A device including an imaging optical unit (9) imaging an object field (5) in an image field (10), a structured mask (7), arranged in the region of the object field (5) via reticle holder (8) displaceable in a reticle scanning direction (21), and a sensor apparatus (25), arranged in the region of the image field (10) via a substrate holder (13) displaceable in a substrate scanning direction (22). The mask (7) has at least one measurement structure (27; 33) to be imaged on the sensor apparatus (25), wherein the sensor apparatus (25) includes at least one line sensor (28) with a multiplicity of sensor elements (29), and affords the possibility of testing the imaging optical unit (9) during the displacement of the substrate holder (13) for exposing a substrate (12) arranged on the substrate holder.