Read Noise
Noise introduced by the process of reading and digitising the sensor's signal, independent of how much light was captured.
Measured in
e⁻ (electrons)
In depth
Read noise sets the bottom of the dynamic range: any signal weaker than it cannot be distinguished from the electronics themselves. Because it is constant regardless of exposure, it dominates in the shadows and is negligible in the highlights. Modern sensors achieve one to two electrons at high gain, which is close to the physical limit.
Examples in practice
A sensor with 1.5 electrons of read noise can distinguish shadow detail that a 6-electron sensor buries entirely.