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Comment by greglindahl | Hacker News Reader

Hang out with astronomers, they do this stuff all the time: poisson noise, dark current, readout noise, readout time, taking thousands of shots and stacking them according to the previous factors.

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DSLRs have come with dark current removal for a few years now. Another fun fact, the sensor used in Canon's G15/16 has a Quantum Efficency of over 60%, meaning that 60% of photons reaching the sensor generate a signal. Most cameras have sensor with QEs between 15 to 25% (but have way larger sensors, so in the end a better SNR than the G16).

I should hope that they subtract off the dark current, astronomers have been doing that since the first time they used a CCD. Dark current is a big limitation to seeing dim things.

That's a much higher QE than the good old days, wow.

Here's a list of QE numbers: http://www.sensorgen.info/

I knew it was over 60%, and I remembered (wrong) as the G15 and G16 having the same sensor. Welp, 77% QE for the G16 and 67% for the G15. There are others with higher QE, though in general those are used in smaller sensors.

DSLRs, while having lower QE numbers, have much larger sensors, and hence a better image quality.

That's incredibly useful, thanks. The only problem is, I don't trust the numbers. How does the Nikon D2X get a QE of 476%?

What does a QE of >100% mean ?

(Sony) Alpha-900 234% 3.6 892070

(Pentax) Q 101% 1.6 8429

There's a few more.

According to Wikipedia "in the event of multiple exciton generation (MEG), quantum efficiencies of greater than 100% may be achieved since the incident photons have more than twice the band gap energy and can create two or more electron-hole pairs per incident photon." https://en.wikipedia.org/wiki/Quantum_efficiency

Pretty fascinating!

In some sense, this is useless. A sensor that counts 50% of incoming photons twice each has more shot noise than a sensor that counts each photon once.

I would take this list with a grain of salt. There are several entries with a QE in the nineties (e. g. the Leice C with 95%) which seems implausible to me. Even more, further down the list are some entries way over 100% (e. g. Nikon D2X with 476%).

Can't edit now, but dark current removal isn't as widespread as you think it should be. Most cameras don't do it at all and will add (actually, subtract) a dark frame when the exposure time goes over a threshold (say, longer than 1 sec exposure) in which the camera takes another new photograph, with the shutter closed and then subtracts that dark frame from the original picture which is still in memory. This is quite useful actually, but not the same quality as dark current removal/supression.

As for those saying about the over 100% QE numbers... I didn't gather those, all I can say that a few years back that list was accepted as generally reasonable measurments.

Besides that, I've read of some imaging techniques which can form images with a really low number of photons (for example [0]), but those generally require a special setup (infrared light, lasers, that sort of stuff). So yeah, those QE's do seem fishy, but it could alse be the result of a photon affecting more than one photosite (or maybe the marketing team hearing about QE and finding a way to raise that number through "smart counting").

[0] http://www.nature.com/articles/ncomms12046

One of the professors in my former department was an astronomer from the US Naval Observatory. He's applying techniques he developed (along with other techniques astronomers use) to get better resolution MRI images.

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