K7 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.12736\,\lambda^2}{\lambda^2 - 0.00720342} + \frac{0.124412\,\lambda^2}{\lambda^2 - 0.0269836} + \frac{0.827101\,\lambda^2}{\lambda^2 - 100.385}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.67 \times 10^{-6} + 2(8.8 \times 10^{-9})\,\Delta T + 3(-2.86 \times 10^{-11})\,\Delta T^2 + \frac{5.42 \times 10^{-7} + 2(7.81 \times 10^{-10})\,\Delta T}{\lambda - 0.172}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$