BK3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.00583\,\lambda^2}{\lambda^2 - 0.00617416} + \frac{0.212096\,\lambda^2}{\lambda^2 - 0.0187793} + \frac{0.898291\,\lambda^2}{\lambda^2 - 88.8735}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[6.74 \times 10^{-6} + 2(1.46 \times 10^{-8})\,\Delta T + 3(-2.37 \times 10^{-11})\,\Delta T^2 + \frac{4.51 \times 10^{-7} + 2(7.53 \times 10^{-10})\,\Delta T}{\lambda - 0.102}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$