SSK51 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.44261\,\lambda^2}{\lambda^2 - 0.00890475} + \frac{0.0828624\,\lambda^2}{\lambda^2 - 0.0400883} + \frac{0.892805\,\lambda^2}{\lambda^2 - 103.153}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.06 \times 10^{-6} + 2(1.23 \times 10^{-8})\,\Delta T + 3(-2.56 \times 10^{-11})\,\Delta T^2 + \frac{4.08 \times 10^{-7} + 2(7.72 \times 10^{-10})\,\Delta T}{\lambda - 0.25}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$