Trifocal lenses

Trifocals are eyeglasses with lenses that have three regions which correct for distance, intermediate (arm's length), and near vision. John Isaac Hawkins developed the trifocal lens in 1827.[1]

"Trifocal" redirects here. For the tensor, see Trifocal tensor.

Trifocals are mostly used by people with advanced presbyopia who 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 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 lenses.

Astigmatism

Astigmatism is a widespread vision condition that can cause people to have blurred vision. It's caused by an imperfection in the cornea or lens of the eyes. Corneal astigmatism is when the cornea isn't evenly curved and smooth. Lenticular astigmatism is when the shape of the lens is distorted. Both conditions can be corrected by using toric contact lenses.[2]

gollark: I think most phone infrastructure uses GPS and maybe a local atomic clock too.
gollark: I'm saying that if it became bad enough that datacentres failed, it would also break other stuff.
gollark: If you just use a pulse per second output from a GPS receiver for generic whatever it's fine. If you want to actually find your position then it would be bad.
gollark: But they do transmit the offset.
gollark: They use TAI, which doesn't have leap seconds at all.

See also

References
  1. Stein, Harold A. (2012). The Ophthalmic Assistant: A Text for Allied and Associated Ophthalmic Personnel (9th ed.). Philadelphia: Elsevier Mosby. p. 205. ISBN 978-1-4557-3346-0.
  2. "Contact lens for Astigmatism". Misaki. Retrieved 2019-12-21.


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