The point where the two axes cross is the centre of the goniophotometer and some means should be provided for marking this position. Since this is in mid-air it is convenient to use an optical device such as a laser fixed to a wall, at the same height as the centre. Then, when it is switched on, its spot of light can be seen shining on the luminaire. It is also a good idea to mark permanently on a wall or other convenient surface the nadir and the point indicating the extended horizontal axis. These can then be used for future reference.