Curt Mobley from my summer course lecture

Transcription

1 This is a placeholder for the web book section on polarization Polari zation Curt Mobley from my summer course lecture

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11 from Ken Voss PhD Dissertation

12 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), , 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)

13 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) no polarizer vertical polarizer this is why you use polarizing sunglasses to remove glare from the water surface and let you see into the water better

14 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

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

16 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.

17 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] (= [S o,s 1,S 2 ]) components of the Stokes vector at 443, 670, 865 nm; get unpolarized radiance at other wavelengths POLDER1 on ADEOS1: sat. solar panel failure after 8 months POLDER2 on ADEOS2: sat. solar panel failure after 7 months ==> don t put your instruments on satellites named Good Bye

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

19 from Chami and Platel, 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? R rs % b b /a = [b bw + b bph + b bnc ]/[a w + a ph + a NC ] M 11 M 12 /M 11 Mueller matrix elements M 11 (the VSF) and M 12 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

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

21 from Chami and Platel, 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?

22 from Chami and Platel, 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?

23 from Chami and Platel, 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 b b. Why?

24 from Chami and Platel, JGR 112 some of their conclusions:

25 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 oceanatmosphere 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