Halftone independent color drift correction

A xerographic printing apparatus comprising:

• an image processing unit adapted to receive image data from a source;
• a printing unit comprising a print engine for printing the image data to define a printed image;
• a color sensor for detecting a color of the printed image printed by the print engine, said color sensor operably connected to the image processing unit to input said color of said printed image to the image processing unit;
• a printed output station for output of the printed image;
• wherein said image processing unit comprises means for compensating for color drift in said printing apparatus when said printing apparatus changes from a first state to a second state, said means for compensating for color drift comprising:
• means for determining a first true tone response curve for a color channel of said printing apparatus when said printing apparatus is in said first state;
• means for determining a first estimated tone response curve for the color channel when said printing apparatus is in said first state;
• means for determining a second estimated tone response curve for the color channel when said printing apparatus is in said second state;
• means for calculating a predicted second true tone response curve for the color channel using said first true tone response curve, said first estimated tone response curve, and said second estimated tone response curve.

A method for color drift correction in a printing device, said method comprising:

• determining a true tone response curve for at least one color channel for a first state of said printing device;
• printing a first plurality of test patches using said at least one color channel when said printer is in said first state;
• measuring a first color output for each of said first plurality of test patches;
• deriving a first plurality of different binary output patterns corresponding respectively to a first plurality of different digital input color levels;
• for each of said first plurality of binary output patterns, deriving a first model comprising a combination of test patches selected from said first plurality of test patches;
• for each first model, calculating an average color output based upon said measured first color output for the test patches from said first plurality of test patches included in said first model;
• using said average output color calculated for each of said first models to derive a first estimated tone response curve for said at least one color channel;
• printing a second plurality of test patches using said at least one color channel when said printer is in a second state that is different from said first state;
• measuring a second color output for each of said second plurality of test patches;
• deriving a second plurality of different binary output patterns corresponding respectively to a second plurality of different digital input color levels;
• for each of said second plurality of binary output patterns, deriving a second model comprising a combination of test patches selected from said second plurality of test patches;
• for each second model, calculating an average color output based upon said measured color output for the test patches of said second plurality of test patches included in said second model;
• using said average output color calculated for each of said second models to derive a second estimated tone response curve for said at least one color channel;
• deriving a second true tone response curve for said at least one color channel using said first and second estimated tone response curves and said first true tone response curve.