N-BALF4 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.31004\,\lambda^2}{\lambda^2 - 0.00796596} + \frac{0.142038\,\lambda^2}{\lambda^2 - 0.0330672} + \frac{0.964929\,\lambda^2}{\lambda^2 - 109.197}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.33 \times 10^{-6} + 2(1.47 \times 10^{-8})\,\Delta T + 3(-1.58 \times 10^{-11})\,\Delta T^2 + \frac{5.75 \times 10^{-7} + 2(6.58 \times 10^{-10})\,\Delta T}{\lambda - 0.195}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$