K11 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.1625\,\lambda^2}{\lambda^2 - 0.0075327} + \frac{0.0582614\,\lambda^2}{\lambda^2 - 0.0359084} + \frac{0.905204\,\lambda^2}{\lambda^2 - 102.405}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.65 \times 10^{-6} + 2(1.98 \times 10^{-8})\,\Delta T + 3(-3.64 \times 10^{-11})\,\Delta T^2 + \frac{4.35 \times 10^{-7} + 2(4.51 \times 10^{-10})\,\Delta T}{\lambda - 0.187}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$