N-LAK7 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.2368\,\lambda^2}{\lambda^2 - 0.00610106} + \frac{0.445052\,\lambda^2}{\lambda^2 - 0.0201388} + \frac{1.01746\,\lambda^2}{\lambda^2 - 90.638}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.4 \times 10^{-6} + 2(1.17 \times 10^{-8})\,\Delta T + 3(2.38 \times 10^{-11})\,\Delta T^2 + \frac{4.96 \times 10^{-7} + 2(4.44 \times 10^{-10})\,\Delta T}{\lambda - 0.107}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$