SFL57 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.88742\,\lambda^2}{\lambda^2 - 0.0145939} + \frac{0.360534\,\lambda^2}{\lambda^2 - 0.0648199} + \frac{2.26189\,\lambda^2}{\lambda^2 - 176.062}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.63 \times 10^{-6} + 2(8.61 \times 10^{-9})\,\Delta T + 3(-9.98 \times 10^{-12})\,\Delta T^2 + \frac{1.1 \times 10^{-6} + 2(1.69 \times 10^{-9})\,\Delta T}{\lambda - 0.293}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$