what is contact lens
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 lens. It is expressed in millimetres. (It should not be confused as being twice the radius of curvature).
ii. Optic zone diameter (OZ) is the dimension of the
central optic zone of lens which is meant to focus
rays on retina.
2. Curves of the lens are as follows :
i. Base curve (BC) or central posterior curve (CPC) is a curve on the back surface of the lens to fit the front surface of cornea.
ii. Peripheral curves. These are concentric to base curve and include intermediate posterior curve (IPC) and peripheral posterior curve (PPC).
These are meant to serve as reservoir of tears and to form a ski for lens movements.
iii. Central anterior curve (CAC) or front curve (FC) is the curve on the anterior surface of the optical zone of the lens. Its curvature determines the
power of contact lens.
iv. Peripheral anterior curve (PAC) is a slope on the periphery of anterior surface which goes up to the edge.
v. Intermediate anterior curve (IAC) is fabricated only in the high power minus and plus lenses. It lies between the CAC and PAC.
3. Edge of the lens. It is the polished and blended union of the peripheral posterior and anterior curves of the lens.
central optic zone of lens which is meant to focus
rays on retina.
2. Curves of the lens are as follows :
i. Base curve (BC) or central posterior curve (CPC) is a curve on the back surface of the lens to fit the front surface of cornea.
ii. Peripheral curves. These are concentric to base curve and include intermediate posterior curve (IPC) and peripheral posterior curve (PPC).
These are meant to serve as reservoir of tears and to form a ski for lens movements.
iii. Central anterior curve (CAC) or front curve (FC) is the curve on the anterior surface of the optical zone of the lens. Its curvature determines the
power of contact lens.
iv. Peripheral anterior curve (PAC) is a slope on the periphery of anterior surface which goes up to the edge.
v. Intermediate anterior curve (IAC) is fabricated only in the high power minus and plus lenses. It lies between the CAC and PAC.
3. Edge of the lens. It is the polished and blended union of the peripheral posterior and anterior curves of the lens.
4. Power of the lens. It is measured in terms of posterior vertex power in dioptres.
6. Tint. It is the colour of the lens.
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