Corrective lenses –refer ICEE ppt: L02 lens

power

P.S.SelvakumarFaculty

Aravind School of Optometry

Corrective lenses are given To correct the refractive errors. To correct the near vision defects. To enhance the vocational vision. To give the exercises to the external

muscles. To correct the squinting eyes.

Corrective lens

Lens type Lens form / power Corrective use

Sphere Convex ( + )Concave (--)

HyperopiaMyopia

Cylinder & Sphero-cylinder

Convex ( + )Concave (--)

Astigmatism

Prism Correcting squinting eyesRelieving from eye strain

Sphere Segment of sphere

which has an equal refractive power in all meridians.

Either in ‘+’ or ‘-’ forms.

Cylinder

Sphero- cylinder Combination of sphere and cylinder. Corrects both simple refractive error

and corneal Sphero-cylinder Lens The front surface is spherical while the

back surface is cylindrical. This combination of front and back surfaces creates a lens prescription of spherical power with a cylinder amount ground on the back surface to correct the two focus points created by the astigmatism.

The cylinder amount on the back surface is ground in the required position, called the axis.

Prism

Index of Refraction The subject of index of refraction is complex and will

only be introduced here. It will be covered in more detail in the course which deals with surface room operations. For now, simply remember that the four quantities which control the refractive power of an ophthalmic lens are:

the curvature of the front surface (closest to the object being viewed);

the curvature of the rear surface (closest to the eye); index of refraction of the material from which the

lens is made; and the thickness of the lens.

Refractive Index

The refractive index is a constant for a given pair of materials. It can be defined as

speed of light in material 1speed of light in material 2This is usually written 1n2 and is the refractive index of material 2 relative to material 1. The incident light is in material 1 and the refracted light is in material 2.

If the incident light is in a vacuum this value is called the absolute refractive index of material 2. This is the value given in data books.

By definition the refractive index of a vacuum is 1. In practice, air makes little difference to the refraction of light with an absolute refractive index of 1.0008, so the value of the absolute refractive index can be used assuming the incident light is in air.

Material Absolute Refractive Index Air 1.0008 Water 1.330 Glass, soda-lime 1.510 Diamond 2.417 Ruby 1.760