PSK3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.887272\,\lambda^2}{\lambda^2 - 0.00469824} + \frac{0.489592\,\lambda^2}{\lambda^2 - 0.0161818} + \frac{1.04865\,\lambda^2}{\lambda^2 - 104.375}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.03 \times 10^{-6} + 2(1.19 \times 10^{-8})\,\Delta T + 3(2.46 \times 10^{-11})\,\Delta T^2 + \frac{3.14 \times 10^{-7} + 2(2.45 \times 10^{-10})\,\Delta T}{\lambda - 0.235}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$