A standard reference plate is configured for insertion into an additive manufacturing apparatus for calibrating an electron beam of the additive manufacturing apparatus. The standard reference plate includes a lower plate and an upper plate being essentially in parallel and attached spaced apart from each other, the upper plate including a plurality of holes. A predetermined hollow pattern is provided inside the holes, and a spacing between the holes and the size of the holes and a distance between the upper plate and the lower plate and a position of an x-ray sensor of the additive manufacturing apparatus with respect to the standard reference plate are arranged so that x-rays emanating from the lower plate, when the electron beam is passing through a hollow part of the hollow pattern, will not pass directly from the lower plate through any one of the holes to the x-ray sensor.