Method for controlling the ink feed of a printing machine for half-tone printing

Claim 1

A method for controlling the ink feed elements of a printing machine operated in half-tone, in particular an offset printing machine, the method comprising the steps of: selecting test regions on an original and corresponding test regions on a printed product printed in printing inks of three chromatic colors and the color black; detecting photoelectrically reflectance values of the selected test regions, wherein for the black printing ink the reflectance is detected in the near infrared spectral range; determining the infrared color density for the black printing ink from the reflectance in the near infrared spectral range; determining from the reflectance values of the test regions the standard color values of the four-color printing; converting via a linear transformation the standard color values of the four-color printing to the standard color values of a three-color printing corresponding to a color locus which is produced by the combined print of only the three chromatic color, the coefficients of the linear transformation being determined empirically as a function of the infrared color density of the black ink; determining the optically effective surface coverage values of the three chromatic colors from the standard color values of the three-color printing; determining the optically effective surface coverage value of the black ink from the infrared color density via an empirically determined relationship; adjusting the settings for ink feed elements of the printing machine according to the differences in the effective surface coverage values between the original and the printed product.

Claim 2

A method for controlling the ink feed elements of a printing machine operated in half-tone, in particular an offset printing machine, the method comprising the steps of: selecting test regions on an original and corresponding test regions on a printed product printed in printing inks of three chromatic colors and the color black; detecting photoelectrically reflectance values of the selected test regions, wherein for the black printing ink the reflectance is detected in the near infrared spectral range; determining the infrared color density for the black printing ink from the reflectance in the near infrared spectral range; determining from the reflectance values of the test regions the standard color values of the four-color printing; converting via a linear transformation the standard color values of the four-color printing to the standard color values of a three-color printing corresponding to a color locus which is produced by the combined print of only the three chromatic color, the coefficients of the linear transformation being determined empirically as a function of the infrared color density of the black ink; determining the optically effective surface coverage values of the three chromatic colors from the standard color values of the three-color printing; determining the optically effective surface coverage value of the black ink from the infrared color density via an empirically determined relationship; adjusting the settings for ink feed elements of the printing machine according to the differences in the effective surface coverage values between the original and the printed product.