N-SK5HTi by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.991464\,\lambda^2}{\lambda^2 - 0.0052273} + \frac{0.495982\,\lambda^2}{\lambda^2 - 0.0172734} + \frac{0.987394\,\lambda^2}{\lambda^2 - 98.3595}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.5 \times 10^{-6} + 2(1.22 \times 10^{-8})\,\Delta T + 3(6.38 \times 10^{-11})\,\Delta T^2 + \frac{2.46 \times 10^{-7} + 2(-3.34 \times 10^{-11})\,\Delta T}{\lambda - 0.278}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$