On-chip polarizationJune 19, 2019
Polarizing cameras have been available for some time and greatly enhance many areas of machine vision and medical imaging. Recent advancements in sensor technology, however, are making polarization more affordable and accessible. We touched on this in our recent blog about bio-inspired artificial vision, but here we take a closer look at the technology.
Polarizing cameras are useful when inspecting transparent objects, reflective or translucent surfaces and to analyse material stress. Application examples include: identifying areas of stress (and therefore possible weakness) in plastic manufacture; imaging through water by eliminating surface reflection in marine applications; classifying objects in low light conditions where improved contrast allows greater recognition in industrial imaging; and differentiating between tissue types to enhance medical diagnoses.
Bayer filters were first patented in 1976 and are still the most commonly-used filters in machine vision color imaging today. The process works by arranging color filters on the pixel array of an image sensor in a particular (Bayer) mosaic, with 50% of the pixels having a green filter, 25% a red filter and 25% a blue filter. The ratio of red, green and blue filters can be altered to adjust resolution and brightness to best fit the task in hand. Algorithms then interpret (“demosaic” or “debayer”) the resulting colour values to produce an interpolated image.
Polarization has conventionally been achieved by the use of polarization filters in front of the camera. Examples include wire grid, dichroic, circular and crystalline polarizers. In such systems, three cameras each with a differently oriented filter may be required, adding to overall cost. Alternatively, three filters can be used with one camera, but the filters need to flipped or rotated so that each one is placed in front of the camera in turn, all at extremely high speed, of course. This option can introduce latency to the vision system and increase the number of moving parts and therefore risk of component failure.
Sony have recently released their IMX250MZR CMOS Pregius sensor, combining Bayer type pattern imaging and polarizers all on one chip. The technology has been recognised by inVISION with one of their ten Top Innovation 2019 Awards and is now being adopted by camera manufacturers. Available products include JAI’s GO-5100MP-USB industrial area scan camera, Imperx’s Camera Link based C2420Z and LUCID’s polarized Phoenix and Triton cameras. Using wire grid polarizers within its design, this sensor moves away from filtering light by the intensity of the wavelength, and instead filters light according to the orientation of the wavelength. The polarizer is built as part of the chip, right on top of the photodiodes and under the on-chip lens layer. Each of the four photodiodes has a different polarization angle: 0, 45, 90 or 135 degrees. This ground-breaking technology enables the sensor to deliver one-shot, real-time, pixel-level resolution polarization information. Moreover, it removes the need for additional components in the vision system, thereby reducing installation cost and improving reliability.
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