N-BAF51 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.51504\,\lambda^2}{\lambda^2 - 0.00942735} + \frac{0.153622\,\lambda^2}{\lambda^2 - 0.0430827} + \frac{1.15428\,\lambda^2}{\lambda^2 - 124.89}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.84 \times 10^{-7} + 2(1.04 \times 10^{-8})\,\Delta T + 3(-1.8 \times 10^{-11})\,\Delta T^2 + \frac{7.01 \times 10^{-7} + 2(8.47 \times 10^{-10})\,\Delta T}{\lambda - 0.219}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$