N-LAF33 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.79653\,\lambda^2}{\lambda^2 - 0.00927313} + \frac{0.311578\,\lambda^2}{\lambda^2 - 0.0358201} + \frac{1.15982\,\lambda^2}{\lambda^2 - 87.3449}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[8.17 \times 10^{-6} + 2(1.24 \times 10^{-8})\,\Delta T + 3(-1.65 \times 10^{-11})\,\Delta T^2 + \frac{7.11 \times 10^{-7} + 2(8.59 \times 10^{-10})\,\Delta T}{\lambda - 0.21}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$