BSL7Y by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.13329\,\lambda^2}{\lambda^2 - 0.00669408} + \frac{0.136897\,\lambda^2}{\lambda^2 - 0.0237392} + \frac{0.703456\,\lambda^2}{\lambda^2 - 70.703}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.6 \times 10^{-6} + 2(1.69 \times 10^{-8})\,\Delta T + 3(-4.86 \times 10^{-11})\,\Delta T^2 + \frac{4.34 \times 10^{-7} + 2(5.54 \times 10^{-10})\,\Delta T}{\lambda - 0.162}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$