Fundamental changes in the design and manufacture of spectacle lenses are allowing us to offer a level of clarity, visual comfort and personalization that we could only dream of just a few years ago. For this, we can thank the integration of computer technology—from aberrometers that measure wavefront distortion to reduce higher-order aberrations to free-form production that allows us to apply any type of curve to a lens surface.
The market continues to expand with new products at a rate that is nothing short of spectacular, especially compared to the early part of my career 30 years ago. Let’s take a look at some of the new lenses, lens treatments and dispensing and processing equipment that have emerged in the past year.
Progressive Addition Lenses
Varilux Physio Enhanced Azio, Essilor.
• Essilor Varilux Physio Enhanced. After studying the ocular anatomy, facial anatomy and reading behavior of more than 200,000 Asian and Indian subjects, Essilor designed a line of lenses to satisfy their unique visual needs. According to the company, these lenses improve acuity and contrast, offer wider fields of view and provide a more natural transition between visual zones than progressive addition lenses (PALs) that are not specifically designed for Asians and Indians.
The lenses also feature Essilor’s Wavefront Advanced Vision Enhancement technology (W.A.V.E. Technology 2). Because this process accounts for variations in pupil size due to age, lighting conditions, prescription and viewing distance, the lenses provide optimal vision at all light levels with fewer higher-order aberrations. The lenses also employ Essilor’s Dual Optix technology (design elements are on both the front and back surfaces) and are free-form processed.
Physio Enhanced multicultural lenses are available in 1.50, Trivex, polycarbonate, 1.60, 1.67 and 1.74 in clear, Transitions VI and Xperio polarized forms (not every material is available in every form). Non-presbyopes can experience similar benefits with the company’s Azio Single Vision lenses.
-Varilux Physio Enhanced Azio. People of Asian descent are frequently myopic. Because their eyes often have longer axial lengths, the optic axes sweep broader areas of the lenses. As a result, lens designs with wider fields of view are beneficial. Essilor’s research also indicates that the less prominent facial characteristics of Asians impact how they wear their frames—leading to differences in vertex distance, pantoscopic tilt and face form, compared to other groups. Finally, the study revealed that Asians tend to hold reading material closer to their eyes than other populations, leading to increased convergence.
-Varilux Physio Enhanced India. People of Indian descent are often hyperopic. Because their eyes usually have shorter axial lengths, the optic axes sweep narrower areas of the lenses. As a result, peripheral distortion can be disturbing, making softer lens designs advantageous. Essilor’s research indicates that the more prominent facial characteristics of Indians impact how they wear their frames—leading to differences in vertex distance, pantoscopic tilt and face form, compared to other groups. Finally, the study revealed that Indians also tend to hold reading material close to their eyes, leading to increased convergence—the opposite of what would be expected in hyperopes.
Varilux Physio Enhanced India, Essilor.
• Hoya Hoyalux iD MyStyle. The newest additions to Hoya’s iD family of PALs, these lenses are available in two levels of personalization—the iD MyStyle and the iD MyStyle Signature. The MyStyle iDentifier, an interactive consultation and ordering tool found on the company’s website, determines the amount of personalization. It accounts for frame parameters, PD, fitting center heights and position-of-wear measurements, as well as the patient’s visual environment, lifestyle preferences and multifocal history. Results are summarized in a personal vision profile that permits the patient to select from two offerings based on their visual and financial needs.
-Varilux Physio Enhanced Fit. This lens brings some degree of personalization to the Physio Enhanced line of PALs. Essilor reports that only 9% of patients fall within the standard values in their database for position-of-wear measurements (vertex distance, pantoscopic tilt and face form). By including these measurements with the PD and fitting center heights, the company claims this lens offers even better acuity and contrast, wider fields of view and easier transition between visual zones than their regular Physio Enhanced PALs. Essilor recommends using their Visioffice system to take the measurements, but you can take them manually or with another manufacturer’s electronic patient measuring devices (EPMDs).Varilux Physio Enhanced Fit lenses are available in 1.50, Trivex and polycarbonate in clear, Transitions VI and Transitions XTRActive forms (not every material is available in every form).
Non-presbyopes can experience similar benefits with Essilor’s Fit Single Vision lenses.
Hoyalux iD MyStyle, Hoya
The iD MyStyle combines select information from the MyStyle iDentifier with fitting data from thousands of patients in the company’s database; the iD MyStyle Signature offers a greater degree of personalization by incorporating all of the information from the MyStyle iDentifier in the customization. The lenses are available in three corridor lengths: 11mm, 14mm and 16mm. Like other iD lenses, the add power is split between the front and back surfaces to reduce distortion and is processed using Hoya’s iD FreeForm Design Technology. iD MyStyle lenses are available in 1.50, Trivex, 1.60 and 1.67 in clear and Suntech photochromic forms.
• ChromaGen Vision ChromaGen Lenses. ChromaGen Vision recently released its second generation of ChromaGen lenses, with a reference guide that instructs O.D.s on how to choose the proper colors for specific disorders. The company has expanded the fitting kit from eight to 16 different colored filters (although they appear neutral gray when worn) and has tapped Pech Optical to manufacture the lenses in the United States. Lenses are available in CR-39 and Trivex in any type of single vision, segmented multifocal or PAL they handle. ChromaGen filters are also available in hydrogel contact lenses.
-Lenses for Color Deficiencies. Approximately 10% of all men and 0.5% of all women have some type of color deficiency, a decreased ability to distinguish the differences between certain colors.1 These people have a hereditary defect that causes the pigment in their retinal cones to have an altered spectral sensitivity, which makes it difficult for them to distinguish red from green and blue from yellow. David Harris, Ph.D., the inventor of ChromaGen lenses, found that when a different colored filter is placed over each of a patient’s eyes, it creates a luminous disparity that produces a cortical rivalry, which aids color discrimination. While not a cure, these lenses can still enhance quality of life and enable some people to qualify for previously inaccessible jobs, such as police work or telephone repair.
-Lenses for Dyslexia. Dyslexia is thought to have a visual component because abnormalities in the magnocellular visual pathway, which controls motion and depth perception, can cause people to experience blurring (independent of visual correction), page shimmering, letter or word movement, word reversal and interference from the spaces between words. After years of working with dyslexics, Dr. Harris and his colleague, Chaaban Zeidan, M.Sc., further developed ChromaGen lenses to treat dyslexia. They found that using a different colored filter over each eye alters the speed at which information is processed in the brain, reducing the symptoms of visual dyslexia. In a study of 75 dyslexic ChromaGen-wearing subjects at the University of Liverpool, researchers noted a 35% improvement in reading speed and accuracy using the Wilkins Rate of Reading Test.2
• Rodenstock Canada Manufaktur. The lenses in this collection are individually made for people with extreme ametropias or accommodative and convergence disorders, and for those who have specific vocational and avocational needs. Depending upon the prescription and lens type, they can be traditionally surfaced, free-form processed or even made by hand.
-Single Vision. All minus lenses are aspheric lenticular in design, available in Lentilux 1.70 glass, and come in sphere powers between -6.00 and -24.00 and cylinder powers up to 6.00. Plus lenses come in two forms. The first is an aspheric lenticular design, available in Perfastar 1.50 plastic, and comes in sphere powers between +8.00 and +22.00 and cylinder powers up to 6.00. The second is a lenticular-only design, available in Starlenti 1.50 plastic, and comes in sphere powers between +6.25 and +26.00 and cylinder powers up to 6.00. The lenses are also available with prism power up to 25.00.
-Occupational Segmented Multifocals. There has been a dwindling number of occupational segmented multifocals from which to choose. This should come as no surprise, as manufacturers have had little incentive to continue to produce or modernize them because O.D.s rarely recommend them. However, they can be a source of additional income if properly marketed.
To address this situation, the Manufaktur line includes segmented bifocals and trifocals that are designed for people whose occupation or hobby requires that their near or intermediate be placed in non-traditional areas of a lens. The Ardis Special Reverse bifocal resembles the Rede-Rite bifocal of my youth (an upside-down Ultex A), with the segment at the top for distance viewing and the major portion of the lens for near viewing. The Ardis Special Trifocal has dual Ultex A-like segments. The FZN has distance at the top, intermediate in the middle and near at the bottom. The ZFN has intermediate at the top, distance in the middle and near at the bottom.
The NFN has near at the top, distance in the middle and near at the bottom. All lenses are available in 1.50 glass in sphere powers between -10.00 and +10.00, in cylinder powers up to 6.00 and in add powers between +0.50 and +7.00. Prism can be incorporated at both distance and near. These lenses are well suited for auto mechanics, carpenters, electricians, librarians, pharmacists, pilots, plumbers and shoe sales people. Because they’re only made in glass, caution should be exercised when recommending them for some occupations.
-Lenses for Scuba Diving. Rodenstock has recognized the needs of the growing number of scuba divers by making lenses that can be mounted in diving masks. These plano concave and plano convex lenses are available in 1.50 or 1.70 glass, come in sphere powers between +10.00 and -10.00 and cylinder powers up to 6.00 and are vertex distance compensated.
-Lenses for Collector Eyewear. Rodenstock has also identified a small group of people who want to read with lorgnettes, opera glasses and monocles. Their unusual shapes and small sizes, as well as their intricate mounting systems, require highly individualized lenses. For these aesthetically conscious people, the company now makes modern lenses for these old-fashioned eyewear options.
|Shamir Relax |
Relax addresses these problems by incorporating an add power of 0.65D in the lower lens area. Shamir claims this reduces accommodative effort by as much as 30%. The lens was designed using the company’s EyePoint Technology and is free-form processed with its Prescriptor software and Direct Lens Technology. It’s available in 1.50, polycarbonate and 1.60 in clear, Transitions and polarized forms (1.50 isn’t available polarized). Although Shamir markets Relax as a single-vision lens, it’s obviously not and requires a minimum fitting height of 16mm.
• Seiko Sportswear Transitions SOLFX. This is the latest member of Transitions Optical’s SOLFX line of photochromic lenses. These lenses are designed to enhance outdoor visual performance by improving color recognition, contrast and depth perception in a variety of lighting conditions. Although it can be used for many activities, Sportwear’s G-15-like color is ideal for walking, running and hiking, Seiko says. It’s available in single-vision aspheric, Perfas-brand PALs and in Seiko’s new Wrap Tech free-form lens and frame packages (in 6- and 8-base curves). Single-vision lenses are available in 1.67, PALs in Trivex, polycarbonate and Wrap Tech lenses in 1.67.
|LifeRx FSV, Vision-Ease |
|ChangeRx, Vision-Ease |
• Carl Zeiss PhotoFusion. In 2010, Carl Zeiss Vision commissioned a market study in Australia, Brazil, China, Germany, Great Britain, Italy and the United States to determine what consumers wanted from photochromic lenses. The 5,896 participants ranked darkness, rapid activation and fade time at the top of their wish lists. The company created a new photochromic process to meet these desires. Zeiss likens its photochromic molecules to soccer balls—in its inactivated state, each molecule resembles a collection of geodesics that intersect to form the pentangular shapes so familiar on a soccer ball.
|PhotoFusion, Carl Zeiss Vision |
Materials and Coatings
• Shanghai Conant Optics Hi-Vex. Distributed in the United Statesby Conant Lens Inc., this lens has an index of refraction of 1.56, which makes it thinner than CR-39 (1.498) and Trivex (1.53), but thicker than polycarbonate (1.59). Its specific gravity is 1.25, so it’s lighter than CR-39 (1.32), but heavier than polycarbonate (1.20) and Trivex (1.11). It has an Abbe value of 46, which means it has less longitudinal chromatic aberration than polycarbonate (31) and Trivex (44), but more than CR-39 (58). Like polycarbonate and Trivex, Hi-Vex meets the requirements of ANSI’s Z87.1 High Velocity Impact Test, making it an appropriate choice for children’s, sports and safety eyewear.
Hi-Vex is also a good option for rimless mountings because the microscopic cracks introduced by drilling won’t cause the holes to expand, which happens in other materials and can lead to lenses loosening and frame misalignment. It shares this advantage with Trivex and some other high-index lens materials, notably 1.67 and 1.70. In addition, the company claims the material is easier to surface and polish than polycarbonate or Trivex. Hi-Vex is available in finished and semi-finished single-vision lenses, a FT 28 bifocal and a short-corridor PAL. Shanghai plans to introduce a polarized lens later this year or in early 2012.
|Mirror Coatings, Ice-Tech Advanced Lens Technologies |
• Safilo Carrera X-Cede. Available exclusively through authorized dealers, this new collection of plano and prescription sunglasses features an innovative non-film polarization technology. Microscopic nanotechnology receptors embedded in the lenses block reflected glare, while simultaneously admitting those light wavelengths that enhance color. Because the polarization and color enhancement elements aremolecularly fused to the lenses, delamination is eliminated.
|Carrera X-Cede, Safilo |
• VedaloHD VedaloRx. After four years of development, VedaloHD recently released VedaloRx prescription sunwear. Although targeted at golfers, they’re suitable for any outdoor activity. VedaloHD’s research demonstrates that the brain has difficulty rapidly defining color and contrast because of areas of overlap and gaps between chromatic receptors. The use of special equalizing filters can create a greater balance. Designed to enhance acuity, depth perception, and color recognition and contrast, VedaloRx lenses utilize the same proprietary tri-stimulus filtering process (HDL-3C technology) found in VedaloHD plano sunglasses. Unlike traditional tinted or coated lenses, the color elements are infused into the lenses in a patented process called “mass tinting.”
Shamir Optical makes VedaloRx lenses exclusively for VedaloHD. They’re available in NXT in single vision and the Autograph II Attitude PAL (both of which are free-form processed), and come in copper-rose and smoke colors in fixed and Varia photochromic forms. The company just expanded its frame selection from eight to 12, with the addition of the new Stritanium collection. These lightweight 18g frames are made of surgical-grade stainless steel infused with titanium. Green may become an option in the future (it’s available in Europe), and a polarized version is currently being tested.
Dispensing and Processing Equipment
• Hoya Vision Care and Optikam Tech Optikam. The two companies recently announced a partnership to benefit patients of independent eye care practitioners in Canada by cross marketing Optikam Tech’s Optikam EPMD with Hoya’s lens technology.
Like other EPMDs, Optikam can take PDs, fitting center heights and position-of-wear measurements, as well as assist patients in selecting frames, lenses and lens treatments. However, only Hoya’s educational materials, lenses and lens treatments are used in this application. The companies hope that the synergy will enhance the patient’s purchasing experience, giving eye care practitioners an edge in today’s competitive market.
• QSpex Premium Lens System. When in-office lens casting systems were first introduced in the 1980s, it was thought that they would have strong market appeal because they could offer O.D.s greater control of the production process at a lower cost.
|QSpex Premium Lens System |
All treatments are applied to the inside surface of each mold, becoming part of the lens during the polymerization process so they can’t crack, craze or peel. Unlike some earlier systems, the QSpex Premium Lens System employs plastic molds that are disposed of after each use. All lenses are made with the proprietary AQuity155 resin, which has an index of refraction of 1.547, a specific gravity of 1.15 and an Abbe value of 40, which blocks 100% of UV light. They’re available in sphere powers between +2.00 and -4.25, in cylinder powers up to 2.00 and in add powers between +1.00 and +3.00. QSpex claims these powers cover about 84% of all prescriptions and estimates 30% to 50% in savings when compared to traditional and free-form lens manufacturing methods.
• Rodenstock Lens Consultation iPad App. Rodenstock heralds this no-charge iPad app as the first of its kind in the industry. It enables O.D.s to provide their patients with a demonstration (including 3-D animation) of the differences between some of their products. With a total of five modules, the application is divided into two areas—one that requires registration and one that doesn’t. For example, in the Professional Consulting module, patients can be shown the differences in fields of view between one of Rodenstock’s traditional PALs and one of its new personalized PALs. This is made all the more impressive because the O.D. can input a patient’s PD and position-of-wear data for a highly individualized demonstration.
Dr. Patrick is assistant professor of optometry at Nova Southeastern University College of Optometry in Ft. Lauderdale, Fla.
1. Haegerström-Portnoy G. Color Vision. In: Rosenbloom AA, Morgan MW (eds.). Principles and Practice of Pediatric Optometry. Philadelphia: JB Lippincott; 1990:449-466.
2. Harris D, Downes J, Latto R. The effect of a new system of haploscopic coloured filters on rate of reading and visual fatigue in dyslexics.
A corrective lens is a lens typically worn in front of the eye to improve vision. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye. Intraocular lenses are surgically implanted most commonly after cataract removal, but can be used for purely refractive purposes.
Prescription of corrective lenses
Main article: Eyeglass prescription
Corrective lenses are typically prescribed by an ophthalmologist or an optometrist. 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). The use of improper corrective lenses may not be helpful and can even exacerbate binocular vision disorders. 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.
Components of a sphero-cylindrical correction
Every corrective lens prescription includes a spherical correction in diopters. Convergent powers are positive (e.g., +4.00 D) and condense light to correct for farsightedness (hyperopia) or allow the patient to read more comfortably (see presbyopia and binocular vision disorders). Divergent powers are negative (e.g., −3.75 D) and spread out light to correct for nearsightedness (myopia). If neither convergence nor divergence is required in the prescription, "plano" is used to denote a refractive power of zero.
The term "sphere" comes from the geometry of lenses. Lenses derive their power from curved surfaces. A spherical lens has the same curvature in every direction perpendicular to the optical axis. Spherical lenses are adequate correction when a person has no astigmatism. To correct for astigmatism, the "cylinder" and "axis" components specify how a particular lens is different from a lens composed of purely spherical surfaces.
Patients with astigmatism need a toric lens to see clearly. The geometry of a toric lens focuses light differently in different meridians. A meridian in this case is a plane that is perpendicular to the optical axis. For example, a toric lens, when rotated correctly, could focus an object to the image of a horizontal line at one focal distance while focusing a vertical line to a separate focal distance.
The power of a toric lens can be specified by describing how the cylinder (the meridian that is most different from the spherical power) differs from the spherical power. Power evenly transitions between the two powers in moving from the meridian with the most convergence to the meridian with the least convergence. For regular toric lenses, these powers are perpendicular to each other and their location relative to vertical and horizontal are specified by the axis component.
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 spherical power describes the most divergent meridian and the cylindrical component describes the most convergent. In the minus cylinder notation, the cylinder power is a number of diopters more divergent than the sphere component. In this convention, the sphere power describes the most convergent meridian and the cylinder component describes the most divergent. Europe typically follows the plus cylinder convention while in the US the minus cylinder notation is used by optometrists and the plus cylinder notation is used by ophthalmologists. Minus cylinder notation is also more 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:
- Add the sphere and cylinder numbers together to produce the converted sphere
- Invert the sign of cylinder value
- Add 90° to axis value, and if the new axis value exceeds 180°, subtract 180° from the result
For example, a lens with a vertical power of -3.75 and a horizontal power of -2.25 could be specified as either -2.25 -1.50 x 180 or -3.75 +1.50 x 090.
The axis defines the location of the sphere and cylinder powers. The name "axis" comes from the concept of generating a cylinder by rotating a line around an axis. The curve of that cylinder is 90° from that axis of rotation. When dealing with toric lenses, the axis defines the orientation of the steepest and flattest curvatures relative to horizontal and vertcal. The "3 o'clock" position is defined as zero, and the 90th meridian is a vertical line. A horizontal line passes through both zero and the 180th meridians. By convention, a horizontal axis is recorded as 180.
In a regular toric lens, the flattest and steepest curvatures are separated by 90°. As a result, the axis of the cylinder is also the meridian with the same power as the recorded sphere power. The cylinder power, as defined above is the power that is most different from the sphere power. Because they are defined relative to each other, it is important to know if the lens is being described in minus cylinder notation, where the sphere power is the most convergent / least divergent power. When using plus cylinder notation, the opposite is true.
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 sphere power (e.g., −3.00 D.S.). This verifies that the prescription is truly spherical rather than the cylinder power being omitted in error.
- correction power is measured in diopters
- by convention, an axis of 90° is vertical, 0° or 180° are horizontal
- if the cylinder power is positive, the lens is most convergent 90° from the axis
- if the cylinder power is negative, the lens is most divergent 90° from the axis
- if the cylinder power is zero, the lens is spherical and has the same power in every meridian
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.
Over the counter correction
Ready-made single-vision reading glasses go by many names, including over-the-counter glasses, ready readers, cheaters, magnifiers, non-prescription readers, or generic readers. They are designed to lessen the focusing burden of near work, such as reading. They are typically sold in retail locations such as pharmacies and grocery stores, but are also available in book stores and clothing retailers. They are available in common reading prescriptions with strengths ranging from +0.75 to +3.50 diopters. While these "magnifiers" do indeed make the image of the viewed object bigger, their main advantage comes from focusing the image, not magnification.
These glasses are not tailored to a person's individual needs. A difference in refractive error between the eyes or presence of astigmatism will not be accounted for. People with little to no need for correction in the distance may find off-the-shelf glasses work quite well for seeing better during near vision tasks. But if the person has a significant need for distance correction, it is less likely that the over the counter glasses will be perfectly effective. Although such glasses are generally considered safe, an individual prescription, as determined by an ophthalmologist or optometrist and made by a qualified optician, usually results in better visual correction and fewer headaches and visual discomfort. Another criticism of over the counter glasses is that they may alleviate symptoms, causing a person to forgo the other benefits of routine vision exams, such as early diagnosis of chronic disease.
Although lenses are normally prescribed by optometrists or ophthalmologists, there is evidence from developing countries that allowing 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.
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 correction for near distances, or else use a multifocal lens (see below).
Reading glasses are single vision lenses designed for near work, and include over the counter glasses. They come in two main styles: full frames, in which the entire lens is made in the reading prescription, and half-eyes, style glasses that sit lower down on the nose. Full frame readers must be removed to see distance clearly, while the distance can be clearly viewed over the top of half-eye readers.
A bifocal is a lens with two sections, separated by a line (see image to the right). Generally, the upper part of the lens is used for distance vision, while the lower segment is used for near vision. The area of the lens that caters to near vision is called the add segment. There are many different shapes, sizes, and positions for the add segment that are selected for functional differences as well as the visual demands of the patient. Bifocals allow people with presbyopia to see clearly at distance and near without having to remove the glasses, which would be required with single vision correction.
Trifocal lenses are similar to bifocals, except that the two focal areas are separated by a third area (with intermediate focus correction) in the middle. This segment corrects the wearer's vision for intermediate distances roughly at arms' length, e.g. computer distance. This lens type has two segment lines, dividing the three different correcting segments.
Progressive addition 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). The lack of any abrupt change in power and the uniform appearance of the lens gives rise to the name "no-line bifocal".
Multifocal contact lenses (e.g. bifocals or progressives) are comparable to spectacles with bifocals or progressive lenses because they have multiple focal points. Multifocal contact lenses are typically designed for constant viewing through the center of the lens, but some designs do incorporate a shift in lens position to view through the reading power (similar to bifocal glasses).
The power or focal length of adjustable or variable focus can be changed to suit the needs of the wearer. A typical application of such a lens is to refocus the correction allowing clear vision at any distance. Unlike with bifocals, near-vision correction is achieved over the entire field of view, in any direction. Switching between distance and near vision is accomplished by re-adjusting the lens, instead of by tilting and/or rotating the head. The need for constant adjustment when the person's attention switches to an object at a different distance is a design challenge of such a lens. Manual adjustment is more cumbersome than bifocals or similar lenses. Automated systems require electronic systems, power supplies, and sensors that increase the cost, size, and weight of the correction.
A corrective lens with a power of zero is called a plano lens. These lenses are used when one or both eyes do not require correction of a refractive error. 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.
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 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/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.
Vertex distance is the space between the front of the eye and the back surface of the lens. In glasses with powers beyond ±4.00D, the vertex distance can affect the effective power of the glasses. A shorter vertex distance can expand the field of view, but if the vertex distance is too small, the eyelashes will come into contact with the back of the lens, smudging the lens and causing annoyance for the patient. A skilled frame stylist will help the patient select a good balance of fashionable frame size with good vertex distance in order to achieve ideal aesthetics and field of view. The average vertex distance in a pair of glasses is 12-14mm. A contact lens is placed directly on the eye and thus has a vertex distance of zero.
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 < 1.60
- High-index: 1.60 ≤ nd < 1.74
- Very high index: 1.76 ≤ nd
This is a general classification. Indexes of nd 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
Disadvantages of increased indices
- Lower Abbe number, meaning, amongst other things, increased chromatic aberration.
- Poorer light transmission and increased backside and inner-surface reflections (see Fresnel reflection equation), increasing the importance of anti-reflective coating.
- 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).
Main article: 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 (beyond ±4.00D). 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 (Vd≤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 (Vd≤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 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. 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 ball-drop impact test, and depending on needed index these seem to currently have"‘best in class" Abbe vs Index (Nd): Glass (2x weight of plastics) or CR-39 (2 mm vs. 1.5 mm thickness typical on newer materials) 58 @ 1.5, Sola Spectralite (firstname.lastname@example.org), Sola Finalite (email@example.com), 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 (Vd=30-32) is very dispersive, but has excellent shatter resistance. Trivex (Vd=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 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.
Lens induced oblique astigmatism
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, although chromatic aberration is often perceived as "blurry vision" in the lens periphery and gives the impression of power error, 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 sometimes 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.
Cosmetics and weight
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 50 mm (2.0 in) across. There are a few adult sizes of 40 mm (1.6 in), and although they are quite rare, can reduce lens weight to about half of the 50 mm versions. 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.
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 corrective lenses, in approximate order of importance are these:
- Choose glasses 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 his or her 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 due to some 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)
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 incorrect numbers for UVA and UVB transmission, as well as other related product type issues.
Glass lenses have become less common owing 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 g/cm³), which is commonly used in telescopes and binoculars, and fluoritecrown glasses such as Schott N-FK51A (nd=1.48656 / Vd=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, 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.).
Plastic lenses are currently the most commonly prescribed lens, owing to their relative safety, low cost, ease of production, and high 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 an exception in that they have inherent scratch resistance.
Trivex was developed in 2001 by PPG Industries for the military as transparent armor. With Hoya Corporation and Younger Optics PPG announced the availability of Trivex for the optical industry in 2001. Trivex is a urethane based pre-polymer. PPG named the material Trivex because of its three main performance properties, superior optics, ultra light weight, and extreme strength.
Trivex is a relative newcomer that possesses 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 may result in slightly thicker lenses depending upon the prescription. Along with polycarbonate and the various high-index plastics, Trivex is a lab favorite for use in rimless frames, owing to the ease with which it can be drilled and its resistance to cracking around the drill holes. One other advantage that Trivex has over polycarbonate is that it can be tinted.
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)
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. A disadvantage is that high-index plastic lenses suffer from a much higher level of chromatic aberrations, which can be seen from their lower Abbe value. 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 particularly suitable for rimless eyeglasses.
These high-refractive-index plastics are typically thiourethanes, with the sulfur atoms in the polymer being responsible for the high refractive index. The sulfur content can be up to 60 percent by weight for an n=1.74 material.
Ophthalmic material property tables
|Material, Plastic||Index (Nd)||Abbe (Vd)||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 Ormix||1.6 ||41||1.3||100% / 100%|
|Polycarbonate||1.586||30||1.20||100% / 100%||10.27||?/1.0||Tegra (Vision-Ease) Airwear (Essilor)|
|MR-8 1.6 Plastic||1.6 ||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 1.665 Plastic||1.665||32||1.35||100% / 100%||?/1.2||Daemyung Optical (Ramia)|
|MR-7 1.67 Plastic||1.67 ||32||1.35||100% / 100%||12.26|
|MR-10 1.67 Plastic||1.67 ||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||1.74 ||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 (Nd)||ABBE (Vd)||Specific Gravity||UVB/ UVA||Reflected light (%)||Minimum thickness typ/min (mm)||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||VisionEase, X-Cel|
|1.7 Glass||1.706||30||2.93||100% / 76%||13.47||X-Cel, VisionEase, Phillips|
|1.8 Glass||1.800||25||3.37||100% / 81%||16.47||X-Cell, Phillips, VisionEase, Zhong Chuan Optical (China)|
|1.9 Glass||1.893||31||4.02||100% / 76%||18.85||Zhong Chuan Optical(China)|
not FDA-approved for sale in USA
Reflected light calculated using Fresnel reflection equation for normal waves against air on two interfaces. This is reflection without an AR coating.
Indices of refraction for a range of materials can be found in the list of refractive indices.
Main article: Optical coating
Main article: Anti-reflective coating
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 UV 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 116° for Crizal Alizé UV).
A UV coating is used to reduce the transmission of light in the ultraviolet spectrum. UV-B radiation increases the likelihood 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; they have UV-cutoff wavelengths just outside the visible range, and do not benefit from the application of a UV coating. Many modern anti-reflective coatings also block UV.
Resists damage to lens surfaces from minor scratches.
Confusing corrective lens industry terminology
Spheric vs. aspheric, atoric, etc.
Lens manufacturers claim that aspheric lenses 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".[verification needed] This qualification is necessary since best-form spherics are always better than aspherics for an ophthalmic lens application. Aspherics are only[verification needed] used for corrective lenses when, in order to achieve a flatter lens for cosmetic reasons, the lens design deviates from best-form sphere; this results in degradation of the visual correction, degradation which can, in some part, be compensated for by an aspheric design. The same is true for atoric and bi-aspheric.
While it is true that aspheric lenses are used in cameras and binoculars, it would be wrong to assume that this means aspherics/atorics result in better optics for eyewear. Cameras and telescopes use multiple lens elements and have different design criteria. Spectacles are made of only one ophthalmic lens, and 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.
It is worth noting that aspheric lenses 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 whether the mathematical and theoretical benefits of aspheric lenses can be implemented in practice in a way that results in 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 of the eye: People prescribed a sphere and a cylinder prescription have astigmatism of the eye and can be given a toric lens to correct it.