LAFN23 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.65263\,\lambda^2}{\lambda^2 - 0.00931143} + \frac{0.139746\,\lambda^2}{\lambda^2 - 0.0391167} + \frac{0.938734\,\lambda^2}{\lambda^2 - 98.2389}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.3 \times 10^{-6} + 2(8.97 \times 10^{-9})\,\Delta T + 3(1.12 \times 10^{-11})\,\Delta T^2 + \frac{4.7 \times 10^{-7} + 2(3.02 \times 10^{-10})\,\Delta T}{\lambda - 0.268}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$