N-LLF1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.27618\,\lambda^2}{\lambda^2 - 0.00911216} + \frac{0.0750285\,\lambda^2}{\lambda^2 - 0.0546801} + \frac{0.942212\,\lambda^2}{\lambda^2 - 102.33}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.85 \times 10^{-6} + 2(1.47 \times 10^{-8})\,\Delta T + 3(-4.03 \times 10^{-12})\,\Delta T^2 + \frac{5.49 \times 10^{-7} + 2(7.1 \times 10^{-10})\,\Delta T}{\lambda - 0.255}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$