In order to position a laser beam in "two dimensions", it is possible either to rotate one mirror along two axes - used mainly for slow scanning systems - or to reflect the laser beam onto two closely spaced mirrors that are mounted on orthogonal axes. Each of the two flat or polygonal mirrors is then driven by a galvanometer or by an electric motor. Two-dimensional systems are essential for most applications in material processing, confocal microscopy, and medical science. Some applications require positioning the focus of a laser beam in "three dimensions". This is achieved by a servo-controlled lens system, usually called a 'focus shifter' or 'z-shifter'. Many laser scanners further allow changing the laser intensity.