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PROGRESS IN OPTICAL TECHNOLOGY DEVELOPMENT FOR
AEROSOL AND CLOUD REMOTE SENSING WITH THE
MULTIANGLE SPECTROPOLARIMETRIC IMAGER (MSPI)
A. Mahler, R.A. Chipman, S.C. McClain
College of Optical Sciences, University of Arizona, Tucson, AZ 85721,USA
D.J. Diner, A. Davis, and N. Raouf
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
1. INTRODUCTION
The Aerosol-Cloud-Ecosytem (ACE) satellite is one of several mission concepts identified in the recent National Research
Council Earth Sciences Decadal Survey [1]. The scientific objectives of this mission include determining the physical
processes governing the impact of aerosols on cloud albedos, lifetimes, microphysics, precipitation, and human health. One
component of the notional ACE payload is a multiangle, multispectral, high-accuracy polarization imager. As part of
NASA's Instrument Incubator Program (IIP), we have been developing enabling technologies in support of a candidate
instrument, the Multiangle SpectroPolarimetric Imager (MSPI) [2]. MSPI is envisioned to contain multiple cameras pointed
at different viewing angles, so its system architecture is conceptually similar to the Terra Multi-angle Imaging
SpectroRadiometer (MISR) [3]. However, the new camera design extends the spectral range to the ultraviolet (UV) and
shortwave infrared (SWIR), increases the image swath width to achieve more rapid global coverage, and adds polarimetric
imaging in selected spectral bands.
Fusion of multispectral, multiangular, and polarimetric observations is important because each brings sensitivity to different
aspects of particle spatial distributions and microphysics. UV wavelengths are sensitive to aerosol absorption and height,
visible/near-infrared (VNIR) wavelengths to fine mode aerosol size distributions, and SWIR wavelengths to coarse mode
aerosol and cloud particle sizes. Multiangular radiances distinguish particle shapes, and improve sensitivity to optical depth,
notably over bright surfaces. Polarimetry is the only means of determining particle real refractive index, providing
compositional information. Moderately high spatial resolution resolves intercloud scales and aerosol gradients in urban
settings. A broad swath is important for providing environmental context to aerosol and cloud spatial relationships. The
technological challenge of integrating all of these attributes into a single instrument is compounded by the need to acquire
accurate multispectral intensity imagery (3% uncertainty) simultaneously with accurate degree of linear polarization (DOLP)
imagery (0.5% uncertainty).
2. KEY TECHNOLOGIES IN THE OPTICAL SYSTEM
Using passive remote sensing to determine aerosol and cloud microphysical parameters necessitates a variety of constraints
to overcome retrieval non-uniqueness, and places stringent design requirements on a cohesive, next-generation passive
satellite imager. The MSPI camera design uses a reflective optical design with optimized mirror coatings to minimize
instrument-induced polarization, introduces a rapid, time-variable retardance into the optical path using tandem photoelastic
modulators (PEMs), and integrates wire-grid polarizers into the focal plane spectral filter assembly. This presentation will
report on progress in fabrication and testing of a brassboard UV/VNIR MSPI camera. Each of the key optical technology
elements is briefly summarized here.
5.1. Reflective optics
A three-mirror, off-axis optical system [4] has been fabricated to accommodate the dual-PEM modulator and to keep
instrumental polarization <1% over a wide spectral range and field of view (FOV). This is important because the MSPI
cameras must operate simultaneously as (a) broadband intensity imagers in which the observed radiance is minimally
sensitive to the polarization state of the scene and (b) polarization imagers in which instrumental perturbations to the Stokes
vector are minimized. Good optical performance over the wide (±31º) cross-track FOV is achieved using a convex spherical
primary mirror and aspheric secondary and tertiary mirrors. An f/5.6 focal ratio balances throughput requirements and the
need to limit the angle of the cone of light passing through the filters and polarizers mounted above the focal plane line
arrays.
5.2. Mirror coatings
We have developed specialized mirror coatings [5] to meet several requirements. Low diattenuation (difference in reflectance
for different polarization orientations) is necessary so that cameras can operate as high-accuracy intensity imagers in the non-
polarization bands. A system diattenuation goal of <1% has been specified. High reflectivity over a broad spectral range
provides good signal-to-noise ratio. Retardance near half-wave in the polarization bands prevents coupling of circular into
linear polarization. The mirrors have been coated and system verification has recently begun.
5.3. Polarization modulator assembly
DOLP equals (Q/I)2(U/I)2, where I is the total intensity, and Q and U describe linear polarization. In MSPI, both Q
and I are obtained from each detector in a pushbroom line array (similarly for U and I), so the ratios q=Q/I and u=U/I are
independent of system transmittance and detector gain variations. This relative polarimetric imaging approach is key to
achieving high accuracy and is accomplished by modulating the polarization signals using PEMs—low-power modulators in
which piezoelectric transducers induce rapidly varying stress birefringence in fused silica elements. These devices have a
wide angle of acceptance and high transmittance from the UV to SWIR. Two PEMs oscillating at slightly different
frequencies result in a system with a tens of kHz “carrier” waveform modulated at a much slower (25 Hz) difference
frequency. In this manner, one cycle of the modulated waveform occurs within each 40-msec “frame” (the time it takes a
satellite in low Earth orbit to fly 275 m). Within the MSPI cameras, the PEMs are placed next to the system stop, where the
ray bundle from the full FOV is most compact. Two quarter wave plates (QWPs) act in concert with the PEMs to modulate Q
and U.
5.4. Spectropolarimetric filters
An integrated spectral filter/polarizer assembly to be located above the pushbroom focal plane detector array is in fabrication.
The full assembly includes a 1.6-mm thick fused silica substrate, blocking layers, filters, wire-grid polarizers, low-reflectivity
masks, and low-reflectivity barriers between channels. The filters are sliced to the long, narrow geometry of the detector
arrays. Our technology development work has demonstrated the wisdom of patterning the polarizers on a separate substrate
prior to integration with the filters. Detailed measurements show that bonding of the polarizers to the filters causes a loss of
transmittance and polarization contrast ratio, but the use of high-contrast stock results in performance that is quite
satisfactory for MSPI.
3. ACKNOWLEDGMENTS
This research is being carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with
NASA, and at the University of Arizona College of Optical Sciences under subcontract with JPL.
4. REFERENCES
[1] NRC (National Research Council), Committee on Earth Science and Applications from Space, Earth Science and
Applications from Space: National Imperatives for the Next Decade and Beyond, The National Academies Press,
Washington, DC, 437 pp., 2007.
[2] Diner, D.J., A. Davis, B. Hancock, G. Gutt, R.A. Chipman, and B. Cairns, “Dual photoelastic modulator-based
polarimetric imaging concept for aerosol remote sensing,” Appl. Opt. 46, 8428-8445, 2007.
[3] Diner, D.J., J.C. Beckert, T.H. Reilly, C.J. Bruegge, J.E. Conel, R.A. Kahn, J.V. Martonchik, T.P. Ackerman, R. Davies,
S.A.W. Gerstl, H.R. Gordon, J-P. Muller, R.B. Myneni, P.J. Sellers, B. Pinty, and M. Verstraete, “Multi-angle Imaging
SpectroRadiometer (MISR) instrument description and experiment overview,” IEEE Trans. Geosci. Remote Sens. 36, 1072-
1087, 1998.
[4] Mahler, A. et al., “Tolerancing and alignment of a three-mirror off-axis telescope,” Presented at SPIE Optics and
Photonics, San Diego, CA, August 2007.
[5] Mahler, A. et al., “Low polarization optical system design,” Presented at SPIE Optics and Photonics, San Diego, CA,
August 2007.
