N-LLF6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.22932\,\lambda^2}{\lambda^2 - 0.00877776} + \frac{0.0755174\,\lambda^2}{\lambda^2 - 0.0480429} + \frac{0.98884\,\lambda^2}{\lambda^2 - 112.136}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.48 \times 10^{-6} + 2(1.16 \times 10^{-8})\,\Delta T + 3(-1.85 \times 10^{-11})\,\Delta T^2 + \frac{5.84 \times 10^{-7} + 2(7.05 \times 10^{-10})\,\Delta T}{\lambda - 0.233}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$