N-PSK53 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.34341\,\lambda^2}{\lambda^2 - 0.00675074} + \frac{0.241418\,\lambda^2}{\lambda^2 - 0.0219911} + \frac{0.952897\,\lambda^2}{\lambda^2 - 103.551}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-9.29 \times 10^{-6} + 2(5.78 \times 10^{-9})\,\Delta T + 3(8.87 \times 10^{-13})\,\Delta T^2 + \frac{4.59 \times 10^{-7} + 2(5.86 \times 10^{-10})\,\Delta T}{\lambda - 0.155}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$