N-LAK33A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.44117\,\lambda^2}{\lambda^2 - 0.00680934} + \frac{0.57175\,\lambda^2}{\lambda^2 - 0.0222292} + \frac{1.16605\,\lambda^2}{\lambda^2 - 80.938}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.63 \times 10^{-6} + 2(1.11 \times 10^{-8})\,\Delta T + 3(-3.92 \times 10^{-12})\,\Delta T^2 + \frac{5.02 \times 10^{-7} + 2(5.08 \times 10^{-10})\,\Delta T}{\lambda - 0.188}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$