F5 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.31045\,\lambda^2}{\lambda^2 - 0.00958633} + \frac{0.196034\,\lambda^2}{\lambda^2 - 0.0457628} + \frac{0.96613\,\lambda^2}{\lambda^2 - 115.012}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.13 \times 10^{-6} + 2(1.65 \times 10^{-8})\,\Delta T + 3(-6.98 \times 10^{-11})\,\Delta T^2 + \frac{1.02 \times 10^{-6} + 2(6.56 \times 10^{-10})\,\Delta T}{\lambda - 0.208}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$