BAF4 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.37493\,\lambda^2}{\lambda^2 - 0.00929554} + \frac{0.146354\,\lambda^2}{\lambda^2 - 0.0440427} + \frac{0.912643\,\lambda^2}{\lambda^2 - 116.608}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[9.36 \times 10^{-7} + 2(1.49 \times 10^{-8})\,\Delta T + 3(-3.89 \times 10^{-11})\,\Delta T^2 + \frac{7.92 \times 10^{-7} + 2(-2.05 \times 10^{-11})\,\Delta T}{\lambda - 0.228}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$