LLF1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.2164\,\lambda^2}{\lambda^2 - 0.00857807} + \frac{0.133665\,\lambda^2}{\lambda^2 - 0.0420143} + \frac{0.883399\,\lambda^2}{\lambda^2 - 107.593}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.25 \times 10^{-7} + 2(1.74 \times 10^{-8})\,\Delta T + 3(-6.12 \times 10^{-11})\,\Delta T^2 + \frac{6.53 \times 10^{-7} + 2(2.58 \times 10^{-10})\,\Delta T}{\lambda - 0.233}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$