Different terms of contact lenses ?

What is a ‘‘blend,’’ and its importance?

To smooth the junction between adjacent curves on the back surface of a lens, blending is performed. 

A blend does not induce a new curve on the posterior surface of a lens. 

Blending is a smoothing of these different curves at their junctions, removing the rigidly demarcated zones of transition to permit better exchange of tear film under the lens.

What are the types of edge design and their purposes?

The shape of the outermost portion (0.2 mm in from the edge) of the contact lens is referred to as the edge contour. 

It is difficult to quantify, so descriptors such as rounded, blunt, sharp, knife edge, square, or tapered are used. 

The edge contour can be modified in order to diminish or augment attachment of the contact lens to the lid in the contact lens fitting process. 

An additional reason for modifying edge contour is to improve patient comfort.

What is a base curve? 

A base curve is the radius of curvature of the central region of the posterior surface of the contact lens, selected based on the curvature of the corneal apex.

What is sagittal depth?

Sagittal depth is the distance between a superficial plane over which the contact lens is placed and the center of the optical zone diameter.

 To increase sagittal depth, one must increase the optic zone diameter or diminish the radius of curvature of the contact lens.

What is the relationship between contact lens center thickness and dioptric power?

Contact lens center thickness varies with lens power (minus contact lenses are thinner), diameter (smaller diameter lenses are thinner), and refractive index (higher refractive index lenses are thinner).

 Central contact lens thickness is important in contact lens fitting, because it may influence stability, flexibility, edge thickness, and oxygen transmission.

What is the wetting angle?

The wetting angle is the angle formed at the surface of the contact lens material when a drop of liquid is placed on the surface .

The extent of the angle is related to the cohesive and adhesive prop- erties of a lens material. When the wetting angle is greater, the material is more hydrophobic. 

For example, polymethylmethacrylate (PMMA) is a relatively hydrophobic material with a wetting angle of approxi-mately 60 degrees.

Side effect of contact lenses.

Preventing Side Effects Caused by contact