Temporal Instability is the third performance parameter of Class AAA standards. It requires that the output light be stable over time in order to ensure that the lamp fluctuations do not distort the measurement of solar cell efficiency. Oriel’s Sol3A easily meets the requirements for temporal instability as defined by the IEC, ASTM, and JIS standards. The IEC 60904-9 (2007) has the most stringent requirements for short term instability with a maximum allowable level of 0.5%. Shown is a typical instability response of 0.369% at 50 ms intervals for an 1.6 kW Sol3A solar simulator.