PCD4 by Hoya

$$n(\lambda) = \sqrt{2.5807 - 0.0109575\,\lambda^2 + 0.0126894\,\lambda^{-2} + 0.000663878\,\lambda^{-4} - 6.48497 \times 10^{-5}\,\lambda^{-6} + 3.47879 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-8.93 \times 10^{-6} + 2(1.28 \times 10^{-8})\,\Delta T + 3(-1.34 \times 10^{-11})\,\Delta T^2 + \frac{5.36 \times 10^{-7} + 2(-8.84 \times 10^{-11})\,\Delta T}{\lambda - 2.93 \times 10^{-9}}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$