Scanner having a merging mark and white reference to correct output of the sensor chip

A scanner comprising:

• multiple sensor chips;
• an optical unit that reduces and focuses an image of a document on the sensor chips;
• a generator that synthesizes images of the document read by the sensor chips, and generates image data;
• and
• a switching unit that can change between and set to the exposure position, to which light from a light source is emitted on the conveyance path of the document, a merging mark surface having a merging mark used as a reference for image synthesis, and a white calibration surface used as a white reference;
• the generator synthesizing images based on sensor chip output when the merging mark surface is at the exposure position, and
• correcting output of the sensor chip based on sensor chip output when the white calibration surface is at the exposure position.