S-BAH28 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.69493\,\lambda^2}{\lambda^2 - 0.0102723} + \frac{0.19289\,\lambda^2}{\lambda^2 - 0.0521188} + \frac{1.56386\,\lambda^2}{\lambda^2 - 137.818}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.8 \times 10^{-6} + 2(1.33 \times 10^{-8})\,\Delta T + 3(-3.17 \times 10^{-11})\,\Delta T^2 + \frac{7.28 \times 10^{-7} + 2(9.35 \times 10^{-10})\,\Delta T}{\lambda - 0.261}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$