Types of contact lenses

Depending upon the nature of the material used in
their manufacturing, the contact lenses can be divided
into following three types:
1. Hard lenses,
2. Rigid gas permeable lenses, and
3. Soft lenses.

1. Hard lenses are manufactured from PMMA
(polymethylmethacrylate). The PMMA has a high
optical quality, stability and is light in

CONTACT LENSES

what is contact lens

Contact lens is an artificial device whose front surface
substitutes the anterior surface of the cornea.
Therefore, in addition to correction of refractive error,
the irregularities of the front surface of cornea can
also be corrected by the contact lenses.
Parts, curves, and nomenclature for contact lens
To understand the contact lens specifications
following standard nomenclature has been recommended
(Fig. 1).

Fig. 1. A contact lens.

1. Diameters of the lens are as follows :
i. Overall diameter (OD) of the lens is the linear measurement of the greatest distance across the physical boundaries of 

ERRORS OF REFRACTION

Emmetropia (optically normal eye) can be defined as a state of refraction, where in the parallel rays of light coming from infinity are focused at the sensitive layer of retina with the accommodation being at rest
(Fig. 1).

Fig. 1. Refraction in an emmetropic eye.

At birth, the eyeball is relatively short, having +2 to +3 hypermetropia. This is gradually reduced until by the age of