Trifocals are

eyeglasses Glasses, also known as eyeglasses (American English), spectacles (Commonwealth English), or colloquially as specs, are Visual perception, vision eyewear with clear or tinted lens (optics), lenses mounted in a frame that holds them in front ...

with

lenses A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

that have three regions which correct for distance, intermediate (arm's length), and near vision.

John Isaac Hawkins John Isaac Hawkins (1772–1855) was a British American inventor and civil engineer who twice emigrated to and ultimately died in the United States, American state of New Jersey. Early life Hawkins was born on March 14, 1772 at Taunton, Somerse ...

developed the trifocal lens in 1827. Trifocals are mostly used by people with advanced

presbyopia Presbyopia is a physiological insufficiency of optical Accommodation (vertebrate eye), accommodation associated with the aging of the human eye, eye; it results in progressively worsening ability to focus clearly on close objects. Also known as ...

who have been prescribed 2

diopter A dioptre ( British spelling) or (American spelling), symbol dpt or D, is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, . It is normally used to express the optical power of a lens or curved mi ...

s 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 Bifocals are eyeglasses with two distinct optical powers correcting vision at both long and short distances. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism. H ...

, but with an additional segment for intermediate vision above the reading section. A common style is the 7×28 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

progressive lens Progressive lenses are corrective lenses used in glasses, eyeglasses to correct presbyopia and other disorders of accommodation reflex, accommodation. They are characterised by a gradient of increasing optical power, lens power, added to the wear ...

es.

References

Corrective lenses 1827 introductions Glasses {{Eye-stub

See also

References