The ColorChecker Calculator is a tool for studying the GretagMacbeth ColorChecker Chart. With it, you may view the spectral reflectance curve of any color, and perform various spectral color calculations.
The small picture of the chart is used as a selector. Simply click on any patch, and its spectrum is displayed along with various tristimulus values computed from it, based on the three pop-up menus at the bottom.
The spectral data used by this calculator came from measurements made in April 1993 from a new ColorChecker chart using a calibrated, X-Rite 938 spectrophotometer. This instrument makes measurements every 20 nm from 400 through 700 nm. These data were interpolated to 5 nm intervals using piecewise cubic Catmull-Rom splines. The standard observers and all reference illuminants are taken from the CIE standards, also at 5 nm intervals covering the range of 400 to 700 nm.
The colorimetric values that are supplied on the data sheet that accompanies the ColorChecker chart are relative to a two-degree standard observer and illuminant C.
All calculations are spectral. No adaptation is used, which makes this calculator an interesting way to compare adaptation algorithms, when used along with the Color Calculator, also found on this site. For example, the ColorChecker "Red" patch has an Lab value of 42.00, 56.64, 28.97 (two-degree observer, D50). The sRGB equivalent is spectrally calculated to be 176, 48, 59. Calculating the sRGB value with the Color Calculator (which uses adaptation to predict the values), gives RGB of 182, 46, 59 with Bradford, and 185, 47, 58 with XYZ scaling. A more complete evaluation of adaptation algorithms may be found here.
It may also be interesting to see how the colors "fit" within the RGB color spaces. For example, the ColorChecker Cyan patch lies outside the sRGB color space (it requires a negative amount of red, which when encoded as an integer in preparation for writing to an image file, cannot be represented), but it lies within the Adobe (1998) color space (all three components are in the range of [0, 255]).
Also note that the reference white and standard observer selections in the ColorChecker Calculator menus do not affect the numeric values of the RGB components because these values are embodied within each of the RGB definitions. For example, sRGB always uses D65, two-degree standard observer regardless of the menu selections you have made.
The "Scaling" checkboxes may be used to scale X, Y, Z by 100 or R, G, B by 255 to accommodate whatever number ranges you are most accustomed to.
The RGB Model pop-up menu is used to change the colorimetric interpretation of the RGB color values. Included in this list are all of the RGB working spaces that ship with Adobe Photoshop, as well as several others that have emerged as a result of research efforts of various individuals (details here).