David Noel
<davidn@aoi.com.au>
Ben Franklin Centre for Theoretical Research
PO Box 27, Subiaco, WA 6008, Australia.
DS915: Polyfibre Universal Glasses
Why do people need glasses?
The human eye has a lens and a cornea at the front. When light from an object being viewed enters the eye, the lens and cornea focus the light onto a point on the retina at the back of the eye. People need glasses when this focussing mechanism does not work exactly to focus accurately on the retina -- when there is some sort of eye defect.
Most eye defects can be corrected by adding an additional lens in front of the eye, so light from an object passes through this lens before the cornea and the eye's own lens. These lenses were traditionally made of glass, and so were called glasses or eyeglasses. They were held by a frame at the right position in front of the face. This frame, containing two lenses, is also called a pair of spectacles.
In more modern times, soft corrective lenses may be placed directly on the eyeball against the cornea and inside the eyelids. These are called contact lenses, and work in the same way as glasses.
There are a number of defects that make the eye unable to focus light quite right. One of the most common causes results from an eye's inability to focus the image directly on the retina. This is the case for three of the most common vision problems: myopia, hyperopia, and astigmatism.
Figure DS915-F1. Correcting eye defects, From [3].
Myopia, commonly known as nearsightedness, is a result of the incoming image being focused in front of the retina. Hyperopia, commonly known as farsightedness, is a result of the incoming image being focused behind the retina. Astigmatism is caused by a distortion (likely from an unsmooth lens and/or corneal surface) that creates a second focal point.
Each of these defects can be corrected by adding spectacle lenses in front of the eyes. Each lens needs to be designed in shape to correct the light path for the particular eye -- the two eyes may have defects which are different between them.
In addition, the natural eye lens can automatically alter its shape so that close or distant objects are each brought into good focus -- this is called "accommodation" of the eye. As people age, the flexibility of their natural lenses reduces, and their eyes become less able to focus objects at different distances, a condition called presbyopia.
This is the common cause of older people "needing glasses" -- instead of their natural eye-lenses accommodating to focus on objects at different distances, their lenses harden and only naturally remain focussed for a particular distance. If this distance is close, they are called short-sighted or near-sighted, if distant, they are called long-sighted.
So short-sighted people may need glasses for driving or viewing distant landscapes, long-sighted people may need glasses for reading. "Bifocal glasses", incorporating two different lens curvatures in the same lens, can be used to get good focus at different distances, depending on which part of the lens is looked through.
So the use of glasses and contact lenses has enabled people with eye defects or loss of natural accommodation to cope with most conditions they encounter in everyday life.
The main drawbacks to the use of glasses is that each lens needed must be specially designed to alleviate the defects in the particular eye involved, and these defects may change during a person's lifetime. So there may be a continuing expense in having glasses made and in updating them as eye changes occur.
In modern western-style nations, these costs are part of life, but in poorer countries people may not be able to afford the costs involved, or there may not be the trained optical specialists and equipment available to cover all needs for glasses.
Pin-hole glasses
Another type of spectacles exist, called pin-hole glasses. Instead of the clear-glass lenses typical of ordinary spectacles, these have opaque plastic ovals with holes drilled into them.
Figure DS915-F2. Pinhole glasses. From [1].
Pin-hole glasses can be helpful if normal optical glasses are unavailable for some reason. They work with any eyes, because they do not depend on focussing light beams through designed lenses. Instead, the pin-holes only let through light coming from a particular narrow path, and as it is restricted to this path it has to reach the eye's retina as a point.
Figure DS915-F3. How pinhole glasses work From [1].
So pin-hole glasses tend only to be used as a last resort. Say you are lost in the jungle without your glasses, and you desperately need to look at the rear of an insect which has stung you, to see if it has a red spot. You find a broad leaf, stab lots of holes through it with a thorn, and peer through the leaf-holes to see the insect clearly.
But pinhole glasses come with a number of drawbacks. The pinholes allow less light to enter the eye, making everything appear dimmer. Peripheral vision is significantly reduced because the pinholes only allow light in from a narrow field in front. This makes them unsafe for activities that require good depth perception and a wide field of view, like driving or operating machinery. And, of course, with their black lenses, they look pretty weird to most people.
Polyfibre Universal Glasses
There is a new design for glasses, called Polyfibre Universal Glasses, which may give the advantages of pin-hole glasses, without their drawbacks. These are based on the Optical Fibres now used very widely in communication lines, carrying computer signals and media signals.
The basis of fibre optics is that very long, thin strands of glass can carry within them beams of light, each of which is "modulated" (has a wave pattern imposed upon it) by a signal (such as an audio wave) and thus carries information.
Fibres can be designed and fabricated with the refractive index of the glass varying across the fibre, being higher in the centre and lower at the fibre surface. The effect of this is that a beam of light entering the end of a fibre is internally reflected to travel along the fibre with almost no loss through the sides, so that the signal it carries can travel a long way -- several hundred metres or more -- and still be preserved.
The fibre also effectively "straightens out" the light entering one end -- light coming out the other end will appear to be from the same small angle as it went in, provided the fibre is kept straight, not bent.
Figure DS915-F4. Fibre optics. From [2].
This is similar to how the light entering one end of a pinhole in pinhole glasses has to come out in the same straight path as it entered.
Optical fibres are typically only about 125 microns across, about the same as a human hair.
The Polyfibre Lens
Optical fibres are typically grouped together into bundles, as in Figure F5, which shows a cross-section across a bundle of 37 fibres.
Figure DS915-F5. Bundle of optical fibres. From [4].
Now imagine imagine the spaces between individual fibres filled out with a clear substance, such as clear plastic or glass, and hundreds of bundles stuck together with the same clear substance. A disc cut across such a group of bundles would look like a clear glass lens, neither convex nor concave.
Figure DS915-F6. Polyfibre lens allows fibre-limited light rays.
Such a polyfibre lens should act like pin-hole glasses, allowing objects viewed through it to be clear for any type of eye. But the fact that the lens is of all clear material means that the image will not be dimmed, as with pin-holes.
Peripheral vision should also be good, particularly if the lens is slightly dished, so that more-peripheral light rays enter the polyfibre lens at a locally perpendicular surface.
Spectacles made with such polyfibre lenses should thus behave as "universal glasses", suitable for vision correction over a wide range of natural vision circumstances. Not needing individual lens designs, they could be manufactured in bulk and supplied to poorer countries lacking in optical suppliers.
At the same time, they are likely to have a somewhat unusual appearance, and be unlikely to supplant conventional optical aids where these can be afforded.
Soft versions of polyfibre lenses should also be possible. for use as contact lenses.
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References and Links
[1]. So how do pinhole glasses work?. https://www.eyerobics.com.au/pinhole_glasses.html .
[2]. Optical Fibers. https://www.coherent.com/news/glossary/optical-fibers .
[3]. The Science of Eyeglasses. https://williamseyeworks.com/the-science-of-eyeglasses/ .
[4]. Industrial Fiber Optics. https://i-fiberoptics.com/fiber-detail.php?id=3330 .
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