What does the ED value stand for in lens sizing?

Study for the NBEO Physiological Optics Test. Prepare with flashcards and multiple-choice questions, each question has hints and explanations. Ace your exam!

Multiple Choice

What does the ED value stand for in lens sizing?

Explanation:
In lens sizing, the ED value stands for the effective diameter—the functional width of the lens that actually sits on the cornea and interacts with the tear film. This measurement reflects the portion of the lens that determines fit, centration, and movement with blinking, not just the physical outer edge. The other terms aren’t used for this purpose: external diameter would imply a simple outer measurement, elliptical diameter suggests a noncircular shape, and estimated diameter implies a guess rather than a defined, functional size.

In lens sizing, the ED value stands for the effective diameter—the functional width of the lens that actually sits on the cornea and interacts with the tear film. This measurement reflects the portion of the lens that determines fit, centration, and movement with blinking, not just the physical outer edge. The other terms aren’t used for this purpose: external diameter would imply a simple outer measurement, elliptical diameter suggests a noncircular shape, and estimated diameter implies a guess rather than a defined, functional size.

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