S-BAL12 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{0.714605\,\lambda^2}{\lambda^2 - 0.00301764} + \frac{0.621993\,\lambda^2}{\lambda^2 - 0.0166505} + \frac{1.22538\,\lambda^2}{\lambda^2 - 143.506}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.77 \times 10^{-7} + 2(1.04 \times 10^{-8})\,\Delta T + 3(-3.79 \times 10^{-11})\,\Delta T^2 + \frac{4.41 \times 10^{-7} + 2(4.08 \times 10^{-10})\,\Delta T}{\lambda - 0.232}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$