Auto exposure error is based on deviation from a nominal luminance value for the neutral grey patch in an X-Rite Colorchecker SG. In a variant calculation, we can also calculate this metric from a 24-patch X-Rite Colorchecker.
A psychophysical study of the subjective quality based on luminance level was how the following model for exposure quality loss was developed:
CPIQ Auto exposure is measured with the Imatest Color/Tone module, which produces a Quality Loss (QL) score in units of Just Noticeable Differences (JND).
Results are available in display #4 2D a*b:
and display #7 B&W density & White Balance:
For more about Imatest’s CPIQ support click here