Black Level Range
Prepared 2017-12-06 by Bill Claff
When the pixel voltage is converted to a Digital Number (DN) by the Analog to Digital Converter (ADC) a reference gound is provided.
In many cameras that voltage is biased so that a black signal (no light), will read out as a value higher than zero.
When no bias is provided; no light, or very low levels of light, can be clipped at an ADC value of zero.
This has no practical effect on photography but clipped values can be a nuisance for making certain sensor measurements such as those to determine read noise.
The amount of bias is stored in the raw image in DNs as the Exif value called BlackLevel.
BlackLevel can be stored as a single value or as separate values for each position in the Color Filter Array (CFA).
There are also more sophisticated Exif tags available to store by row or column but they are seldom used.
Processing software uses this value (these values) to determine what to consider as pure black.
If BlackLevel does not truly agree with the bias used then the pure black
level will be off.
Since BlackLevel is often an integer it is almost never precisely correct.
A possible photographic consequence is that color balance may be incorrect at very low light levels.
For sensor researchers BlackLevel discrepancies can complicate constructing the low end of the Photon Transfer Curve (PTC).
Because of noise sources such as thermal noise (dark current), it's not unusual for the BlackLevel value to be lower than the actual black level.
This is of little consequnce unless the difference varies greatly between the color channels.
One of the measurements I make is to determine the BlackLevel Range.
BlackLevel Range is the difference between the maximum BlackLevel
discrepancy (difference between measured black level and Exif BlackLevel) and
the minimum BlackLevel discrepancy.
A value of zero is perfect, as values approach or exceed one, color balance issues ar low light levels may arise.
The measurement is determined using raw data from a minimumof 16 black images.
BlackLevel Range appears as a column in the sort-table table in Sensor Heatmaps.