This document discusses characterizing bifacial solar cells and achieving inter-laboratory comparability of results. It proposes using a black reference chuck with 0% reflectance as the standard, as it is easy to define and realize, and improves comparability between labs. A simple model is presented to estimate the impact of bifaciality based on a cell's spectral response and the chuck's reflectance properties. Experimental tests show good agreement with the model for some surfaces but higher currents for surfaces like anodized aluminum that reflect light more diffusely. Defining a reference standard is important for accurately comparing bifacial cell efficiencies between laboratories.