S-NBM51 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.37023\,\lambda^2}{\lambda^2 - 0.0087192} + \frac{0.177666\,\lambda^2}{\lambda^2 - 0.0405726} + \frac{1.30515\,\lambda^2}{\lambda^2 - 112.703}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.21 \times 10^{-6} + 2(1.49 \times 10^{-8})\,\Delta T + 3(-3.78 \times 10^{-11})\,\Delta T^2 + \frac{5.35 \times 10^{-7} + 2(6.88 \times 10^{-10})\,\Delta T}{\lambda - 0.25}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$