Trifocals lenses have the distance prescription power on upper portion of lens, a lined segment with reading prescription power on bottom portion and an intermediate (computer distance) prescription power in middle portion.
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About Glass Lenses:
While glass lenses are extremely scratch resistant, they are also a lot heavier than plastic or polycarbonate. In addition, they are not as impact resistant as plastic or polycarbonate (see disclaimer below), even though they are safety hardened in accordance with FDA regulations.
Notice: Glass lenses are treated for impact resistance and tested in accordance with the Food and Drug Administration requirements (21 CFR, Sec 801.410). They are not shatterproof, nor will they provide an unbreakable shield against eye injury. The continued impact resistance of the lenses depends on how well they are protected against surface scratches and pits. For your own protection, lenses with surface marks should be replaced immediately, since they will no longer meet basic impact resistance in accordance with ANSI Z87.1 and should not be used when safety eyeglasses are required.