BALKN3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.11632\,\lambda^2}{\lambda^2 - 0.0068411} + \frac{0.157261\,\lambda^2}{\lambda^2 - 0.0257049} + \frac{0.91403\,\lambda^2}{\lambda^2 - 105.919}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[9.7 \times 10^{-7} + 2(9.21 \times 10^{-9})\,\Delta T + 3(-4.28 \times 10^{-11})\,\Delta T^2 + \frac{3.74 \times 10^{-7} + 2(3.3 \times 10^{-10})\,\Delta T}{\lambda - 0.262}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$