SSK3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.39916\,\lambda^2}{\lambda^2 - 0.00828661} + \frac{0.158924\,\lambda^2}{\lambda^2 - 0.0354515} + \frac{0.991913\,\lambda^2}{\lambda^2 - 115.228}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.94 \times 10^{-6} + 2(1.36 \times 10^{-8})\,\Delta T + 3(-6.41 \times 10^{-11})\,\Delta T^2 + \frac{5.32 \times 10^{-7} + 2(3.68 \times 10^{-10})\,\Delta T}{\lambda - 0.224}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$