LF7 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.2688\,\lambda^2}{\lambda^2 - 0.00919554} + \frac{0.156697\,\lambda^2}{\lambda^2 - 0.0447257} + \frac{0.887676\,\lambda^2}{\lambda^2 - 106.82}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.11 \times 10^{-6} + 2(1.56 \times 10^{-8})\,\Delta T + 3(-1.79 \times 10^{-11})\,\Delta T^2 + \frac{7.99 \times 10^{-7} + 2(8.55 \times 10^{-10})\,\Delta T}{\lambda - 0.229}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$