The polarization of light is finding its way into the awareness of the ocean optics community, and this Applied Optics article is a great contribution in this direction – this is really applied optics!

The fundamental data product that characterizes ocean color is the so-called spectral water-leaving radiance, L_w. This contribution is due to sunlight that enters the ocean surface and then leaves it again following scattering by molecules and particles in the water. From knowledge of L_w, water constituents such as the chlorophyll concentration can be estimated. At sea, L_w is determined from measurements of the upwelling radiance from the sea surface and the sky radiance. The reflected skylight has to be removed from the upwelling radiance by means of the sea surface reflectance factor ρ which depends on the viewing geometry. The measured sky radiance is typically an order of magnitude greater than the water-leaving radiance, thus it is crucial to use the correct value of ρ for the estimation of the latter.

For many years now Curtis Mobley has dedicated his research to light in water. His classical approach for the determination of the surface reflectance is based on wind-dependent wave slope distributions according Cox-Munk statistics and unpolarized ray tracing. In the present work, he shows that this approach is actually not adequate for the accurate estimation of the water-leaving radiance from above-water measurements. Indeed, the utilization of more realistic sea surfaces and polarized ray tracing yields improved values of ρ. The sea surface wave description used here is sophisticated and incorporates full resolution of wave elevations and slope variance. The new numerical values of ρ, which were made available online, depend on sensor viewing angle, sun zenith, and wind speed. It is very helpful to have this correction factor for all sun-viewing geometries, since it is not always possible to measure under optimal angles with respect to the sun, e.g. for permanently installed measurement systems on ferries or even for manually adjustable sensors. Furthermore, it is good to hear that after this revision, the established “optimal” viewing directions used by the community of ϑ _v ≈ 40° and φ_v ≈ 135° (relative to the sun’s azimuthal direction) remain a reasonable choice.

But the author shows more than revisited radiance reflectance factors. He underlines the importance of polarization effects in radiative transfer theory, since we also know that polarization has some importance to many sea creatures for orientation and food detection. Up to now, radiometric measurements at sea are normally made with sensors that are not sensitive to polarization. Over the last years however, new sensors were developed, e.g. cameras for measuring polarized spectral radiance distributions in water and above the surface or in-situ and laboratory instruments that measure the polarized volume scattering function. This gives us information on the Mueller calculus and the Stokes vectors, which represent the polarization of light and its change through the marine environment. It is a challenge for modern models to handle the vector radiative transfer equation. But the degree of polarization of the light field can strongly impair the accuracy of the results from the scalar radiative transfer equation. And in addition, valuable new information can be obtained by exploiting polarized light.

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