The construction platform is submerged step by step into a tank full of light-sensitive liquid plastic (photopolymer). In each step, mirrors are used to guide a laser beam to the points where it hardens the plastic. The finished component is lifted out of the tank. Depending on the printing method used, the component then has to be hardened further and the support structures, which are printed along with the part, are removed.
Stereolithography has been known for a long time and features a very high accuracy of the 3D printed object. Among all 3D printing technologies, it also offers the best surface quality and appearence.