Polarization - A Property of Light
Application engineers can take advantage of polarization cameras 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, the special polarized sensors used inside polarization cameras can 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.
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.
Existing Polarization Solutions
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. The polarizer array layer is placed on-chip and is an air-gap nano wire-grid coated with an anti-reflection material that suppresses flaring and ghosting. This on-chip placement reduces polarization crosstalk and improves extinction ratios.
The polarizer array is comprised of four different angled polarizers (90°, 45°, 135° and 0°) which are placed on each pixel. Every block of four pixels makes up a calculation unit. The relationship between the different directional polarizers in this innovative 4 pixel block design allows the calculation of both the degree and direction of polarization.
Above: Sony’s 4 Pixel Block Polarizer Design
Sony’s IMX250MZR 4 pixel block, known as a calculation unit, allows for the detection of all linear angles of polarized light, not just 90°, 45°, 135° and 0°. This is possible through comparing the rise and fall in intensities transmitted between each pixel in the 4 pixel block.
Above: The 90°, 45°, 135° and 0° pixels are each measuring polarized light (rotating red line) in relation to their wire-grid axis. The percentage represents the rise and fall of light transmission (100% equals maximum transmission). By comparing each pixel with each other the sensor can detected all angles of linear polarized light.
The polarizer array is positioned on-chip as opposed to on-glass. With the position of the polarizer array being on-chip and below the micro lens, Sony’s polarization sensor is able to reduce crosstalk from polarized angles being incorrectly detected by the wrong pixel.
Above: 0° polarized light is entering the pixel meant to detect 90° and will be incorrectly detected as 90°. This crosstalk error happens because the polarization array is placed above the micro lens.
Above: Sony’s polarization sensor reduces the chance of crosstalk thanks to the polarizer array being placed on-chip. The 0° polarized light is unable to enter the pixel meant to detect only 90°.
An extinction ratio represents the relationship between the maximum and minimum amount of transmitted polarized light through a polarizer. For a linear wire-grid polarizer, the maximum transmission of polarized light occurs perpendicular to the wire-grid axis. As this perpendicular angle of polarization rotates 90°, we hit the minimum transmission point which occurs parallel to the wire-grid axis. No polarizer is perfect. At the angle of maximum transmission there will be some loss and at the angle of minimum transmission there will be some unwanted angles that transmits through. High extinction ratios allow better detection of a desired polarized angle without other polarized angles mixing in.
Above: The maximum transmission (T max) and minimum transmission (T min) are used to calculate the extinction ratio
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 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.