The present invention relates to a building platform (100) for building up a workpiece (200) in layers or continuously by stereolithography, comprising: a rear illumination device (101-1) for illuminating a layer (103-n) from a rear side; a photosensor (105) for detecting a light intensity of a light from a front illumination device (101-2) through the layer (103-n) and/or a material and/or a deflecting mirror; and a control device (107) for activating the rear illumination device (101-1) when the detected light intensity of the front illumination device (101-2) is above a predetermined threshold.