LAK10 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.67483\,\lambda^2}{\lambda^2 - 0.00849325} + \frac{0.222669\,\lambda^2}{\lambda^2 - 0.0330463} + \frac{1.13957\,\lambda^2}{\lambda^2 - 80.4373}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.95 \times 10^{-6} + 2(8.33 \times 10^{-9})\,\Delta T + 3(1.68 \times 10^{-11})\,\Delta T^2 + \frac{4.84 \times 10^{-7} + 2(7.28 \times 10^{-10})\,\Delta T}{\lambda - 0.221}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$