BAFN6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.36719\,\lambda^2}{\lambda^2 - 0.00882821} + \frac{0.10908\,\lambda^2}{\lambda^2 - 0.0438732} + \frac{1.02108\,\lambda^2}{\lambda^2 - 113.586}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.34 \times 10^{-6} + 2(1.34 \times 10^{-8})\,\Delta T + 3(-5.5 \times 10^{-11})\,\Delta T^2 + \frac{4.95 \times 10^{-7} + 2(3.62 \times 10^{-10})\,\Delta T}{\lambda - 0.265}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$