N-PK52A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.02961\,\lambda^2}{\lambda^2 - 0.005168} + \frac{0.188051\,\lambda^2}{\lambda^2 - 0.0166659} + \frac{0.736488\,\lambda^2}{\lambda^2 - 138.964}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.97 \times 10^{-5} + 2(-5.5 \times 10^{-9})\,\Delta T + 3(5.28 \times 10^{-12})\,\Delta T^2 + \frac{3.6 \times 10^{-7} + 2(2.45 \times 10^{-10})\,\Delta T}{\lambda - 0.172}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$