Editing Corrective lens (section)

==Lens optical profile==
[[Image:Lens2.png|thumb|right|upright=1.5|Common lens optical profiles]]
Although corrective lenses can be produced in many different profiles, the most common is ophthalmic or convex-concave. In an ophthalmic lens, both the front and back surfaces have a positive radius, resulting in a positive/convergent front surface and a negative/divergent back surface. The difference in curvature between the front and rear surface leads to the corrective power of the lens. In [[hyperopia]] a convergent lens is needed; therefore, the convergent front surface overpowers the divergent back surface. For [[myopia]] the opposite is true: the divergent back surface is greater in magnitude than the convergent front surface. To correct for [[presbyopia]], the lens, or section of the lens, must be more convergent or less divergent than the person's distance lens.

[[File:Basecurve power.png|thumb|left|upright=1.2|Constant corrective power with different [[base curve]]s require varying the back surface curvature]]
The [[base curve]] (usually determined from the profile of the front surface of an ophthalmic lens) can be changed to result in the best optic and cosmetic characteristics across the entire surface of the lens. [[Optometrist]]s may choose to specify a particular base curve when prescribing a corrective lens for either of these reasons. A multitude of mathematical formulas and professional clinical experience has allowed optometrists and lens designers to determine standard base curves that are ideal for most people. As a result, the front surface curve is more standardized and the characteristics that generate a person's unique prescription are typically derived from the geometry of the back surface of the lens.

===Bifocals and trifocals===
Bifocals and trifocals result in a more complex lens profile, compounding multiple surfaces. The main lens is composed of a typical ophthalmic lens. Thus the base curve defines the front surface of the main part of the lens while the back surface geometry is changed to achieve the desired distance power. The "bifocal" is a third spherical segment, called an ''add segment'', found on the front surface of the lens. Steeper and more convergent than the base curve, the add segment combines with the back surface to deliver the person's near correction. Early manufacturing techniques fused a separate lens to the front surface, but modern processes cut all the geometry into a single piece of lens material. There are many locations, profiles, and sizes of add segments typically referred to as segment type. Some "seg type" examples include Flat top, Kryptok, Orthogon, Tillyer Executive, and Ultex A. Trifocals contain two add segments to achieve a lens that corrects the person's vision for three distinct distances.

The optical center of the add segment may be placed on the lens surface or may hang off into an empty space near the lens surface. Although the surface profile of a bifocal segment is spherical, it is often trimmed to have straight edges so that it is contained within a small region of the overall lens surface.

===Progressive lens===
The [[Progressive lens|progressive addition lens]] (PAL, also commonly called a no-line or varifocal lens) eliminates the line in bi/tri-focals and is very complex in its profile. PALs are a continuously variable [[parametric surface]] that begins using one spherical surface base curve and ends at another, with the radius of curvature continuously varying as the transition is made from one surface to the other. This shift in curvature results in different powers being delivered from different locations on the lens.
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