SSK50 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.45563\,\lambda^2}{\lambda^2 - 0.00870104} + \frac{0.113576\,\lambda^2}{\lambda^2 - 0.0378429} + \frac{0.969521\,\lambda^2}{\lambda^2 - 110.109}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.42 \times 10^{-7} + 2(1.18 \times 10^{-8})\,\Delta T + 3(-2.3 \times 10^{-11})\,\Delta T^2 + \frac{5.02 \times 10^{-7} + 2(5.74 \times 10^{-10})\,\Delta T}{\lambda - 0.22}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$