Nikon finally gets the noise down.

Started Nov 17, 2007 | Discussions thread
DRG Veteran Member • Posts: 5,217
Re: Collect photons twice as fast? I don't get it.

Don_D wrote:

Eric Sorensen wrote:

Please explain this - I'm confused.

It has been determined by John and others that the D3's sensor has a
2x higher quantum efficiency than the 5D's sensor. That is, for each
photon that strikes the D3's sensor, twice number of electron hole
pairs, and therefore twice the charge is generated compared to the
5D's sensor.

Not exactly. What you describe can't happen at the fundamental level. Two electron-hole pairs from one photon requires that the first generated pair then go on to generate additional ones through some form of amplification within the device (such as impact ionization) or circuit. This offers no shot noise advantage.

QE (what John describes) relates to what fraction of photons reaching the sensor actually make it through the surface films (including color filters), enter the optically-active region of the pixel and are absorbed to become an electron-hole pair. Factors that act against this are reflection or absorption in the surface films or a thin optically-active region that allows too many photons to pass through without being absorbed (that is, too thin compared with the aborption length). In particularly bad sensors (none of which are likely in current product), a significant number of electron-hole pairs can actually recombine before the charge has a chance to be read, defeating the point of the pair having been generated in the first place. This requires a very poor design or poor silicon quality (by the standards of the best sensors), however.


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