Featuring light polarization
What is light polarization?
At macroscopic scale, a light ray is not described only by its intensity, color or spectrum. It carries another information, called polarization, related to the orientation of the electromagnetic waves. When the ray hits a surface, reflections and transmission coefficients will depend on its polarization state. In return, the polarization state is modified by the material.
Light polarization in nature
Polarization effects are very common. The Rayleigh diffusion of sunlight by the atmosphere, which causes the blue sky, creates polarized light. Any reflection on a smooth surface, such as water, ice or glass, is polarized as soon as the angle of incidence is non-normal. Combined together, this can makes mountain lakes under blue sky darker. Reflections on tree leaves, rainbows, iridescence of insect wings… all these phenomena imply strong polarization effects.
This is well known by the photographer : by using a polarizing filter, he can attenuate or emphasize polarized light, just by rotating the filter. Darkening the blue sky, removing the glare caused by reflections on water, improving the vibrancy of leaves : all these effects are possible without using any computer image post-processing!
Light polarization in technology
Light polarization is at the heart of many technologies, such as liquid crystal displays (LCD), 3D theaters or sunglasses. Light polarization is also used in sciences : for instance, polarized microscopy helps seeing grains in minerals
Light polarization in Ocean
Ocean uses Stokes-Mueller formalism in all its light calculations. This means it may support any kind of free-space, incoherent light polarization phenomenon, including circular and elliptical polarizations. This covers any possible type of material seen at macroscopic scale.
Currently, polarized BSDFs are used by Fresnel, Complex Fresnel and Polarized Tabulated interface laws. In the two first, the polarization Mueller matrix is computed automatically. For the Polarized Tabulated law, the user provides the s and p coefficients.
The skylight environment model is also polarized. It computes the polarization components from the incident and sun directions, according to Rayleigh’s laws.
Why is this important?
The error introduced by simple non-polarized algorithms can be very significant, especially when dealing with specular reflections such as on glass or water. It may be as high as 50% in these cases.
With advanced materials such as inferential thin-film coated surfaces, the error will not only impact reflection lightness, but also the color. The result may be completely wrong regarding colorimetric predictions.
Taking polarization into account is mandatory for simulating polarization-related phenomenons. For instance, colored fringes in tempered coated glass, commonly called ‘anisotropy’ or ‘quenching marks’, is caused by glass birefringence, which turns it into a polarizing filter. Modelling this effect will be the topic of a future post.
The following picture allows comparing a scene rendered with non-polarized optics (left) and general polarization (right). By moving the mouse over the picture, you can change the boundary between both images. You can see on the curved glass part in the right, that colorimetry and lightness are both significantly impacted.