In a method for optically checking a large-area three-dimensional object an illumination surface of parallel light beams is generated using a light source, a large-area three-dimensional object is partly or completely illuminated by the illumination surface, and a change of the parallel light beams of the illumination surface through the object is detected, wherein a projection surface is arranged on an object face opposite the light source, and the object shadow cast on the projection surface by the illumination surface is detected using an optical detection device. The strip-shaped light curtain is produced with a Fresnel lens. The shadow is detected using an optical lens system or a CCD camera. The dimensions of the object parallel to the projection surface are ascertained and compared with specified reference values. The projection surface is an opaque panel made of a translucent material or a surface-treated glass pane.