We now take a closer look at light and find out that I haven’t been telling you the whole truth

1. Polari zation We now take a closer look at light and find out that I haven’t been telling you the whole truth Curt Mobley Maine 2007

2. Questions from Lab 1: • 1) Why did putting plastic in between the polarizing filters let more light through? Why not the same effect with glass? • 2) Where did the scattered light go when the laser was rotated by 90 deg? • 3) Why was the flashlight beam polarized for scattering at 90 deg but not for forward scattering? • Answering these questions requires a more exact description of light. (Understanding these phenomena requires a lot of physics.) • What is this stuff good for in optical oceanography and remote sensing?

3. Other reasons to study polarization: From the IOCCG: Position: 3-year PhD Position funded by CNES (French Spatial Agency) Location: ELICO, Wimereux, France Date Posted: 26 February 2007 Description: Characterisation of the oceanic particulate matter from in situ and satellite (POLDER) measurements of the polarized light field. Many studies based on field measurements as well as on theoretical or radiative transfer calculations highlighted that the polarization of the underwater light field is very sensitive to particle microphysics. However, whereas the polarized light field is now extensively used in aerosol (Deuzé et al, 2001) or cloud (Goloub et al, 2000) remote sensing studies, it has rarely been exploited from space borne observations of ocean color…. From Applied Optics (2005): Moderate-Resolution Imaging Spectroradiometer ocean color polarization correction Gerhard Meister, Ewa J. Kwiatkowska, Bryan A. Franz, Frederick S. Patt, Gene C. Feldman, and Charles R. McClain The polarization correction for the Moderate-Resolution Imaging Spectroradiometer (MODIS) instruments on the Terra and Aqua satellites is described. <snip> The effect on the water-leaving radiances can exceed 50%.

8. Q1: Why did plastic let more light through?

10. Q3: Why was projector light polarized at 90 deg, but not at 0?

12. Q2: Where did the laser light go when the laser was rotated?

13. from Ken Voss’ PhD Dissertation

14. Fun with Polarization (1) Using polarization to improve visibility in a passive imaging system Physics: light scattered by small particles is partially polarized at 90 deg scattering angles from Schechner et al, AO 42(3), 511-525, 2003. this is why you use a polarizing filter to improve your photographs of distant mountains, or to darken the sky (at right angles to the sun)

15. vertical polarizer no polarizer Fun with Polarization (2) Using polarization to improve remove reflections from water or glass Physics: light reflected by dielectrics (water, glass, etc., but not metals) is partially horizontally plane polarized (totally polarized at the Brewster angle) this is why you use polarizing sunglasses to remove glare from the water surface and let you see into the water better

16. Fun with Polarization (3) Using circular polarization to improve visibility in an active imaging system Physics: circularly polarized light changes handedness (R to L or L to R) whenever the light is reflected use of circular polarization increased the visibility range from 4 to 8 attenuation lengths from Gilbert and Pernika, Ocean Optics, vol. III

17. Fun with Polarization (4) Using circular polarization to identify scarab beetles from George Kattawar note: only scarab beetles do this!

18. Fun with Polarization (5) The French POLDER (POLarization and Directionality of the Earth’s Reflectance) sensors on the Japanese ADEOS satellites used polarization to reduce atmospheric path radiance and sea surface reflectance. polarization was used for land and atmosphere retrievals, not for ocean color, but as we’ll see….

19. POLDER • wide field of view optics looks at multiple spots on the earth’s surface, thus can look at the same spot from up to 14 different viewing directions as the satellite passes overhead (samples the VSF at different scattering angles) • linear polarizers measure [I,Q,U] (= [So,S1,S2]) components of the Stokes vector at 443, 670, 865 nm; get unpolarized radiance at other wavelengths POLDER1 on ADEOS1: 1996--sat. solar panel failure after 8 months POLDER2 on ADEOS2: 2003--sat. solar panel failure after 7 months ==> don’t put your instruments on satellites named “Good Bye”

20. RGB of total radiance RGB of polarized radiance (RGB) = (865,670,443 nm) Brewster angle or backscatter direction sun glint POLDER Example image Note that most of the polarized radiance seen by the satellite is at blue wavelengths because of high polarization by atmospheric (Rayleigh) scattering

21. M11 M12/M11 from Chami and Platel, 2007. JGR 112 • How much improvement can be obtained in IOP retrievals if directional and polarization information is available (as in POLDER)? • Can algal and non-algal contributions to IOPs be separated? Rrs bb/a = [bbw + bbph + bbNC]/[aw + aph + aNC] Mueller matrix elements M11 (the VSF) and M12 show differences for different particle types (indices of refraction) and size distributions. Implies that polarization and multi-angle views can help separate biological and mineral particles

22. from Chami and Platel, 2007. JGR 112 • used a polarized ocean RT model to generate ~10,000 polarized Rrs for various IOPs and viewing directions • trained a NN using half of the Rrs spectra • validated retrievals using the other half of the spectra • 3 versions of simulated retrievals: • Case S (Standard): used only nadir-viewing unpolarized Rrs for retrievals • Case M (Multidirectional): used unpolarized Rrs but at multiple viewing directions • Case MP (Multidirectional and Polarized): used polarized Rrs and multiple viewing angles

23. from Chami and Platel, 2007. JGR 112 • for retrievals of total absorption, adding multiple viewing directions improved the retrievals. Why? • adding polarization did not improve absorption retrievals by much. Why not?

24. from Chami and Platel, 2007. JGR 112 • for retrievals of total backscatter, adding multiple viewing directions improves the retrievals, and adding polarization cuts the error almost in half again. Why?

25. from Chami and Platel, 2007. JGR 112 • can always retrieve total absorption better than can retrieve the individual components (phyto vs non-phyto). Ditto for total vs component backscatter and scattering coefs. Why? • the errors in retrieval of total scattering coef b always higher than the errors in retrieval of backscatter coef bb. Why?

26. from Chami and Platel, 2007. JGR 112 some of their conclusions:

27. Well then, if polarization is so useful, why isn’t there a polarized version of HydroLight? • people/funding agencies haven’t been asking for it • would require a major rewrite of the code, for which I don’t have the time or funding • really should develop a coupled ocean-atmosphere RT code with polarization--a really big project (for example, MODTRAN, the atmospheric equivalent of HydroLight, does not include polarization) • even if we had these codes today, we don’t have the IOP inputs needed to run them: people rarely measure even the VSF, and they *never* measure the full Mueller scattering matrix or Stokes vector radiance in the ocean

28. Florideans inspecting a tidewater glacier in SE Greenland