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The Clue

Eyeglass prescription measurement (7 letter answer)

The Answer

DIOPTER

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Notes

The answer DIOPTER is so rare that it appears less than once every 2500 puzzles.

Related Clues

Unit of refractive power of a lens

Definition

DIOPTER as a noun:

1. (diopter, dioptre) = a unit of measurement of the refractive power of a lens which is equal to the reciprocal of the focal length measured in meters; used by oculists