P-LAK35 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.39324\,\lambda^2}{\lambda^2 - 0.0071596} + \frac{0.418883\,\lambda^2}{\lambda^2 - 0.0233637} + \frac{1.04381\,\lambda^2}{\lambda^2 - 88.3284}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.9 \times 10^{-6} + 2(7.99 \times 10^{-9})\,\Delta T + 3(7.76 \times 10^{-12})\,\Delta T^2 + \frac{5.64 \times 10^{-7} + 2(6.57 \times 10^{-10})\,\Delta T}{\lambda - 0.185}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$