K-PSFn4(M) by Sumita

$$n(\lambda) = \sqrt{3.2299 - 0.0113892\,\lambda^2 + 0.0471924\,\lambda^{-2} + 0.00244871\,\lambda^{-4} - 6.48867 \times 10^{-5}\,\lambda^{-6} + 2.10451 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.02 \times 10^{-6} + 2(2.01 \times 10^{-8})\,\Delta T + 3(-1.33 \times 10^{-10})\,\Delta T^2 + \frac{1.73 \times 10^{-6} + 2(1.55 \times 10^{-9})\,\Delta T}{\lambda - 0.277}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$