Image-based BRDF measurements on spherical material samples present a great opportunity to shorten significantly the acquisition time with respect to more traditional, non-multiplexed measurement methods for isotropic BRDFs. However, it has never been analyzed deeply, what measurement accuracy can be achieved in such a setup; what are the main contributing uncertainty factors and how do they relate to calibration procedures. In this paper we present a new set of isotropic BRDF measurements with their radiometric and geometric uncertainties acquired within such an imaging setup. We discuss the most prominent optical phenomena that affect measurement accuracy and pave the way for more thorough uncertainty analysis in forthcoming image-based BRDF measurements. Our newly acquired data with their quantified uncertainties will be helpful for comparing the quality and accuracy of the different experimental setups and for designing other such image-based BRDF measurement devices.