S-BAH27 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.68939\,\lambda^2}{\lambda^2 - 0.0103598} + \frac{0.133081\,\lambda^2}{\lambda^2 - 0.0533982} + \frac{1.41166\,\lambda^2}{\lambda^2 - 126.516}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.96 \times 10^{-6} + 2(1.33 \times 10^{-8})\,\Delta T + 3(-3.48 \times 10^{-11})\,\Delta T^2 + \frac{5.66 \times 10^{-7} + 2(7.35 \times 10^{-10})\,\Delta T}{\lambda - 0.278}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$