BAF50 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.63007\,\lambda^2}{\lambda^2 - 0.0100983} + \frac{0.135169\,\lambda^2}{\lambda^2 - 0.045458} + \frac{1.1533\,\lambda^2}{\lambda^2 - 121.435}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.41 \times 10^{-6} + 2(6.5 \times 10^{-9})\,\Delta T + 3(6.78 \times 10^{-11})\,\Delta T^2 + \frac{6.13 \times 10^{-7} + 2(1.04 \times 10^{-9})\,\Delta T}{\lambda - 0.249}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$