TIF6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.40069\,\lambda^2}{\lambda^2 - 0.0109426} + \frac{0.134903\,\lambda^2}{\lambda^2 - 0.0708667} + \frac{0.924097\,\lambda^2}{\lambda^2 - 99.4792}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.69 \times 10^{-5} + 2(-7.02 \times 10^{-9})\,\Delta T + 3(-1.85 \times 10^{-11})\,\Delta T^2 + \frac{1.01 \times 10^{-6} + 2(1.39 \times 10^{-9})\,\Delta T}{\lambda - 0.305}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$