LAK16A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.57608\,\lambda^2}{\lambda^2 - 0.0077808} + \frac{0.36765\,\lambda^2}{\lambda^2 - 0.0259316} + \frac{1.1649\,\lambda^2}{\lambda^2 - 82.3723}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.17 \times 10^{-6} + 2(1.17 \times 10^{-8})\,\Delta T + 3(-1.26 \times 10^{-11})\,\Delta T^2 + \frac{4.73 \times 10^{-7} + 2(5.65 \times 10^{-10})\,\Delta T}{\lambda - 0.211}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$