BAF9 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.51019\,\lambda^2}{\lambda^2 - 0.00918095} + \frac{0.133637\,\lambda^2}{\lambda^2 - 0.041645} + \frac{0.953994\,\lambda^2}{\lambda^2 - 108.049}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.75 \times 10^{-6} + 2(1.36 \times 10^{-8})\,\Delta T + 3(-1.07 \times 10^{-11})\,\Delta T^2 + \frac{7.01 \times 10^{-7} + 2(8.67 \times 10^{-10})\,\Delta T}{\lambda - 0.208}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$