PK3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.14966\,\lambda^2}{\lambda^2 - 0.00670136} + \frac{0.147546\,\lambda^2}{\lambda^2 - 0.0227276} + \frac{1.03052\,\lambda^2}{\lambda^2 - 103.152}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.08 \times 10^{-6} + 2(1.29 \times 10^{-8})\,\Delta T + 3(-2.65 \times 10^{-11})\,\Delta T^2 + \frac{4.37 \times 10^{-7} + 2(7.07 \times 10^{-10})\,\Delta T}{\lambda - 0.138}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$