BALF50 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.3738\,\lambda^2}{\lambda^2 - 0.00871951} + \frac{0.104741\,\lambda^2}{\lambda^2 - 0.0397153} + \frac{1.00909\,\lambda^2}{\lambda^2 - 114.462}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.43 \times 10^{-6} + 2(1.21 \times 10^{-8})\,\Delta T + 3(-3.76 \times 10^{-11})\,\Delta T^2 + \frac{6.38 \times 10^{-7} + 2(7.95 \times 10^{-10})\,\Delta T}{\lambda - 0.184}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$