SF57 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.81651\,\lambda^2}{\lambda^2 - 0.0143704} + \frac{0.428894\,\lambda^2}{\lambda^2 - 0.0592801} + \frac{1.07186\,\lambda^2}{\lambda^2 - 121.42}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[7.26 \times 10^{-6} + 2(1.88 \times 10^{-8})\,\Delta T + 3(-5.14 \times 10^{-11})\,\Delta T^2 + \frac{1.96 \times 10^{-6} + 2(1.79 \times 10^{-9})\,\Delta T}{\lambda - 0.276}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$