N-SK15 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.30418\,\lambda^2}{\lambda^2 - 0.00695051} + \frac{0.285841\,\lambda^2}{\lambda^2 - 0.0232024} + \frac{0.974782\,\lambda^2}{\lambda^2 - 99.0169}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.92 \times 10^{-7} + 2(1.2 \times 10^{-8})\,\Delta T + 3(-2.96 \times 10^{-12})\,\Delta T^2 + \frac{4.66 \times 10^{-7} + 2(5.16 \times 10^{-10})\,\Delta T}{\lambda - 0.179}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$