. If there are mirrors, these will have to be tilted to achieve this. A check should be made that when the laser beam is directed downwards and the goniophotometer arm is moved into the nadir position, the laser spot still lies in the centre of the photocell. Any straying indicates that there is bending in the arm or mirror. CIE 121 and most national standards allow an error of 0.5 ° in measurement.