P-PK53 by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.960317\,\lambda^2}{\lambda^2 - 0.00531033} + \frac{0.340437\,\lambda^2}{\lambda^2 - 0.0175073} + \frac{0.777866\,\lambda^2}{\lambda^2 - 106.875}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.65 \times 10^{-5} + 2(-5.14 \times 10^{-10})\,\Delta T + 3(-2.02 \times 10^{-11})\,\Delta T^2 + \frac{4.11 \times 10^{-7} + 2(4.17 \times 10^{-10})\,\Delta T}{\lambda - 0.208}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$