LAF22A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.85985\,\lambda^2}{\lambda^2 - 0.0109833} + \frac{0.219921\,\lambda^2}{\lambda^2 - 0.0508017} + \frac{1.34141\,\lambda^2}{\lambda^2 - 114.556}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.26 \times 10^{-6} + 2(1.42 \times 10^{-8})\,\Delta T + 3(-4.07 \times 10^{-11})\,\Delta T^2 + \frac{7.05 \times 10^{-7} + 2(2.45 \times 10^{-10})\,\Delta T}{\lambda - 0.265}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$