SFL6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.78922\,\lambda^2}{\lambda^2 - 0.0135164} + \frac{0.328427\,\lambda^2}{\lambda^2 - 0.062273} + \frac{2.01639\,\lambda^2}{\lambda^2 - 168.015}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.26 \times 10^{-6} + 2(7.41 \times 10^{-9})\,\Delta T + 3(-1.89 \times 10^{-11})\,\Delta T^2 + \frac{1.02 \times 10^{-6} + 2(1.62 \times 10^{-9})\,\Delta T}{\lambda - 0.288}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$