Color calibration

1. A method, comprising:

determining that a print color established for printing a reference color by a printing system using multiple colors has a measured difference from the reference color by more than a color consistency threshold;

in response to determining that the print color measurably differs from the reference color by more than the color consistency threshold, creating a color calibration chart including a plurality of colors that are variations of the print color, each color measurably differing from the print color by less than the measured difference;

measuring the colors in the calibration chart; and

selecting from the calibration chart the color for printing the reference color which has a measurement value indicative of a color consistency within a predefined distance from a reference value for the print color, calibrating the printing system for printing the reference color using the multiple colors by replacing the print color with the selected color for printing the reference color; and

printing an image including the reference color by using the color selected from the calibration chart for the reference color.

2. The method of claim 1 , wherein the plurality of colors that are variations of the print color in the calibration chart are generated by printing a plurality of color patches having increasing color differences from the print color.

3. The method of claim 1 , wherein the colors that are variations of the print color are independent of the ICC profile of the printing system.

4. The method of claim 1 , wherein the colors in the color calibration chart depend on a location of the print color on a gamut.

5. The method of claim 1 , wherein the colors in the calibration chart are photospectrometrically measured by a photospectrometer of the printing system.

6. The method of claim 1 , wherein selecting the color from the calibration set comprises analyzing the vector contribution of the color difference between the measurement value and the reference value.

7. The method of claim 1 , comprising, in case there is no measurement value indicative of the color consistency within the predefined distance from the reference value, creating a further calibration chart having a plurality of variations of the print color generated with color differences from the print color which are smaller than the color differences in the preceding calibration chart, and repeating the measuring and the selecting.

8. The method of claim 1 , comprising obtaining a reference value for the print color, the reference value representing the reference color.

9. The method of claim 8 , wherein obtaining the reference value comprises: printing the reference color; photospectrometrically measuring the printed reference color; and storing the measurement result as the reference value.

10. The method of claim 9 , wherein the reference color is printed using colors of the printing system obtained from a lookup table associating the reference color with the colors of the printing system for printing the reference color.

11. The method of claim 8 , wherein the creating, the measuring and the selecting is performed in response to a predefined event.

12. The method of claim 11 , wherein the predefined event comprises a lapse of a predefined time interval after obtaining the reference value for the print color or after the last printout of the reference color, or a modification of the printing system, or a change of environmental parameters when compared to the environmental parameters at the time of obtaining the reference value.

13. The method of claim 1 , comprising: based on a physical representation of the reference color, creating a color chart having a plurality of colors, the plurality of colors including the color for printing the reference color by the printing system and a plurality of variations of the color; visually evaluating the color chart by a user and selecting from the chart one of the colors to be used for printing the reference color; and storing the selected color as the print color for printing the reference color.

14. A non-transitory computer-readable medium having instructions which, when executed by a processor, cause to processor to:

determine that a print color established for printing a reference color by a printing system using multiple colors has a measured difference from the reference color by more than a color consistency threshold;

in response to determining that the print color measurably differs from the reference color by more than the color consistency threshold, create a color calibration chart including a plurality of colors that are variations of the print color, each color measurably differing from the print color by less than the measured difference;

measure the colors in the calibration chart; and

select from the calibration chart the color for printing the reference color which has a measurement value indicative of a color consistency within a predefined distance from a reference value for the print color, calibrating the printing system for printing the reference color using the multiple colors by replacing the print color with the selected color for printing the reference color,

wherein an image including the reference color is printed by using the color selected from the calibration chart for the reference color.

15. A multiple color printing system, comprising:

a print head;

a measurement device; and

a controller to cause the print head to:

determining that a print color established for printing a reference color by a printing system using multiple colors has a measured difference from the reference color by more than a color consistency threshold;

in response to determining that the print color measurably differs from the reference color by more than the color consistency threshold print a color calibration chart including a plurality of colors that are variations of the print color, each color measurably differing from the print color by less than the measured difference;

cause the measurement device to measure the colors in the calibration chart, and to select from the calibration chart the color for printing the reference color which has a measurement value indicative of a color consistency within a predefined distance from a reference value for the print color, calibrating the printing system for printing the reference color using the multiple colors by replacing the print color with the selected color for printing the reference color,

wherein the multiple color printing system prints an image including the reference color is printed by using the color selected from the calibration chart for the reference color.