Polarization and Its Applications in Vision
In many industrial applications, visual inspection poses significant challenges due to low contrast or high reflectivity imaging conditions. Polarization cameras can aid in discovering hidden material properties and provide higher visual clarity than standard color and monochrome cameras.
Polarizing cameras can filter out unwanted reflections or glare and enhance contrast by colorizing polarized light angles. Different materials used in products can reflect and alter the characteristics of light. While regular color and monochrome sensors detect the intensity and wavelength of incident light, specialized polarization sensors used internally in polarization cameras can detect and filter light polarization angles reflected, refracted, or scattered from surfaces.
What is Polarization?
Polarization of Polarizers
Polarization is a fundamental property of light that describes the direction of oscillation of the electric field of light. Most light sources, such as the sun, emit unpolarized light. Unpolarized light vibrates randomly in directions perpendicular to the direction of travel. For light to be polarized, randomly oriented vibrations are removed or converted into linear, circular, or elliptical electromagnetic waves.
A polarizer is an optical filter that allows light waves of a specific direction to pass through while blocking light waves of other directions. It can filter a beam of undefined or mixed polarized light into a beam of well-defined polarized light, called polarized light. Common types of polarizers are linear polarizers and circular polarizers. Reflection Polarization
Unpolarized light can be polarized through surface reflection from non-metallic surfaces. Metallic surfaces reflect incident light, whether polarized or unpolarized, without significant polarization. Other materials, such as the semi-transparent surfaces of glass, plastic, and water, reflect and polarize a certain amount of light back into the environment.
Because the reflectance coefficient of light with electric fields parallel to the plane of incidence is zero at some angle between 0° and 90°, the reflected light at that angle is linearly polarized, with its electric field vector perpendicular to the plane of incidence and parallel to the reflecting surface. This angle is called the polarization angle or Brewster's angle. At other angles, the reflected light is partially polarized. Refraction Polarization
Polarization can also occur through the refraction of light. When a beam of light enters a different material from another material, refraction occurs. The path of the light beam changes its direction at the surfaces of the two materials. The refracted light beam acquires a degree of polarization. In most cases, polarization occurs in a plane perpendicular to the surface.
Iceland Spar is a rather rare form of the mineral calcite that refracts incident light into two different paths. The light is split into two beams upon entering the crystal. Subsequently, if an object is observed through an Iceland Spar crystal, two images will be seen. These two images are the result of the double refraction of light. Both refracted beams of light are polarized, one in the direction parallel to the surface and the other in the direction perpendicular to the surface. Because both refracted light beams are polarized in the vertical direction, a polarizing filter can be used to completely block one of the images. If the polarization axis of the filter is aligned perpendicular to the plane of polarization of the light, the light is completely blocked by the filter; meanwhile, the second image appears as bright as possible. If the filter is rotated 90 degrees in either direction, the second image will reappear, and the first image will disappear. Applications of Polarization
The applications of polarization have long been used in machine vision inspection for detecting stress, identifying objects, and reducing glare from transparent objects. Typical setups involve installing one or more external polarizing filters between the target object, light source, and camera. Various configurations are available for measuring material stress, enhancing contrast, and analyzing surface quality defects.
Stress Detection
When polarized light passes through a transparent material, the angle of incidence of polarized light is converted into different angles through different stress regions within the object. By assigning colors to specific polarization angles, defects and stress regions can be visualized.
Reducing Reflections
Objects reflect light, making surface detection difficult. Polarizing filters can be used to reduce reflections and glare in food and packaging inspection applications.
Enhancing Contrast
Under low-light conditions, contrast can be improved by detecting the polarization angle of objects. The example above demonstrates how contrast can be improved through conventional imaging under low light.
Scratch Detection
Similar to stress detection, it is difficult to identify certain defects and scratches using traditional imaging. Polarization imaging can be used to detect scratches on transparent materials.
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