An apparatus for forming at least one three-dimensional article through successive fusion of parts of a powder bed, which parts corresponds to successive cross sections of the three-dimensional article, the apparatus comprising: a powder distributor configured for evenly distributing a layer of powder on top of a work table provided inside a build chamber; and at least one high energy beam source emanating at least one high energy beam configured for fusing the powder layer in selected locations corresponding to the cross section of the three-dimensional article, wherein the apparatus further comprising at least one target area arranged spaced apart from the layer of powder for emanating light when irradiated by the at least one high energy beam.