Adjustable-focus eyeglasses
Encyclopedia
Adjustable focus eyeglasses are prescription eyeglasses
with an adjustable focal length. They compensate for refractive errors (such as presbyopia
) by providing variable focusing, allowing users to adjust them for desired distance or prescription, or both.
Current Bifocals
and progressive lenses
are static, in that the user has to change their eye position to look through the portion of the lens with the focal power corresponding to the distance of the object. This usually means looking through the top of the lens for distant objects and down through the bottom of the lens for near objects. Adjustable focus eyeglasses have one focal length, but it is variable without having to change where one is looking.
Possible uses for such glasses are to provide cheap glasses for 3rd world countries, or to accommodate for presbyopia
.
Electro-optical often uses liquid crystal
s as the active medium.) Applying an electric current to the liquid changes the refraction of the liquid.
Early work on opto-mechanical methods was done by Martin Wright
. Opto-mechanical spectacles allow focus control by the wearer via movement of a small slider located on top of the bridge. The user adjusts the lens for optical clarity at the desired distance. They are a combination of rigid and flexible lenses that can change prescription to enable sharp focus at different distances (from infinity up to 13"). The appropriate addition range depends on the user’s level of refractive error. A tiny mechanism, actuated by the slider, simultaneously controls both flexible lenses to assure appropriate near vision tracking in both eyes.
Another type of opto-mechanical lens is the design of Joshua Silver
, and uses liquid pressure against a diaphragm to control focus of a lens. Stephen Kurtin also has a product based on what appears to be a related design called TruFocals.
, in any direction. Distance vision corrections are made by re-adjusting the lens for distance, instead of by tilting and/or rotating the head to view object through the best part of the lens for the distance. Adjustable focus lenses, like single-focus lenses, also reduce image-jump and spatial distortion in the field of view associated with traditional multi-focal lenses. Additionally, the ideal near-vision correction can be achieved with precision, because the variable lenses emulate the focusing action of the youthful (non-presbyopic) eye.
Glasses
Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...
with an adjustable focal length. They compensate for refractive errors (such as presbyopia
Presbyopia
Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...
) by providing variable focusing, allowing users to adjust them for desired distance or prescription, or both.
Current Bifocals
Bifocals
Bifocals are eyeglasses with two distinct optical powers. Bifocals are most commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism.-History:...
and progressive lenses
Progressive lenses
Progressive spectacle lenses, also called progressive addition lenses , progressive power lenses, graduated prescription lenses, and varifocal or multifocal lenses, are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation...
are static, in that the user has to change their eye position to look through the portion of the lens with the focal power corresponding to the distance of the object. This usually means looking through the top of the lens for distant objects and down through the bottom of the lens for near objects. Adjustable focus eyeglasses have one focal length, but it is variable without having to change where one is looking.
Possible uses for such glasses are to provide cheap glasses for 3rd world countries, or to accommodate for presbyopia
Presbyopia
Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...
.
Methods
There are currently two basic methods to achieve variable focal length: electro-optical and opto-mechanical.Electro-optical often uses liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
s as the active medium.) Applying an electric current to the liquid changes the refraction of the liquid.
Early work on opto-mechanical methods was done by Martin Wright
Martin Wright (bioengineer)
Basil Martin Wright was a British bioengineer who invented several notable medical instruments, including the peak flow meter and Graseby syringe driver.- References :...
. Opto-mechanical spectacles allow focus control by the wearer via movement of a small slider located on top of the bridge. The user adjusts the lens for optical clarity at the desired distance. They are a combination of rigid and flexible lenses that can change prescription to enable sharp focus at different distances (from infinity up to 13"). The appropriate addition range depends on the user’s level of refractive error. A tiny mechanism, actuated by the slider, simultaneously controls both flexible lenses to assure appropriate near vision tracking in both eyes.
Another type of opto-mechanical lens is the design of Joshua Silver
Joshua Silver
Professor Joshua D Silver is a UK physicist whose discoveries have included a new way to change the curvature of lenses, with significant application for the low-cost manufacture of corrective lenses....
, and uses liquid pressure against a diaphragm to control focus of a lens. Stephen Kurtin also has a product based on what appears to be a related design called TruFocals.
Advantages
Unlike with bifocals, near-vision correction is achieved over the entire field of viewField of view
The field of view is the extent of the observable world that is seen at any given moment....
, in any direction. Distance vision corrections are made by re-adjusting the lens for distance, instead of by tilting and/or rotating the head to view object through the best part of the lens for the distance. Adjustable focus lenses, like single-focus lenses, also reduce image-jump and spatial distortion in the field of view associated with traditional multi-focal lenses. Additionally, the ideal near-vision correction can be achieved with precision, because the variable lenses emulate the focusing action of the youthful (non-presbyopic) eye.
Disadvantages
The adjustable focus mechanism is controlled by a mechanism located on the glasses, requiring adjustment every moment the user wants to change from seeing near to far (or vice versa).See also
- Adaptive opticsAdaptive opticsAdaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
have been suggested as another potential for adjustable eyeglasses. - Intraocular lensIntraocular lensAn intraocular lens is an implanted lens in the eye, usually replacing the existing crystalline lens because it has been clouded over by a cataract, or as a form of refractive surgery to change the eye's optical power. It usually consists of a small plastic lens with plastic side struts, called...
"CrystaLens" replaces the normal eye lens with an adjustable one that is adjusted by using the eye's focusing muscles to focus. - ElectrowettingElectrowettingElectrowetting is the modification of the wetting properties of a surface with an applied electric field.-History:...
is a technology used to electrically adjust the path of light - Bates methodBates MethodThe Bates method is an alternative therapy aimed at improving eyesight. Eye-care physician William Horatio Bates attributed nearly all sight problems to habitual strain of the eyes, and felt that glasses were harmful and never necessary...