PARAMETERS OF CONTACT LENS
1. Power:
It is determined by curvature of anterior surface of optic zone of contact lens. It is not related directly with thickness of lens.
It is calculated by doing refraction and taking help of conversion table.
2. Base curve:
It is decided by curvature of anterior surface of cornea as determined by keratometery.
Base curve of soft contact lens is made flatter than flat K of cornea because the overall diameter of soft contact lens is always greater than greatest diameter of cornea.
However diameter of semisoft/hard contact lens is smaller than diameter of cornea so its base curve is kept same as flat K of cornea.
3. Overall diameter:
It depends upon diameter of cornea which can be taken either by a caliper or a simple scale.
4. Thickness:
More is the thickness of lens lesser will be the oxygen permeability.
In addition it makes the lens heavier and it tends to fall thus making the fit flatter.
Most of the manufacturing companies
keep this parameter of contact lens constant.
5. Optic zone diameter:
It is usually kept constant, however it can be increased or decreased depending upon size of pupil.
6. Edges:
They should be smooth.
7. Water content:
More is the water content, more is the oxygen permeability so that person can wear the contact lens for a longer duration.
However, it becomes thicker, heavier and more pliable with increasing water content.
Accordingly, parameters have to be set to have alignment fitting.
8. Oxygen permeability:
It depends on DK value of material,
thickness of lens and water content of lens.
Higher DK value means more oxygen permeability.
More thickness of lens means
lesser oxygen permeability.
High water content means higher oxygen permeability.
High oxygen permeability increases wearing period of contact lens.
Tags:
Contact lens