  SrAs$_3$ is a unique nodal-line semimetal that contains only a single nodal
ring in the Brillouin zone, uninterrupted by any trivial bands near the Fermi
energy. We performed axis-resolved optical reflection measurements on SrAs$_3$
and observed that the optical conductivity exhibits flat absorption up to 129
meV in both the radial and axial directions, confirming the robustness of the
universal power-law behavior of the nodal ring. Furthermore, in conjunction
with model and first-principles calculations, the axis-resolved optical
conductivity unveiled fundamental properties beyond the flat absorption,
including the overlap energy of the topological bands, the spin-orbit coupling
gap along the nodal ring, and the geometric properties of the nodal ring such
as the average ring radius, ring ellipticity, and velocity anisotropy. In
addition, our temperature-dependent measurements revealed a spectral weight
transfer between intraband and interband transitions, indicating a possible
violation of the optical sum rule within the measured energy range.

Strongly Correlated Electrons