Instead, it was curving upward, with significant errors in the interpolated percent reflectance values of the black, neutral gray, and white tiles. I found that the initially calibrated raw ColorChecker image did not produce a linear response when I used the published colorimetric 8-bit sRGB values. As a check, I plotted the measurements of all 6 black, neutral gray, and white tiles against the nominal 8-bit sRGB colorimetric values published by X-Rite here: I calibrated a raw ColorChecker image using three of the neutral gray tiles (black, neutral 5, and neutral 8), and measured the percent reflectance of the ColorChecker chart using the function micaToolbox>Camera calibration>Measure chart. Another possible option is to refer to the spectral measurements of the ColorChecker chart found in the ImageJ/plugins/Cone Mapping/Charts folder, but this would require calculating a weighted average of the reflectance measurements over the range of wavelengths for each channel, which in any case represent a unique ColorChecker target. The guide currently recommends measuring the user’s calibration targets against a known reference (ideally), but many users might not have access to the reference standard and tools for measurement, and instead rely on published ColorChecker colorimetric values for calibration.
They must be converted to a linear percent reflectance scale for calibration. It might be worth adding a caution to the User Guide not to use the nominal 8-bit colorimetric values of the ColorChecker chart published by X-Rite to calculate the percent reflectance values of the tiles used for calibration, because they aren’t on a linear scale.
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