N-LAF21 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.87135\,\lambda^2}{\lambda^2 - 0.00933322} + \frac{0.250783\,\lambda^2}{\lambda^2 - 0.0345638} + \frac{1.22049\,\lambda^2}{\lambda^2 - 83.2405}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.11 \times 10^{-6} + 2(1.13 \times 10^{-8})\,\Delta T + 3(-2.07 \times 10^{-11})\,\Delta T^2 + \frac{5.88 \times 10^{-7} + 2(6.32 \times 10^{-10})\,\Delta T}{\lambda - 0.199}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$