eyeglasseswhere the lenses have 3 regions to correct for distance, intermediate (arm's length), and near vision. They are mostly used by people with advanced presbyopiawho have been prescribed 2 diopters or more of reading addition. The intermediate addition is normally half the reading addition. So, for someone with a distance prescription of -4 diopters and a reading addition of +3, the reading portion of their trifocals would have a net power of -1, and the intermediate segment would be -2.5 diopters.
Trifocal lenses are made in similar styles to
bifocals, but with an additional segment for intermediate vision above the reading section. A common style is the 7x28 flat-top or D-shaped segment, 28 mm wide, with a 7 mm high intermediate segment. Larger intermediate segments are available, and are particularly useful for people who spend a lot of time using computers.
Trifocals are becoming rarer as more people choose to wear
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trifocal — ☆ trifocal [trī fō′kəl; ] also, and for n. usually [, trī′fō΄kəl ] adj. adjusted to three different focal lengths n. a lens like a bifocal but with an additional narrow area ground to adjust the intermediate focus for objects at c. 76 cm ( c. 30… … English World dictionary
trifocal — Having three foci. See t. lens. * * * tri·fo·cal ( )trī fō kəl adj 1) having three focal lengths 2) of an eyeglass lens having one part that corrects for near vision, one for intermediate vision (as at arm s length), and one for distant vision… … Medical dictionary
trifocal — I. adjective Date: 1826 having three focal lengths II. noun Date: 1899 1. plural eyeglasses with trifocal lenses 2. a trifocal glass or lens … New Collegiate Dictionary
trifocal — /truy foh keuhl, truy foh /, adj. 1. Optics. having three foci. 2. (of an eyeglass lens) having three portions, one for near, one for intermediate, and one for far vision. n. 3. trifocals, eyeglasses with trifocal lenses. [1945 50; TRI + FOCAL] * … Universalium
trifocal — adjective (of a pair of glasses) having lenses with three parts with different focal lengths. noun (trifocals) a pair of trifocal glasses … English new terms dictionary
trifocal — tri•fo•cal [[t]traɪˈfoʊ kəl, ˈtraɪˌfoʊ [/t]] adj. 1) opt (of a lens) having three foci 2) (of an eyeglass lens) having three portions, one for near, one for intermediate, and one for far vision 3) trifocals, eyeglasses with trifocal lenses •… … From formal English to slang
trifocal — adj. & n. adj. having three focuses, esp. of a lens with different focal lengths. n. (in pl.) trifocal spectacles … Useful english dictionary
trifocal — adjective Of or pertaining to corrective lenses that have three different powers per eye. See Also: bifocal, trifocals … Wiktionary
trifocal — adj. concerning an eyeglass lens with three portions (for seeing far, intermediate and near); having three focal lengths … English contemporary dictionary
trifocal — tri·focal … English syllables