LAK9G15 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.28774\,\lambda^2}{\lambda^2 - 0.00557542} + \frac{0.518245\,\lambda^2}{\lambda^2 - 0.022368} + \frac{26.1756\,\lambda^2}{\lambda^2 - 1892.25}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.19 \times 10^{-6} + 2(1.16 \times 10^{-8})\,\Delta T + 3(-7.71 \times 10^{-12})\,\Delta T^2 + \frac{4.82 \times 10^{-7} + 2(4.5 \times 10^{-10})\,\Delta T}{\lambda - 0.193}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$