trifocal
Of or pertaining to corrective lenses that have three different powers per eye.
trifocal: three focal zones in one lens
A trifocal lens contains three distinct optical powers ground into a single piece of glass or plastic, allowing the wearer to focus at three different distances without changing eyewear. The lens is divided into zones: the main distance correction occupies the upper portion for far vision, an intermediate zone in the middle addresses mid-range work, and a reading segment at the bottom provides near focus. Each zone has its own optical prescription, typically differing by 1 to 3 diopters between adjacent powers.
The intermediate zone distinguishes trifocals from bifocals, which have only distance and near corrections. This middle power (usually around 40 percent of the lens height) was added to reduce the image jump and accommodation demand when looking from distance to reading. It proved especially useful for professionals who spent time at computer screens or workbenches at arm's length, where bifocal wearers experienced significant visual discontinuity.
Optical and manufacturing considerations
The three powers create visible lines on the front of the lens: a primary line separating distance from intermediate, and a secondary line between intermediate and near. These segment lines become more pronounced with higher prescriptions and remain a cosmetic concern despite decades of optical development. The transition between zones causes slight image swim or distortion at the edges of each power band, a tradeoff manufacturers accept for the broader functional vision range.
Trifocals are produced by fusing or molding separate lens elements together, or by surfacing multiple optical curves onto a single substrate. CR-39 plastic dominates in budget segments; higher-index materials like polycarbonate or glass reduce lens thickness for strong prescriptions. The reading segment typically sits 12 to 14 millimeters below the top of the lens, calibrated for standard reading distance of around 40 centimeters.
Progressive lenses, which provide continuous power graduation across the entire lens surface, have largely displaced trifocals in retail optical markets since the 1980s. However, trifocals remain common in industrial eyewear, specialty protective lenses, and in regions where cost is the primary constraint. Workers in technical trades sometimes prefer trifocals for their sharper optical definition at each working distance compared to the softer transitions of progressives.