MSPI: The Multiangle Spectro-Polarimetric Imager

The Multiangle SpectroPolarimetric Imager (MSPI) is a new type of polarimetric camera jointly designed and fabricated by the University of Arizona and the Jet Propulsion Laboratory. MSPI is well-suited for a variety of Stokes imaging applications from the UV through the Visible, NIR, and SWIR which demand polarimetric precision of better than 0.5% at time scales between DC and 25 Hz. The telescope is a three-mirror off-axis anastigmat, and images are built up through a pushbroom scan. Polarization modulation is provided by a novel implementation of dual photoelastic modulators set between quarterwave plates. The polarization is analyzed by an integrated assembly of rows of linear polarizers and dielectric filters applied directly above the focal plane array.

Two versions of MSPI are currently in operation. GroundMSPI is a portable ground-based instrument operating at the University of Arizona’s College of Optical Sciences. AirMSPI, a similar instrument with modifications suitable for aircraft flights, operates at JPL. AirMSPI has made two successful ER-2 flights, and more are planned. Both GroundMSPI and AirMSPI operate from the UV to the near IR, with polarimetric channels at 470 nm, 660 nm, and 865 nm and additional intensity-only channels at 355 nm, 380 nm, 445 nm, 555 nm, and 935 nm. Both instruments are fully operational, providing polarimetric images typically at 1536 x 5097 pixel resolution. A third instrument is under construction that extends the wavelength range to 2.1 microns.

Polarized _UA_1
Panoramic set of four cropped MSPI polarization images of the University of Arizona south horizon and sky acquired by Ground MSPI. Polarization orientation is encoded in hue, and degree of linear polarization is encoded in saturation.

Description and Specifications

The three mirror anastigmat images a ±15° field onto a silicon CMOS focal plane containing thirteen lines of 1536 pixels of 10 micron pitch. The GroundMSPI instrument is housed in a rotating cylinder. Low polarization coatings were optimized for the mirrors, and the system was fully polarization ray traced. Dielectric filters select eight wavelength bands, three of which are covered by horizontal and 45° wire grid polarizers to form polarimetric bands. At each polarimetric pixel, the intensity modulation produced by the photoelastic modulators, quarter wave plates, and polarizer has the form of a Bessel function whose parameters depend on incident Stokes vector components I and Q for the horizontal polarizer, and I and U for the 45° polarizer. Because I and Q (or I and U) are obtained simultaneously from a single pixel, the polarimeter does not suffer from the same artifacts as a conventional division of aperture or division of time polarimeter. In particular, edge artifacts prevalent in many polarimeter images are absent with MSPI.

MSPI schematic: Three curved mirrors form an off-axis astigmat. The large blocks are photoelastic modulators placed between achromatic quarter wave retarders. Filters and polarizers are integrated on the focal plane assembly.
 AOLP, DOLP, Intensity (left to right)


MSPI and Climate Study

MSPI has received over $7M in NASA instrument incubator funding in support of critical instrumentation for aerosol science. Climate scientists have highlighted multispectral polarimetry as a key capability for retrieving aerosol parameters in the atmosphere. The NAS Decadal Survey of 2007 identified an aerosol-cloud-ecosystem (ACE) mission for obtaining polarimetric data from an orbiting platform. In the Earth’s overall energy balance equation (radiative forcing), the uncertainties in the aerosol contribution are larger than the rest of the contributors combined, making this a particularly important area of study. The MSPI prototype development has been supported in order to explore a mission mapping the Earth from a sun-synchronous Low Earth Orbit with a multi-angle aerosol polarimeter utilizing seven MSPI cameras arrayed at different angles.

Other Applications

MSPI is available for future fieldwork and collaborative imaging studies. A partial list of topics for which spectropolarimetric measurements are relevant includes:

  • Aerosol and atmospheric studies
  • Test bed for surface scattering studies
  • Test bed for object identification studies
  • Classification of objects and estimation of cardinal orientation
  • Dependence of polarization/scattering on wavelength and angle
  • Imaging through haze
  • Imaging through foliage
  • Imaging through water
  • Identification of sub-resolution objects due to anomalous polarization signatures
  • Polarimetric scattering models of natural vs. manmade scenes
Angel Lee, from Cheyenne Eagle Butte High School, collects MSPI images as part of a Research Experience for Teachers (RET) program.