A plumb bob is dropped from the centre of the spindle used for turning the luminaire through C angles and its position marked on the floor (Figure 13(a)). If the gantry is used at more than one height, all the nadirs should be coincident. Moreover, when the gantry is loaded with the heaviest luminaire to be tested, there should be no significant deviation from the position marked on the floor when the gantry is unloaded