Corrective lens - AbsoluteAstronomy.com

A corrective lens is a lens

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

worn in front of the eye

Human eye

The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

, mainly used to treat myopia

Myopia

Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

, hyperopia

Hyperopia

Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...

, astigmatism

Astigmatism (eye)

Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

, and presbyopia

Presbyopia

Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...

. Glasses

Glasses

Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

or "spectacles" are worn on the face a short distance in front of the eye. Contact lens

Contact lens

A contact lens, or simply contact, is a lens placed on the eye. They are considered medical devices and can be worn to correct vision, for cosmetic or therapeutic reasons. In 2004, it was estimated that 125 million people use contact lenses worldwide, including 28 to 38 million in the United...

es are worn directly on the surface of the eye. Intraocular lens

Intraocular lens

An intraocular lens is an implanted lens in the eye, usually replacing the existing crystalline lens because it has been clouded over by a cataract, or as a form of refractive surgery to change the eye's optical power. It usually consists of a small plastic lens with plastic side struts, called...

es are surgically implanted most commonly after cataract

Cataract

A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light...

removal, but recently for purely refractive purposes. Myopia (near-sightedness) requires a divergent lens, whereas hyperopia (far-sightedness) requires convergent lens.

Prescription of corrective lenses

Corrective lenses are typically prescribed

Eyeglass prescription

An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist or ophthalmologist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient.If an examination indicates that...

by an optometrist

Optometry

Optometry is a health care profession concerned with eyes and related structures, as well as vision, visual systems, and vision information processing in humans. Optometrists, or Doctors of Optometry, are state licensed medical professionals trained to prescribe and fit lenses to improve vision,...

. The prescription consists of all the specifications necessary to make the lens. Prescriptions typically include the power specifications of each lens (for each eye). Strengths are generally prescribed in quarter-diopter steps (0.25 D) because most people cannot generally distinguish between smaller increments (e.g., eighth-diopter steps / 0.125 D).

Sphere component

Each power specification includes a spherical correction in diopters. Convergent powers are positive (e.g., +4.00 D) and condense light to correct for farsightedness (hyperopia

Hyperopia

) or allow the patient to read more comfortably (see presbyopia

Presbyopia

and binocular vision disorders). Divergent powers are negative (e.g., −3.75 D) and spread out light to correct for nearsightedness (myopia

Myopia

). If neither convergence nor divergence is required in the prescription, "plano" is used to denote a refractive power of zero.

Cylinder component

If a patient has an astigmatism

Astigmatism

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...

, the patient needs two different correction powers in two different meridians (horizontally and vertically for example). This is specified by describing how the cylinder (the meridian that is most different from the spherical power) differs from the sphere power. Power evenly transitions between the two powers as you move from the meridian with the most convergence to the meridian with the least convergence or most divergence.

There are two different conventions for indicating the amount of cylinder: "plus cylinder notation" and "minus cylinder notation" In the former, the cylinder power is a number of diopters more convergent than the sphere power. That means the sphere power describes the most divergent meridian and the cylinder component describes the most convergent. In the latter, the cylinder power is a number of diopters more divergent than the sphere component. Thus the sphere power describes the most convergent meridian and the cylinder component describes the most divergent. Europe typically follows the plus cylinder convention while the US typically follows the minus cylinder convention. Minus cylinder notation is also most common in Asia, although either style may be encountered there. There is no difference in these forms of notation and it is easy to convert between them: simply add the sphere and cylinder numbers together to produce the converted sphere -- then change the sign of the cylinder number. For example a lens with {sphere, cylinder} of (-2.25, -1.50} would convert to {-3.75, +1.50}.

Axis component

The axis defines the location of the sphere and cylinder powers. The sphere is almost always 90° from the cylinder. (This is regular astigmatism, which is by far more common than irregular astigmatism where separations are other than 90°). Vertical is the 90th meridian and horizontal is both zero and the 180th meridians. The axis is the meridian 90° away from the cylinder power. To rationalize this, think of a soft drink can, or other cylindrical object. Note how the curvature is 90° away from its axis (the line it would spin around if you were to roll it).

Since the cylinder and sphere powers almost always separated by 90°, the axis is also the location of the sphere component. If the lens is spherical (there is no cylinder component) then there is no need for an axis. A prescription like this is written with D.S. (diopters sphere) after the spherical power (e.g., −3.00 D.S.).

Sphero-cylindrical correction "in focus"

In summary, regardless of whether plus or minus cylinder notation is used, the sphere describes the correction power on the axis and the cylinder describes the change in correction power 90º from the axis.
As described above:

this is true for the case of regular astigmatism.
correction power is measured in diopters
by convention, an axis of 90° is vertical, 0° or 180° are horizontal
if the cylinder is positive, the lens is most convergent 90° from the axis
if the cylinder is negative, the lens is most divergent 90° from the axis

Sample prescription

So, a prescription of −1.00 +0.25 x 180 describes a lens that has a horizontal power of −1.00 D and a vertical power of −0.75. (The same prescription written in minus cylinder notation: −0.75 −0.25 x 090)

Other considerations

Single vision lenses correct for only distance or near vision. Patients with presbyopia

Presbyopia

or other disorders of accommodation

Accommodation reflex

The accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object , comprising coordinated changes in vergence, lens shape and pupil size...

often benefit from corrections for both distance and near vision (see Lens Types below). Infrequently, prism

Prism correction

Eye care professionals use prism correction as a component of some eyeglass prescriptions. A lens with prism correction displaces the image, which is used to treat muscular imbalance or other conditions that cause errors in eye orientation. Prism correction is measured in prism dioptres...

and base curve values may also be specified to correct for binocular vision disorders.

Over the counter correction

Over the counter glasses are available without a prescription. They are used to lessen the focusing burden of near work and, in some cases, mild farsightedness (hyperopia

Hyperopia

). While these "magnifiers" as they are often called do indeed make the image of the viewed object bigger, the main advantage comes from the power of the lens. Over the counter readers are spherical corrective lenses of varying powers(commonly +1.00 D to +4.00 D). These lenses bend light coming from near objects making it more similar to light coming from distant objects.

These glasses are not as tailored to the specific needs of the patient. A difference in refractive error between the eyes or presence of astigmatism will not be accounted for. The use of improper corrective lenses may not help or could even exacerbate binocular vision disorders. Over the counter glasses do not usually correct for distance vision. Eyecare professionals (optometrists and ophthalmologists) are trained to determine the specific corrective lenses that will provide the clearest, most comfortable and most efficient vision, avoiding double vision and maximizing binocularity. They can test to see if over the counter corrective lenses are appropriate.

Single vision

Single vision has the same optical focal point or correction over the entire area of the lens.

Single lenses in CR-39 plastic come finished and unfinished. An unfinished lens is ground at the time your prescription is filled and the base curve is called a compensated base curve due to the fact that the lab tech can use a wide range of base curves to make the lens. A finished lens on the other hand, has been made using the exact base curve to match the prescription and the natural curve of the eyeball and is therefore called a correct base curve.

Bifocal

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

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, e.g. computer distance. This lens type has two segment lines, dividing the three different correcting segments.

Progressive

Progressive or varifocal provide a smooth transition from distance correction to near correction, eliminating segment lines and allowing the viewing of all intermediate distances.

Adjustable focus

Adjustable or variable focus

Adjustable-focus eyeglasses

Adjustable focus eyeglasses are prescription eyeglasses with an adjustable focal length. They compensate for refractive errors by providing variable focusing, allowing users to adjust them for desired distance or prescription, or both....

dynamically adjusts focal length to allow clear vision at any distance. It is especially useful for treating the loss of accommodation common in presbyopia

Presbyopia

Aspheric

Aspheric lens

An aspheric lens or asphere is a lens whose surface profiles are not portions of a sphere or cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens....

es are typically designed to give a thinner lens, and also distort the viewer's eyes less as seen by other people, producing better aesthetic appearance. Older texts may refer to lenticular lens

Lenticular lens

A lenticular lens is an array of magnifying lenses, designed so that when viewed from slightly different angles, different images are magnified...

polynomial designs, and advantages of curvatures that do not produce Scotoma

Scotoma

A scotoma is an area of partial alteration in one's field of vision consisting of a partially diminished or entirely degenerated visual acuity which is surrounded by a field of normal - or relatively well-preserved - vision.Every normal mammalian eye has a scotoma in its field of vision, usually...

, or a blind spot

Blind spot

Blind spot may refer to:In ophthalmology:*Scotoma, an obscuration of the visual field*Optic disc, also known as the anatomical blind spot, the specific region of the retina where the optic nerve and blood vessels pass through to connect to the back of the eye*Blind spot , also known as the...

Plano

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. 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.

Lens optical profile

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

Hyperopia

and presbyopia

Presbyopia

a convergent lens is needed, therefore the convergent front surface overpowers the divergent back surface. For myopia

Myopia

the opposite is true: the divergent back surface is greater in magnitude than the convergent front surface.

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.

A much more detailed web-published document with technical images and diagrams may be found here:

Bifocals, Trifocals, and Progressive-Addition Lenses by H. Jay Wisnicki, M.D., Focal Points: Clinical Modules for Ophthalmologists, by the American Academy of Ophthalmologists, Volume 17, Number 6, June 1999 (Section 3 of 3)

Bifocals and trifocals

Bifocals and trifocals result in a more complex lens profile, compounding multiple spherical surfaces. The main lens is composed of two spherical surfaces, and the bifocal adds a third spherical segment, working with the back side spherical surface.

The bifocal spherical segment is called an add segment, but is cut into the lens in the manufacturing process and is not an actual separate piece of material glued onto main lens.

There are many profiles and sizes of segments and the method that they are combined with the distance corrective lens has to do with the lens material and segment type. Some examples include Flat top, Kryptok, Orthogon, Tillyer Executive, Ultex A.

The optical center of the bifocal 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 lens surface.

Progressive lens

The progressive addition lens (PAL, also commonly called a progressive lens) eliminates the line in bi/tri-focals and are very complex in their profile. They are a continuously variable parametric surface

Parametric surface

A parametric surface is a surface in the Euclidean space R3 which is defined by a parametric equation with two parameters. Parametric representation is the most general way to specify a surface. Surfaces that occur in two of the main theorems of vector calculus, Stokes' theorem and the divergence...

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.

Lens shape

Corrective lenses can be produced in many different shapes, from a simple circular lens, to wide, squat rectangular fashion glasses, to full eye lenses that cover as much of the visual space as possible.

For lower power lenses, the wearer may choose a lens shape that is limiting in the corrective capability but which allows for trendy fashion shapes.

Higher power patients with naturally poor vision may opt for a larger full-eye lens simply because without the lens they can not see clearly, and a small lens leads to tunnel vision

Tunnel vision

Tunnel vision is the loss of peripheral vision with retention of central vision, resulting in a constricted circular tunnel-like field of vision.- Medical / biological causes :Tunnel vision can be caused by:...

with blurriness in the surrounding space beyond the edges of the lens.

Bifocal, trifocal, and progressive lenses generally require a tall lens shape to accommodate the different bands of lens focus. If the lens is too squat these bands form very narrow strips, as if looking at the world through a clearly focused mailbox slot within the bands of focus.

Refractive index

In the UK and the US, the refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

is generally specified with respect to the yellow He

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

-d Fraunhofer line, commonly abbreviated as n_d. Lens materials are classified by their refractive index, as follows:

Normal index - 1.48 ≤ n_d < 1.54
Mid-index - 1.54 ≤ n_d < 1.60
High-index - 1.60 ≤ n_d < 1.74
Very high index - 1.76 ≤ n_d

This is a general classification. Indexes of n_d values that are ≥ 1.60 can be, often for marketing purposes, referred to as high-index. Likewise, Trivex and other borderline normal/mid-index materials, may be referred to as mid-index.

Advantages of higher indices

Thinner, sometimes lighter lenses (See below).
Improved UV protection over CR-39 and glass lenses.

Disadvantages of increased indices

Lower Abbe number
Abbe number
In physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...

meaning, amongst other things, increased chromatic aberration.
Poorer light transmission and increased backside and inner-surface reflections (see Fresnel reflection
Fresnel equations
The Fresnel equations , deduced by Augustin-Jean Fresnel , describe the behaviour of light when moving between media of differing refractive indices...

equation) increasing importance of anti-reflective coating
Anti-reflective coating
An antireflective or anti-reflection coating is a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. This improves the efficiency of the system since less light is lost. In complex systems such as a telescope, the reduction in reflections also...

.
Manufacturing defects have more impact on optical quality.
Theoretically, off-axis optical quality degrades (oblique astigmatic error). In practice this degradation should not be perceptible - current frame styles are much smaller than they would have to be for these aberrations to be noticeable to the patient, the aberration occurring some distance away from the optical centre of the lens (off-axis).

Abbe number

Of all of the properties of a particular lens material, the one that most closely relates to its optical performance is its dispersion

Dispersion (optics)

In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...

, which is specified by the Abbe number

Abbe number

In physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...

. Lower Abbe numbers result in the presence of chromatic aberration

Chromatic aberration

In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...

(i.e., color fringes above/below or to the left/right of a high contrast object), especially in larger lens sizes and stronger prescriptions (±4D

Dioptre

A dioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres . It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at metre...

or greater). Generally, lower Abbe numbers are a property of mid and higher index lenses that cannot be avoided, regardless of the material used. The Abbe number for a material at a particular refractive index formulation is usually specified as its Abbe value.

In practice, a change from 30 to 32 Abbe will not have a practically noticeable benefit, but a change from 30 to 47 could be beneficial for users with strong prescriptions that move their eyes and look ‘off-axis’ of optical center of the lens. Note that some users do not sense color fringing directly but will just describe 'off-axis blurriness'.
Abbe values even as high as that of (V_d≤45) produce chromatic aberrations which can be perceptible to a user in lenses larger than 40 mm in diameter and especially in strengths that are in excess of ±4D. At ±8D even glass (V_d≤58) produces chromatic aberration that can be noticed by a user. Chromatic aberration is independent of the lens being of spherical, aspheric, or atoric design.

The eye’s Abbe number is independent of the importance of the corrective lens’s Abbe, since the human eye:

Moves to keep the visual axis close to its achromatic axis, which is completely free of dispersion (i.e., to see the dispersion one would have to concentrate on points in the periphery of vision, where visual clarity is quite poor)
Is very insensitive, especially to color, in the periphery (i.e., at retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

l points distant from the achromatic axis and thus not falling on the fovea
Fovea
The fovea centralis, also generally known as the fovea , is a part of the eye, located in the center of the macula region of the retina....

, where the cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

s responsible for color vision are concentrated. See: Anatomy and Physiology of the Retina.)

In contrast, the eye moves to look through various parts of a corrective lens as it shifts its gaze, some of which can be as much as several centimeters off of the optical center. Thus, despite the eye's dispersive properties, the corrective lens's dispersion cannot be dismissed. People who are sensitive to the effects of chromatic aberrations, or who have stronger prescriptions, or who often look off the lens’s optical center, or who prefer larger corrective lens sizes may be impacted by chromatic aberration. To minimize chromatic aberration:

Try to use the smallest vertical lens size that is comfortable. Generally, chromatic aberrations are more noticeable as the pupil moves vertically below the optical center of the lens (e.g., reading or looking at the ground while standing or walking). Keep in mind that a smaller vertical lens size will result in a greater amount of vertical head movement, especially while performing activities that involve short and intermediate distance viewing, which could lead to an increase in neck strain, especially in occupations involving a large vertical field of view.
Restrict the choice of lens material to the highest Abbe value at acceptable thickness. The oldest most basic commonly used lens materials also happen to have the best optical characteristics at the expense of corrective lens thickness (i.e., cosmetics). Newer materials have focused on improved cosmetics and increased impact safety, at the expense of optical quality. Lenses sold in the USA must pass the Food and Drug Administration
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...

ball-drop impact test, and depending on needed index these seem to currently have ‘best in class’ Abbe vs Index (N_d): Glass (2x weight of plastics) or CR-39
CR-39
CR-39, or allyl diglycol carbonate , is a plastic polymer commonly used in the manufacture of eyeglass lenses. The abbreviation stands for “Columbia Resin #39,” because it was the 39th formula of a thermosetting plastic developed by the Columbia Resins project in 1940.The first commercial use of...

(2 mm vs. 1.5 mm thickness typical on newer materials) 58 @ 1.5, Sola Spectralite (47@1.53), Sola Finalite (43@1.6), and Hoya Eyry (36 @ 1.7). For impact resistance safety glass is offered at a variety of indexes at high Abbe number, but is still 2x the weight of plastics. Polycarbonate (V_d=30-32) is very dispersive, but has excellent shatter resistance. Trivex (V_d=43 @ 1.53), is also heavily marketed as an impact resistant alternative to Polycarbonate, for individuals who don’t need polycarbonate’s index. Trivex is also one of the lightest materials available.
Use contact lenses in place of eyeglasses. A contact lens rests directly on the surface of the cornea and moves in sync with all eye movements. Consequently the contact lens is always directly aligned on center with the pupil and there is never any off-axis misalignment between the pupil and the optical center of the lens.

Power error (-D corrections for myopia)

Power error is the change in the optical power

Optical power

Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

of a lens as the eye looks through various points on the area of the lens. Generally, it is least present at the optic center and gets progressively worse as one looks towards the edges of the lens. The actual amount of power error is highly dependent on the strength of the prescription as well as whether a best spherical form of lens or an optically optimal aspherical form was used in the manufacture of the lens. Generally, best spherical form lenses attempt to keep the ocular curve between four and seven diopters.

Lens induced oblique astigmatism (+D corrections for presbyopia)

As the eye shifts its gaze from looking through the optical center of the corrective lens, the lens induced astigmatism value increases. In a spherical lens, especially one with a strong correction whose base curve is not in the best spherical form, such increases can significantly impact the clarity of vision in the periphery.

Minimizing power error and lens induced astigmatism

As corrective power increases, even optimally designed lenses will have distortion that can be noticed by a user. This particularly affects individuals that use the off-axis areas of their lenses for visually demanding tasks. For individuals sensitive to lens errors, the best way to eliminate lens induced aberrations is to use contact lenses. Contacts eliminate all these aberrations since the lens then moves with the eye.

Barring contacts, a good lens designer doesn’t have many parameters which can be traded off to improve vision. Index has little effect on error. Note that, chromatic aberration is often perceived as ‘blurry vision’ in the lens periphery giving the impression of power error, although this is actually due to color shifting. Chromatic aberration can be improved by using a material with improved ABBE. The best way to combat lens induced power error is to limit the choice of corrective lens to one that is in the best spherical form. A lens designer determines the best-form spherical curve using the Oswalt curve on the Tscherning ellipse. This design gives the best achievable optical quality and least sensitivity to lens fitting. A flatter base-curve is sometime selected for cosmetic reasons. Aspheric or atoric design can reduce errors induced by using a suboptimal flatter base-curve. They cannot surpass the optical quality of a spherical best-form lens, but can reduce the error induced by using a flatter than optimal base curve. The improvement due to flattening is most evident for strong farsighted lenses. High myopes (-6D) may see a slight cosmetic benefit with larger lenses. Mild prescriptions will have no perceptible benefit (-2D). Even at high prescriptions some high myope prescriptions with small lenses may not see any difference, since some aspheric lenses have a spherically designed center area for improved vision and fit.

In practice, labs tend to produce pre-finished and finished lenses in groups of narrow power ranges to reduce inventory. Lens powers that fall into the range of the prescriptions of each group share a constant base curve. For example, corrections from -4.00D to -4.50D may be grouped and forced to share the same base curve characteristics, but the spherical form is only best for a -4.25D prescription. In this case the error will be imperceptible to the human eye. However, some manufacturers may further cost-reduce inventory and group over a larger range which will result in perceptible error for some users in the range who also use the off-axis area of their lens. Additionally some manufacturers may verge toward a slightly flatter curve. Although if only a slight bias toward plano is introduced it may be negligible cosmetically and optically. These optical degradations due to base-curve grouping also apply to aspherics since their shapes are intentionally flattened and then asphericized to minimize error for the average base curve in the grouping.

Self refraction

Although lenses are normally prescribed by optometrists or opthalmologists, there is evidence from developing countries that allow people to select lenses for themselves produces good results in the majority of cases and is less than a tenth of the cost of prescription lenses. In a study published by Zhang in 2011 of 648 young Chinese people (mean age 14.9 years), differences between subjective refraction (formal testing by an ophthalmologist) and self refraction (allowing the children to choose the lenses that suited them best) were very close. With self refraction, 97% of participants were able to achieve a visual acuity of 6/7.5 or better, compared to 99% by subjective refraction.

Reducing lens thickness

Note that the greatest cosmetic improvement on lens thickness (and weight) is had from choosing a frame which holds physically small lenses. The smallest of the popular adult lens sizes available in retail outlets is about 50mm across. There are a few adult sizes of 40mm and although they are quite rare, can reduce lens weight to about half of the 50mm versions. See the diagram opposite, for a simplified graphical explanation of how smaller sizes with the same radius of curvature can greatly reduce thickness. The curves on the front and back of a lens are ideally formed with the specific radius of a sphere. This radius is set by the lens designer based on the prescription and cosmetic consideration. Selecting a smaller lens will mean less of this sphere surface is represented by the lens surface, meaning the lens will have a thinner edge (myopia) or center (hyperopia). A thinner edge reduces light entering into the edge, reducing an additional source of internal reflections.

Extremely thick lenses for myopia can be beveled to reduce flaring out of the very thick edge. Thick myopic lenses are not usually mounted in wire frames, because the thin wire contrasts against the thick lens, to make its thickness much more obvious to others.

Index can improve the lens thinness, but at a point no more improvement will be realized. For example, if an index and lens size is selected with center to edge thickness difference of 1 mm then changing index can only improve thickness by a fraction of this. This is also true with aspheric design lenses.

The lens's minimum thickness can also be varied. The FDA ball drop test (5/8" 0.56 ounce steel ball dropped from 50 inches) effectively sets the minimum thickness of materials. Glass or CR-39 requires 2.0 mm, but some newer materials only require 1.5 mm or even 1.0 mm minimum thickness.

Weight

Material density typically increases as lens thickness is reduced by increasing index. There is also a minimum lens thickness required to support the lens shape. These factors results in a thinner lens which is not lighter than the original. There are lens materials with lower density at higher index which can result in a truly lighter lens. These materials can be found in a material property table. Reducing frame lens size will give the most noticeable improvement in weight for a given material. Ways to reduce the weight and thickness of spectacle lenses, in approximate order of importance are these:

Choose spectacle frames with small lenses; that is to say, so that the longest measurement across the lens at any angle is as short as possible. This gives the greatest advantage of all.
Choose a frame that allows the pupil to occupy the exact middle point of the lens.
Choose a lens as near round as possible. These are less commonly found than other shapes.
Choose as high a refractive index for the lens material as cost permits.

It is not always possible to follow the above points, because of the rarity of such frames, and the need for more pleasing appearance. However, these are the main factors to consider if ever it should become necessary and possible to do so.

Facial distortion and social stigma

Eyeglasses for a high-diopter nearsighted or farsighted person cause a visible distortion of their face as seen by other people, in the apparent size of the eyes and facial features visible through the eyeglasses.

For extreme nearsightedness the eyes appear small and sunken into the face, and the sides of the skull can be visible through the lens. This gives the wearer the appearance of having a very large or fat head in contrast with their eyes.
For extreme farsightedness the eyes appear very large on the face, making the wearer's head seem too small.

Either situation can result in social stigma

Social stigma

Social stigma is the severe disapproval of or discontent with a person on the grounds of characteristics that distinguish them from other members of a society.Almost all stigma is based on a person differing from social or cultural norms...

for children and adults due to apparent unattractiveness or ugliness of the wearer caused by these facial distortions. This can result in low self-esteem of the eyeglass wearer and lead to difficulty in making friends and developing relationships.

People with very high power corrective lenses can benefit socially from contact lenses because these distortions are minimized and their facial appearance to others is normal. Aspheric/atoric eyeglass design can also reduce minification and magnification of the eye for observers at some angles.

Optical crown glass (B270)

Refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

(n_d): 1.52288
Abbe value
Abbe number
In physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...

(V_d): 58.5
Density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

: 2.55 g/cm³ (the heaviest corrective lens material in common use, today)
UV cutoff: 320 nm
- Please note: Schott B270 is an optical glass used in precision optics. It is NOT an ophthalmic glass. Schott ophthalmic glass types are S-1 and S-3. The issue here is an incorrect value for UVA and UVB transmission, as well as other related product type issues. ***

Glass lenses have become less common in recent years due to the danger of shattering and their relatively high weight compared to CR-39

CR-39

CR-39, or allyl diglycol carbonate , is a plastic polymer commonly used in the manufacture of eyeglass lenses. The abbreviation stands for “Columbia Resin #39,” because it was the 39th formula of a thermosetting plastic developed by the Columbia Resins project in 1940.The first commercial use of...

plastic lenses. They still remain in use for specialised circumstances, for example in extremely high prescriptions (currently, glass lenses can be manufactured up to a refractive index

Refractive index

of 1.9) and in certain occupations where the hard surface of glass offers more protection from sparks or shards of material. If the highest Abbe value is desired, the only choices for common lens optical material are optical crown glass and CR-39.

Higher quality optical-grade glass materials exist (e.g., Borosilicate crown glasses

Borosilicate glass

Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...

such as BK7 (n_d=1.51680 / V_d=64.17 / D=2.51 g/cm³), which is commonly used in telescopes and binoculars, and fluorite

Fluorite

Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...

crown glasses

Crown glass (optics)

Crown glass is type of optical glass used in lenses and other optical components. It has relatively low refractive index and low dispersion...

such as Schott N-FK51A (n_d=1.48656 / V_d=84.47 / D=3.675 g/cm³), which is 16.2 times the price of a comparable amount of BK7, and are commonly used in high-end camera lenses). However, one would be very hard pressed to find a laboratory that would be willing to acquire or shape custom eyeglass lenses, considering that the order would most likely consist of just two different lenses, out of these materials. Generally, V_d values above 60 are of dubious value, except in combinations of extreme prescriptions, large lens sizes, a high wearer sensitivity to dispersion, and occupations that involve work with high contrast elements (e.g., reading dark print on very bright white paper, construction involving contrast of building elements against a cloudy white sky, a workplace with recessed can or other concentrated small area lighting, etc.).

Plastic (CR-39)

Refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

(n_d): 1.498 (standard)
Abbe value
Abbe number
In physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...

(V_d): 59.3
Density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

: 1.31 g/cm³
UV cutoff: 355 nm

Plastic lenses are currently the most commonly prescribed lens, due to their relative safety, low cost, ease of production, and outstanding optical quality. The main drawbacks of many types of plastic lenses are the ease by which a lens can be scratched, and the limitations and costs of producing higher index lenses. CR-39 lenses are the exception to the plastics in that they have inherent scratch resistance.

Trivex

Refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

(n_d): 1.532
Abbe value
Abbe number
In physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...

(V_d): 43–45 (depending on licensing manufacturer)
Density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

: 1.1 g/cm³ (the lightest corrective lens material in common use)
UV cutoff: 380 nm

Trivex is a relative newcomer that possesses the UV blocking properties and shatter resistance of polycarbonate

Polycarbonate

PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...

while at the same time offering far superior optical quality (i.e., higher Abbe value) and a slightly lower density. Its lower refractive index of 1.532 vs. polycarbonate's 1.586, however, may result in slightly thicker lenses. Along with polycarbonate and the various high-index plastics, Trivex is a lab favorite for use in rimless frames, due to the ease with which it can be drilled as well as its resistance to cracking around the drill holes. One other advantage that Trivex has over polycarbonate is that it can be easily tinted.

Polycarbonate

Refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

(n_d): 1.586
Abbe value
Abbe number
In physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...

(V_d): 30
Density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

: 1.2 g/cm³
UV cutoff: 385 nm

Polycarbonate is lighter weight than normal plastic. It blocks UV rays, is shatter resistant and is used in sports glasses and glasses for children and teenagers. Because polycarbonate is soft and will scratch easily, scratch resistant coating is typically applied after shaping and polishing the lens. Standard polycarbonate with an Abbe value of 30 is one of the worst materials optically, if chromatic aberration intolerance is of concern. Along with Trivex and the high-index plastics, polycarbonate is an excellent choice for rimless eyeglasses. Similar to the high-index plastics, polycarbonate has a very low Abbe value which may be bothersome to individuals sensitive to chromatic aberrations.

High-index plastics (thiourethanes)

Refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

(n_d): 1.600–1.740
Abbe value
Abbe number
In physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...

(V_d): 42–32 (higher indexes generally result in lower Abbe values)
Density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

: 1.3–1.5 (g/cm³)
UV cutoff: 380–400 nm

High-index plastics allow for thinner lenses. The lenses may not be lighter, however, due to the increase in density vs. mid- and normal index materials. Despite being popular with customers due to their thinner appearance, high-index lenses also suffer from a much higher level of chromatic aberrations due to their lower Abbe value. For people with strong prescriptions, the significant reduction in thickness may warrant the reduction in optical quality. Aside from thinness of the lens, another advantage of high-index plastics is their strength and shatter resistance, although not as shatter resistant as polycarbonate. This makes them another excellent choice for rimless eyeglasses. As raw materials of High-index plastic lenses, thiourethane based materials such as Mitsui Chemicals' MR SERIEShttp://www.mitsuichem.com/special/mr/index.htm are widely used.

Ophthalmic material property tables

Material, Plastic	Index (N_d)	ABBE (V_d)	Specific Gravity (g/cm³)	UVB/ UVA	Reflected light (%)	Minimum thickness typ/min (mm)	Note
CR-39 Hard Resin	1.49	59	1.31	100% / 90%	7.97	?/2.0
PPG Trivex (Average)	1.53	44	1.11	100% / 100%	8.70	?/1.0	PPG, Augen, HOYA, Thai Optical, X-cel, Younger
SOLA Spectralite	1.54	47	1.21	100% / 98%	8.96	(also Vision 3456 (Kodak)?)
Essilor Ormex	1.56http://catalogue.essilor.ca/en-ca/Gammes/Simple%20Vision/Organique/Hyp%E9ral%20Ormex/	37	1.23	100% / 100%	9.52
Polycarbonate	1.586	30	1.20	100% / 100%	10.27	?/1.5	Tegra (Vision-Ease) Airwear (Essilor) FeatherWates (LensCrafters)
MR-8 1.6 Plastic	1.6http://www.mitsuichem.com/special/mr/index.htm	41	1.30	100% / 100%	10.43
MR-6 1.6 Plastic	1.6	36	1.34	100% / 100%	10.57
MR-20 1.6 Plastic	1.60	42	1.30	100% / 100%
SOLA Finalite	1.60	42	1.22	100% / 100%	10.65
MR-7 1.67 Plastic	1.67http://www.mitsuichem.com/special/mr/index.htm	32	1.35	100% / 100%	12.26
MR-10 1.67 Plastic	1.67http://www.mitsuichem.com/special/mr/index.htm	32	1.37	100% / 100%	12.34
Nikon 4 Plastic NL4	1.67	32	1.35	100% / 100%
Hoya EYRY	1.70	36	1.41	100% / 100%	13.44	?/1.5
MR-174 1.74 Plastic	1.74http://www.mitsuichem.com/special/mr/index.htm	33	1.47	100% / 100%	14.36		Hyperindex 174 (Optima)
Nikon 5 Plastic NL5	1.74	33	1.46	100% / 100%
Tokai	1.76	30	1.49	100% / 100%

Material, Glass	Index (N_d)	ABBE (V_d)	Specific Gravity	UVB/ UVA	Reflected light (%)	Note
Crown Glass	1.525	59	2.54	79% / 20%	8.59
PhotoGray Extra	1.523	57	2.41	100% / 97%	8.59
1.6 Glass	1.604	40	2.62	100% / 61%	10.68	Zeiss Uropal, VisionEase, X-Cel
1.7 Glass	1.706	30	2.93	100% / 76%	13.47	Zeiss Tital, X-Cel, VisionEase, Phillips
1.8 Glass	1.800	25	3.37	100% / 81%	16.47	Zeiss Tital, X-Cell, Phillips, VisionEase,Zhong Chuan Optical(China)
1.9 Glass	1.893	31	4.02	100% / 76%	18.85	Zeiss Lantal, Zhong Chuan Optical(China) not FDA-approved for sale in USA

Reflected light calculated using Fresnel reflection equation

Fresnel equations

The Fresnel equations , deduced by Augustin-Jean Fresnel , describe the behaviour of light when moving between media of differing refractive indices...

for normal waves against air on two interfaces. This is reflection without an AR coating.

Compilations of manufacturer material data can be found at opticampus, firstvisionmedia, and eyecarecontacts. Additional information on branding can be found at eyetopics.

Indices of refraction for a range of materials can be found in the List of indices of refraction.

Anti-reflective

Anti-reflective coatings help to make the eye behind the lens more visible. They also help lessen back reflections of the white of the eye as well as bright objects behind the eyeglasses wearer (e.g., windows, lamps). Such reduction of back reflections increases the apparent contrast of surroundings. At night, anti-reflective coatings help to reduce headlight glare from oncoming cars, street lamps and heavily lit or neon signs.

One problem with anti-reflective coatings is that historically they have been very easy to scratch. Newer coatings, such as Crizal Alizé with its 5.0 rating and Hoya's Super HiVision with its 10.9 rating on the COLTS Bayer Abrasion Test (glass averages 12–14), try to address this problem by combining scratch resistance with the anti-reflective coating. They also offer a measure of dirt and smudge resistance, due to their hydrophobic

Hydrophobe

In chemistry, hydrophobicity is the physical property of a molecule that is repelled from a mass of water....

properties (110° water drop contact angle

Contact angle

The contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...

for Super HiVision and 112° for Crizal Alizé).

Ultraviolet protection

A UV coating is used to reduce the transmission of light in the ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

spectrum. UV-B

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

radiation increases the likelihood of cataracts, while long term exposure to UV-A

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

radiation can damage the retina

Retina

The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

. DNA damage from UV light is cumulative and irreversible. Some materials, such as Trivex and Polycarbonate naturally block most UV light and do not benefit from the application of a UV coating.

Spheric vs. aspheric, atoric, etc.

Lens manufacturers claim that aspheric lens

Aspheric lens

es improve vision over traditional spheric lenses. This statement could be misleading to individuals who do not know that the lenses are being implicitly compared to "a spheric flattened away from best-form for cosmetic reasons". This qualification is necessary since best-form spherics are always better than aspherics for an ophthalmic lens application. Aspherics for corrective lenses are only used to attempt to improve the degradation caused by deviating from best-form sphere resulting from making a flatter lens for cosmetic reasons. The same applies for atoric and bi-aspheric.

It is true that aspheric lenses are used in cameras and binoculars. It would be wrong to assume that this means aspherics/atorics are a sign of good optics in eyewear. Cameras and telescopes use multiple lens elements and have different design criteria. Spectacles are made of only one ophthalmic lens. The best-form spheric lens has been shown to give the best vision. In cases where best-form is not used, such as cosmetic flattening, thinning, or wrap-around sunglasses, an aspheric design can reduce the amount of induced optical distortions.

The problem with aspheric lenses is that they are a broad category. A lens is made of two curved surfaces, and an aspheric lens is a lens where one or both of those surfaces is not spherical. Further research and development is being conducted to determine if the mathematical and theoretical benefits of aspheric lenses can actually lead to better vision correction.

Optical aberrations of the eye lens vs. corrective lens

Optical terms are used to describe error in the eye's lens and the corrective lens. This can cause confusion, since "astigmatism" or "ABBE" has drastically different impact on vision depending on which lens has the error.

Astigmatism disambiguation

Astigmatism of the eye: People prescribed a sphere and a cylinder prescription have astigmatism of the eye

Astigmatism (eye)

and can be given a toric lens

Toric lens

A toric lens is a lens with two different powers in two orientations perpendicular to each other. One of the lens surfaces is shaped like a "cap" from a torus , while the other one usually is spherical...

to correct it.

Astigmatism

Astigmatism

of the corrective lens: This phenomenon is called lens-induced oblique astigmatism error (OAE) or power error and is induced when the eye looks through the ophthalmic lens at a point oblique to the optical center (OC). This may become especially evident beyond -6D.

Example: A patient with astigmatism (or no astigmatism) of the eye and a high prescription may notice astigmatism of the lens (OAE) when looking through the corner of their glasses.

Aspheric and atoric disambiguation

An ophthalmic "aspheric lens" specifically refers to a subclass of aspheric lens

Aspheric lens

. Designs referring to "flatter" curves are trading off optical quality for cosmetic appearance. An aspheric lens attempts to correct the error induced by flattening the lens by using a non-spheric lens shape. Typically the design focuses on reducing the error (OAE) across the horizontal and vertical lens axis edges. This can be primarily beneficial to farsighted individuals, whose lenses have a thick center.

An atoric lens design refers to a lens with more complex aspheric lens design. An atoric lens design can address error over more corners of the lens, not just the horizontal and vertical axis.

A toric

Toric lens

lens is designed to compensate for the patients with astigmatism. Even though this technically requires an "aspheric" lens, "aspheric" and "atoric" are reserved for lenses which correct errors induced by cosmetic lens flattening.

U.S. legal requirements for prescriptions

In the U.S., laws at the federal and state level govern the provision and effective dates of prescriptions for contact lenses and eyeglasses. (These consumer protection laws grew out of the practice of tying lens purchases to prescription services.) Federal law requires that eyeglass and contact lens prescriptions be given to every consumer, and that the prescriptions be for a minimum of one year. (FTC Section 456.2 “Separation of examination and dispensing” was reviewed in 2004: FTC 2004 review of section 456.2)).

State laws vary. For example, California law also requires prescriptions to be provided to clients whether demanded or not. Eyeglass prescriptions must be for a minimum of 2 years, and contact prescriptions must be for a minimum of 1 year. See Business and Professions Code Section 2541.1.

External links

MR Series Website (Mitsui Chemicals Inc)

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Prescription of corrective lenses

Sphere component

Cylinder component

Axis component

Sphero-cylindrical correction "in focus"

Sample prescription

Other considerations

Over the counter correction

Single vision

Bifocal

Trifocal

Progressive

Adjustable focus

Aspheric

Plano

Lens optical profile

Bifocals and trifocals

Progressive lens

Lens shape

Refractive index

Advantages of higher indices

Disadvantages of increased indices

Abbe number

Power error (-D corrections for myopia)

Lens induced oblique astigmatism (+D corrections for presbyopia)

Minimizing power error and lens induced astigmatism

Self refraction

Reducing lens thickness

Weight

Facial distortion and social stigma

Optical crown glass (B270)

Plastic (CR-39)

Trivex

Polycarbonate

High-index plastics (thiourethanes)

Ophthalmic material property tables

Anti-reflective

Ultraviolet protection

Spheric vs. aspheric, atoric, etc.

Optical aberrations of the eye lens vs. corrective lens

Astigmatism disambiguation

Aspheric and atoric disambiguation

U.S. legal requirements for prescriptions

See also

External links