BALF4 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.29376\,\lambda^2}{\lambda^2 - 0.00780228} + \frac{0.158139\,\lambda^2}{\lambda^2 - 0.0321349} + \frac{0.947443\,\lambda^2}{\lambda^2 - 109.159}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.23 \times 10^{-6} + 2(1.35 \times 10^{-8})\,\Delta T + 3(-1.92 \times 10^{-11})\,\Delta T^2 + \frac{6.91 \times 10^{-7} + 2(8.72 \times 10^{-10})\,\Delta T}{\lambda - 0.143}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$