![Bifocals with separate lenses. In this case, the Swedish ethnologist Jan-Ojvind Swahn [sv]. Jan-Ojvind Swahn vid doktorspromovering 2006-06-02.jpg](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Jan-%C3%96jvind_Swahn_vid_doktorspromovering_2006-06-02.jpg/220px-Jan-%C3%96jvind_Swahn_vid_doktorspromovering_2006-06-02.jpg)
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.
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.
Benjamin Franklin is generally credited with the invention of bifocals. He decided to saw his lenses in half so he could read the lips of speakers of French at court, the only way he could understand them. [1] Historians have produced some evidence to suggest that others may have come before him in the invention; however, a correspondence between George Whatley and John Fenno, editor of the Gazette of the United States , suggested that Franklin had indeed invented bifocals, and perhaps 50 years earlier than had been originally thought. [2] On the contrary, the College of Optometrists concluded:
John Isaac Hawkins, the inventor of trifocal lenses, coined the term bifocals in 1824 and credited Benjamin Franklin.[ citation needed ]
In 1955, Irving Rips of Younger Optics created the first seamless or "invisible" bifocal, a precursor to progressive lenses. [4] This followed Howard D. Beach's 1946 work in "blended lenses", [5] [6] O'Conner's "Ultex" lens in 1910, [7] and Isaac Schnaitmann's single-piece bifocal lens in 1837. [8]
Original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper. Up until the beginning of the 20th century two separate lenses were cut in half and combined in the rim of the frame. The mounting of two half-lenses into a single frame led to a number of early complications and rendered such spectacles quite fragile. A method for fusing the sections of the lenses together was developed by Louis de Wecker at the end of the 19th century and patented by John Louis Borsch Jr. (1873–1929) [9] in 1908. In 1915, Henri (Henry) A. Courmettes (1884-1969), a French immigrant to the US, patented the “Flat Top” (or “D Segment”) reading portion of the bifocal. [10] The advantages were wide reading area, less prismatic effects and no image jump between distance and close viewing. This was first introduced in mass production by the Univis Lens Co. of Dayton, OH. in 1926. [11] In 1935, Courmettes went on to patent the Tilted Bifocal Lens, [12] in 1936, a method of grinding two prescriptions simultaneously on that Tilted Bifocal Lens, [13] and in 1951, the Cataract Bifocal Lens. [14]
Today most bifocals are created by moulding a reading segment into a primary lens and are available with the reading segments in a variety of shapes and sizes.
Bifocals can contribute to falls, [15] cause headaches, and even dizziness for some wearers. Adaptation to the small field of view offered by the reading segment of bifocals can take some time, as the user learns to move either the head or the reading material rather than the eyes. Computer monitors are generally placed directly in front of users and can lead to muscle fatigue due to the unusual straight and constant movement of the head. [16] This trouble is mitigated by the use of monofocal lenses for computer use.
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Research continues in an attempt to eliminate the limited field of vision in current bifocals. New materials and technologies may provide a method which can selectively adjust the optical power of a lens. Researchers have constructed such a lens using a liquid crystal layer applied between two glass substrates.[ citation needed ]
The aquatic larval stage of the diving beetle Thermonectus marmoratus has, in its principal eyes, two retinas and two distinct focal planes that are substantially separated (in the manner of bifocals) to switch their vision from up-close to distance, for easy and efficient capture of their prey, mostly mosquito larvae. [17]
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 and spectacles, are vision eyewear with clear or tinted 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, known as temples or temple pieces, that rest over the ears.
Myopia, also known as near-sightedness and short-sightedness, is an eye disease where light from distant objects focuses in front of, instead of on, the retina. As a result, distant objects appear blurry while close objects appear normal. Other symptoms may include headaches and eye strain. Severe myopia is associated with an increased risk of macular degeneration, retinal detachment, cataracts, and glaucoma.
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 eye care.
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.
Refractive surgery is an optional eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea (keratomileusis), lens implantation or lens replacement. The most common methods today use excimer lasers to reshape the curvature of the cornea. Refractive eye surgeries are used to treat common vision disorders such as myopia, hyperopia, presbyopia and astigmatism.
Anisometropia is a condition in which a person's eyes have substantially differing refractive power. Generally, a difference in power of one diopter (1D) is the threshold for diagnosis of the condition. Patients may have up to 3D of anisometropia before the condition becomes clinically significant due to headache, eye strain, double vision or photophobia.
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.
Vision therapy (VT), or behavioral optometry, is an umbrella term for alternative medicine treatments using eye exercises, based around the pseudoscientific claim that vision problems are the true underlying cause of learning difficulties, particularly in children. Vision therapy has not been shown to be effective using scientific studies, except for helping with convergence insufficiency. Most claims—for example that the therapy can address neurological, educational, and spatial difficulties—lack supporting evidence. Neither the American Academy of Pediatrics nor the American Academy of Ophthalmology support the use of vision therapy.
A phoropter or refractor is an ophthalmic testing device. It is commonly used by eye care professionals during an eye examination, and contains different lenses used for refraction of the eye during sight testing, to measure an individual's refractive error and determine their eyeglass prescription. It also is used to measure the patients' phorias and ductions, which are characteristics of binocularity.
Retinoscopy is a technique to obtain an objective measurement of the refractive error of a patient's eyes. The examiner uses a retinoscope to shine light into the patient's eye and observes the reflection (reflex) off the patient's retina. While moving the streak or spot of light through the pupil across the retina, the examiner observes the relative movement of the reflex or manually places lenses over the eye to "neutralize" the reflex.
An eye care professional is an individual who provides a service related to the eyes or vision. It is any healthcare worker involved in eye care, from one with a small amount of post-secondary training to practitioners with a doctoral level of education.
Patricia Era Bath was an American ophthalmologist and humanitarian. She became the first female member of the Jules Stein Eye Institute, the first woman to lead a post-graduate training program in ophthalmology, and the first woman elected to the honorary staff of the UCLA Medical Center. Bath was the first African-American to serve as a resident in ophthalmology at New York University. She was also the first African-American woman to serve on staff as a surgeon at the UCLA Medical Center. Bath was the first African-American woman doctor to receive a patent for a medical purpose. A holder of five patents, she founded the non-profit American Institute for the Prevention of Blindness in Washington, D.C.
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.
Adjustable focus eyeglasses are eyeglasses with an adjustable focal length. They compensate for refractive errors by providing variable focusing, allowing users to adjust them for desired distance or prescription, or both.
Konrad Pesudovs is an Australian optometrist and outcomes researcher in ophthalmology; recognised as the leading optometrist researcher worldwide in terms of H-Index and total citations. He is SHARP Professor of Optometry and Vision Science at the University of New South Wales (2020-). He was the Foundation Chair of Optometry and Vision Science at Flinders University from 2009 to 2017.
The optometer was a device used for measuring the necessary spherical and/or cylindrical corrections to be prescribed for eyeglasses, from the middle of the 18th century until around 1922, when modern instruments were developed. The term, coined in 1738 by W. Porterfield to describe his Scheiner slit optometer, and used for 200 years to describe many different inventions to measure refractive error of the eye, has completely fallen out of usage today as the task of measuring eyes for spectacles is done with modern instruments, such as the phoropter.
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.
In ophthalmology, accommodative excess occurs when an individual uses more than normal accommodation for performing certain near work. Accommodative excess has traditionally been defined as accommodation that is persistently higher than expected for the patient's age. Modern definitions simply regard it as an inability to relax accommodation readily. Excessive accommodation is seen in association with excessive convergence also.
The Jackson cross cylinder (JCC) is an instrument used by ophthalmologists, orthoptists and optometrists in their routine eye examination, particularly in determination of corrective lens power in patients with astigmatism. It is also used for testing near point of the eye.
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