Multilevel density compensation for digital printing machines
A method corrects color density unevenness during a printing operation on an inkjet printing machine having printing nozzles and a computer. The computer modifies the number or size of the drops to be applied to a printing substrate to attain nominal color density values after the processing of a rasterized print image. For different actual tone values, it is determined which target tone value is required to attain a nominal coloration by measuring a test image for every printing nozzle using a color measurement device, and, based thereon, an area coverage-dependent compensation profile is determined and saved on the computer to be applied in the printing operation to calculate the number and size of the drops on the computer.
1. A method for correcting color density unevenness during a printing operation on an inkjet printing machine having printing nozzles and a computer, which comprises the steps of:
modifying, via the computer, a number or size of drops to be applied to a printing substrate after a processing of a rasterized print image to attain nominal color density values;
determining, for different actual tone values, which target tone value is required to attain a nominal coloration by measuring a test image for every printing nozzle using a color measurement device; and
determining an area coverage-dependent compensation profile based on the target tone value and saving the area coverage-dependent compensation profile on the computer to be applied during the printing operation to calculate the number and/or the size of the drops on the computer.
2. The method according to claim 1 , wherein the computer applies the area coverage-dependent compensation profile by calculating a required number and/or the size of the drops in real time during the printing operation.
3. The method according to claim 1 , wherein the drops exist in two sizes and the computer calculates a required number of the drops in the two sizes.
4. The method according to claim 3 , wherein the drops exist in the two drop sizes S and M.
5. The method according to claim 1 , which further comprising carrying out, via the computer, a number of steps to calculate the area coverage-dependent compensation profile.
6. The method according to claim 5 , which further comprises performing the following steps of:
calculating an intensity profile;
masking white lines;
adapting the intensity profile;
allocating image dots to the printing nozzles; and
calculating nominal values.
7. The method according to claim 1 , which further comprises:
calculating an area coverage wedge from a print original in digital form; and
calculating, via the computer, the number and/or the size of the drops in dependence on area coverage and saving a result in a form of a drop mixture table.
8. The method according to claim 1 , which further comprises calculating, via the computer, an actual area coverage and a nominal area coverage for every nozzle.
9. The method according to claim 8 , which further comprises calculating, via the computer, a compensation factor based on the nominal area coverage and the actual area coverage.