Solar simulator is one of the essential equipment for studying solar cells. Solar Simulator is equipment irradiating light very similar to sun light. And its specifications are determined by industrial standards.
Spectral Matching - How is a solar simulator spectrum similar to sun light?
Non-Uniformity of Irradiance - How is a uniformity of irradiation area?
Temporal Instability - How is irradiance stable against time?
User chooses a solar simulator depending on your purpose of research and your budget.