Do any image sensors measure per-pixel time-to-saturation to increase dynamic range?
Asked 11/10/2022
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Conventional image sensors read each pixel’s accumulated charge after a fixed exposure time, so very bright areas can clip while dark areas are still barely above noise. Could a sensor instead detect when each pixel reaches a threshold and record the time-to-threshold for bright pixels, while using a normal readout for pixels that never reach that threshold during the exposure? In theory this seems like it could extend dynamic range, though it might create motion-related artifacts. Are there commercial or experimental sensor designs that use this kind of per-pixel timing approach?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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If you want to use the logged time as an indicator of exposure (relative brightness) you would also need to incorporate another way of converting that data compared to the relatively simple ratiometric voltage conversion the ADC does in current designs. And that conversion would have to happen in parallel with the ADC (secondary ADC?) and be combined afterwards digitally as they are two separate measures/values/units. So you are also increasing the processing load and introducing a secondary data stream/pipeline.
The current approach is to use dual gain architecture in bright scenarios where photosites are more likely to saturate (secondary capacitor in parallel)...
I.e. when a low ISO is selected the photosite's FWC is increased so that the pixels are less likely to reach saturation. The slight disadvantage this has is that the larger FWC also has a higher conversion gain requirement (it's somewhat less sensitive to lower light levels).
The other current direction is increasing ISO invariance and smaller pixels (i.e. reducing the minimum conversion gain requirement and reducing/offsetting camera generated noise levels). In this way you can simply underexpose an image by using a lower ISO and prevent any pixels from saturating in the first place; and the resulting image/data can be digitally manipulated (brightened in camera/post) with no negative effect.
I don't really see where your idea provides significant benefits over the current directions/processes...
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
3y ago
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In principle, yes—but it’s not a simple drop-in improvement.
A time-to-threshold design would need extra per-pixel logic plus a separate way to convert and combine that timing data with normal pixel readout. That means more silicon area, more processing, and a second data pipeline, rather than the relatively simple readout/ADC flow used in conventional sensors.
In practice, camera makers more commonly increase usable dynamic range with techniques like dual-gain / dual-conversion-gain sensor architectures. At low ISO, the pixel full-well capacity can be increased so bright areas are less likely to saturate.
There has also been research into related ideas that avoid simple saturation behavior:
- modulo camera concepts, where pixels are reset and keep accumulating instead of just clipping
- quanta image sensors, which aim to count individual photons rather than rely on conventional analog charge measurement
So the basic idea is real and has been explored experimentally, but mainstream commercial sensors generally use other methods because they’re more practical to implement.
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