PSK2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.17976\,\lambda^2}{\lambda^2 - 0.00647988} + \frac{0.246687\,\lambda^2}{\lambda^2 - 0.0204067} + \frac{0.989457\,\lambda^2}{\lambda^2 - 95.2625}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.02 \times 10^{-6} + 2(1.29 \times 10^{-8})\,\Delta T + 3(-7.05 \times 10^{-12})\,\Delta T^2 + \frac{4.3 \times 10^{-7} + 2(6.11 \times 10^{-10})\,\Delta T}{\lambda - 0.141}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$