an apparatus for optically scanning a document to generate digital image data corresponding to local optical densities of the document including a fixed glass plate , one side of which defines an original image plane , a transporting mechanism for transporting the document in a sub - scanning direction in the original image plane , at least two linear sensors situated in a main scanning direction , each having an array of opto - electric converters and each provided with a lens which images on the sensor a linear observation area , situated in the original plane , wherein the observation areas of the sensors are substantially in line and partially overlapping at the ends in the main scanning direction . an aligning element is situated between the lenses and the original image plane at a short distance from the latter in a position such that its perpendicular projection onto the original image plane is situated within the overlapping ends of two adjoining observation areas , wherein the aligning element has a shape such that for each sensor in whose observation area it is situated the aligning element has a width in the main scanning direction which monotonically increases or decreases as a function of the sub - scanning direction .