S-TIL25 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.32123\,\lambda^2}{\lambda^2 - 0.00952091} + \frac{0.123825\,\lambda^2}{\lambda^2 - 0.0516063} + \frac{1.43685\,\lambda^2}{\lambda^2 - 149.065}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.03 \times 10^{-6} + 2(1.32 \times 10^{-8})\,\Delta T + 3(-4.09 \times 10^{-11})\,\Delta T^2 + \frac{6.01 \times 10^{-7} + 2(5.93 \times 10^{-10})\,\Delta T}{\lambda - 0.278}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$