S-TIH18 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.59922\,\lambda^2}{\lambda^2 - 0.0116469} + \frac{0.259532\,\lambda^2}{\lambda^2 - 0.0584825} + \frac{2.12455\,\lambda^2}{\lambda^2 - 186.928}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.15 \times 10^{-6} + 2(1.28 \times 10^{-8})\,\Delta T + 3(-3.62 \times 10^{-11})\,\Delta T^2 + \frac{7.62 \times 10^{-7} + 2(1.01 \times 10^{-9})\,\Delta T}{\lambda - 0.306}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$