SSK4A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.38336\,\lambda^2}{\lambda^2 - 0.00787155} + \frac{0.187125\,\lambda^2}{\lambda^2 - 0.0289352} + \frac{0.944171\,\lambda^2}{\lambda^2 - 105.742}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.65 \times 10^{-6} + 2(1.37 \times 10^{-8})\,\Delta T + 3(-1.89 \times 10^{-11})\,\Delta T^2 + \frac{5.4 \times 10^{-7} + 2(5.46 \times 10^{-10})\,\Delta T}{\lambda - 0.192}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$