LASF33 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.90477\,\lambda^2}{\lambda^2 - 0.0117757} + \frac{0.248374\,\lambda^2}{\lambda^2 - 0.0528083} + \frac{1.18109\,\lambda^2}{\lambda^2 - 108.155}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.85 \times 10^{-6} + 2(8.94 \times 10^{-9})\,\Delta T + 3(-5.97 \times 10^{-12})\,\Delta T^2 + \frac{8.69 \times 10^{-7} + 2(1.09 \times 10^{-9})\,\Delta T}{\lambda - 0.262}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$