LAKL12 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.28115\,\lambda^2}{\lambda^2 - 0.0063644} + \frac{0.482608\,\lambda^2}{\lambda^2 - 0.0213764} + \frac{1.16554\,\lambda^2}{\lambda^2 - 93.2772}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.26 \times 10^{-6} + 2(1.05 \times 10^{-8})\,\Delta T + 3(-8.4 \times 10^{-12})\,\Delta T^2 + \frac{4.57 \times 10^{-7} + 2(5.86 \times 10^{-10})\,\Delta T}{\lambda - 0.21}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$