1. A method of processing signals from a first a second and a third image sensor arranged in a large format optical scanner that provide respective image signals recorded at respective areas of a scanned medium, where a portion of the respective areas is recorded by two of the respective image sensors, wherein image data from at least a first and second overlapping area and a non-overlapping area are present in a signal from the second image sensor; wherein the image sensors comprise individual image sensor elements; the method comprising: determining a first and a second level of intensity from image signals, at the first overlapping area, from the first and the second image sensor respectively; determining a third and a fourth level of intensity from image signals, at the second overlapping area, from the second and the third image sensor respectively; determining a first compensation value by processing the first level of intensity and the second level of intensity to determine a discrepancy between the first and second levels of intensity at the first overlapping area; determining a second compensation value by processing the third level of intensity and the fourth level of intensity to determine a discrepancy between the third and fourth levels of intensity at the second overlapping area; generating a set of compensation values for the individual image sensor elements of the second image sensor at the non-overlapping area by performing an interpolation between the first and second compensation values; applying the compensation values, determined from the levels of intensity of image signals at the overlapping areas, to image data from overlapping and non-overlapping areas to provide compensated image data to compensate for a discrepancy between levels of intensity from the first and the second image sensor and to compensate for a discrepancy between levels of intensity from the second and the third image sensor, wherein the above steps are performed at the same time as a document is being scanned.