Compensating for drift and sensor proximity in a scanning sensor, in color calibrating incremental printers

An apparatus for printing an image hardcopy on a printing medium;

• said apparatus comprising:
• at least one printhead for marking on such medium;
• a processor for controlling the at least one printhead to discharge inkdrops in a pattern to form such image;
• and
• means for color-calibrating the at least one printhead;
• said means comprising:
• portions of the processor for operating the at least one printhead and the carriage to form a color-calibration test pattern, said test pattern being formed on such medium adjacent to at least one reference area,
• at least one light source for scanning across the test pattern and the at least one area to illuminate the pattern and the at least one area,
• a sensor for scanning across the pattern and at least one area, with the at least one source, to measure illuminated colors in the test pattern and the at least one area,
• means for interpreting measurement signals from the sensor, to correct the sensor output signals for drift due to incomplete warmup;
• said interpreting means comprising processor portions for:
• isolating measurement-signal segments representing the at least one area to establish a tonal-reference calibration level, and
• applying the calibration level to correct the measurement signals due to the measured illuminated colors.

An economical apparatus for printing an image hardcopy on a printing medium, and for obtaining near-colorimetric quality although said apparatus has inexpensive components;

• said apparatus comprising:
• at least one printhead for marking on such medium, said at least one printhead being subject to marking tolerances that require color calibration;
• at least one processor having portions for controlling the at least one printhead to discharge inkdrops in a pattern to form such image;
• and
• means for color-calibrating the at least one printhead;
• said means comprising:
• portions of the processor for operating the at least one printhead to form a color-calibration test pattern, said test pattern being formed on such printing medium adjacent to at least one reference area, of such printing medium, that provides a tonal-reference calibration level,
• plural light-emitting diodes for scanning across the test pattern and the at least one reference area to illuminate the pattern and the at least one area, temperature dependence in the diodes leading to drift of illumination level during warmup,
• a sensor for scanning across the pattern and at least one area, with the diodes, to measure illuminated colors in the test pattern and to measure the at least one reference area, whereby the illumination drift leads to drift of measurement signals from the sensor,
• an analog-to-digital converter for receiving the measurement signals, nonlinearities in the converter making measurements of small signal differences on a large signal pedestal undesirable, wherefore the diodes are used in alternation rather than continuously, and therefore never fully complete warmup, and
• means for compensating for incomplete diode warmup;
• said compensating means comprising portions of the processor for interpreting output signals from the converter, to correct the converter output signals for drift due to said incomplete warmup;
• said interpreting portions comprising processor portions for:
• isolating converter-signal segments representing tonal-reference calibration level, and
• applying the tonal-reference calibration-level segments to correct the measurement signals due to the measured illuminated colors;
• whereby the apparatus accommodates the printhead tolerances and diode temperature dependence, and avoids the converter nonlinearities.

An apparatus for printing an image hardcopy on a printing medium;

• said apparatus comprising:
• at least one printhead for marking on such medium;
• a processor for controlling the at least one printhead to discharge inkdrops in a pattern to form such image;
• and
• means for color-calibrating the at least one printhead;
• said means comprising:
• portions of the processor for operating the at least one printhead to form a color-calibration test pattern on such medium,
• at least one light source for scanning across the test pattern to illuminate the pattern at plural scan positions,
• a sensor for scanning across the pattern, with the at least one source, to measure illuminated colors at the scan positions,
• guide means establishing a spacing between such printing medium and at least a portion of the sensor;
• said guide means being subject to tolerances that lead to nonuniformity of the spacing, at the scan positions,
• means for interpreting measurement signals from the sensor, to correct the sensor output signals for variation due to said nonuniformity of the spacing;
• said interpreting means comprising processor portions for:
• also scanning the sensor across an unprinted region of the medium to obtain respective unprinted-medium tonal-reference calibration levels for the scan positions,
• isolating measurement-signal segments representing the tonal-reference calibration levels for the scan positions, and
• applying the isolated signal segments to correct the measurement signals due to the measured illuminated colors.