SK18A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.53857\,\lambda^2}{\lambda^2 - 0.00863201} + \frac{0.0985934\,\lambda^2}{\lambda^2 - 0.0367156} + \frac{1.0231\,\lambda^2}{\lambda^2 - 100.118}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.83 \times 10^{-7} + 2(1.31 \times 10^{-8})\,\Delta T + 3(-1.22 \times 10^{-11})\,\Delta T^2 + \frac{5.56 \times 10^{-7} + 2(5.92 \times 10^{-10})\,\Delta T}{\lambda - 0.146}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$