LAK33 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.45797\,\lambda^2}{\lambda^2 - 0.00680545} + \frac{0.555404\,\lambda^2}{\lambda^2 - 0.0225253} + \frac{1.19939\,\lambda^2}{\lambda^2 - 82.7543}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.57 \times 10^{-6} + 2(1.16 \times 10^{-8})\,\Delta T + 3(-7.29 \times 10^{-11})\,\Delta T^2 + \frac{6.01 \times 10^{-7} + 2(1.59 \times 10^{-10})\,\Delta T}{\lambda - 0.114}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$