A method for detecting defects when forming a three-dimensional article through successive fusion of parts of a powder bed, which parts corresponds to successive cross sections of the three-dimensional article, said method comprising the steps of: a. providing a model of said three dimensional article, b. providing a first powder layer on a work table, c. directing an energy beam over said work table causing said first powder layer to fuse in selected locations according to said model to form a first cross section of said three-dimensional article, d. providing a second powder layer on said work table, e. directing the energy beam over said work table causing said second powder layer to fuse in selected locations according to said model to form a second cross section of said three-dimensional article, wherein said second layer is bonded to said first layer, characterized in that said method further comprising the steps of: f. capturing at least one first heat image of at least a first fusion zone of said first powder layer after having fused selected portions of said first powder layer, g. capturing at least one second heat image of at least a second fusion zone of said second powder layer, wherein said second fusion zone is at least partly overlapping said first fusion zone, h. comparing said first and second heat images with corresponding layers in said model, i. detecting a defect in the three-dimensional article if a deviation in said first heat image with respect to said model is at least partially overlapping a deviation in said second heat image with respect to said model, where the defect is in the form of cavities or irregularities in the fused layer which are large enough for the camera to capture a deviation. a. providing a model of said three dimensional article, b. providing a first powder layer on a work table, c. directing an energy beam over said work table causing said first powder layer to fuse in selected locations according to said model to form a first cross section of said three-dimensional article, d. providing a second powder layer on said work table, e. directing the energy beam over said work table causing said second powder layer to fuse in selected locations according to said model to form a second cross section of said three-dimensional article, wherein said second layer is bonded to said first layer, characterized in that said method further comprising the steps of: f. capturing at least one first heat image of at least a first fusion zone of said first powder layer after having fused selected portions of said first powder layer, g. capturing at least one second heat image of at least a second fusion zone of said second powder layer, wherein said second fusion zone is at least partly overlapping said first fusion zone, h. comparing said first and second heat images with corresponding layers in said model, i. detecting a defect in the three-dimensional article if a deviation in said first heat image with respect to said model is at least partially overlapping a deviation in said second heat image with respect to said model, where the defect is in the form of cavities or irregularities in the fused layer which are large enough for the camera to capture a deviation.