1 Reply Latest reply on May 16, 2009 10:41 AM by MadManChan2000

    RGB/Lab value in ACR at different illuminants

    Marco N. Level 1

      Is there a way to calculate the ideal RGB/Lab value starting from RGB/Lab D50 value.

       

      For example I have my Colorchecker D50 Lab value. Can I calculate the ideal value rendered at 6000K? I have try via Bradford adaptation, but it seems a bad idea.

       

      Thank you

        • 1. Re: RGB/Lab value in ACR at different illuminants
          MadManChan2000 Adobe Employee

          If you only have a D50-relative XYZ/RGB/Lab value for a patch of your ColorChecker, you can approximate the appearance of that patch under another illuminant (e.g., D65) using chromatic adaptation. The linear Bradford adaptation is one way to do this, as you've mentioned. The closer the spectral characteristics of the source and destination illuminations, the better the approximation. Adapting to D65 works reasonably well, but less well when adapting to, say, illuminant A.

          Ideally, you would not only have a D50-relative XYZ/RGB/Lab value for your patch, but spectral information: specifically, spectral reflectance samples across the visible range -- roughly 380 nm to 730 nm. From the spectral reflectance data of the patch, the spectrum of an illuminant (e.g., A or D65, or one that you have measured), and the CIE standard observer function data (usually 2-degree), you can very accurately compute the XYZ coordinates, and hence the Lab values relative to that illuminant -- no chromatic adaptation needed.

          If you're using a device like an Eye-One Pro to measure your ColorChecker, you can save out the spectral data (not just the derived Lab values).

          Relevant sites:

          http://www.brucelindbloom.com/Eqn_Spect_to_XYZ.html

          http://www.cis.rit.edu/mcsl/online/cie.php

          Eric