the light and enable the monitoring photocell to be housed away from the heat of the lamp.
Another solution is to keep the luminaire at its correct aiming angle during the test and to rotate mirrors about the luminaire, as in Figure 9. Strictly, a different set of data is
required for each aiming angle.
Because, as already stated, the test may need to be conducted with the photocell at a large distance from the luminaire, readings taken on the bare lamp for calibration purposes may be too small to achieve good accuracy. Provided the photocell is sensitive enough, it may be possible to overcome this problem by the use of different ranges on the photocell current measuring or indicating device. Alternatively, provision can be made for bringing the photocell closer to the goniophotometer. If a large receptor is used, a check should be made that its angle of subtense at the luminaire is not too great.
An alternative approach is to use an accurate illuminance meter. The luminous flux of the test lamp should be measured so that all readings can be multiplied by a factor to correct for any
deviation from the nominal light output of the lamp.