We describe a calibration method for calibrating one or more optical elements of an additive layer manufacturing apparatus useable for producing a three-dimensional workpiece, the method comprising: projecting, using the one or more optical elements, an optical pattern onto a material in order to prepare, from said material, solidified material layers using an additive layer manufacturing technique to form a test sample; determining a geometry of the test sample; comparing the determined geometry with a nominal geometry to generate calibration data; and calibrating the one or more optical elements using said calibration data.