SK1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.23998\,\lambda^2}{\lambda^2 - 0.00676461} + \frac{0.30883\,\lambda^2}{\lambda^2 - 0.0233831} + \frac{1.03366\,\lambda^2}{\lambda^2 - 114.586}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.93 \times 10^{-6} + 2(1.6 \times 10^{-8})\,\Delta T + 3(-9.64 \times 10^{-11})\,\Delta T^2 + \frac{5.38 \times 10^{-7} + 2(5.24 \times 10^{-10})\,\Delta T}{\lambda - 0.175}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$