A method is provided for forming a three-dimensional article through successive fusion of parts of a powder bed. The method includes the steps of: applying a first powder layer on a work table； directing an electron beam from an electron beam source over the work table, the directing of the electron beam causing the first powder layer to fuse in first selected locations according to a pre-determined model, so as to form a first part of a cross section of the three dimensional article, and intensity modulating X-rays from the powder layer or from a clean work table with a patterned aperture modulator and a patterned aperture resolver, wherein a verification of at least one of a size, position, scan speed, or shape of the electron beam is achieved by comparing detected intensity modulated X-ray signals with saved reference values.
