What does a lens clock primarily measure on a spectacle lens?

A lens clock is a specialized tool used in optics to measure the surface curvature of a spectacle lens. It functions by using a set of calibrated probes that touch the lens surface and take measurements at specific points. The readings obtained from the lens clock can inform the eye care professional about the radii of curvature of both the front and back surfaces of the lens. Understanding the curvature is essential for ensuring that the lens fits into the frame properly and that it meets the optical prescriptions needed for the wearer.

The other options, while relevant in the context of lens characteristics, do not pertain directly to the primary function of a lens clock. For instance, while the thickness of a lens can affect how it is fabricated and its weight can impact comfort, these factors are not the direct focus of a lens clock's measurement capabilities. Similarly, the index of refraction refers to how light travels through different materials and is determined through other methodologies, rather than a lens clock measurement. Thus, the lens clock's primary purpose is accurately measuring surface curvature, positioning it as the correct answer in this context.