SK15 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.29173\,\lambda^2}{\lambda^2 - 0.00686358} + \frac{0.298393\,\lambda^2}{\lambda^2 - 0.0228704} + \frac{0.955518\,\lambda^2}{\lambda^2 - 97.4779}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.85 \times 10^{-7} + 2(1.17 \times 10^{-8})\,\Delta T + 3(-4.64 \times 10^{-12})\,\Delta T^2 + \frac{4.77 \times 10^{-7} + 2(5.5 \times 10^{-10})\,\Delta T}{\lambda - 0.17}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$