This paper presents an overview of the improvements made since 2014 to the primary reference cell calibrations performed at the National Renewable Energy Laboratory (NREL). Reducing the measurement uncertainty of the calibrations was the primary goal of the improvements, which were used during the recent calibrations conducted in the autumn of 2016. Changes and improvements have been made to the spectral irradiance and short-circuit current measurements, spectral irradiance modeling, and reference cell temperature corrections. Almost 6000 points were collected on 38 reference cells. Analysis of the results demonstrated reductions of expanded uncertainty from $\pm 1.0-1.2\%$ in previous years down to $\pm 0.4-0.5\%$ for crystalline-Si reference cells.
