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Incorporating Water into Polarimetric Simulations

Oftentimes water can be a dominant signature in a polarimetric image dataset. Incorporation of water into a DIRSIG simulated polarimetric scene can be accomplished a few different ways, namely treating the water as (1) a volumetric medium having both surface and bulk medium optical properties or (2) a surface, reflecting only material described by a micro-facet based BRDF.



This example has three boxes of water demonstrating the differences between the water medium material properties (with a flat and a wavy surface) and the microfacet surface water material. Note that treating water as a medium permits 1st surface reflection and transmission as well as bulk material radiative transfer, whereas the micro-facet BRDF water material only accounts for 1st surface reflected radiance effects.

The water medium material utilizes well defined inherent optical properties of water that are contained within the DIRSIG model (validated here) and is well suited for closed volumetric shapes such as boxes, cylinders and spheres. However other geometries (such as the wavy surface) can be appropriate when they are well bounded by other materials. This water medium material is also appropriate for modeling signatures of underwater objects, but may require more advanced ray tracing setups (such as photon mapping) for scenarios with direct solar illumination.

The micro-facet water surface material included here utilizes complex index of refraction values for water at coarse spectral resolution to accomplish a 1st surface reflection that is radiometrically accurate and the similar to the water medium material. The primary difference is the water medium assumes a perfectly specular, delta function BRDF, whereas the microfacet water surface has a specular reflectance lobe with a finite width and height dictated by the user supplied root-mean-square surface slope. This particular material configuration for water uses an INTERPOLATION_METHOD=0 setting to ignore the supplied emissivity file (which is why it is called dummy.ems) and implement the pBRDF model with a linear spectral interpolation between reference wavelengths for which the BRDF parameters are supplied (in the water.fit file).

A zip archive of this demonstration scene can be downloaded from here.

Comments

Unknown said…
the IEEE link (text is validated "here") is dead
Michael Gartley said…
thanks for noting this, apparently the link required a IEEE subscription to access the PDF. We updated the post to now have a link to the PhD Dissertation documenting the validation efforts.
https://ritdml.rit.edu/handle/1850/12787
Ocean said…
Is there any way to have Fresnel reflection/transmission in addition to a BRDF function that accounts for unresolved wave roughness?
Unknown said…
Ellipsometry is an optical technique for investigating the dielectric properties (complex refractive index or dielectric function) of thin films. Ellipsometry measures the change of polarization upon reflection or transmission and compares it to a model.Take a look at-ellipsometry

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