PSK52 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.23651\,\lambda^2}{\lambda^2 - 0.00627579} + \frac{0.296555\,\lambda^2}{\lambda^2 - 0.0190341} + \frac{0.938996\,\lambda^2}{\lambda^2 - 97.6591}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.49 \times 10^{-6} + 2(7.58 \times 10^{-9})\,\Delta T + 3(6.67 \times 10^{-12})\,\Delta T^2 + \frac{4.22 \times 10^{-7} + 2(6.2 \times 10^{-10})\,\Delta T}{\lambda - 0.158}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$