Reducing artifacts using alternating light source power levels
A printhead in a digital printing includes an array of light sources for exposing a photosensitive medium moving past the printhead. Artifacts are reduced in a high-speed print mode by defining first and a second power level control values that differ by at least 20%. The light sources used to print odd-numbered image pixels are activated responsive to the first power level control value and the light sources used to print even-numbered image pixels are activated responsive to the second power level control value.
1. A method for controlling a printhead in a digital printing system to reduce artifacts in a high-speed print mode, the printhead including an array of light sources for exposing a photosensitive medium moving past the printhead at a defined velocity, the light sources being spaced apart by a light-source spacing in a cross-track direction, wherein in the high-speed print mode, image pixels are printed in a pixel printing time such that an in-track printed pixel spacing in an in-track direction is larger than the light-source spacing, comprising:
defining a first power level control value and a second power level control value that differs from the first power level control value by at least 20%;
receiving print data for an image to be printed, wherein the print data includes lines of image data, each line of image data including a one-dimensional array of image pixels having pixel code values; and
using the printhead to print each line of image data to form a printed image, each image pixel being printed with at least one corresponding light source, wherein a pixel code value for the image pixel controls an exposure time for the corresponding light source, and wherein the light sources used to print odd-numbered image pixels are activated responsive to the first power level control value and the light sources used to print even-numbered image pixels are activated responsive to the second power level control value,
wherein the first and second power level control values are defined using an iterative process where a difference between the first and second power level control values and are increased until corresponding print artifacts are decreased to an acceptable level.
2. The method of claim 1 , wherein the printed image has a reduced level of artifacts compared to a nominal printed image formed using the digital printing system where the first power level equals the second power level.
3. The method of claim 2 , wherein the artifacts are streak artifacts.
4. The method of claim 1 , wherein the first power level control value is used to control an overall current level or voltage level supplied to the odd-numbered light sources, and a second power level is used to control an overall current level or voltage level supplied to the even-numbered light sources.
5. The method of claim 1 , wherein the first power level control value is used to control chip-dependent control values for driver chips used to control the odd-numbered light sources, and a second power level is used to control chip-dependent control values for driver chips used to control the even-numbered light sources.
6. The method of claim 1 , wherein each image pixel is printed with a single corresponding light source.
7. The method of claim 1 , wherein each image pixel is printed with two adjacent light sources.
8. A method for controlling a printhead in a digital printing system to reduce artifacts in a high-speed print mode, the printhead including an array of light sources for exposing a photosensitive medium moving past the printhead at a defined velocity, the light sources being spaced apart by a light-source spacing in a cross-track direction, wherein in the high-speed print mode, image pixels are printed in a pixel printing time such that an in-track printed pixel spacing in an in-track direction is larger than the light-source spacing, comprising:
defining a first power level control value and a second power level control value that differs from the first power level control value by at least 20%;
receiving print data for an image to be printed, wherein the print data includes lines of image data, each line of image data including a one-dimensional array of image pixels having pixel code values; and
using the printhead to print each line of image data to form a printed image, each image pixel being printed with at least one corresponding light source, wherein a pixel code value for the image pixel controls an exposure time for the corresponding light source, and wherein the light sources used to print odd-numbered image pixels are activated responsive to the first power level control value and the light sources used to print even-numbered image pixels are activated responsive to the second power level control value,
wherein the first and second power level control values are defined using an automated process which includes:
printing a test target using a plurality of different combinations of first and second power level control values;
evaluating the printed test targets using an automated artifact detection algorithm to determine corresponding artifact levels;
fitting a parametric function to the determined artifact levels, wherein the parametric function relates the first and second power level control values to the corresponding artifact level; and
using the parametric function to select first and second power level control values that produce an acceptable artifact level.