S-TIL27 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.31433\,\lambda^2}{\lambda^2 - 0.00950404} + \frac{0.1123\,\lambda^2}{\lambda^2 - 0.0524113} + \frac{1.4139\,\lambda^2}{\lambda^2 - 148.43}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.72 \times 10^{-6} + 2(1.09 \times 10^{-8})\,\Delta T + 3(-4.25 \times 10^{-11})\,\Delta T^2 + \frac{5.94 \times 10^{-7} + 2(5.57 \times 10^{-10})\,\Delta T}{\lambda - 0.276}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$