PK50 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.20726\,\lambda^2}{\lambda^2 - 0.00683503} + \frac{0.0774485\,\lambda^2}{\lambda^2 - 0.0252755} + \frac{0.770552\,\lambda^2}{\lambda^2 - 84.1707}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.43 \times 10^{-6} + 2(1.09 \times 10^{-8})\,\Delta T + 3(-7.07 \times 10^{-12})\,\Delta T^2 + \frac{3.59 \times 10^{-7} + 2(5.38 \times 10^{-10})\,\Delta T}{\lambda - 0.182}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$