SK11 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.17964\,\lambda^2}{\lambda^2 - 0.00680282} + \frac{0.229817\,\lambda^2}{\lambda^2 - 0.0219737} + \frac{0.93579\,\lambda^2}{\lambda^2 - 101.513}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.14 \times 10^{-6} + 2(1.27 \times 10^{-8})\,\Delta T + 3(-7.21 \times 10^{-11})\,\Delta T^2 + \frac{3.51 \times 10^{-7} + 2(5.41 \times 10^{-10})\,\Delta T}{\lambda - 0.238}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$