Mastering thin film measurements on a grand scale as a daily routine

25% of all measurements performed in 2010 at CalLab PV Modules, Fraunhofer ISE's laboratory for module measurements, were of thin film PV modules. This contribution outlines the daily procedures that are applied to guarantee minimum uncertainties and explains the two major differences in uncertainty estimation for thin film and crystalline silicon PV modules: the spectral mismatch factor and stability issues. A clear distinction is made between pure measurement uncertainties, i. e. uncertainty of the module power at time of measurement, and the uncertainty of the current state of the module in terms of metastable STC power. Pure measurement uncertainties can be minimized by using a reference cell with equal wavelength limits as the module under test. We demonstrate this theoretically and on the basis of measurements with different reference cells. As a result, we found that pure measurement uncertainties of ±2% for single junction technologies are realistic. This is also reflected in results of an international round robin. However, more effort is still needed to control metastability effects.