Why doesn’t sensor full well capacity match the listed e-/ADU gain and ADC bit depth?
Asked 7/13/2017
7 views
2 answers
0
A camera datasheet lists these EMVA 1288 parameters:
- ADC: 14 bits
- Full well capacity: 17,273 e-
- Gain: 0.17 e-/ADU
At first glance, I expected the gain to be approximately full well divided by the ADC range, i.e. 17,273 / 2^14 ≈ 1.05 e-/ADU. But that does not match the listed 0.17 e-/ADU.
Why aren’t these numbers directly related? Is full well capacity separate from the ADC scaling, or is the EMVA gain measured in a different way?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
3
The well depth is a property of the sensor. In your example, the incident photons are converted into electrons with a ratio corresponding to the quantum efficiency and when the pixel has accumulated 17 273e-, the generated extra e- are not accepted anymore in the "fully booked" pixel.
ADC and gain are properties of the camera electronics design and are explaining how these damned electrons will be counted after transfer from the sensor.
ADC of 14 bits means that the electronics will be able to produce a digital signal in between 0 and 16 383 gray levels. And, finally, the gain of 0.17 is telling you that each 0.17e- is generating one digital level in 16bit. Then, the actual displayed gray level can be calculated by using the bit ratio between 16 and the actual converter.
Nothing is preventing a camera from being poorly designed or with an ADC converter not fitted to the selected sensor. Here, a saturated pixel with 17273e- will generate 101 606 digital levels (16 bits) which makes 25 401 gray levels in 14bits (divided by 4 for the 2 bits difference). Since 14 bits are limited to 16 383 levels, it means that your electronics will saturate the signal far before your sensor pixel will be saturated.
To put it simply, you would see no difference if your pixel was saturating at 11 140e-. Just try to redo the calculation with this new saturation value and you should see a good correlation with the gain.
Originally by user79404. Source · Licensed CC BY-SA 4.0
user79404
7y ago
0
Generated from our catalog & community — verify before relying on it.
These values are related, but not by a simple full-well ÷ ADC-count formula.
Full well capacity is a sensor property: how many electrons a pixel can hold before saturating.
ADC bit depth is an electronics property: how many digital output levels the camera can encode.
The EMVA 1288 “gain” is not simply derived from full well and ADC range. It is an experimentally estimated system conversion gain using the photon-transfer method, and it also reflects the camera’s readout/electronics behavior and noise assumptions. In your example, that’s why 17,273 / 2^14 does not equal the quoted 0.17 e-/ADU.
The manufacturer’s support note also matters: the EMVA document reports gain using 16-bit data, even if the camera ADC itself is 14-bit. Using 2^16 gives 17,273 / 65,536 ≈ 0.26 e-/ADU, which is closer, but still not exact because the EMVA gain is measured rather than taken from a simple ratio.
So yes: full well, ADC resolution, and quoted e-/ADU are distinct specifications, and the EMVA gain should be treated as a measured conversion factor for the complete camera system, not just the sensor well depth divided by digital counts.
Recommended products
UniqueBot
AI9y ago