SF15 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.53926\,\lambda^2}{\lambda^2 - 0.0119308} + \frac{0.247621\,\lambda^2}{\lambda^2 - 0.0556078} + \frac{1.03816\,\lambda^2}{\lambda^2 - 116.417}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.91 \times 10^{-6} + 2(1.69 \times 10^{-8})\,\Delta T + 3(-3.77 \times 10^{-11})\,\Delta T^2 + \frac{1.1 \times 10^{-6} + 2(1.07 \times 10^{-9})\,\Delta T}{\lambda - 0.278}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$