LASF14A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.96557\,\lambda^2}{\lambda^2 - 0.0107142} + \frac{0.255069\,\lambda^2}{\lambda^2 - 0.0490296} + \frac{1.41244\,\lambda^2}{\lambda^2 - 92.2399}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.13 \times 10^{-6} + 2(1.22 \times 10^{-8})\,\Delta T + 3(-1.59 \times 10^{-11})\,\Delta T^2 + \frac{6.61 \times 10^{-7} + 2(9.49 \times 10^{-10})\,\Delta T}{\lambda - 0.257}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$