S-BAL2 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.30924\,\lambda^2}{\lambda^2 - 0.00838874} + \frac{0.114137\,\lambda^2}{\lambda^2 - 0.0399436} + \frac{1.17882\,\lambda^2}{\lambda^2 - 140.258}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.92 \times 10^{-6} + 2(1.14 \times 10^{-8})\,\Delta T + 3(-3.59 \times 10^{-11})\,\Delta T^2 + \frac{5.24 \times 10^{-7} + 2(7.01 \times 10^{-10})\,\Delta T}{\lambda - 0.247}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$