BAK4 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.22966\,\lambda^2}{\lambda^2 - 0.00723553} + \frac{0.191513\,\lambda^2}{\lambda^2 - 0.0277771} + \frac{0.95689\,\lambda^2}{\lambda^2 - 109.687}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.85 \times 10^{-6} + 2(1.29 \times 10^{-8})\,\Delta T + 3(-1.93 \times 10^{-11})\,\Delta T^2 + \frac{4.92 \times 10^{-7} + 2(7.7 \times 10^{-10})\,\Delta T}{\lambda - 0.218}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$