SK12 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.16983\,\lambda^2}{\lambda^2 - 0.00652887} + \frac{0.29745\,\lambda^2}{\lambda^2 - 0.0214034} + \frac{0.965848\,\lambda^2}{\lambda^2 - 105.09}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.09 \times 10^{-6} + 2(9.88 \times 10^{-9})\,\Delta T + 3(1.38 \times 10^{-11})\,\Delta T^2 + \frac{5.87 \times 10^{-7} + 2(9.86 \times 10^{-10})\,\Delta T}{\lambda - 0.105}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$