Measurements of QY in silicon use one of two methods. The first method, used by Kuschnerus et al., is to compare the detector QE to the measured or modeled transmission, assuming that QE accounts for any discrepancy between them.24,25 This method provides an upper limit of QY, since it relies on the assumption that the PDs have small internal losses and that the silicon-oxide interface has limited absorption. The assumption regarding absorption of the oxide fails below 160 nm, but these results provide reasonable QY for longer wavelengths. An alternative method21,26 uses a photon transfer curve (PTC) taken at relevant wavelengths. This method exploits the fact that the mean and variance of the detector signal scale as QY and , respectively, violating the behavior of the photon shot noise, which is Poisson distributed. PTC measurements of QE are accurate at very high energies (e.g., x-ray), where the e/h pairs generated per photon are strongly peaked around a mean value, i.e., Fano noise is relatively low. In the UV, Fano noise contributes significantly to the signal variance, which biases the QY measurement [see Eq. (30) in McCullough et al.23]. QY calculated using the photon transfer method can be well below those calculated using the reflectance method (e.g., versus at 200 nm) and can change the apparent QE of a detector significantly, as seen in Fig. 7.