SSK1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.42691\,\lambda^2}{\lambda^2 - 0.00831215} + \frac{0.141196\,\lambda^2}{\lambda^2 - 0.033437} + \frac{0.986005\,\lambda^2}{\lambda^2 - 109.701}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.22 \times 10^{-6} + 2(1.23 \times 10^{-8})\,\Delta T + 3(-1.74 \times 10^{-11})\,\Delta T^2 + \frac{4.51 \times 10^{-7} + 2(3.49 \times 10^{-10})\,\Delta T}{\lambda - 0.251}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$