Are ideal white balance coefficients intensity dependent?

[tlkvstepan]

Hello,

Thank you for amazing work. I have a question related to white balance. I have following piece of code

reference_color_checker = colour.CCS_COLOURCHECKERS['ColorChecker24 - After November 2014']
reference_swatches = colour.XYZ_to_RGB(
        XYZ=colour.xyY_to_XYZ(list(reference_color_checker.data.values())),
        illuminant=reference_color_checker.illuminant,
        colourspace=colour.models.RGB_COLOURSPACE_sRGB,
        apply_cctf_encoding=False,
    )

Last six swatches in the color chart correspond to gray patches with different intensity. Therefore, they all should lead
to same color balancing coefficients. However, they all producing slightly different coefficients:

What can be the reason for this behavior?

[KelSolaar]

The swatches are not exactly neutral so the computed white balancing factors will change as a function of the swatches you are using:

print(reference_swatches[-6:])

[[ 0.87919095  0.88476747  0.8349529 ]
 [ 0.58443959  0.59212352  0.58458201]
 [ 0.35767777  0.36706043  0.36528718]
 [ 0.19008669  0.19086038  0.1898278 ]
 [ 0.08593528  0.08873843  0.08978779]
 [ 0.03135966  0.03149993  0.03231098]]

[tlkvstepan]

Thank you for reply! What is the reason? Is it due to non-ideal manufacturing process of the chart?

[KelSolaar]

Yes, producing a truly neutral reflectance across the Visible Spectrum at an affordable price point is hard!

[tlkvstepan]

Thank you!