BAK5 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.02975\,\lambda^2}{\lambda^2 - 0.00584732} + \frac{0.356259\,\lambda^2}{\lambda^2 - 0.0204774} + \frac{0.956057\,\lambda^2}{\lambda^2 - 115.24}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.08 \times 10^{-7} + 2(1.06 \times 10^{-8})\,\Delta T + 3(9.48 \times 10^{-11})\,\Delta T^2 + \frac{3.95 \times 10^{-7} + 2(-2.33 \times 10^{-10})\,\Delta T}{\lambda - 0.252}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$