How much of digital sensor noise is thermal?

by feetwet   Last Updated June 28, 2016 08:07 AM

Is all digital sensor noise thermal?

And what is the relationship between temperature and noise? For example, as we cooled a CMOS towards absolute zero, would we discover that high-ISO images would be less noisy? Would they converge towards a truly noiseless image at maximum ISO and absolute zero?

Answers 2

No. The noise is in the photon count, and it is the square root of the number of photons. So if you get 9 photons, the noise level is 3 (33%); if you get 10000 photons, the noise level is 100 (1%).

Photons are not perfectly distributed over space, but randomly, and even at perfect 0 K, different receiver buckets will catch different amounts of photons each time you try.

June 27, 2016 22:31 PM

There is dark current, as inkista wrote in a comment, and there's quantization noise at the digitizer, and there's gain noise in the analog amplifiers, and there's readout noise ( slightly less than 100% of collected charge is extracted).

When people write "scene-limited" or "photon -limited" noise, what they mean is that the photon shot noise is much greater than the combination of all the electronics noise sources. You can never do better than being photon-limited. Luckily, if you read up a bit on energy per photon, you'll see that an SNR of sqrt(number of photons) is a very large number -- unless you're imaging distant galaxies or some such :-)

Carl Witthoft
Carl Witthoft
June 28, 2016 12:09 PM

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