Camera spectral sensitivity functions are obtained either by measurements in the laboratory or via estimation algorithms. A procedure for camera spectral sensitivity measurement is explained in this article, which aims to provide high accuracy ground-truth values with known uncertainty. The measurements were carried out on a Nikon D5100 camera at the National Physical Laboratory, London. We use the obtained ground truth spectral sensitivity functions to evaluate the performance of some estimation algorithms reported in the literature. We conclude that the estimated sensitivities are not as accurate as the ground truth spectral sensitivities and further improvements are required. We validate previous work, which shows that a known camera basis provides a powerful constraint for estimation. We have made the ground truth spectral sensitivities measured at NPL along with the detailed uncertainty levels available online for the community to use as a reference data set.