LLF6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.20067\,\lambda^2}{\lambda^2 - 0.00851611} + \frac{0.103689\,\lambda^2}{\lambda^2 - 0.0418805} + \frac{0.954739\,\lambda^2}{\lambda^2 - 114.99}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.54 \times 10^{-6} + 2(1.84 \times 10^{-8})\,\Delta T + 3(-3.65 \times 10^{-11})\,\Delta T^2 + \frac{5.59 \times 10^{-7} + 2(4.53 \times 10^{-10})\,\Delta T}{\lambda - 0.25}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$