N-LAK22 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.1423\,\lambda^2}{\lambda^2 - 0.00585779} + \frac{0.535138\,\lambda^2}{\lambda^2 - 0.0198546} + \frac{1.04088\,\lambda^2}{\lambda^2 - 100.834}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.36 \times 10^{-6} + 2(1.49 \times 10^{-8})\,\Delta T + 3(-1.29 \times 10^{-11})\,\Delta T^2 + \frac{3.41 \times 10^{-7} + 2(2.09 \times 10^{-10})\,\Delta T}{\lambda - 0.262}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$