LAK28 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.50442\,\lambda^2}{\lambda^2 - 0.00719666} + \frac{0.474121\,\lambda^2}{\lambda^2 - 0.0249143} + \frac{1.17784\,\lambda^2}{\lambda^2 - 83.1443}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.01 \times 10^{-6} + 2(1.12 \times 10^{-8})\,\Delta T + 3(-1.08 \times 10^{-11})\,\Delta T^2 + \frac{4.68 \times 10^{-7} + 2(3.34 \times 10^{-10})\,\Delta T}{\lambda - 0.226}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$