N-BAK4HT by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.28835\,\lambda^2}{\lambda^2 - 0.00779981} + \frac{0.132818\,\lambda^2}{\lambda^2 - 0.0315631} + \frac{0.945395\,\lambda^2}{\lambda^2 - 105.966}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.06 \times 10^{-6} + 2(1.44 \times 10^{-8})\,\Delta T + 3(-2.23 \times 10^{-11})\,\Delta T^2 + \frac{5.46 \times 10^{-7} + 2(6.05 \times 10^{-10})\,\Delta T}{\lambda - 0.189}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$