K50 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.01888\,\lambda^2}{\lambda^2 - 0.00594871} + \frac{0.266823\,\lambda^2}{\lambda^2 - 0.0215167} + \frac{0.906672\,\lambda^2}{\lambda^2 - 102.14}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.69 \times 10^{-6} + 2(1.3 \times 10^{-8})\,\Delta T + 3(-5.26 \times 10^{-11})\,\Delta T^2 + \frac{5.48 \times 10^{-7} + 2(7.11 \times 10^{-10})\,\Delta T}{\lambda - 0.164}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$