N-KZFS4 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.35055\,\lambda^2}{\lambda^2 - 0.00876282} + \frac{0.197576\,\lambda^2}{\lambda^2 - 0.0371767} + \frac{1.09963\,\lambda^2}{\lambda^2 - 90.3867}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.81 \times 10^{-6} + 2(1.16 \times 10^{-8})\,\Delta T + 3(-7.99 \times 10^{-12})\,\Delta T^2 + \frac{6.2 \times 10^{-7} + 2(7.94 \times 10^{-10})\,\Delta T}{\lambda - 0.205}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$