N-SF15 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.57056\,\lambda^2}{\lambda^2 - 0.0116507} + \frac{0.218987\,\lambda^2}{\lambda^2 - 0.0597857} + \frac{1.50824\,\lambda^2}{\lambda^2 - 132.709}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-7.15 \times 10^{-7} + 2(1.04 \times 10^{-8})\,\Delta T + 3(-2.62 \times 10^{-11})\,\Delta T^2 + \frac{8.56 \times 10^{-7} + 2(1.29 \times 10^{-9})\,\Delta T}{\lambda - 0.281}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$