Polarization - A Property of Light
Application engineers can take advantage of polarized sensors by filtering unwanted reflection or glare as well as enhancing contrast by colorizing polarized angles of light. Different materials used in products can reflect and alter the properties in light. Whereas normal colour and mono sensors detect the intensity and the wavelength of incoming light, polarized sensors detect and filter angles of polarization from light reflected, refracted, or scattered off surfaces. To understand some of the benefits of Sony’s polarized sensor, let us first expand on what polarized light is.
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Good Vibrations: Unpolarized to Polarized
Polarization is a fundamental property of light and describes the direction in which the electric field of light oscillates. The majority of light sources, such as the sun, emit unpolarized light. Unpolarized light has vibrations at randomly oriented directions perpendicular to the direction of travel. For light to be polarized randomly oriented vibrations are removed or transformed into either a linear, circular or elliptical electromagnetic wave. For the examples below, we will only discuss how unpolarized light is linearly polarized.
Polarization from Polarizers
When this light hits a linear polarizer, such as the vertical and horizontal polarizers in the example below, the angled vibrations are filtered out with only vertical or horizontal vibrations passing through. When light vibration is restricted to one plane it is called linearly polarized. There are different types of polarizers with the most common ones being crystalline, dichroic, film, and wire-grids. In the example below the line patterns on the film polarizers represent their polarizing angles.
Polarization from Reflection
Unpolarized light can be polarized through surface reflection of non-metallic surfaces. Metallic surfaces reflect the polarization of the incident light, either polarized or unpolarized and do not significantly polarize. Other materials such as semi transparent surfaces of glass, plastics, and water reflect and polarize a certain amount of light back into the environment. This reflected light causes undesirable glare depending on the position of the user or camera. However, because the reflected light is polarized perpendicularly to the plane of incidence, it can be removed by using a polarizer aligned parallel to the plane of incidence.
Polarization from Refraction
The passing of light through one medium to another, known as refraction, can also cause some of the unpolarized light to become polarized. The amount of light that is polarized from refraction depends on how near or far it is from Brewster’s angle (a 90° relationship between the reflected and refracted light). Transparent materials such as glass, plastics, and water can partially polarize refracted light to the plane of incidence.
Taking Advantage of Polarization
The applications of polarization have long been used in machine vision inspection to detect stress, inspect objects, and reduce glare from transparent objects. The typical setup would require one or more external polarizer plates between the target object, light source and camera. Various setups can be used to measure material stress, enhance contrast, and analyze surface quality for dents or scratches.
Sony's First Polarized Sensor: How It Works
Sony expands their sensor technology leadership beyond visible imaging with their first ever polarized sensor. Built upon their Pregius 5.0 MP IMX250 CMOS sensor, the new IMX250MZR (mono) sensor incorporates a layer of polarizers above the photodiodes. Four different angled polarizers (90°, 45°, 135° and 0°) are placed on each pixel with every block of four pixels making up a calculation unit. The relationship between the different directional polarizers in this innovative 4- pixel block design is able to calculate both the degree and direction of polarization.
Above: Sony’s 4 Pixel Block Polarizer Design
Sony’s IMX250MZR polarizer array layer is an air-gap nano wire-grid coated with an anti-reflection material that suppresses flaring and ghosting. The polarizer array is positioned on-chip as opposed to on-glass. Because of this placement, the on-chip polarizer is closer to the photodiode and produces high extinction ratios.
Phoenix using Sony's IMX250MZR CMOS
With the help of polarization cameras, many material properties that were impossible to identify with conventional RGB sensors can now be easily acquired. The 5 MP global shutter sensor with a pixel size of 3.45µm is based on the popular IMX250 Sony Pregius CMOS mono sensor with the addition of an on-chip nanowire polarizing layer that provides excellent extinction ratios. Lucid’s new Phoenix camera featuring Sony’s IMX250MZR CMOS polarized sensor provides on-camera processing using the four directional filters and outputs both the intensity and polarized angle of each image pixel. With the combining of these two innovative products, polarization can now be a compact and cost-effective way to solve imaging challenges and uncover hidden material properties to better perform inspection and classification.