S-TIL26 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.31066\,\lambda^2}{\lambda^2 - 0.00968898} + \frac{0.0941903\,\lambda^2}{\lambda^2 - 0.0527763} + \frac{1.23293\,\lambda^2}{\lambda^2 - 133.296}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[8.85 \times 10^{-7} + 2(1.27 \times 10^{-8})\,\Delta T + 3(-3.61 \times 10^{-11})\,\Delta T^2 + \frac{5.87 \times 10^{-7} + 2(6.12 \times 10^{-10})\,\Delta T}{\lambda - 0.27}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$