SK4 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.39389\,\lambda^2}{\lambda^2 - 0.00769147} + \frac{0.164511\,\lambda^2}{\lambda^2 - 0.0271946} + \frac{0.963522\,\lambda^2}{\lambda^2 - 99.2758}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.49 \times 10^{-7} + 2(1.22 \times 10^{-8})\,\Delta T + 3(1.33 \times 10^{-13})\,\Delta T^2 + \frac{4.58 \times 10^{-7} + 2(5.08 \times 10^{-10})\,\Delta T}{\lambda - 0.173}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$