N-PSK53A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.38122\,\lambda^2}{\lambda^2 - 0.00706416} + \frac{0.196746\,\lambda^2}{\lambda^2 - 0.0233251} + \frac{0.886089\,\lambda^2}{\lambda^2 - 97.4847}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-9.28 \times 10^{-6} + 2(7.19 \times 10^{-9})\,\Delta T + 3(1.45 \times 10^{-12})\,\Delta T^2 + \frac{4.06 \times 10^{-7} + 2(3.17 \times 10^{-10})\,\Delta T}{\lambda - 0.19}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$