FK5HTi by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.909362\,\lambda^2}{\lambda^2 - 0.00520142} + \frac{0.279077\,\lambda^2}{\lambda^2 - 0.0158938} + \frac{0.891813\,\lambda^2}{\lambda^2 - 95.9109}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-7.47 \times 10^{-6} + 2(1.58 \times 10^{-8})\,\Delta T + 3(-1.23 \times 10^{-11})\,\Delta T^2 + \frac{3.58 \times 10^{-7} + 2(4.03 \times 10^{-10})\,\Delta T}{\lambda - 0.164}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$