I am not sure what really the term "sensitivity" means when characterizing image sensors, so I will describe what I am looking for in the sensor at the following conditions:
- the exposure time is very short
- the intensity if light is very low
Therefore, I need a sensor that will allow me to "see" very dim light at short exposure times. To compare the sensors (assuming equal spectral responses, exposure times), I decided that the most sensitive one will be the one that will "see" the dimmest light.
One problem is that different manufacturers provide different information/specs for their product, so it is difficult to compare them and choose the best one for application. I took one example and tried to calculate it's minimum amount that must reach the pixel before it can be detected. Please, correct me if I am wrong:
- I use "Output due to dark current" and "Conversion Gain" values to determine how many electrons converted from light are needed to reach the threshold above which they will become "detected": 6mV/3.4uV/e = 1765 electrons.
- I find conversion efficiency at a desired wavelength (let it be 700nm): Given that the QE(675nm) is 60%, I find that at the 700nm QE=60% * 0.87 = 52.2%
- Thus, the minimum amount of photons that must strike the pixel before they become detected is 1765e/.522e/ph= 3381 photons
Now, I wish to compare this sensor to S11639, however, there is not information to calculate the number of photons as in the first example: - The amount of electrons needed is: 0.4mV/25uV/e= 16 electrons! That is 110x less than for the first sensor. Now, there is no QE plot for this sensor. The only thing I can do is assume that at 700nm it has a low QE value, say 20%. But even with this bad QE value the number of photons needed to reach detectable range is 16e/.2e/ph= 80 photons - muuuch less than in the first case. Thus is it right to assume that the second sensor is much more suitable for my application with very low light intensities and exposure times because it needs less photons to reach the detection threshold? Are my calculations correct? Anything that I am missing?