A lens clock is used to measure which aspect of a lens?

• A. PD
• B. Lens thickness
• C. Frame color
• D. Base curve of a lens

Answer: (D)

A lens clock measures the curvature of a lens surface, specifically its base curve. When you place the clock on the lens, the scale shows the amount of rise or the radius of curvature of the lens front (or back, depending on how you use it). Knowing the base curve helps in lens fabrication and frame fitting because it describes how curved the lens is, which affects edge thickness, proper seating in the frame, and compatibility with the prescription. PD is determined with a pupillometer or PD ruler, not a lens clock, and frame color or simple thickness measurements aren’t what the clock reads. So the lens clock is all about the lens’s base curve.