PSK54 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.31533\,\lambda^2}{\lambda^2 - 0.00719144} + \frac{0.163047\,\lambda^2}{\lambda^2 - 0.0229837} + \frac{0.691495\,\lambda^2}{\lambda^2 - 91.1848}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.82 \times 10^{-5} + 2(4.07 \times 10^{-9})\,\Delta T + 3(-8.7 \times 10^{-12})\,\Delta T^2 + \frac{4.38 \times 10^{-7} + 2(5.64 \times 10^{-10})\,\Delta T}{\lambda - 0.154}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$