UK50 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.06592\,\lambda^2}{\lambda^2 - 0.00640858} + \frac{0.219843\,\lambda^2}{\lambda^2 - 0.0225474} + \frac{0.900983\,\lambda^2}{\lambda^2 - 101.921}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.7 \times 10^{-6} + 2(1.1 \times 10^{-8})\,\Delta T + 3(-9.04 \times 10^{-11})\,\Delta T^2 + \frac{5.64 \times 10^{-7} + 2(1.09 \times 10^{-9})\,\Delta T}{\lambda - 0.131}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$