KZFS7A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.53916\,\lambda^2}{\lambda^2 - 0.0102662} + \frac{0.209619\,\lambda^2}{\lambda^2 - 0.0471105} + \frac{0.936171\,\lambda^2}{\lambda^2 - 71.6979}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[6.24 \times 10^{-6} + 2(1.37 \times 10^{-8})\,\Delta T + 3(-3.58 \times 10^{-11})\,\Delta T^2 + \frac{7.02 \times 10^{-7} + 2(7.76 \times 10^{-10})\,\Delta T}{\lambda - 0.241}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$