Abstract: A scanner is provided with photosensors and a stationary registration mark placed on each side of the reading window. The registration mark is thus located in the reading field of the photosensors as long as no document is presented for scanning within the reading window. A document which arrives within the reading window is interposed between the registration mark and the photosensors. A comparison circuit coupled with the photosensors delivers a document-reading start signal when the reading signal no longer corresponds to a registration-mark reading signal stored in memory during a preceding reading operation, that is, when the front edge of the document begins to enter the reading window.