What is contact lenses spherical and aspherical geometry? How does it influence comfort?

The comfort of a contact lens is due to its geometry and particularly to the type of its edge, thickness and diameter as well. It is easy to imagine the the diameter affects the width of a contact lens and consequently the surface of the cornea the lens covers. As concerns thickness, the more contact lenses are thick the more water they contain and release throughout the day. On the other hand, referring to the edge, there are two big geometric groups: spherical and aspherical contact lenses.

Every part of the contact lens contributes to a particular aspect of its use. The interior and concave part which touches the cornea contributes to comfort; the external and convex side is where optical corrections concentrate the most. And what about the edge?

For spherical contact lens we mean lenses whose surfaces are part of a sphere. They may have a monocurve edge for which the optical (that is to say, the visual correction) arrives to the end of the geometry that provides in each case rounded edges not to overwhelm the delicate surface of the cornea. This type of geometry can also provide more curvature and then be bicurve, or multicurve and have a gentler edge. To date, the single curved spherical lenses is considered prehistoric and out of fashion. The new forms of spherical lenses increasingly provide lenses with more curves, which improves the tear replacement by loosening the pressure. As we increase the number of flange and bends, it improves significantly the adaptability and therefore the feeling of greater comfort.

Aspherical lenses remind the form of a parabola or hyperbola whose internal surface is characterized by a gradual flattening of the center towards the periphery of the contact lens itself. Toric, bifocal (a focal center of the curve, a different focus in the device) and multifocal (progressive change of the focal gradually from the center to the device for the concentric logic) lenses belong to the the aspherical lens family.

In both cases they are soft lenses. Moreover, the type of production influences the geometry: bicurve soft lenses are usually produced by turning or stamping, those produced by centrifugation are aspherical.

*This content has been translated and written by staff non-medical so may be subject to different interpretations. The source are public and proposed in simple language


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