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Abstract

A method of calibrating a printhead, for printing two-dimensional bit-mapped images having a number of pixels per row, is disclosed for printheads (1) having a row of printing channels (5). During printing, in order to cause volumes of charged particulate concentrations of one of a number of predetermined volume sizes to be ejected from selected ejection channels of the printhead to form printed pixels, control pulse values of respective predetermined amplitude and duration, as determined by respective image pixel bit values, are applied to the selected printing channels. The calibration method comprises providing an image (50) that causes each channel of the printhead to be driven with the same pulse value, and printing one or more test prints of the image. The pulse value for all channels is then varied (101) in a set of defined steps within the test print or between the test prints and the optical density of the test print or test prints measured (102) at positions arranged on a grid (51) to obtain data of print density and pulse value at positions across the printhead. A desired tone reproduction curve (52) is pre-selected for the print process represented by optical density versus image grey level. Then pulse values are calculated (104) from the measured test print or test prints that are estimated to produce the desired values of print density corresponding to selected values of image grey level and which may include non-printing pulse values, and the pulse value for each of said positions across the printhead for each of said image grey levels is recorded in memory (105,106).

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First Claim