Base-10 logarithm of the ratio of the null fluctuation noise to background noise against wavelength for a fringe tracking RMS of δ = 5 nm. The background noise is chosen to be the maximum of stellar leakage and zodiacal light for that given wavelength. Plotted for two different stars and varying amounts of beam combiner optical error. The black dashed line divides the upper region where the combiner is dominated by instrumental errors induced by fluctuations in the null depth, and the lower region where the instrument is photon limited by either stellar leakage or the zodiacal background.