British English spelling of diopter, a unit of optical power equal to the reciprocal of the focal length in meters, used in prescribing lenses.
Alternative British spelling of the Greek-derived 'diopter,' following British conventions of using '-re' endings instead of '-er' for certain scientific terms.
British and American optical prescriptions use different spellings for the same unit—a British optician writes 'dioptre' while an American one writes 'diopter,' but your glasses have exactly the same power either way.
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