Method and apparatus for sensing a register discrepancy in a multi-color printed item, and register control system and register marks

A method for sensing a register discrepancy (r) in a multi-color printed item, the position of at least one register mark (1, 1′) that is associated with one color being sensed with respect to the position of at least one register mark (2, 2′) that is associated with at least one further color, and any register discrepancy (r) being ascertained from discrepancies in the actual relative position (3) with respect to the reference relative position (4), comprising the steps of:

• providing a system wherein possible register discrepancy between colors (r), for which the sum of adjacent individual densities (5, 6) is not equal to the total density (7) when the colors are superimposed, as ascertained by the fact that first the individual densities (5, 6) of the colors and their total density (7) when printed onto one another is measured;
• measuring the density (5, 6, 7) of the register marks (1, 1′, 2, 2′) in a measurement window (8, 9);
• and
• determining a degree of overlap from the measured density values (5, 6, 7), of the register marks (1, 1′, 2, 2′) in the measurement window (8, 9), and using the degree of overlap to determine from that the register discrepancy (r), is measured.