Printhead calibration and printing

A method of calibrating a printhead for printing two-dimensional bit-mapped images having a number of pixels per row, the printhead having a row of printing channels, wherein the volume of marking fluid ejected from each printing channel in use is independently controlled by respective control pulses, such that for a given printing channel the volumes of marking fluid ejected are determined by pulse values of the control pulse and wherein the pulse values are determined by respective image pixel bit values, and wherein the pulse value required to eject a given volume of marking fluid may vary between printing channels of the printhead, the calibration method comprising:

• providing an image that, when printed by an uncalibrated printhead, causes the printhead to print a plurality of rows, wherein each row is oriented parallel to the printing channels of the printhead and wherein, to print each row, each printing channel of the printhead is driven with the same pulse value, and wherein the pulse value with which the printing channels are driven varies between the rows of the image in a set of defined steps,
• printing one or more test prints of said image,
• measuring the optical density of the test print or test prints at positions arranged on a grid to obtain data of optical print density and pulse value at positions across the printhead,
• selecting a desired tone reproduction curve for the print process represented by optical density versus image grey level, wherein the tone reproduction curve specifies a desired optical density associated with different pixel bit values,
• calculating, for each printing channel, pulse values from the measured test print or test prints that are estimated to produce the desired values of optical print density corresponding to selected pixel bit values as specified by the tone reproduction curve and which may include non-printing pulse values, and
• recording in memory, for each printing channel across the printhead, the pulse value for each of said printing channels required to produce the desired optical density for the selected pixel bit values as specified by the tone reproduction curve.