A sheet edge detection device (20) wherein the signal threshold may be calibrated without interrupting the sheet transport. A detector unit (30) comprising a detector (31) and an emitter (32) emitting at a constant intensity generates signal data as a sheet (S) is transported along the detector (31). While the sheet (S) travels further on a calibration transport path section (CS), a processor (40) processes the signal data to calibrate the signal threshold by deriving a first signal level corresponding to a state wherein no sheet (S) is present between the detector unit (30) and by deriving a second signal level corresponding to a state wherein a sheet (S) covers the detector (31). From said signal levels the signal threshold is derived. This signal threshold is then compared to delayed signal data to determine the position of a point on a sheet edge of the sheet (S) before said sheet (S) reaches the upstream end of the calibration transport section (CS).