N-LAK14 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.50781\,\lambda^2}{\lambda^2 - 0.00746099} + \frac{0.318867\,\lambda^2}{\lambda^2 - 0.0242025} + \frac{1.14287\,\lambda^2}{\lambda^2 - 80.9565}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.68 \times 10^{-6} + 2(1.15 \times 10^{-8})\,\Delta T + 3(-1.44 \times 10^{-11})\,\Delta T^2 + \frac{3.72 \times 10^{-7} + 2(5.53 \times 10^{-10})\,\Delta T}{\lambda - 0.226}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$