Corrective lens
From Wikipedia, the free encyclopedia
A corrective lens is a lens worn on or before the eye, used to treat myopia, hyperopia, astigmatism, and presbyopia. The most common types of corrective lenses are eyeglass lenses and contact lenses. Intraocular lenses are also beginning to become common. Myopia (near sightedness) requires biconcave or diverging lenses, whereas hypermetropia (far sightedness) requires biconvex or converging lenses.
Corrective lenses are usually prescribed by an optometrist. The prescription consists of the necessary corrections for refraction errors in each eye, individually for distance vision and near vision, for a total of four possible correction specifications. Each specification can include a spherical correction in diopters for near/far sightedness and/or presbyopia, a cylindrical correction in diopters combined with the cylinder axis in degrees, correcting for any cylindrical deformation of the eye (i.e., astigmatism). Infrequently, prism and base values may also be specified to correct for a muscular imbalance and/or errors in eye orientation.
In some cases, mild farsightedness can be treated with simple magnifying lenses or commodity reading glasses that can be purchased without a prescription. However, an optometrist may need to prescribe a correction for both eyes or each eye, individually, allowing lenses to be custom ground to the patient's specific needs. Usually, the amount of correction necessary for both eyes is similar, although in rare cases the prescriptions can differ by a wide margin.
Patients with presbyopia or other disorders of accommodation often benefit from bifocals, or lenses with separate sections ground to different prescriptions for different circumstances. 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 hypermetropia would have one section of the lens with a certain converging power and another section with a greater power for close-up work.
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[edit] Eyeglass lenses
[edit] Lens types
- Single vision - Has the same optical correction over the entire area of the lens
- Bifocal - The upper part of the lens is generally used for distance vision, while the lower part is used for near vision. A segment line separates the two
- Trifocal - Similar to bifocals, except that the two bifocal areas are separated by a third middle area with no correction, used for intermediate distance vision. This lens type has two segment lines, dividing the three different correcting segments.
- Progressive - Provide a smooth transition in a bifocal or trifocal lens, eliminating the characteristic segment lines that plague those lens types
[edit] Lens shapes
Although corrective lenses can be produced in many different shapes, the most common single vision shapes are convex-concave/plano-concave for the treatment of myopia and biconvex for the treatment of presbyopia. The difference in curvature between the front and rear surface leads to the corrective power of the lens.
Bifocals and trifocals result in more complex lens shapes that are a combination of the common shapes. Progressive lenses which try to eliminate the line in bi/tri-focals, further dull the ability to categorize the shape.
Optically, base curves in the best spherical form, for a particular prescription, result in the best possible optical characteristics across the entire surface of the lens. Cosmetically, a more plano outer surface, especially for convex-concave lenses, may be desired, despite a reduction in optical quality at points distant from the optical center of the lens.
[edit] Refractive index
In the UK and the US, the refractive index is generally specified with respect to the yellow He-d Fraunhofer line, commonly abbreviated as nd. Lens materials are classified by their refractive index, as follows:
- Normal index - 1.48 ≤ nd < 1.54
- Mid-index - 1.54 ≤ nd < 1.64
- High-index - 1.64 ≤ nd < 1.74
- Very high index - 1.74 ≤ nd
Despite this classification, nd values that are ≥1.60 are often, especially for marketing purposes, referred to as high-index. Likewise, Trivex and other borderline normal/mid-index materials, are often referred to as mid-index.
[edit] Optical quality
[edit] 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, which is specified by the Abbe number. Lower Abbe numbers result in the presence of chromatic aberration (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 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, Abbe values even as high as that of Trivex (Vd≤45) produce noticeable chromatic aberrations in lenses larger than 40mm in diameter and especially in strengths that are in excess of ±4D. This is true even in aspheric and atoric lens designs. There seems to be a lot of confusion in the literature concerning the impact on the corrective lens wearer of a low Abbe value, since the materials in the human eye give it an Abbe number of 43-58, with the correct value likely being toward the higher bound of that interval. This is misleading however, 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 retinal points distant from the achromatic axis and thus not falling on the fovea, where the cone cells 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. People who are sensitive to the effects of chromatic aberrations, have stronger prescriptions, and prefer larger corrective lens sizes, should:
- 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 CR-39 (Vd=58), crown glass (Vd=59), and potentially Trivex Vd)=43-45). This will result in a thicker and for CR-39 and glass lenses, heavier lens, however. 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.
[edit] Power error (-D corrections for myopia)
Power error is the change in the 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.
[edit] 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.
[edit] Minimizing power error and lens induced astigmatism
The best way to combat lens induced aberrations is to limit the choice of corrective lens to one that is in the best spherical form. An alternative is to select an aspherical or atoric lens shape, when possible, which not only makes the lens thinner, but also corrects for power error and lens induced astigmatism at points away from the optical center.
Unfortunatelly, labs tend to produce pre-finished and finished lenses in groups of narrow power ranges. Lens powers that fall into the range of the prescriptions of each group share a constant base curve. This base curve is only best for the power in the middle of the range of the group. This makes it less probable that a true best spherical form of a lens will be available for a particular strength, unless one's prescription happens to fall in the middle of such a range. 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. This grouping applies to both spherical and aspherical lenses. To make matters worse, the grouping of spherical lenses can take cosmetic factors, such as a more plano front surface into account, thus making it impossible to get a lens in best spherical form, regardless of strength. The impact of cosmetic compromises applies less to aspherics, which are generally thinner and have a more plano (i.e., flatter) outer surface.
[edit] Lens materials
[edit] Optical crown glass (B270)
- Refractive index (nd): 1.52288
- Abbe value (Vd): 58.5
- Density: 2.55 g/cm3 (the heaviest corrective lens material in common use, today)
- UV cutoff: 320 nm
Glass lenses have become less common in recent years due to the danger of shattering and their relatively high weight compared to CR-39 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 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 such as BK7 (nd=1.51680 / Vd=64.17 / D=2.51 kg/m3), which is commonly used in telescopes and binoculars, and fluorite crown glasses such as Schott N-FK51A (nd=1.48656 / Vd=84.47 / D=3.675 kg/m3), 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, Vd 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...).
[edit] Plastic (CR-39™)
- Refractive index (nd): 1.498 (standard)
- Abbe value (Vd): 59.3
- Density: 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 are the ease by which a lens can be scratched, and the limitations and costs of producing higher index lenses.
[edit] Trivex™
- Refractive index (nd): 1.532
- Abbe value (Vd): 43 - 45 (depending on licensing manufacturer)
- Density: 1.1 g/cm3 (the lightest corrective lens material in common use)
- UV cutoff: 380 nm
Trivex™ is a relative newcomer that posseses the UV blocking properties and shatter resistance of polycarbonate 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, if desired.
[edit] Polycarbonate
- Refractive index (nd): 1.586
- Abbe value (Vd): 30
- Density: 1.2 g/cm3
- UV cutoff: 385 nm
Lighter weight than normal plastic. Less tendency to irritate your nose or leave red marks on your nose where the glasses touch your nose. Polycarb blocks UV rays, is shatter resistant and is used in sports glasses and glasses for children and teenagers. Polycarb is soft and will scratch easily. You must get a scratch coating on a polycarb 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.
[edit] High-index plastics (polyurethanes)
- Refractive index (nd): 1.640 - 1.740
- Abbe value (Vd): 42 - 32 (higher indexes generally result in lower Abbe values)
- Density: 1.3 - 1.5 (g/cm3)
- UV cutoff: 380 nm - 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 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.
[edit] Lens coatings
[edit] 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-relective 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 properties (110° water drop contact angle for Super HiVision™ and 112° for Crizal® Alizé™).
[edit] Ultraviolet protection
A UV coating is used to reduce the transmission of light in the ultraviolet spectrum. UV-B radiation increases the likelyhood of cataracts, while long term exposure to UV-A radiation can damage the retina. 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.
[edit] Scratch resistant
Highly recommended, especially for polycarbonate and softer materials, to make lenses last longer.
