The concept is mainly referenced in the book Baidu Baike and Machine Vision Algorithms and Applications.
The concept of polarized light
Light is a kind of electromagnetic wave, electromagnetic wave is transverse wave, and the plane formed by the direction of vibration and the direction of light wave is called the vibration plane, as shown in the figure below
The direction of vibration is perpendicular to the direction of light propagation, so it can be imagined that the direction of vibration is all the vertical directions around a circle of the direction of propagation;
Natural light
Generally speaking, the vibration of the light wave vector (direction) of the light wave emitted by the light source is randomly oriented perpendicular to the direction of propagation, but statistically speaking, the distribution of the light wave vector in all possible directions in space can be Regarded as equal opportunities, their sum is symmetrical with the propagation direction of light. It follows that the light vector has axial symmetry, both are distributed, and the amplitude of vibration in each direction is the same. This kind of light is called natural light.
Classification of polarized light
A. Fully polarized light
1. Linearly polarized light: The direction of light vibration is not in all directions, but along a certain direction, its magnitude changes with the change of the phase, and the direction does not change;
2. Elliptical polarized light: The light vector is constantly rotating, and its size and direction change regularly with time;
3. Circularly polarized light: The light vector is constantly rotating, its size does not change, but the direction changes regularly with time;
B. Partially polarized light: On the plane perpendicular to the light propagation direction, it contains the light vector of each vibration direction, but the light vibration is more pronounced in a certain direction;
Polarized light output
Simply put, it is caused by the reflection of light. The reflection on the surface of the metal or insulating material makes the light partially polarized. The polarization is mainly caused by diffuse reflection and specular reflection, but in fact it is mainly determined by specular reflection;
Application in life
Stereo movie
When shooting a three-dimensional movie, two cameras are used. The lenses of the two cameras are equivalent to two human eyes. They respectively take two portraits of the same object at the same time, and project the two portraits on the screen at the same time. If you try to make the audience see only one of the images with one eye, you can make the audience feel three-dimensional. For this reason, during the screening, a polarizer is placed on each lens of the two projectors, and the polarization directions of the two polarizers are perpendicular to each other. The audience wears glasses made of polarizers, and the polarization direction of the left eye polarizer is The polarization direction of the right-eye polarizer is the same as the polarization direction on the left side camera, and the polarization direction of the right-eye polarizer is the same as the polarization direction on the right side camera. In this way, the two images on the screen are viewed through two eyes and formed in the human mind. Three-dimensional images.
Application in machine vision
In visual inspection, the reflection of metal and insulating surfaces will produce partial polarization, causing specular reflection, that is, reflection, which affects the inspection.
So we can add a polarization filter in front of the camera and adjust the direction to suppress partial polarization. However, since unpolarized light is only partially polarized after reflection, it is better to suppress some reflections by first turning the light into polarized light, and then illuminating the surface of the object. In this way, we use two polarizers, one on the lens, and the light source. In the previous one, the polarizer installed on the lens functions as an analyzer (detecting polarized light), and the polarizer on the light source functions as a polarizer (turning natural light into polarized light). In this way, as long as the analyzer and the polarizer are at 90 degrees, the polarized light caused by reflection will be completely suppressed.
