We note that, within this treatment, the TE is independent of the volume density of fluorescers. This simple expression is plotted in Fig. 6. In this figure, the core index is the same as that of the Saint Gobain fiber, 1.6. The maximum cladding index is also the same as the Saint Gobain fiber, 1.49, and the minimum index is that of an air cladding, 1. The TE of the Saint Gobain fiber is $\u223c13%$, while for an air cladding, it is $\u223c61%$. The indices between these two are not included just to exhibit a trend; down to $\u223c1.3$, they can be achieved with various degrees of fluorination of conventional polymers, for which $ncl/nco=0.8125$. Indices $<1.3$ to values $<1.1$ can be achieved by creating nanopores (i.e., small empty spaces) in a material such as silica.^{11}