What is a hallmark of principal meridians?

The hallmark of principal meridians is that they represent the shortest and longest radii of curvature on a toric lens or in the context of astigmatism. In optics, particularly when dealing with lenses, the principal meridians correspond to the orientations along which the lens has the maximum and minimum power. This characteristic is crucial for understanding how the lens will interact with light and how it corrects vision for astigmatic patients.

When analyzing a toric lens, the two principal meridians are perpendicular to each other and are fundamentally important in defining the lens's refractive characteristics. This knowledge is vital for practitioners fitting contact lenses, as the orientation and design of the lens must take into account these meridians for effective vision correction.

This answer highlights the significance of understanding the principal meridians in the context of contact lens fitting and vision correction, reinforcing the concept that they play a crucial role rather than being trivial or merely a geometric consideration.