Patent ID: 7551775

Claim:
A method of evaluating an image of a predetermined extract of a printed product, comprising: using a processor to perform, recording an image of a white reference using a camera for acquiring images of the predetermined extract; deriving basic reference data only once, at the outset of a print job before color is applied to the printing material, for evaluating the extract from intensity signals of the image of the white reference; during each subsequent evaluation of an image of the extract acquired by the camera, deriving correction data from predetermined reference areas within the extract; generating a set of corrected reference data based on the basic reference data and the correction data by: calculating a first interpolation function for the basic reference data and a second interpolation function for the correction data; for a given location in the extract, determining a ratio of the second interpolation function over the first interpolation function for said given location; and multiplying a value of the basic reference data for the given location with the ratio to generate a value for corrected reference data for said given location; and combining intensity signals recorded from predetermined measuring areas with the corrected reference data to provide corrected intensity signals for the predetermined measuring areas.