Black Level
The numerical value a sensor reports for a completely unexposed photosite, subtracted during processing to establish true black.
Measured in
ADU
In depth
Sensors are deliberately biased above zero so that noise can swing in both directions without being clipped, which would skew the average. Masked photosites around the array measure this offset for every frame, allowing it to be subtracted accurately. An incorrect black level produces either milky shadows or crushed, colour-shifted blacks.
Examples in practice
A camera reporting a black level of 512 in a 14-bit file subtracts that value before any tone curve is applied.