The aim of this paper is to study the reproducibility of measurements in LED lamps. For research purposes as well as for standardized tests, there is a need for measurements to be reproducible so that results can be accurately compared. The knowledge regarding which parameters and test conditions will influence the results is therefore crucial. This paper aims at identifying how changes in supply voltage and ambient conditions impact the results and defining a methodology to determine when an LED lamp is considered stable. The paper describes the differences in illuminance and active power under different ambient conditions and supply voltages for several LED lamps for indoor use. Variations in illuminance and active power are also studied within the stabilisation time. A consistent methodology for temperature measurement is explained and used in this paper.