S-NBH52 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.50306\,\lambda^2}{\lambda^2 - 0.00999022} + \frac{0.221716\,\lambda^2}{\lambda^2 - 0.0450328} + \frac{1.84496\,\lambda^2}{\lambda^2 - 163.722}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.88 \times 10^{-6} + 2(5.19 \times 10^{-9})\,\Delta T + 3(-3.15 \times 10^{-11})\,\Delta T^2 + \frac{7.29 \times 10^{-7} + 2(9.66 \times 10^{-10})\,\Delta T}{\lambda - 0.25}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$