Type Of Spectacle Lenses


   There are variety of spectacle lenses in the market catering need for different professions, life style and budget of the patient.

Features of Lenses

 
Refractive index
Increasing the refractive index reduces the volume of lens making lens flatter. In minus lenses it reduces edge thickness and in plus lenses it reduces central thickness. The lenses with higher index is known as Hi Index lens.

 
Specific Gravity
It is density or weight of lens per cubic cm. High Index lenses are heavier because of high specific gravity but reduced volume compensates for increased weight.

Lens Materials

 
Glass
This is the most common used lens.

 
Plastic lenses
Plastic lenses has lesser refractive index than glass and specific gravity is more than glass.

Advantage over glasses:
         Light Weight.
         Impact Resistance.

Disadvantage over glasses:
         Increased thickness.
         Warpage and increased scratches.
        
Instability  at High temperatures.

  Polycarbonates
Advantages:
         Impact Resistant.
         Thinner than glass lenses.
         Lighter weight.
         Built in U V protection.

Recommended for
:
         Children.
         As safety eye glass.
         Sports Wear.

Lens designs

 Lenticular
In this high power of lens is in central smaller diameter reducing peripheral thickness.

Advantage:
         Less weight.
         Less peripheral prismatic effect.

Disadvantages:
         Poor cosmetic effect.
         Small field of view.

 Aspheric
In this the centre of curvature changes from centre to periphery.

Advantages:
         Decreased central thickness so cosmetically better.
         Lesser weight.
         Less prismatic effect.
         Increased field of vision.


Other Options

 U.V.Protection: This property can be increased by some additives.
 
Scratch Resistance Coating: Reduces the scratches in plastic lenses.
 
Anti Reflection Coating:

It is a coating given on the surface of the lens ,which could be of plastic or glass base.The compound used is mostly Magnesium base oxides and fluorides in some cases.The purpose of this coating is to cut off unwanted reflections and glare from different sources thus enhancing visual also enhances the cosmetic appeal of the person. The commonly used tints are green or  is the most preferred owing to its wavelength and cosmetic appeal. This coating is chiefly recommended for people involved in computer work, stage performers and also who are involved in night driving. The longevity of the coating on the surface of the lens depends on the quality of the anti reflection coating given.

 Hard Multi Coating:

This is an 8 layered combination of multiple coating given principally to the plastic lenses.This coating comprises antireflection coating, the scratch resistant coating , the top hydrophobic coat (which prevents water getting smudged on the surface of the lens) and zirconium oxide coating to increase the toughness of the lens. This not only enhances the optical performance of the lens but also the life of the lens. 

 Polaroid Lenses:

These are special type of lenses that cut off unwanted reflections and glare in a particular desired means that these lenses have the ability to specifically cut off reflections in either horizontal or vertical plane like glare of sun from road and is available both in sunglasses and in prescription lenses are specifically indicated for people who are involved in snow skating, driving and in those kind of activities where they get exposed to reflections from a particular plane. It also benefits people who work in furnaces and other kind of work station design where they are exposed to sources of light which bring down the visual performance . These lenses cut off the unwanted reflections only in a desired plane as opposed to an anti reflection coated lens or an hard multicoated lens which cuts off reflections in all the planes.

 Photochromic Lenses: Becomes dark colored in bright light, so act as sunglasses and U.V.Protecter.

        Infrared Blocking lenses: Infrared wavelengths are not known to cause any harm to eye so these glasses are of no known advantage.
        Blue blocking lenses: Lenses that block blue light are usually amber colored and make the surroundings look yellow or orange. The tint     
                   makes distant object appear clearer, especially in snow and haze. These are good for skiers , hunters, boaters and pilots.



Bifocals / Trifocals / Multifocals

 Bifocals:

In presbyopic cases you need separate power for distance and near. It is not convenient to carry two glasses. When you are reading you will have to take change glasses for distance vision to see distant objects and vice versa.

Disadvantages of bifocals are:

You have to get used to seeing near from reading portion and look down from distance portion while walking. Falling of eye sight at the junction of two powers will cause difficulty in vision.

With bifocals reading power is given for best vision at 13 to 14 inches (may change with need of person) range about 9 to 22 inches. But person wanting to see at different distances will find problem.


 Trifocals:
In these cases you have three power center one is for intermediate distance.This helps if one has to work on table near and far.

 Multifocals / Progressive:
In these cases the lens has multiple power to cover all distances as power changes progressively.It thus gives good vision for all distances. 

Further there is no demarcation line giving better look. 



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