BK8 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.05499\,\lambda^2}{\lambda^2 - 0.00620952} + \frac{0.225825\,\lambda^2}{\lambda^2 - 0.0201593} + \frac{1.00519\,\lambda^2}{\lambda^2 - 106.22}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.33 \times 10^{-6} + 2(1.28 \times 10^{-8})\,\Delta T + 3(-8.27 \times 10^{-11})\,\Delta T^2 + \frac{2.89 \times 10^{-7} + 2(3.34 \times 10^{-10})\,\Delta T}{\lambda - 0.267}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$