It's common in my industry for reference/archival photographs to be taken with a combined scale, grey, and colour checker card (often with rectification targets too). In most cases, the standard is a four-panel colour checker. A common card people use is this, or something like it. Photograph of a pocket reference scale - this version is credit card sized, with an 8 centimetre scale, a 20 millimetre scale, a three-panel grey card, four colour reference panels, and a rectification target. The reference colours are red, yellow, green, and blue.

I have been reviewing a collection of unprocessed RAW photos at my current company, and they have cards like this. I've discovered, however, that no one at the company knows how to process RAW photos, and they just use the camera JPEG outputs, so I am working on processing the backlog of RAW photos. I am used to using a 24 or 48 colour checker, but have not used four-colour checkers before, and I cannot find any information on using them for colour correction.

In particular, I need to know how to make use of these in Darktable (v4.2.1). I have the CMYK values of the cards used, and have converted them to the RGB and hex values. But I cannot work out a way to calibrate the colour using these cards other than by eye.

Can anyone suggest a workflow for this scenario?

  • \$\begingroup\$ Can't you just use the white balance's eyedropper on the gray patch to get accurate WB? \$\endgroup\$
    – dandavis
    Sep 15 at 21:52
  • \$\begingroup\$ @dandavis that's for white balance correction. I'm interested in colour calibration, which is what colour checkers are for. See the Calibrite ColorChecker Passport for an example of a 24-panel colour calibration unit. \$\endgroup\$
    – OBeighton
    Sep 17 at 12:24
  • \$\begingroup\$ I don't see how the color would be off if the white balance is good. Usually calibration is applied to outputs like monitors and printers, not photos. If you want realistic color, toe in the levels to paper white and black, blur 10px to kill noise, then check that the color squares are reflective of the pure rgb values. If there is a cast, use the color correct to bump each channel up/down a notch or three until the eyedropper rgb number match what you expect from the card. Then you can apply the same corrections to each photo in the batch/session. \$\endgroup\$
    – dandavis
    Sep 18 at 23:39


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