S-TIL1 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.25089\,\lambda^2}{\lambda^2 - 0.00883921} + \frac{0.0997973\,\lambda^2}{\lambda^2 - 0.0482685} + \frac{1.20584\,\lambda^2}{\lambda^2 - 137.415}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.21 \times 10^{-6} + 2(1.02 \times 10^{-8})\,\Delta T + 3(-4.37 \times 10^{-11})\,\Delta T^2 + \frac{5.67 \times 10^{-7} + 2(5.97 \times 10^{-10})\,\Delta T}{\lambda - 0.265}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$