SFL56 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.75126\,\lambda^2}{\lambda^2 - 0.0133187} + \frac{0.301567\,\lambda^2}{\lambda^2 - 0.0621436} + \frac{1.89211\,\lambda^2}{\lambda^2 - 157.884}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.3 \times 10^{-6} + 2(8.4 \times 10^{-9})\,\Delta T + 3(-2.28 \times 10^{-11})\,\Delta T^2 + \frac{1 \times 10^{-6} + 2(1.52 \times 10^{-9})\,\Delta T}{\lambda - 0.287}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$