N-LF5 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.33273\,\lambda^2}{\lambda^2 - 0.00971526} + \frac{0.112285\,\lambda^2}{\lambda^2 - 0.0534456} + \frac{1.10203\,\lambda^2}{\lambda^2 - 118.652}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.75 \times 10^{-7} + 2(1.01 \times 10^{-8})\,\Delta T + 3(-2.71 \times 10^{-11})\,\Delta T^2 + \frac{6.64 \times 10^{-7} + 2(8.3 \times 10^{-10})\,\Delta T}{\lambda - 0.257}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$