S-TIL6 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.17702\,\lambda^2}{\lambda^2 - 0.00771088} + \frac{0.127958\,\lambda^2}{\lambda^2 - 0.0411325} + \frac{1.3474\,\lambda^2}{\lambda^2 - 154.532}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.53 \times 10^{-7} + 2(1.01 \times 10^{-8})\,\Delta T + 3(-3.81 \times 10^{-11})\,\Delta T^2 + \frac{4.89 \times 10^{-7} + 2(4.59 \times 10^{-10})\,\Delta T}{\lambda - 0.277}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$