Method for verifying characteristics of an electron beam

A method for forming a three-dimensional article through successive fusion of parts of a powder bed, which parts correspond to successive cross sections of the three-dimensional article, said method comprising the steps of:

• providing a model of said three-dimensional article;
• applying a first powder layer on a work table;
• directing an electron beam from an electron beam source over said work table, said directing of said electron beam causing said first powder layer to fuse in first selected locations according to said model, so as to form a first part of a cross section of said three dimensional article, detecting X-rays produced by the electron beam, and intensity modulating the X-rays from said 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.