ULTRAN30 by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.730713\,\lambda^2}{\lambda^2 - 0.00314818} + \frac{0.636952\,\lambda^2}{\lambda^2 - 0.012991} + \frac{0.856177\,\lambda^2}{\lambda^2 - 143.605}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.78 \times 10^{-5} + 2(-3.07 \times 10^{-9})\,\Delta T + 3(1.31 \times 10^{-11})\,\Delta T^2 + \frac{3.87 \times 10^{-7} + 2(4.58 \times 10^{-10})\,\Delta T}{\lambda - 0.159}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$