UV++ Blue Blockers
Ideally, all people should use a form of UV protection in their eyewear to prevent harmful UV damage that comes from the sun. Our UV++ Lenses also block blue light that comes from smartphones, tablets and computers. Blue light exposure before bedtime can disturb sleep patterns. It’s also believed to contribute to macular degeneration.
Single vision lenses correct for only one distance. If they correct for far distance, the person must accommodate to see clearly up close. If the person cannot accommodate, they may need a separate pair of single vision glasses for near distances, or else use a multifocal lens (see below).
With a bifocal, the upper part of the lens is generally used for distance vision, while the lower part is used for near vision. Usually, a segment line separates the two. Typically a person with myopia would have one section of a prescription lens that has a certain diverging power while another section of the lens would have a lower diverging power for close-up work. Similarly a person with hyperopia would have one section of the lens with a certain converging power and another section with a greater power for close-up work.
Trifocal lenses are similar to bifocals, except that the two focal areas are separated by a third middle area with intermediate focus correction, used for intermediate vision, roughly at arms' length, e.g. computer distance. This lens type has two segment lines, dividing the three different correcting segments.
Progressive or varifocal lenses provide a smooth transition from distance correction to near correction, eliminating segment lines and allowing clear vision at all distances, including intermediate (roughly arms' length).
Some people with good natural eyesight like to wear eyeglasses as a style accessory, or want to change the appearance of their eyes using novelty contact lenses. For these people, no power or acuity correction is required. Some may need a prescription in one lens in their spectacles, but not in the other. In this case the lens is simply a placeholder that does nothing, with equal parallel curved surfaces. This is referred to as a plano lens.
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 surface 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.
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. Optometrists 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 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.
The progressive addition lens (PAL, also commonly called a no-line or varifocal lens) eliminates the line in bi/trifocals 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.
What types of eyeglass lenses are available?
As technology advances so, too, do eyeglass lenses. In the past, eyeglass lenses were made exclusively of glass. Today, most eyeglasses are made of high-tech plastics. These new lenses are lighter, do not break as easily as glass lenses, and can be treated with a filter to shield your eyes from damaging ultraviolet light.
The following modern eyeglass lenses are lighter, thinner, and more scratch-resistant than glass lenses or the older, common plastic lenses.
The type of vision problem that you have determines the shape of the eyeglass lens. For example, a lens that is concave, or curves inward, is used to correct nearsightedness, while a lens that is convex, or curves outward, is used to correct farsightedness. To correct astigmatism, which is caused by distortions in the shape of the cornea, a cylinder shaped lens is frequently used. Simply put, the eyeglass lens is a tool you use to focus light appropriately onto your retina.
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