Image Sensor sensitivity
Sensitivity in image sensors can also be expressed in terms of "photon efficiency", which at a fundamental level, is a measure of how many of the photons striking the area of a pixel actually get converted into detectable electron-hole pairs in the photodiode junction. These accumulate over the duration of the exposure to become the total charge for that pixel.

The other factor you need to consider is the "dark current", which essentially are electron-hole pairs created by things other than the light, such as the temperature of the sensor, cosmic rays (and other ionizing radiation) and leakage. This sets a lower limit on the level of light you'll be able to detect.

Datasheets for sensors will specify these parameters, which will then allow you to determine whether they're suitable for your particular application.