BAL15Y by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.28348\,\lambda^2}{\lambda^2 - 0.00790901} + \frac{0.102801\,\lambda^2}{\lambda^2 - 0.0305971} + \frac{0.40461\,\lambda^2}{\lambda^2 - 46.5268}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.61 \times 10^{-6} + 2(1.32 \times 10^{-8})\,\Delta T + 3(-4.12 \times 10^{-11})\,\Delta T^2 + \frac{4.65 \times 10^{-7} + 2(5.14 \times 10^{-10})\,\Delta T}{\lambda - 0.201}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$