Introduction to Eyeglasses

Eyeglasses are used for around 500 years for vision correction and fashion. In these years quality and usability of eyeglasses has increased very much. Now in days 1 in 5 people of the USA is using eyeglasses. So we can imagine the scale in which eyeglasses are used in these days.

Eyeglasses in these days are widely used for fashions as well as for the vision corrections. The eyeglasses with powers are known as prescription eyeglasses. Eyeglasses consists of two basic parts, frame and lens.
Frames are used to hold the lens. Frame can be made up of metal or plastics etc. Other part of eyeglass is lens. Lenses are generally made up of CR39, Polycarbonates, glasses and many different types of plastics. Lens can be prescribed according to need of the wearer and recommendations of doctor

Uses of Eyeglasses

The most important use of Eyeglasses is vision correction. There are many people around us, suffers from shortsightedness, they need concave lenses to correct the vision and see far distance easily. Some other type of people suffers from Long-Sightedness, they need convex lenses to see near objects. There is one other class of people who suffers from both type of vision defects. These people need either bifocal or progressive lenses to cure there vision. Apart from vision correction there are many more uses of Eyeglasses.

Many of us are fond of driving bikes. While driving the bike we must wear eyeglasses for the safety of our eyes. There are many different small particles of stones, dust, and chemical pollutants that emerge from factories, always present in the environment.

When people drive the bike, the speed of bike is normally averaging between 60-80 km/hr and even more. Imagine what will happen if a small dust particle hit our eyes at speed of 60-80 km/hr. It may damage our eyes. Sometimes it may make us blind. So I request all people to wear eyeglasses with strong lens material while driving your motorbike.

One other use of Eyeglasses is in sunshine. Many people find direct sunlight to bright to be comfortable. This specially occurs while people take sunbath in beach or try to read new paper in sunshine. There are many such outdoor activities in which people find sunlight is more than usual. It has been recommended to always wear the sunglasses in strong sunshine to protect eyes from ultraviolet ray, which can lead the development for cataract in eyes. Sunglasses have colored lenses, which prevents strong light from reaching our eyes.

One other use of eyeglasses for safety comes while working in front of the computers. If we work long hours of day in front of the computer, then at the end of the day we feel tiredness and sometimes pain in our eyes. Wearing anti-glare eyeglasses reduce the tiredness and pain even if we work in front of computers for long hours.

Anti glare eyeglasses or less glare eyeglasses are also useful while driving in night.

Eyeglass Frames

Introduction:

Eyeglasses are normally used for vision correction. They are also the symbol of fashion. The Frames of Eyeglasses play important role in looks and flexibility of eyeglasses. Frames can be classified in 3 main categories. These are, Plastic Frames, Metal Frames and Rimless Frame. There are many other types of frames are there. Metal and plastic frames also can be subdivided into many different categories

Plastic Frames:

Some type of plastic frames are very flexible and light in weight, these frames are very popular to sports person as they suit very good for there needs. Instead of breaking in the pressure these frame bends slightly and return to original shape when pressure released. Due to flexibility the glasses grip better on the face. Plastic frames are very fashionable also. This frame is widely used in the entertainment industry.

Metal Frames:

Metal Frames are rigid and heavier in weight as compared to the plastic frames. Metal frames are more popular in the eyeglasses which are used for the vision correction. They are less noticeable as compared to plastic frames. But due the rigid in nature they are more prone to be damaged. In some metal frames there is spring loaded hinges to grip glasses on the face better.

Rimless Frames:

Rimless frame contain frames only at temple and bridge. There is not frame around glasses. Due to this rimless frame are least noticeable on face. There is a other type of frame called semi-Rimless frame or half rimless in which half of the glass is bounded by frame.

The biggest advantage of rimless frames are that, glasses in these frames can be figured in any shape like oval, rectangle and even heart and stars. That's why these frame also very fashionable. The rimless frames can be made bifocal and progressive more easily as compared plastic and metal frame. The disadvantage of rimless frames is that they can be broken very easily. These frames are comparatively expansive.

Eyeglass Lenses

In the past, the only material that was used for the manufacturing of lenses is Crown glass. But now in days many different types of lens materials are used, like CR-39 Plastic, polycarbonate, mid-index plastic, spectra-light and 1.6 index plastic, 1.66 index plastic. Almost all plastics and polycarbonate are lighter in weight as compared to crown glass.

The lens material used for the eyeglasses depends upon the life style and prescription of patient. It also depends upon the recommendation of eyewear professional and face measurements of the patients. Here is the comparison between different types of lens materials
Crown Glass: This is the oldest know material for preparation of eyeglasses. It is cheapest among all glass material. The disadvantage of glass is that it can bee easily broken. And also it is heavier than all other materials used for making of eyeglasses. The refractive index of crown glass is 1.52. Good lenses have higher refractive index. Larger will the refractive index, thinner the size of lens used for making eye-glasses. The refractive index of crown glass is greater than CR39 plastic but less than others
CR39 Lenses: CR39 basically is a plastic polymer which is used for the making of eyeglasses. The important advantage of CR39 plastic is that is transparent in visible spectrum and almost completely opaque in the ultraviolet range. Ultraviolet rays are very harmful to our eyes and even for our body. CR39 ovoid the ultraviolet rays from reaching eyes. CR39 also has high abrasion resistance, in fact the highest abrasion/scratch resistance of any uncoated optical plastic. Weight of the CR39 is around half as compared to the glasses, but the refractive index is slightly higher.

C39 plastic is best suited material for formation of color lenses. Very wide range of colors can be achieved by dying the surface of cr39 material. CR39 is reasonably cheaper. It is most popular now in days for making of eyeglasses. Different type of coating like anti-refractive coating and photochromic layer can be easily added on CR39 plastic, making them best material for eyeglasses.

Polycarbonate: The polycarbonates are the thermoplastic polymer. The advantage of polycarbonate lenses are that they are very strong and can't be easily broken. Polycarbonate lenses are also dyed in any tint color. Hence they are very popular for making sunglasses. The refractive index of polycarbonate is high.

Classification of lenses according to focus/vision

Single vision Lenses

Single vision lenses are used for only one type of vision correction either myopia or hyperopia. As people grow in age eye's crystalline lens loses elasticity, resulting in presbyopia, which limits their ability to change focus. Here need for corrective lenses arises for near vision correction, convex lens is used and for far vision correction concave lenses are used. Apart from old age there can be many other reason for vision defect like works which make pressure on eyes and genetic reasons.

Some People around us suffer from both Shortsightedness and Long Sightedness. In order to correct eyesight such people has 3 options. First is use of two separate eyeglasses for each vision, second is use of bifocal lenses and third is using progressive lenses. In first option, people required to keep two eyeglasses one for near vision and other for the far vision. But this option is not very comfortable. No one wants to carry two glasses. Also changing glasses many times a day is disturbing.

Multi-Focal Lenses

Multifocal lenses are the type of lenses, which have more than on focus point or more than one lens powers. Multifocal lenses are used by the people who need to correct both near vision and far vision. Advantage of multi-focal lenses is that people don't need to keep 2 eyeglasses one for near vision and one for the far vision.

Bifocal Lenses, Advantages and disadvantages

Second Option is two use the bifocal lenses. Bifocal lenses have lower part with positive power to see near vision and upper part with negative power to see far distance. Both powers are clearly separated by a line called Bifocal line. For making bifocal lenses two lenses prepared separately and then joined. Bifocal lens provide the user to correct both type of vision with a single eyeglass. But bifocal lenses have some complications. The bifocal division of lenses causes headache and even dizziness in some people. Sometimes it takes time to adapt bifocal lenses to people, as they have to move their head to adjust for near and far vision. Bifocal lens wearer has many difficulties in working in front of computer for the long hours due to unusual angle and constant movement of the head.

Progressive Lenses:

Progressive lenses can also be used to correct both type of vision. Progressive lenses are also called graduated lenses and varifocal lenses. Power in the progressive lenses increases gradually from top to bottom. In other words top of the lens has least power and it increases to bottom at a regular and very small steps. It doesn't have a bifocal line.

Advantages

Progressive lenses are best suited to provide distance vision from the top and near vision from the bottom. Progressive lenses are better for working in front of computer monitor as compared to bifocal lenses. Progressive lenses provide the facility to adjust the different power for special needs. In this type of lenses people needs to adjust very small head position to change from near vision to far vision. Progressive lens also avoids the discontinuity that is generated by bifocal lenses.

Disdvantages

Progressive lenses suffer the disadvantage that the power progression creates regions of aberration away from the optic axis, this leads to poor resolution of vision. As the lenses combine a range of powers in a single surface there are also geometric distortions to the visual field. This geometric distortion is annoying to many people. Some people needs around 15 days time to adapt to progressive lenses and some people never able to adapt the progressive lenses.

Classification of lenses according to lens type
Clear Lenses/Eyeglasses

Clear LensesLenses of eyeglasses without colors are known as clear eyeglasses. The clear eyeglasses are most common eyeglasses in use. Clear lenses suits for office working and reading type of jobs. It is recommended to use only clear lenses while you are indoors. It can be also used outside if the intensity of light is not very much. But in strong sunshine, where we need to protect our eyes against ultra-violets rays, we need colored lenses.

Sunglasses, Tinted Lenses, Colored Eyeglasses
Sunglasses are the special type of eyeglasses in which lens is colored or darken in order to prevent strong light reaching to eyes. Sunglasses also known as tinted glasses. The popular firms for manufacture of sunglasses are Ray-Ban, Dior, and Armani etc. The sunglasses can be used as safely lens and they are also a fashion symbol. Sunglasses are the symbol for many celebrities.

Many people like to spend hour in beach or snow. They like the activities like sunbath and reading in the sunshine. But sometimes sunshine is very bright and uncomfortable for eyes. Wearing sunglasses in the sunlight saves eyes from temporary loss of vision and prevents ultraviolet rays to reach eyes. Excessive exposure to ultraviolet light(in short UV light) can cause many short-term and long term eye-problems like photokeratitis, snow blindness, cataracts and many other diseases.

Sportsperson also like to wear Sunglasses. In outdoor games they are normally need to play in sunshine where sunglasses are very necessary. Sunglasses also save the sportsperson eyes from injuries, small dust particles and small mosquitoes which keep flying in atmosphere.
There is a category of sunglasses known as water-sunglasses which are made according to needs of swimmers and sea-divers. These glasses are shatter proof & impact resistant lenses.

Sunglasses are made of flexible frame and durable lens materials. Normally Crown glasses generally not used as lens material for sunglasses. Most popular lenses for sunglasses are CR39 plastic lenses and Polycarbonate lenses. Both are very light as compared to glass. CR39 is almost opaque to UV Rays and can be easily dyed to various colors. Polycarbonate lenses are quality lenses, are light weight and have very high refractive index. Polycarbonate lenses are expensive but have many features.

Sunglasses frame are made up of nylon, acetate and metals, for sports wear sunglasses frame should be flexible and light in weight.

Sunglasses are fashion symbols too. People especially young boys and girl like to wear sunglasses. Many celebrities wore the sunglasses. There are many different brands of sunglasses available. Famous of which are Aviator, Mirrorshades, Onassis glasses etc. Aviators are sunglasses with an oversized teardrop-shaped lens and thin metal frames.

Mirrorshades are sunglasses with a mirrored coating on the surface

Polarized Lenses

Introduction

Polarized lenses are very popular now days. Polarized lenses have the property that they eliminate the glare that was produced from reflection of light from water surface and roads. Polarized lenses are colored, hence they are a type of sunglasses and can be used as fashion symbol.

These lenses are very useful for fish
erman and boater. Besides these two professions many sportsman like golfer, jogger and biker find polarized lenses useful. Polarized lenses are also very useful for driving, as they remove the glare produced due reflection of light from hood of the car and flat road surface.

Working of polarized lenses

The light which are reflected from snow, water, roads, and any other surface which reflects most of the light falling on it, are not itself polarized. This type of light have both horizontal and vertical components. The polarized lenses are vertically oriented and they block horizontally polarized wave. This reduces the annoying glare which sometimes proved quite dangerous. Polarized lenses make vision sharper and better color perception as opposed to simply darkening the lenses.

Polarization

All electromagnetic waves propagating in free space or in a uniform material of infinite extent have electric and magnetic fields perpendicular to the direction of propagation. In polarization, the electric field is described and the magnetic field is ignored since it is to the electric field and proportional to it.
Considering light is traveling in z direction, electric field can be divided into two components along x and y directions. These components characteristics may differ. For example they may not have same amplitude, and they may have some phase difference. Due to this they may not reach there maxima and minima at the same time. The shape traced out in a fixed plane by the electric vector as such a plane wave passes over it, is a called of the polarization state. Polarization can be linear, circular and elliptical type.

Photochromatic lenses and Eyeglasses

Introduction

Photochromic lenses have the property that they automatically darken on exposer to ultra-violet rays. The amount of darkness depends upon the intensity of light. If light is very strong, photochromic lenses become almost opaque. When wearer of photochromic lens comes to indoor, lenses comeback to their original color and darkness disappears. In other words the photochromic lenses have the property to control light falling on eyes by changing its darkness and hence unable the wearer to see comfortable in any intensity of light. All photochromic lenses block 100% or ultra-violet light. Photochromic lenses are also called photochromatic lenses.

Building Blocks

The substances responsible for photochromic property of a lens are microcrystalline silver halides (usually silver chloride). These substances embedded into glass version of lenses to make them photochromic.

Photochromic lens contains millions of silver halides crystals. When exposed to sunlight, UV rays transform the crystals into light-absorbing particles. As these particles absorb light, the lens darkens and the amount of light passing through the lens is reduced.

When the lenses are removed from sunlight and the stimulating UV radiation is no longer present, the crystals return to their original orientation and the lenses become clear. Plastic lenses depend upon organic photo chromatic molecules for their reversible darkening. The main reason this lens darken in the sunlight and not in indoors is that indoor does not contain ultraviolet rays. The photochromic lenses come back to their original color due to thermal process. Because of this reason some lenses doesn't work well in hot temperatures.

More on photochromic lenses.

The transition lenses are today most popular photochromic lenses. Transition lenses are manufactured by transition optical. Transitions apply a thin layer of photochromic particles in front of the surface of lenses instead of distributing light-sensitive particles throughout the lens material. The light sensitive material penetrate lens evenly and up to depth of 0.15 millimetres.

Ad-ons to Eyeglasses

Eyeglasses are used for vision care as well as used for fashion. Eyeglasses are also used to save our eyes from sun-shine and strong light. There are many types of lenses available in market. We can make our eyeglass lens more valuable by adding some extra coating to eyeglasses. The commonly used Add-ons to lenses are Anti-Reflective Coating and Anti-Scratch Coating. Let's discuss each one by one.

Anti Reflective Coating

This coating is also called anti-glare. Anti reflective coating removes the light that hits the back surface of the eyeglass and bounces into eye. This reflection cause glare when an eyeglass wearer photographed using camera. Anti-reflective coating is made of a very hard thin film that is layered on the lens. It practically removes all the reflections. Anti reflective coating keeps eye less tired even after long hour work in front of the computer monitor.

Anti Scratch Coating

Normally the light-weight lenses like plastics are more prone to scratching. Anti-scratch coating is a scratch resistant coating that can be applied to lenses and many surfaces. An anti-scratch coating is applied by dipping the lenses into a silicon-based solvent. This coating does not affect the function of the lens, but greatly reduces the chances of reflections.

Eyeglass Prescription

Even the use of prescription eyeglasses is very common, many people are not aware of parameters of prescription eyeglasses. Let's discuss each parameter of prescription eyeglasses one by one.

OD, OS and OU:

In Latin Right eye is called Oculus Dexter and in short OD. Similarly Left eye is Oculus sinister (OS) . In other words in prescription Right Eye and Left eye referred as OD and OS. The term OU (oculi uterque) is also commonly used to refer both eyes.

DV and NV:

DV is an abbreviation for distance vision. DV specifies the some part of the prescription designed primarily to improve far vision. In a bifocal lens, this generally indicates what is to be placed in the top segment N.V. is an abbreviation for near vision. This may represent a single-vision lens prescription to improve near work, or the reading portion of a bifocal lens

Sphere, SPH, or S:

Sphere is associated with curvature of the eye lens. A + ve number indicates you are farsighted and a - ve indicates you are nearsighted. Spherical correction in eyeglass corrects the refractive error of the eye by adding or subtracting refractive strength

Cylinder, CYL, or C:

Cylinder is associated with irregular curvatures in the cornea and eye shape that cause-focusing distortions associated with astigmatism. Cylindrical (toric) lenses have specific curvatures and orientation that compensate for eye surface irregularities.

Axis:

Axis is related to how the lens will be oriented in degrees to help determine whether, in focusing, images will be elongated or constricted to help offset vision defects associated with astigmatism.

Add:

Add is used for people with both farsighted and nearsightedness, this number will indicate the nearsighted power in there vision. Add indicates that either lens is bifocal or progressive

PD/ Pupillary Distance:

Pupillary Distance is the distance between the centers of two pupils (center black dots between the eyeball).