Method of compensating for resistance tolerances in printing a multi-tone picture

A method of compensating for resistance variations in printing elements in a printing head for the printing of dots to obtain a multi-tone picture, said printing elements being arranged in a row and said printing elements using a predetermined number m of tone grades of desired optical density taken from a number n of physically available tone grades in which n>m, the method comprising the steps of printing every dot with a tone grade whose available optical density specific to the printing element resistance is closest to the desired optical density, and compensating for the deviation of the available optical density from the desired optical density by combining the desired optical density with the deviation of the available optical density for a directly neighboring physically available tone grade, such that a mean value of the actual optical density of consecutive dots corresponds to the desired optical density;

• further comprising the steps of (a) predetermining a number k of successive resistance ranges such that the resolution thereof, with reference to the nominal value of resistance, reliably falls below the resolution capacity of the human eye, (b) measuring a resistance value of each printing element, (c) assigning each printing element to one of the k resistance ranges, (d) assigning a value to each required optical density and to each available optical density, (e) defining the difference between the values of the required optical density and the actually available optical density as a mean deviation for every printing element, wherein steps (a) to (e) are each carried out once, (f) selecting one of the available tone grades whose optical density neighbors the required optical density for each dot to be printed as a function of the actual value of the mean deviation, and (g) determining the mean deviation for the following print dot, wherein steps (f) and (g) are carried out cyclically.