Determination of thresholds to detect missing printing nozzles

A method for detecting defective printing nozzles in an inkjet printing machine having a computer, which method comprises the steps of:

• printing a multi-row nozzle test chart for detection purposes, the multi-row nozzle test chart having a specified number of horizontal rows of equidistant vertical lines printed periodically and disposed underneath one another, wherein in every row of the multi-row nozzle test chart periodically only respective printing nozzles of a print head of the inkjet printing machine contribute to a first element of the multi-row nozzle test chart that correspond to the specified number of the horizontal rows;
• printing an area coverage element in such a way that the area coverage element and the multi-row nozzle test chart are geometrically positioned relative to one another, with the printed area coverage element having the same width as the multi-row nozzle test chart and the area coverage element and multi-row nozzle test chart being printed immediately one after another on the printing substrate, and such that every equidistant vertical line of the nozzle test chart element printed by a single nozzle is allocated to a specific region in the area coverage element;
• recording both the area coverage element and the multi-row nozzle test chart by means of at least one image sensor;
• evaluating both the area coverage element and the multi-row nozzle test chart by means of the computer;
• identifying the defective printing nozzles by evaluating a recorded nozzle test chart by means of the computer;
• allocating defects in the area coverage element to the printing nozzles in the multi-row nozzle test chart by means of the computer,
• evaluating parameters of allocated printing nozzles in the nozzle test chart as a function of the defects in the area coverage element by means of the computer, the parameters defining a range of values from which the computer derives thresholds for every allocated printing nozzle;
• and
• using the thresholds for detecting the defective printing nozzles, wherein an allocation of detected defects in the area coverage element by means of the computer is achieved on a basis of deviations at a corresponding location transverse to a printing direction in the multi-row nozzle test chart, wherein the printing nozzles in the multi-row nozzle test chart that are allocated to the detected defects are always those that, under consideration of the parameters to be evaluated, are most likely to cause the defect.