BK10 by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.888308\,\lambda^2}{\lambda^2 - 0.00516901} + \frac{0.328964\,\lambda^2}{\lambda^2 - 0.016119} + \frac{0.984611\,\lambda^2}{\lambda^2 - 99.7575}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.28 \times 10^{-6} + 2(1.62 \times 10^{-8})\,\Delta T + 3(4.95 \times 10^{-11})\,\Delta T^2 + \frac{4.22 \times 10^{-7} + 2(8.57 \times 10^{-10})\,\Delta T}{\lambda - 0.1}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$