Polarization Understanding
2023-6-25 21:40:48
“This is a summary of my understanding of polarization and cross-polarization”.
The transition of light through a medium will be affected by the vibration of the electron cloud of the medium: the vibration direction of reflected or transmitted light is the vibration direction of the medium electron cloud. If we look macroscopically, the sunlight or light beams from bulbs are the combination of light from different polarized directions, therefore we say the sunlight is ‘unpolarized’, as the figure shows.
Now we talk about where the highlight comes from in the images. Most of them, the specular. The specular is the light reflected in the same direction as the incidence light. Of course, the angle we observing is closer to the specular angle, the scene is getting brighter.(Source)
Another part is diffuse. Due to the roughness of the object’s surface, the reflected light will be in different directions. The combination of diffuse and ambient reflection constitutes the basic colour of the object.
In the natural world, there are many phenomena of highlight, such as the reflection of water surface and glass reflection of buildings.
Here people thought of using polarized glasses to diminish the highlight. Before that, we should know about the polarizer and Fresnel effect. Polarizer is a medium that can transmit light like glass. When light contacts the polarizer, the light in all directions will be deformed to the direction of the polarizer. We should mind that the light in different directions also have the component in the polarizer direction, so that’s why we look through a polarizer and rotate it, the brightness of the scene is constant, and just a little darker than the real scene. (Source)
the Fresnel equations simulate the phenomena of the reflected light. That is, the incidence light could be decomposed into s and p polarization, the polarization state of reflected light will be affected by the viewing angle. When viewing at Brewster angle(the reflected light is perpendicular to the refracted light), the reflected light is completely polarized, the polarization direction is perpendicular to the plane consisting of the light source(s polarized light), incidence point and viewer. Therefore, we only need to use the polarizer perpendicular to the s polarized light then we can reduce the highlight. As for the viewing angle not as Brewster angle, there is still a domain part of s polarized light, therefore the polarized glasses is still useful.
Lastly, the cross-polarization. If we set a polarizer in front of the light source, and then another polarizer perpendicular to it in front of the camera, then we could capture the cross-polarized image. That is because although the light near the Brewster angle will be polarized as s polarization, there are still many other angles of light to generate highlight(specular and angle near specular but not at Brewster angle). The specular reflection does not change the polarization state of light.(Ref)Therefore we use a cross-polarizer could reduce the most parts of highlights in the image.