S-NBH56 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.85191\,\lambda^2}{\lambda^2 - 0.0132733} + \frac{0.431103\,\lambda^2}{\lambda^2 - 0.0585945} + \frac{3.45278\,\lambda^2}{\lambda^2 - 239.357}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.95 \times 10^{-8} + 2(7.56 \times 10^{-9})\,\Delta T + 3(-3.2 \times 10^{-11})\,\Delta T^2 + \frac{1.01 \times 10^{-6} + 2(1.32 \times 10^{-9})\,\Delta T}{\lambda - 0.299}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$