, A. C., Kennerly, S. W., Little, J. W., Shafer T. A, Mears, C. L., Schaake, H. F., Winn, M., Taylor, M., and Uppal, P. N., “Comparison of HgCdTe and quantum-well infrared photodetector dual-band focal plane arrays,” Opt. Eng. 42, 30–46 (2003). [31] HyPAT II User’s Manual, version 4.04 , Pacific Advance Technology, Buellton, CA. (2003). [32] http://www.patinc.com/products/index.htm [33] Marinelli, W. J., Gittins, C. M., Gelb, A. H., Green, B. D., “A Tunable Fabry-Perot Etalon-Based LongWavelength Infrared Imaging Spectroradiometer,” Appl. Opt. 38, 2594–2604 (2000).
