Accurate aerosol polarization measurements

At SRON, together with partners from knowledge institutes and industry, we have developed the SPEX family of instruments for aerosol remote sensing comprising high-accuracy multi-angle, multi-spectral polarimeters. All instruments employ the technique of spectral modulation, invented in the group of Prof. Christoph Keller (Leiden University), to determine the state of linear polarization.

Smart technology The development of SPEX-modulation technology will make it possible to measure polarization with an unprecedented accuracy. This opens up the way for detailed characterization of aerosol properties, allowing e.g. to discriminate between various man-made and natural aerosol types. This makes SPEX an enabling technology for breakthrough in science for society.

Industrialization SRON, Airbus Defence & Space Netherlands (ADS-NL), and TNO have devised a space instrument SPEXlite by combining the SPEX polarization modulation optics and a miniaturized yet highly performing spectrometer designed by TNO and derived from TROPOMI. SPEXlite is a full performance-wide swath instrument, where each viewing angle has its own opto-mechanical-detector unit.

Next to SPEXlite the SPEXone CubeSat-type instrument concept has been designed, having the same performance with a narrower 100 km swath. We apply a novel multi-viewing-angle telescope design, where the images of all viewing angles are projected on a single detector. The combination of the new designs of SPEXlite and SPEXone and the heritage of Dutch knowledge institutes and industry on atmospheric instruments allow for fast industrial production.


Figure 1. Light paths through the SPEX optical subsystem.
Figure 2. SPEX polarization pre-optics. Top: schematics. Bottom: mechanical design.
Figure 3. Principle of Spectral Modulation.
Figure 4. SPEX airborne payload as configured for the ER-2 aircraft.
Figure 5. In-flight SPEX spectra over two particular scenes. Top plot is over ocean, bottom plot over forest. Each plot shows in red and green two modulation spectra, from which the spectral radiance is obtained by taken the sum which is shown in black. The blue line is the degree of linear polarization derived from the depth of the modulations. Over ocean, light is strongly polarized due to Fresnel reflection onto the water surface. Over forest, the depth of the modulations is much smaller, and the radiance displays a prominent rise above 750nm. The Oxygen-A absorption band near 760 nm is prominent in both spectra.

SPEX prototype

Each member of the SPEX family of multi-angle spectropolarimeters has the following optical functions (see Figure 1): spectral modulation, multi-angle imaging, and spectrophotometry. To fulfill these functions, together with Dutch partners, we have built the SPEX prototype instrument that forms the optical core of SPEX airborne.

In SPEX, spectral radiance and state of linear polarization are measured in the visual band of 400-800nm as a continuous function of wavelength. Spectral resolution is 3 nm for radiance, and 10-20 nm for polarization. The instrument has nine fixed viewports, separated by 14 degrees, covering an angular range from -56 to +56 degrees along the ground track. Each viewport has a moderate swath of 7 degrees in the across-track direction, with a projected field of view of 2.4 km at nadir to 4.5 km at fore and aft viewports when applied in SPEX airborne on the ER-2 (see below). Conceptually, the instrument acts as nine separate pushbroom spectrometers, which produce nine overlapping strips of data on the ground. In this way, a multi-angular view is obtained of ground scenes when the aircraft flies over it.

SPEX employs the method of spectral modulation (addressed below) to measure the degree of linear polarization (DoLP) of sunlight scattered by the Earth’s atmosphere and the particles therein. The degree of linear polarization is encoded onto the spectrum in the form of sinusoidal variation of the intensity with wavelength. One of the advantages of this method is that it provides a measurement of the spectral radiance and polarization with a single measurement. This feature is essential in obtaining high-accuracy polarization performance. The instrument is entirely passive – there are no moving parts.

Optical design
The SPEX optical subsystem consists of nine identical pre-slit optical elements – the polarization encoders (see Figure 2) - and a spectrometer. The interface between the two is an array of slits. The nine pre-slit polarization encoders are responsible for encoding the linear degree of polarization of the incoming light. Light passes the polarization encoder optics first, ensuring that no instrument polarization corrupts the signal. Two modulated beams exiting each of the nine polarization encoders are focused onto a straight array of eighteen slits. The slit-array is the entrance to the spectrometer. In the spectrometer, light is collimated via a spherical mirror, then folded via a flat mirror, and spectrally dispersed by a holographic grating. Light is subsequently brought into re-focus by a set of nine lenses onto a 2k x 2k CCD detector. Hence a single detector is used for recording modulations of nine viewports.

Spectral Modulation
SPEX' spectropolarimetry works by the principle of spectral modulation (see the references at the bottom of this page). The degree and angle of linear polarization of the incoming light are encoded in a sinusoidal modulation of the flux spectrum (see Figures 2 and 3). Spectral modulation is achieved by three optical elements in series.

Light first passes a quarter wave retarder (QWR). The next element is an achromatic Multiple Order Retarder (MOR), and the final component of the polarization optical subsystem is a Polarizing Beam Splitter (PBS). Light passing through the MOR undergoes a wavelength-dependent phase. The PBS acts as a linear polarizer; the projection of the state of polarization onto the polarizer axis determines the intensity of the light passing through. The net effect is a sinusoidal modulation of the intensity spectrum, where the amplitude scales with the degree of linear polarization (DoLP), and the phase determines the angle of linear polarization (AoLP). The PBS splits light in two beams with orthogonal polarization, whereby the modulations are precisely in anti-phase. The function of the QWR is to change linearly polarized light with polarization axis along the optical axes of the MOR into circularly polarized light. The MOR is a composite of two slabs of birefringent crystals: MgF2 and Quartz. The combined retarder has a retardance of ~23m, resulting in a modulation period of 7nm at 400nm, and 20nm at 800nm. The thickness ratio of the two components is such that the composite is independent of temperature to first order. DoLP and AoLP are derived by determining the depth and phase of the modulation spectrum, while the spectral radiance is obtained from the sum of the modulation spectra. Hence the state of polarization and spectral radiance are derived from a single measurement. This is an extremely powerful advantage, unique to SPEX, and essential in obtaining the required high-accuracy polarimetry.

SPEX airborne

SPEX airborne has been designed and built to enable SPEX measurements from an aircraft. The basis of the opto-mechanical system of SPEX airborne is formed by the SPEX prototype.

SPEX-airborne objectives
SPEX airborne participates in campaigns with the main scientific objective to better quantify the impact of aerosol on climate and air quality. For that purpose scientific research themes like aerosol-cloud interactions and the development of experimental retrieval algorithms to determine aerosol layer height are addressed. The SPEX-airborne instrument is designed with the objective to retrieve aerosol properties such as Aerosol Optical Thickness (AOT), Single Scattering Albedo (SSA), and mean particle sizes, and refractive index are determined from recorded multi-angle, spectral radiance and polarimetric data. SPEX airborne will also be used in European campaigns to quantify the impact of aerosol on the light path in very high accuracy retrievals of greenhouse gases, in particular CO2.

With SPEX airborne on the NASA ER-2, the objectives are to:

  • demonstrate end-to-end instrument performance using spectral modulation up to and including the retrieval of detailed aerosol microphysical and optical properties, as a stepping stone to a space-based spectropolarimeter for global aerosol observation and characterization with unprecedented accuracy
  • participate in scientific multi-instrument aerosol campaigns, including other polarimeter instruments, like the Research Scanning Polarimeter (NASA-GISS), airMSPI(NASA-JPL) and PACS (UMBC), and aerosol and cloud lidars
  • perform instrument cross-validation, including polarimetry data and derived data products
  • improve aerosol data products.

SPEX-airborne configuration for the NASA ER-2
SPEX airborne comprises the optical subsystem SPEX plus camera, two electronic boxes, and heaters (see Figure 4). The Instrument Power Unit (IPU) is the electrical and digital interface to the aircraft platform. It distributes power from the platform over the payload’s subsystems, and channels aircraft-navigation data to the Instrument Control Unit (ICU). The ICU handles extraction and storage of camera images, collects, monitors, and stores housekeeping and navigation data, and handles exceptions. The optical subsystem SPEX is mounted onto a mechanical structure which allows alignment relative to the aircraft. The ICU plus IPU together drive a thermal control loop designed to maintain SPEX at a constant temperature. From calibration measurements performed at JPL facilities it is found that SPEX airborne has a DoLP error of less than 0.2% over the entire wavelength range A typical flight result is shown in Figure 5.