KZFSN4 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.37994\,\lambda^2}{\lambda^2 - 0.0089116} + \frac{0.168497\,\lambda^2}{\lambda^2 - 0.0405334} + \frac{0.874886\,\lambda^2}{\lambda^2 - 69.6628}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[6.01 \times 10^{-6} + 2(1.3 \times 10^{-8})\,\Delta T + 3(-7.94 \times 10^{-11})\,\Delta T^2 + \frac{4.05 \times 10^{-7} + 2(7.23 \times 10^{-10})\,\Delta T}{\lambda - 0.272}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$