S-BAL3 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.29367\,\lambda^2}{\lambda^2 - 0.00800368} + \frac{0.13244\,\lambda^2}{\lambda^2 - 0.0354711} + \frac{1.10197\,\lambda^2}{\lambda^2 - 134.517}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.7 \times 10^{-6} + 2(1.05 \times 10^{-8})\,\Delta T + 3(-3.78 \times 10^{-11})\,\Delta T^2 + \frac{4.4 \times 10^{-7} + 2(5.55 \times 10^{-10})\,\Delta T}{\lambda - 0.262}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$