ZK1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.09192\,\lambda^2}{\lambda^2 - 0.006555} + \frac{0.22346\,\lambda^2}{\lambda^2 - 0.0243713} + \frac{0.869353\,\lambda^2}{\lambda^2 - 102.338}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.39 \times 10^{-6} + 2(1.51 \times 10^{-8})\,\Delta T + 3(-2.19 \times 10^{-11})\,\Delta T^2 + \frac{5.33 \times 10^{-7} + 2(6.38 \times 10^{-10})\,\Delta T}{\lambda - 0.186}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$