S-NBM52 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.39799\,\lambda^2}{\lambda^2 - 0.00946114} + \frac{0.172238\,\lambda^2}{\lambda^2 - 0.0444078} + \frac{1.21743\,\lambda^2}{\lambda^2 - 105.314}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.03 \times 10^{-6} + 2(1.22 \times 10^{-8})\,\Delta T + 3(-1.45 \times 10^{-11})\,\Delta T^2 + \frac{5.86 \times 10^{-7} + 2(5.86 \times 10^{-10})\,\Delta T}{\lambda - 0.261}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$