P-SK57 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.31053\,\lambda^2}{\lambda^2 - 0.00740877} + \frac{0.169376\,\lambda^2}{\lambda^2 - 0.0254563} + \frac{1.10988\,\lambda^2}{\lambda^2 - 107.751}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.6 \times 10^{-6} + 2(9.4 \times 10^{-9})\,\Delta T + 3(-2.3 \times 10^{-11})\,\Delta T^2 + \frac{4.9 \times 10^{-7} + 2(5.96 \times 10^{-10})\,\Delta T}{\lambda - 0.178}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$