This paper focuses on the stability and consistency of intensity measurements of WLED phosphors. The quality of phosphor products are determined by the measured intensities, but in practice it is often found that intensity measurements cannot be replicated. This problem is due to the instability of test conditions (fluctuating light source power, different paring of quartz lids, etc.) A Monte Carlo ray-tracing model is applied to illustrate the differences in faculae caused by tiny quartz lid slips. The research presented would be particularly beneficial for researchers and manufactures of phosphor.