  Anti-reflective coatings (ARCs) are used on the vast majority of solar
photovoltaic (PV) modules to increase power production. However, ARC longevity
can vary from less than 1 year to over 15 years depending on coating quality
and deployment conditions. A technique that can quantify ARC degradation
non-destructively on commercial modules would be useful both for in-field
diagnostics and accelerated aging tests. In this paper, we demonstrate that
accurate measurements of ARC spectral reflectance can be performed using a
modified commercially-available integrating-sphere probe. The measurement is
fast, accurate, non-destructive and can be performed outdoors in full-sun
conditions. We develop an interferometric model that estimates coating
porosity, thickness and fractional area coverage from the measured reflectance
spectrum for a uniform single-layer coating. We demonstrate the measurement
outdoors on an active PV installation, identify the presence of an ARC and
estimate the properties of the coating.
