Method and apparatus for calculating metameric colors

1. A method for selecting a metameric pair of first and second color patches having colorant values for the purpose of visually assessing fading of a printed product under a specific illuminant, the method comprising the steps of:

(a) determining a fading model by printing a characterization target with a printer, measuring the characterization target under the specific illuminant at a first time T=0 and after a time interval at a second time T=1, and calculating corresponding first and second printer models;

(b) selecting a first set of colorant values for the printer in a printer colorant space, the first set of colorant values corresponding with a first color patch;

(b) converting the first set of colorant values into a first set of color coordinates in an XYZ color space using the first printer model;

(c) converting the first set of color coordinates in the XYZ color space into multiple second sets of colorant values that are different from the first set of colorant values using an inverse of the first printer model;

(d) converting the multiple second sets of colorant values into multiple second sets of color coordinates in the XYZ color space using the second printer model;

(e) converting the first set of color coordinates in the XYZ color space into a first set of color values in a CIELAB color space and converting the multiple second sets of color coordinates in the XYZ color space into multiple second sets of color values in the CIELAB color space;

(f) determining a difference between the first and second sets of color values in the CIELAB color space; and

(g) selecting one of the second sets of colorant values that yields a maximum difference between the first and the second sets of color values in the CIELAB color space and that corresponds with a second color patch.

2. The method according to claim 1 , wherein the first set of colorant values is selected within a colorant gamut so that the difference between the first and second sets of colorant values in the CIELAB color space is maximized.

3. The method according to claim 1 , further comprising the step of printing the metameric pair of first and second color patches having the first set of colorant values and the selected second set of colorant values.

4. The method according to claim 2 , further comprising the step of printing the metameric pair of first and second color patches having the first set of colorant values and the selected second set of colorant values.

5. The method according to claim 3 , further comprising the step of visually assessing a color difference between the metameric pair of first and second color patches.

6. The method according to claim 4 , further comprising the step of visually assessing a color difference between the metameric pair of first and second color patches.

7. A non-transitory digital data carrier comprising computer code instructions for carrying out, when the computer code instructions are run on a computer, the steps according to claim 1 .