A method for forming a 3-dimensional article through successive fusion of parts of a powder bed, which parts correspond to successive cross sections of the 3-dimensional article, said method comprising the steps of: providing (step 410) a model of said 3-dimensional article; applying (step 420) a first powder layer on a work table, directing (step 430) an electron beam (370, 110) 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, and intensity modulating (step 440) 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 and/or shape of the electron beam is achieved by comparing detected intensity modulated X-ray signals (121-127, 511, 521) with saved reference values, wherein said verification comprising the steps of: arranging a patterned aperture resolver (140, 240, 240', 515, 525, 630, 740) having at least one opening (145, 241-243, 241'-243', 745) in front of at least one X-ray detector (150, 250, 510, 525, 650, 750), where said at least one opening is facing towards said at least one X-ray detector; arranging a patterned aperture modulator (130, 230, 230', 530, 540, 640, 730) between said patterned aperture resolver and a substrate (170, 270, 570, 670, 770) and at a predetermined distance from said patterned aperture resolver (B, P) and said substrate (A, R), where said patterned aperture modulator having a plurality of openings (131-137, 231-236, 231'-233', 735) in at least a first direction; scanning an electron beam in at least a first direction on said substrate for generating X-rays to be received by said at least one X-ray detector; and verifying at least one of a position, size and shape of said electron beam by comparing a detected intensity modulation of said X-ray signal by said detector with corresponding reference values, wherein said electron beam is said to be verified if a deviation of said detected signal and said reference value is smaller than a predetermined value. providing (step 410) a model of said 3-dimensional article; applying (step 420) a first powder layer on a work table, directing (step 430) an electron beam (370, 110) 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, and intensity modulating (step 440) 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 and/or shape of the electron beam is achieved by comparing detected intensity modulated X-ray signals (121-127, 511, 521) with saved reference values, wherein said verification comprising the steps of: arranging a patterned aperture resolver (140, 240, 240', 515, 525, 630, 740) having at least one opening (145, 241-243, 241'-243', 745) in front of at least one X-ray detector (150, 250, 510, 525, 650, 750), where said at least one opening is facing towards said at least one X-ray detector; arranging a patterned aperture modulator (130, 230, 230', 530, 540, 640, 730) between said patterned aperture resolver and a substrate (170, 270, 570, 670, 770) and at a predetermined distance from said patterned aperture resolver (B, P) and said substrate (A, R), where said patterned aperture modulator having a plurality of openings (131-137, 231-236, 231'-233', 735) in at least a first direction; scanning an electron beam in at least a first direction on said substrate for generating X-rays to be received by said at least one X-ray detector; and verifying at least one of a position, size and shape of said electron beam by comparing a detected intensity modulation of said X-ray signal by said detector with corresponding reference values, wherein said electron beam is said to be verified if a deviation of said detected signal and said reference value is smaller than a predetermined value. arranging a patterned aperture resolver (140, 240, 240', 515, 525, 630, 740) having at least one opening (145, 241-243, 241'-243', 745) in front of at least one X-ray detector (150, 250, 510, 525, 650, 750), where said at least one opening is facing towards said at least one X-ray detector; arranging a patterned aperture modulator (130, 230, 230', 530, 540, 640, 730) between said patterned aperture resolver and a substrate (170, 270, 570, 670, 770) and at a predetermined distance from said patterned aperture resolver (B, P) and said substrate (A, R), where said patterned aperture modulator having a plurality of openings (131-137, 231-236, 231'-233', 735) in at least a first direction; scanning an electron beam in at least a first direction on said substrate for generating X-rays to be received by said at least one X-ray detector; and verifying at least one of a position, size and shape of said electron beam by comparing a detected intensity modulation of said X-ray signal by said detector with corresponding reference values, wherein said electron beam is said to be verified if a deviation of said detected signal and said reference value is smaller than a predetermined value.