S-TIM28 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.54271\,\lambda^2}{\lambda^2 - 0.0113925} + \frac{0.217114\,\lambda^2}{\lambda^2 - 0.0579225} + \frac{1.81904\,\lambda^2}{\lambda^2 - 167.697}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.08 \times 10^{-6} + 2(1.07 \times 10^{-8})\,\Delta T + 3(-3.65 \times 10^{-11})\,\Delta T^2 + \frac{7.81 \times 10^{-7} + 2(1.09 \times 10^{-9})\,\Delta T}{\lambda - 0.291}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$