Methods and apparatus for determining a total colorant limit for digital imaging devices
View Patent ↗Methods and apparatus are provided for calibrating a digital color imaging device to a printing press by determining a total colorant limit, per-channel colorant limits, and channel linearization tables using colorimetric and/or spectrophotometric techniques. In addition, for digital color imaging devices that use multi-hue colorants, methods and apparatus are provided for determining distribution functions for the multi-hue colorants as a function of input values.
1. A system for calibrating a digital color imaging device, the system comprising:
a color output device for printing an image file including a test pattern, the test pattern comprising a plurality of strips, each strip comprising a plurality of test patches, the test patches for each strip comprising scales of percentages of a fixed number of at least two C, M, Y, and K colorants;
a measurement device that measures each of the test patches for colorimetric data;
a processor for calculating the total colorant limit by:
receiving the colorimetric data from the measurement device based upon the colorimetric data;
selecting one patch in each strip with a minimum lightness;
determining, for each patch in each strip that was selected by the processor as having a minimum lightness, a total area coverage (TAC), the TAC comprising a sum of the percentages of colorants used to produce the patch;
selecting a maximum TAC from the TAC determined for each patch in each strip that was selected by the processor as having the minimum lightness; and
the processor setting the total colorant limit to the maximum TAC selected by the processor.
2. The system of claim 1 wherein the patches include at least one 100% single-colorant patch for each colorant and the processor generates a per-channel colorant limit by:
determining a minimum limit for each colorant,
determining a maximum limit for black by identifying a colorant value for a black patch with a minimum lightness,
determining a maximum limit for a colorant other than black by identifying a colorant value of a printed patch having one of a maximum chroma and a colorant where oversaturation beings, and
an optimal limit for each colorant.
3. The system of claim 2 , wherein the minimum colorant limit for each colorant is based on a comparison between the measured colorimetric values of the single-colorant patch and a colorimetric value of a press profile.
4. The system of claim 1 , wherein the colorants comprise multi-hue colorants.
5. The system of claim 2 ,
wherein the processor generates an optimal limit for each colorant by determining the colorant values for interpolated patches between minimum and maximum colorants, calculating a chroma value for each interpolated patch, determining a deviation between calculated and measured chroma values, vector-correcting chroma values for interpolated patches, and identifying an interpolated patch having a minimum chroma.
6. The system of claim 1 , wherein the instrument comprises a spectrophotometer.
7. The system of claim 1 , wherein the instrument comprises a spectrocolorimeter.
8. The system of claim 1 , wherein the colorimetric measurements comprise CIELAB data values.
9. The system of claim 1 , wherein the colorimetric measurements comprise CIEXYZ data values.
10. The system of claim 1 , wherein the processor generates a linearization table by calculating a plurality of tonal response values from the measured colorimetric values, retrieving colorimetric values from a press profile, calculating a plurality of midtone tonal response values from retrieved colorimetric values, calculating a tonal response of a press for a range of input values, and mapping the press input values to equivalent input values for the color output device using the tonal response values.
11. The system of claim 4 , wherein the processor determines distribution functions for the multi-hue colorants.
12. The system of claim 11 , wherein the processor determines a distribution function for each multi-hue colorant by calculating a tonal response from the measured colorimetric values, calculating a tonal response of a press for a range of input values, calculating a range of light distribution, and creating a table of light to normal tonal responses.
13. A computer-implemented method for calibrating a digital color imaging device, the method comprising:
printing a test pattern, the test pattern comprising a plurality of strips, each strip comprising a plurality of test patches, the test patches for each strip comprising scales of percentages of a fixed number of at least two C, M, Y, and K colorants;
measuring each of the test patches for colorimetric data;
receiving the colorimetric data from the measurement device;
selecting, with a processor, one patch in each strip with a minimum lightness;
based on the patches selected in the previous step, determining, with the processor, a total area coverage (TAC), the TAC comprising a sum of the percentages of colorants used to produce the patch;
based on the patches for which TAC was determined in the previous step, selecting, with the processor, the patch with a maximum TAC; and
setting, with the processor, the total colorant limit to the maximum TAC that was determined in the previous step.
14. The method of claim 13 , wherein the patches include at least one 100% single-colorant patch for each colorant and further comprising generating a per-channel colorant limit by:
determining a minimum limit for each colorant,
determining a maximum limit for black by identifying a colorant value for a black patch with a minimum lightness,
determining a maximum limit for a colorant other than black by identifying a colorant value of a printed patch having one of a maximum chroma and a colorant where oversaturation begins, and an optimal limit for each colorant.
15. The method of claim 14 , wherein the minimum colorant limit for each the minimum colorant limit for each colorant is based on a comparison between the measured colorimetric values of the single-colorant patch and a colorimetric value of a press profile.
16. The method of claim 13 , wherein the colorants comprise multi-hue colorants.
17. The method of claim 14 , further comprising the step of:
generating an optimal limit for each colorant by determining the colorant values for interpolated patches between minimum and maximum colorants, calculating a chroma value for each interpolated patch, determining a deviation between calculated and measured chroma values, vector-correcting chroma values for interpolated patches, and identifying an interpolated patch having a minimum chroma.
18. The method of claim 13 , wherein colorimetrically measuring comprises measuring the printed test pattern using a spectrophotometer.
19. The method of claim 13 , wherein colorimetrically measuring comprises measuring the printed test pattern using a spectrocolorimeter.
20. The method of claim 13 , wherein the colorimetric measurements comprise CIELAB data values.
21. The method of claim 13 , wherein the colorimetric measurements comprise CIEXYZ data values.
22. The method of claim 13 , further comprising generating a linearization table by calculating a plurality of tonal response values from the measured colorimetric values, retrieving colorimetric values from a press profile, calculating a tonal response of a press for a range of input values, and mapping the press input values to equivalent input values for the color output device using the tonal response values.
23. The method of claim 16 , further comprising generating a distribution function for each multi-hue colorant.
24. The method of claim 23 , wherein generating a distribution function for each multi-hue colorant comprises calculating a tonal response from the measured colorimetric values, calculating a tonal response of a press for a range of input values, calculating a range of light distribution, and creating a table of light to normal tonal responses.