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| Figure 4 (from the paper linked below): Realized CMOS test chip: (a) photograph of the packaged chip, (b) screenshot of the layout. |
German researchers have developed a pixel design with the potential for massively increased dynamic range. Their design, reported in the ‘Advances in Radio Science’ journal isn’t limited by the point at which it saturates, meaning it can continue to capture more highlight data when other sensors would become overwhelmed.
Unlike conventional CMOS chips, their ‘self-resetting pixel’ doesn’t simply ‘clip’ when it becomes saturated, instead, it resets and has a circuit that counts how many times it’s had to reset during the exposure. It also contains a conventional analog-to-digital conversion circuit, so it is also able to measure the remaining charge at the end of the exposure.
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| Figure 2 (from the linked paper above): The working principle of the self-reset pixel. |
This would mean that you don’t need to limit your exposure to protect highlight data and can instead set an optimal exposure for capturing your subject, safe in the knowledge that this won’t result in blown-out highlights. In their paper, the researchers from Institut für Mikroelektronik Stuttgart created a series of test pixels with different designs, and will now focus on the one that gave the most linear response to different light levels, both in terms of its reset characteristics and its conventional ADC mode.
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| Figure 1 (from the linked paper): Schematics of the analog and digital parts of one pixel cell and a global control for all pixel cells. |
Before you get too excited, though, this work is still at a fairly early stage and is primarily focused on video for industrial applications, though lead researcher Stefan Hirsch tells us: ‘basically it should also be possible to use for still images.’
At present, the additional counting circuitry ends up meaning the light-sensitive photodiode in each pixel is very small, making up just 13% of the surface area of huge 53?m pixels. A move to a stacked CMOS design, with the circuitry built as different layers, would increase this, with potential for 20?m pixels with more of the area being light-sensitive. A three-layer design could allow still smaller pixels. For perspective, the pixels in the 12MP Full-Frame a7S II are around 8.5?m, so there would need to be a lot of work done to find a way to produce a sensor useful as a consumer video or stills camera.
