BAC4 by Hoya

$$n(\lambda) = \sqrt{2.41966 - 0.00833293\,\lambda^2 + 0.0153974\,\lambda^{-2} - 0.000173917\,\lambda^{-4} + 6.21832 \times 10^{-5}\,\lambda^{-6} - 3.06213 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[9.31 \times 10^{-6} + 2(1.09 \times 10^{-8})\,\Delta T + 3(-5.68 \times 10^{-11})\,\Delta T^2 + \frac{4.67 \times 10^{-7} + 2(7.99 \times 10^{-10})\,\Delta T}{\lambda - 0.248}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$