SK7 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.03745\,\lambda^2}{\lambda^2 - 0.00537369} + \frac{0.504633\,\lambda^2}{\lambda^2 - 0.0182926} + \frac{0.989114\,\lambda^2}{\lambda^2 - 102.526}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.28 \times 10^{-6} + 2(1.36 \times 10^{-8})\,\Delta T + 3(-9.51 \times 10^{-12})\,\Delta T^2 + \frac{4.2 \times 10^{-7} + 2(4.35 \times 10^{-10})\,\Delta T}{\lambda - 0.189}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$