N-PK52 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.00814\,\lambda^2}{\lambda^2 - 0.00501968} + \frac{0.209435\,\lambda^2}{\lambda^2 - 0.0162477} + \frac{0.781686\,\lambda^2}{\lambda^2 - 152.389}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.08 \times 10^{-5} + 2(-7.86 \times 10^{-9})\,\Delta T + 3(1.51 \times 10^{-11})\,\Delta T^2 + \frac{3.21 \times 10^{-7} + 2(3.02 \times 10^{-10})\,\Delta T}{\lambda - 0.194}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$