N-LAK33B by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.42289\,\lambda^2}{\lambda^2 - 0.00670283} + \frac{0.593661\,\lambda^2}{\lambda^2 - 0.0219416} + \frac{1.16135\,\lambda^2}{\lambda^2 - 80.7408}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.77 \times 10^{-6} + 2(1.24 \times 10^{-8})\,\Delta T + 3(1.22 \times 10^{-11})\,\Delta T^2 + \frac{5.19 \times 10^{-7} + 2(6.02 \times 10^{-10})\,\Delta T}{\lambda - 0.184}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$