Additive layer manufacturing apparatus with process monitoring facility

An additive layer manufacturing apparatus 10 in vacuum chamber 12 housing 11, with at least one window 23 for monitoring a region 13 of the chamber in which additive layer manufacturing is to be carried out by an electron beam column 17 generating an electron beam for action on successive layers 16 of fusible powder material, particularly metallic material e.g. a Ti-Al alloy, to produce an article 15. To combat the problem of deposition on the window of material escaping from the layer as gaseous phase constituents in the high temperature vacuum, the apparatus additionally comprises an optical element e.g. a prism or lens 26 to block and intercept such liberated gaseous phase constituents, to shelter the window(s) 23A, 23B, or to guard and protect other elements such as a refractive prism or mirror 26C which directs light to the associated window 23C; while allowing light to pass through. To prevent condensation of the intercepted constituents a heating element 26 is included with the shielding optical elements, e.g. around the perimeter. Behind the windows sensors 25, e.g. cameras, using lasers or the IR spectrum, may be located. Optical characteristics of the elements are preferably maintained up to at least 700 degrees Celsius.