A stereolithography apparatus (1) for generating a three-dimensional object (2), comprising: an optical unit (4) for projecting an image towards the photocurable substance (3) for hardening the photocurable substance (3) deposited in the focus layer (5); a control unit (6), characterized by further comprising: a detection unit (7) which comprises: a detection means (8) movably arranged in a detection region (9) for detecting during the generation process or in a generation-pause the image projected by the optical unit (4) and for outputting a detection signal; and a first driving means (10) for moving the detection means (8) into or out of the detection region (9), wherein the optical unit (4) comprises: a second driving means (11) linked to the optical unit (4) for moving the focus layer (5) into or out of the detection region (9), wherein the control unit (6) adjusts the optical unit (4) and/or modifies the image to be projected based on the signal indicative of the detected image.