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A corrective lens is a transmissive optical device that is worn on the eye to improve visual perception. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye. Intraocular lenses are surgically implanted most commonly after cataract removal but can be used for purely refractive purposes.
Glasses, also known as eyeglasses or spectacles, are vision eyewear with lenses mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms that rest over the ears.
A dioptre or diopter, symbol dpt, is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dpt = 1 m−1. It is normally used to express the optical power of a lens or curved mirror, which is a physical quantity equal to the reciprocal of the focal length, expressed in metres. For example, a 3-dioptre lens brings parallel rays of light to focus at 1⁄3 metre. A flat window has an optical power of zero dioptres, as it does not cause light to converge or diverge. Dioptres are also sometimes used for other reciprocals of distance, particularly radii of curvature and the vergence of optical beams.
Bifocals are eyeglasses with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism.
Optometry is a specialized health care profession that involves examining the eyes and related structures for defects or abnormalities. Optometrists are health care professionals who typically provide comprehensive primary eye care.
Presbyopia is physiological insufficiency of accommodation associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Also known as age-related farsightedness, it affects many adults over the age of 40. A common sign of presbyopia is difficulty reading small print which results in having to hold reading material farther away. Other symptoms associated can be headaches and eyestrain. Different people will have different degrees of problems. Other types of refractive errors may exist at the same time as presbyopia. This condition is similar to hypermetropia or far-sightedness which starts in childhood and exhibits similar symptoms of blur in the vision for close objects.
An optician is an individual who fits eyeglasses or contact lenses by filling a refractive prescription from an optometrist or ophthalmologist. They are able to translate and adapt ophthalmic prescriptions, dispense products, and work with accessories. There are several specialties within the field.
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. The term comes from the Greek α- (a-) meaning "without" and στίγμα (stigma), "a mark, spot, puncture".
An Intraocular lens (IOL) is a lens implanted in the eye usually as part of a treatment for cataracts or for correcting other vision problems such as short sightedness and long sightedness, a form of refractive surgery. If the natural lens is left in the eye, the IOL is known as phakic, otherwise it is a pseudophakic lens. Both kinds of IOLs are designed to provide the same light-focusing function as the natural crystalline lens. This can be an alternative to LASIK, but LASIK is not an alternative to an IOL for treatment of cataracts.
Progressive lenses are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation. They are characterised by a gradient of increasing lens power, added to the wearer's correction for the other refractive errors. The gradient starts at the wearer's distance prescription at the top of the lens and reaches a maximum addition power, or the full reading addition, at the bottom of the lens. The length of the progressive power gradient on the lens surface depends on the design of the lens, with a final addition power between 0.75 and 3.50 dioptres. The addition value prescribed depends on the level of presbyopia of the patient. In general the older the patient, the higher the addition. They are also known as multifocal lenses, progressive addition lenses (PAL), varifocal lenses, progressive power lenses, graduated prescription lenses, or progressive spectacle lenses.
Refractive error, also known as refraction error, is a problem with focusing light accurately on the retina due to the shape of the eye and or cornea. The most common types of refractive error are near-sightedness, far-sightedness, astigmatism, and presbyopia. Near-sightedness results in far away objects being blurry, far-sightedness and presbyopia result in close objects being blurry, and astigmatism causes objects to appear stretched out or blurry. Other symptoms may include double vision, headaches, and eye strain.
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies. In this, distances vary for individuals from the far point—the maximum distance from the eye for which a clear image of an object can be seen, to the near point—the minimum distance for a clear image. Accommodation usually acts like a reflex, including part of the accommodation-vergence reflex, but it can also be consciously controlled. The main ways animals may change focus are:
An autorefractor or automated refractor is a computer-controlled machine used during an eye examination to provide an objective measurement of a person's refractive error and prescription for glasses or contact lenses. This is achieved by measuring how light is changed as it enters a person's eye.
An aspheric lens or asphere is a lens whose surface profiles are not portions of a sphere or cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens.
Convergence insufficiency is a sensory and neuromuscular anomaly of the binocular vision system, characterized by a reduced ability of the eyes to turn towards each other, or sustain convergence.
The amplitude of accommodation is the maximum potential increase in optical power that an eye can achieve in adjusting its focus. It refers to a certain range of object distances for which the retinal image is as sharply focussed as possible.
Aniseikonia is an ocular condition where there is a significant difference in the perceived size of images. It can occur as an overall difference between the two eyes, or as a difference in a particular meridian. If the ocular image size in both eyes are equal, the condition is known as iseikonia.
Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the ciliary muscle in a relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and eye lens and the axial length of the eye balance out, which focuses rays exactly on the retina, resulting in perfectly sharp distance vision. A human eye in a state of emmetropia requires no corrective lenses for distance; the vision scores well on a visual acuity test.
Laser blended vision is a sophisticated laser eye treatment which is used to treat presbyopia or other age-related eye conditions. It can be used to help people that simply need reading glasses, and also those who have started to need bifocal or varifocal spectacle correction due to ageing changes in the eye. It can be used for people who are also short-sighted (myopia) or long-sighted (hyperopia) and who also may have astigmatism.
Irvin M. Borish was an American optometrist who is widely considered "The Father of Modern Optometry". Even though he entered the field of optometry because his family could only afford to pay for two years of college, he left a lasting impression in the field. He wrote one of the most renowned textbooks of optometry, Clinical Refraction. He worked to create several educational and research institutions for optometry. He also lobbied tirelessly to establish optometry as a mainstream medical practice. His contribution to optometry has been recognized through prestigious awards and recognition from his peers.
References
- ↑ Hanlon, Mike (April 15, 2006). "Eyeglasses with adaptive focus". New Atlas. Retrieved 2013-05-31.
- ↑ US Patent 3,598,479 and Trans Ophthal Soc UK (1978) 98, 84
- ↑
- Focusspecs – appears to be an example of this Archived 2020-02-22 at the Wayback Machine
- ↑ US patent is Berreman 4,190,330
- ↑ Bakalar, Nicholas (September 26, 2011). "Self-Adjustable Eyeglass Lenses". The New York Times . Retrieved 2014-09-03.
- ↑ "Adjustable-Focus Glasses Can Replace Bifocals". Slashdot. 2009-08-04. Retrieved 2013-05-31.
- ↑ "see comments". Science.slashdot.org. 2009-08-04. Retrieved 2013-05-31.
- Markoff, John (August 3, 2009). "Making Eyeglasses That Let Wearers Change Focus on the Fly". The New York Times.