Apical zone of the cornea, corneal apex
The steepest part of the cornea, normally including its geometric center, usually 3–4 mm in diameter.
The curvature of the central posterior surface of the lens, which is adjacent to the cornea, described by its radius of curvature (mm). Base curves and peripheral curvatures of contact lenses are chosen to achieve a good fit of the lens to the cornea.
Diameter (chord diameter)
The width of the contact lens, from edge to edge. The diameter of soft contact lenses, for example, ranges from 13 mm to 15 mm, whereas that of rigid gas-permeable (RGP) lenses ranges from 9 mm to 10 mm.
The central area of the contact lens. The curvature of its anterior surface is designed to yield the desired refractive power of the lens.
Secondary curves of the posterior lens surface away from the center, nearer the lens edge. These curves are flatter than the central posterior “base” curve to approximate the normal flattening of the peripheral cornea and achieve a desired fit. Junctions between central and more peripheral curvatures are smoothed or “blended” or may be continuously graduated.
Sagittal depth or vault
The distance between the center of the posterior surface to the plane of the edges of the lens. If the diameter of the lens is held constant, the sagittal depth decreases as the base curve radius increases.
The lens formed by the tears that fill in the space between a contact lens and the cornea
The wettability of a lens surface. A low wetting angle means water will spread over the surface, increasing surface wettability, whereas a high wetting angle means that water will bead up, decreasing surface wettability. A lower wetting angle (greater wettability) generally translates into better lens comfort and vision.