|Understanding image sensor characteristics
||[Jun. 17th, 2010|10:25 pm]
Can you explain image sensor specs to me? I am trying to understand where the brightness limit is at which shot noise dominates, and understand how to compare sensor specs.
Forgetting for a moment about dark current and photo-response non-uniformity (PRNU), I see a pixel on a sensor as taking in a certain number of photons and turning them into a certain DN number, (zero to 255 for 8-bit images). Now, ideally you might have a 32- or 64-bit sensor so you could actually count the number of electrons that built up, but for whatever reason, that's not how these things work. Instead, you build up electrons and then use an A-to-D converter (with its own gain and offset) to get the DN. This gain is the ISO setting on a camera.
Why, then, do sensor specs talk of "sensitivity"? I'm fine with the notion that the A-to-D converter will have some noise, either in how many electrons it can count as one or in its output; that makes sense. But what does it mean to have a sensitivity of 8.40 V/(lx·s)? I read that as the conversion from lx·s to volts... but isn't that captured by the quantum efficiency times the fill factor? That is, before we digitize the counts, doesn't QE·FF completely define how much signal there is to digitize?
Along the same lines, why all this talk of volts at all? If each photon that excites an electron is bumping it over a potential, then won't each electron be charged to the same voltage, rather than building up voltage? Wouldn't they be building up coulombs not volts?
In one spec, I see "conversion gain" in microvolts per electron (17.6 μV/e−); is that the number of volts of charge a photosite gets for each electron?
I see saturation charge of 62,500 e−. I assume that's the well depth. Is that to say that the maximum charge on a photosite is 17.6×10−6 V · 62,500 = 1.1 V? Does that correspond to "Output signal amplitude" of 1 V?
Sensitivity is given either in Vm2/(W·s) or V/(lx·s). If I understand the conversion gain above, then I can use it to convert these to electrons per lux-second. Is that right?
I feel like I would really just like to know the PDF of the random variable that is the output DN as a function of exposure time, light flux, and amplifier gain, so I can approximate an image I would get from a given sensor, but these specs don't seem to make that easy.