“…It should be noted that other impurity centres in diamond, which show a more suitable spectral power distribution for calibration purposes, are currently under investigation. These are, e.g., the colours centres based on Silicon [34][35][36][37], Germanium [38,39], Tin [40][41][42][43], Fluorine [44,45], Helium [46] impurities and Lead vacancy centre [47][48][49][50], see also the overview article from Moreva et al [51]. In [52], Vaigu et al successfully used a single-photon source based on a SiV-centre in nanodiamond with an emission bandwidth of Δλ FWHM ≈ 2 nm for the determination of the detection efficiency of a Si-SPAD detector, however, a direct calibration against a reference detector could not be carried out, because of the low photon rate of approx.…”