Additive manufacturing apparatus and method

A laser solidification apparatus for building objects by layerwise solidification of powder material, the apparatus comprising a build chamber containing a build platform, a device for depositing layers of powder material on to the build platform, an optical unit for directing a laser beam to selectively solidify areas of each powder layer, the optical unit comprising tiltable mirrors for steering the laser beam onto the powder layer, movable focussing optics for focussing the laser beam on the powder layer, an actuator for driving movement of the movable focusing optics and a controller arranged to control movement of the focusing optics synchronously with movement of the tiltable mirrors to maintain the focus of the laser beam in the same plane as a deflection angle of the laser beam changes, a photodetector arranged for detecting characteristic radiation, generated by interaction of material in the layer with the or a further laser beam, the photodetector being arranged to detect the characteristic radiation that is collected by the tiltable mirrors and the movable focussing optics, a beam splitter arranged to reflect the laser beam and transmit the characteristic radiation, and a heat dump for capturing laser light that is transmitted through the beam splitter.