LLF1HTi by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.2251\,\lambda^2}{\lambda^2 - 0.00870432} + \frac{0.125156\,\lambda^2}{\lambda^2 - 0.0427325} + \frac{0.892237\,\lambda^2}{\lambda^2 - 108.05}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.55 \times 10^{-7} + 2(1.41 \times 10^{-8})\,\Delta T + 3(-3.32 \times 10^{-11})\,\Delta T^2 + \frac{6.74 \times 10^{-7} + 2(6.27 \times 10^{-10})\,\Delta T}{\lambda - 0.227}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$