What is the term for removing plastic from the bottom of a lens to create a flat edge that will control rotation?

The term used for removing plastic from the bottom of a lens to create a flat edge that will control rotation is truncation. This process specifically involves cutting away part of the lens material to produce a flat surface, which helps stabilize the lens and minimize unwanted movement when it is in position on the eyewear frame. Truncation is often used for lenses with special designs or prescriptions that require precise fitting and alignment to ensure optimal performance and comfort for the wearer.

In contrast, flattening generally refers to making an existing surface level or even but does not imply the creation of a specific flat edge for rotation control. Edge polishing involves smoothing the edges of a lens after it has been shaped, enhancing appearance and comfort without addressing the specific issue of lens rotation. Profiling refers to shaping the lens to specific dimensions or curvatures but is more focused on overall lens design rather than the particular removal of material to create flat edges. Thus, truncation is the accurate choice when discussing the removal of material for the particular purpose of controlling lens rotation.