BALF5 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.23751\,\lambda^2}{\lambda^2 - 0.00808113} + \frac{0.116758\,\lambda^2}{\lambda^2 - 0.0349868} + \frac{0.853086\,\lambda^2}{\lambda^2 - 110.314}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.42 \times 10^{-7} + 2(1.35 \times 10^{-8})\,\Delta T + 3(-7.72 \times 10^{-11})\,\Delta T^2 + \frac{3.46 \times 10^{-7} + 2(4.85 \times 10^{-10})\,\Delta T}{\lambda - 0.293}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$