What term refers to the real measured or sagged curve that exists on the surface of a lens?

The term that refers to the real measured or sagged curve that exists on the surface of a lens is the true base curve. This curve is essential in optics, as it defines the actual shape of the lens surface, which impacts how light is refracted and how the lens interacts with the eye.

The true base curve is derived from direct measurements and reflects the lens's physical characteristics, making it crucial for providing accurate prescriptions and ensuring comfort for the wearer. Accurate matching of the true base curve to individual eye geometry can significantly enhance visual performance and comfort.

On the other hand, terms like effective base curve, measured base curve, and calculated base curve relate to other aspects of lens design or theoretical constructs rather than directly describing the actual curvature of the lens itself. These may focus on different definitions or approximations used during lens design or fitting processes but do not capture the specific, physical curvature formed during the lens manufacturing process.