What does ED stand for in box system measurements?

• A. Edge Diameter
• B. Effective Diameter
• C. External Diameter
• D. Estimated Diameter

Answer: (B)

In measurements used for box system sizing, ED stands for Effective Diameter—the portion of the lens that actually covers and interacts with the corneal surface during wear. The idea is to capture the functional width of the lens, not just its outer boundary. This is important because the effective diameter determines how well the lens centers, moves with blinking, and provides proper coverage of the cornea and optical zone. If you compare ED to the corneal or horizontal visible iris diameter, you get a better sense of fit and stability than you would from the physical edge or an approximate size. Other terms like Edge Diameter, External Diameter, or Estimated Diameter describe outer or assumed measurements that don’t reflect the lens’s functional coverage on the eye, so they’re not the standard parameter used in box system measurements.