L-TIM28P by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.53013\,\lambda^2}{\lambda^2 - 0.0114237} + \frac{0.245362\,\lambda^2}{\lambda^2 - 0.0555102} + \frac{1.78233\,\lambda^2}{\lambda^2 - 179.074}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-6.17 \times 10^{-6} + 2(5.7 \times 10^{-9})\,\Delta T + 3(-3.8 \times 10^{-11})\,\Delta T^2 + \frac{9.99 \times 10^{-7} + 2(1.21 \times 10^{-9})\,\Delta T}{\lambda - 0.26}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$