The experimental measured rates of retinal discrete dark noise in Luo et al’s paper [2] are converted to the unit of (cm−2 s−1) to be compared with the rates of UPE.

The rates are calculated as Rcm = 108 Rμm in which where Sμm is the area of the measured segment (in terms of μm2), Scm is the area of the measured segment (in terms of cm2), N is the number of pigments in the measured segment, Rp is the rate constant per pigment (in terms of s−1), Rμm is the rate constant in segment (in terms of μm−2 s−1), and Rcm is the rate constant in segment (in terms of cm−2 s−1). The results indicate that the obtained rates for dark noise are of the order of UPE rate (i.e. a few, up to 104 photons/(cm2s)). [18–24, 27, 31–37, 41, 42] This results indicate that the retinal discrete dark noise can be potentially due to spontaneous cellular UPE (or biophotons) in photoreceptor cells.