S-BAL50 by Ohara

$$n(\lambda) = \sqrt{2.39827 - 0.0102228\,\lambda^2 + 0.0121104\,\lambda^{-2} + 0.000377676\,\lambda^{-4} - 2.7339 \times 10^{-5}\,\lambda^{-6} + 1.57019 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.64 \times 10^{-6} + 2(1.93 \times 10^{-8})\,\Delta T + 3(4.39 \times 10^{-12})\,\Delta T^2 + \frac{5.12 \times 10^{-7} + 2(8.07 \times 10^{-10})\,\Delta T}{\lambda - 0.162}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$