S-TIM1 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.44964\,\lambda^2}{\lambda^2 - 0.0112094} + \frac{0.122986\,\lambda^2}{\lambda^2 - 0.0596266} + \frac{1.38067\,\lambda^2}{\lambda^2 - 138.178}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.39 \times 10^{-7} + 2(1.37 \times 10^{-8})\,\Delta T + 3(-4.03 \times 10^{-11})\,\Delta T^2 + \frac{6.89 \times 10^{-7} + 2(6.08 \times 10^{-10})\,\Delta T}{\lambda - 0.284}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$