BAF52 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.41632\,\lambda^2}{\lambda^2 - 0.00928783} + \frac{0.117331\,\lambda^2}{\lambda^2 - 0.044991} + \frac{0.934115\,\lambda^2}{\lambda^2 - 114.126}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.17 \times 10^{-6} + 2(1.2 \times 10^{-8})\,\Delta T + 3(-6.97 \times 10^{-11})\,\Delta T^2 + \frac{5.51 \times 10^{-7} + 2(-9.24 \times 10^{-11})\,\Delta T}{\lambda - 0.258}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$