Polarization measurements for CLARREO

1. Polarization measurements for CLARREO Brian Cairns, Michael Mishchenko, Andrew Lacis

2. Benefits of polarimetry • Why use polarimetry: • Measure something that is sensitive to the state of the climate. • The variation of polarization with scattering angle and wavelength is sensitive to aerosol particle size, refractive index and shape.

3. Benefits of polarimetry • Polarization is sensitive to the aerosol load over land, even over urban areas (c, Mexico City). • Not only can the aerosol burden be identified, but the spectral and angular signature in the polarized reflectance is sensitive to the complex refractive index (1.54 + i 0.027) and the single scattering albedo (0.865).

4. Benefits of polarimetry It is accurate, multi-spectral, multi-angle radiance and polarization measurements that provide the sensitivity to aerosols over a wide dynamic range.

5. Benefits of polarimetry Polarized observations of clouds are sensitive to the cloud droplet size distribution (rainbow), the cloud top pressure (side scattering in the blue UV) and aerosols above the cloud (side scattering in the red/NIR/SWIR).

6. Benefits of polarimetry • Why use polarimetry: • Make the measurements with sufficient accuracy that the sensitivity can be used. • Polarization is a relative measurement that can be made extremely accurately and stably calibrated on orbit. • Polarimeters can also be highly accurate radiometers and the methods for creating an accurate radiometric standard on orbit are discussed elsewhere at this workshop.

7. Benefits of polarimetry • In the case of the Aerosol Polarimetry Sensor two polarimetric calibrators are used, one providing a source of (very) weakly polarized light and the other providing a source of strongly polarized light (using crystal and wire grid polarizers). • Approach already demonstrated accuracy of better then 0.1% at a single shot level over clouds for the source of zero polarization.

8. Benefits of polarimetry • Why use polarimetry: • Minimize impacts of surface. • Polarized reflectance of land surfaces is grey, shape of polarized BRDF of oceans is similar whether the water is blue or green. • Allows surface and atmosphere to be characterized with minimal contamination of the signal from one in the other.

9. Benefits of polarimetry • Polarized reflectance of land surfaces is grey, with contrast.

10. Benefits of polarimetry • Polarized reflectance of the ocean body also has a weaker and very different dependence on Chlorophyll concentration to that of the reflectance.

11. Benefits of polarimetry • This facilitates the separation of surface and atmospheric effects and the accurate characterization of the surface. BEFLUX is the ground based estimate of the total solar directional hemispheric reflectance at the DoE ARM SGP CF. The RSP estimate of DHR comes from a single snapshot (i.e. instantaneous) while the MODIS processing stream uses sixteen days of data to reduce the effects of aerosols, clouds and increase angular sampling.

12. Benefits of polarimetry • Why use polarimetry: • Measure something that is sensitive to the state of the climate. • The variation of polarization with scattering angle and wavelength is sensitive to aerosol particle size, refractive index and shape. • Make the measurements with sufficient accuracy that the sensitivity can be used. • Polarization is a relative measurement that can be made extremely accurately and stably calibrated on orbit. • Minimize impacts of surface. • Polarized reflectance of land surfaces is grey, shape of polarized BRDF of oceans is similar whether the water is blue or green. • Allows surface and atmosphere to be characterized with minimal contamination of the signal from one in the other.

13. Measuring polarization • How to make polarization measurements: • Sequential measurements pay a high penalty in terms of accuracy.

14. Measuring polarization Er El Detector, Retarder,  Polarizer,  Stokes Vector Degree of Linear Polarization, Angle of Polarization

15. Measuring polarization APS/RSP as an example of how to make polarization measurements: • The differences required to calculate Q and U are differences between orthogonal polarization states, so if we measure these orthogonal states such that they are looking at the same scene at the same time we can effectively eliminate “false” polarization. • This can be done very simply using a Wollaston prism in the collimated beam of a relay telescope. Wollaston prism - splits beam into orthogonal polarizations Objective Field Stop Collimator Dichroics

16. Measuring polarization APS/RSP as an example of how to make polarization measurements: • Using dichroic beam splitters you can make measurements for multiple spectral bands in a single telescope (3 in the case of APS). • Use one telescope for Q and one telescope for U. • If we are measuring a total of 9 bands this means we need 3 telescopes for Q and 3 telescopes for U for a total of 6 telescopes.

17. Measuring polarization APS/RSP as an example of how to make polarization measurements: • Crossed mirrors, if identical, introduce no polarization into the scene polarized radiance and allow the telescope fields of view to be scanned across the earth either across track like MODIS, or along track as is planned for APS. • One polarization experiences an s, then a type p reflection, while the other experiences a p then an s type reflection. Polarization induced by scan mirror assembly of RSP was not measurable <<0.1%. RSP mirror alignment Scanner uses matched mirrors illuminated at 45° with reflection planes at 90° to one another

18. Measuring polarization How to make polarization measurements: • Only the Wollaston prism approach to spectral separation has heritage (RSP, PPR, OCPP) and high accuracy as of October 2008. It is feasible to use a single spectrometer to provide the spectral analysis of orthogonal polarizations. • Spectral encoding has been demonstrated (HySPAR) but the accuracy of the present implementation is at the 1% level. This polarization analysis approach can be part of an imaging spectrometer as longs as the spectrometer has sufficiently high spectral resolution to measure the spectral fringes in which the polarization is encoded. • Rotating polarizers provide limited accuracy but extreme simplicity. • Dual PEMs with polarizers (MSPI) may meet 0.5% DoLP accuracy but has no heritage. • Spectral analysis/detection • Spectrometer (HySPAR) • Bandpass filters (APS, RSP, POLDER, MSPI) • Scene Definition • Camera (MSPI, HySPAR, POLDER) • Relay Telescope and Scanner (APS, RSP) • Polarization separation • Wollaston (RSP, APS) • PEM+Polarizer (MSPI) • Spectral encoding (HySPAR) • Polarizers (POLDER)

19. Measuring polarization • New Tools • Plasmonic/Photonic Hybrid Crystals • Optical effects scale with structural feature sizes, so devices can be designed for every part of the spectrum from the deep UV through the microwave.

20. Measuring polarization • New Tools • Polarizing beam splitter gratings that are integrated with the detectors. • Uses a subwavelength metal grating fabricated directly on the surface of a standard HgCdTe or InSb substrate. Photons of different polarizations are separated by being swept to adjacent but different grooves within the grating. They are then swept through the grooves and into the substrate, where they are detected separately because of their spatial separation. Grating also functions as a wavelength filter, and multiple wavelengths can be selected for if an interpenetrating groove design is used.

21. Summary • Why use polarimetry: • Measure something that is sensitive to the state of the climate. • Measurements can be made with sufficient accuracy that the sensitivity can be used. • Minimize impacts of surface on observations of atmosphere and allow for its effects to be characterized. • Making polarization measurements: • There are aircraft instruments that have demonstrated the capability to make highly accurate polarimetric measurements using Wollaston prisms for the polarization analysis. The APS is expected to demonstrate similar (better) performance on orbit. • Implementations other than the Wollaston prism approach have been used in aircraft instruments (POLDER, HySPAR) with varying degrees of success.