Optical Repair Glossary for Prescription Glasses and Sunglasses

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A divergence from the normal; the formation of a blurred image because of physical defects in the eye or lens.
Achromatic Lens:
A lens that combines a plus lens and a minus lens to decrease chromatic aberration.
One of the first synthetic fibers, common material used in eyeglass frames and parts.
Addition (Add Power):
The difference in vertex power between the reading or intermediate portion of a multifocal lens and its distance portion, also referred to as add power.
Lessened vision in an eye with no physical damage; not fixable by optical devices. Also called "Lazy Eye", reduce vision because of lack of use of one eye.
Attributed to anisometropia, which when correcting this condition it creates a difference in magnification of object between eyes. This is caused by a difference between the magnification or de-magnification between corrective lenses, creating a discrepancy.
Uneven refractive powers in the two eyes. Usually hereditary, but may also be attributed to cataracts and/or trauma. Patients with this condition usually refer to contact lenses, which move with the eyes limiting the prismatic effect that may be caused by a spectacle lens.
A condition where one eye is hyperopic and the other eye is myopic. A form of anisometropia.
Anti-Reflection Coating:
Single or multi-layered application of magnesium fluoride to lens to lessen reflected light. (see our Anti-reflective coatings page for an in depth look at AR coatings)
The vertex of a pyramid, cone or angle.
A lens that departs a little from a fixed radius of curvature and consequently, spherical aberrations are reduced.
Aspheric Design Hi-Index for Plus Lenses:
Flatten the base of higher plus prescriptions hence reducing the magnified appearance of the eyes (bug-eyed effect); creating an aesthetically pleasing lens.
Astigmatic Keratotomy (AK):
is a surgical procedure performed to correct astigmatism
Irregularly shaped cornea causes light images to focus on two points in the eye, blurring images
Automated Lamellar Keratectomy (ALK):
is a relatively new procedure performed for extremely nearsighted patients. In this procedure, a flap of corneal surface tissue is folded back, a layer of the central corneal tissue is removed, and the surface flap is replaced.
AVN ( anti-visual noise):
A chemically treated lens that helps to reduce eyestrain, headache, blurred vision, color perception changes, and/or dry itching burning irritated eyes related with VDT (Video Display Terminals) use.
An imaginary straight line passing through a body with respect to symmetry of the body or about which the body rotates or corresponding to a diametric dimension of the body.
Balance Lens:
Usually requested for patients with little or no vision with one eye. A balance lens is created to suit cosmetic appearance by creating a lens for the impaired eye with a similar thickness and appearance of the receptive eye.
Thickest point of a prism.
Base Curve:
Dictates the surface power for a side of a lens (usually the front) into which a range of prescriptions can be surfaced generating the desired lens power. The higher the plus power the higher the base curve. The higher the minus power the flatter the base curve.
Lens with two concave surfaces.
Lens with two convex surfaces.
Bifocal/multifocal contact lenses:
Contact lens with two areas to look through with differing prescriptions for seeing near and far objects.
Bifocal glasses:
Spectacles with two viewing zones, one on top and one on the bottom, separated by a visible line, for seeing both near and far objects. Invented in 1784 by Benjamin Franklin.
The usage of both eyes at the same time, so that each retinal perception for so that each retinal perception adds to the end image.
Combination of two prescriptions into a single lens enabling observer to see different distances).
Tool to find thickness, diameter, caliber and distance between surfaces of lenses.
Carbon Fiber:
A very strong and lightweight synthetic fiber made especially by carbonizing acrylic fiber at high temperatures. Used in making eyeglass frames and parts.
Partial or full loss of transparency of the crystalline lens within the eye. Cataracts usually have to mature before being treated.
Center Of Rotation:
Point around which the human eye was thought to rotate; not identifiable because of several variables.
Chromatic Aberration:
Uneven refraction of various wavelengths of light creating colored fringes around images.
Color blindness:
Congenital or acquired impairment of color distinction. This term is most commonly used when referring to color vision deficiency. Patients affected with this deficiency have the ability to see color, but have trouble distinguishing between colors.
A spherical aberration causing a comet shaped blur; created from the rays of an off axis point of light in the object plane.
Computer lenses:
Near or mid-range variable focus lenses that are specifically designed for frequent computer work.
Contact lens:
A thin plastic lens designed to fit over the cornea, usually for the correction of refractive error.
An inward movement of both eyes simultaneously; caused by the rectus muscles pulling both eyes toward each other.
Transparent surface that covers the pupil and iris; gives eye most optical power.
Crystalline lens:
Natural lens of the eye that is normally transparent and resilient; Held behind the pupil by fibers.
Cutting line:
"Archaic term" The new term is Horizontal Lens Bisetor HLB. Straight line going through the geometric center of spectacle lens in the axis of the Cartesian coordinate system.
Solid mass formed into the shape of a column/cylinder. A cylinder shaped form is ground into a lens most commonly used to help with the correction of astigmatism.
Daily wear contact lenses:
Contact lenses designed to be worn only during waking hours; removed, disinfected and stored for the next day's use.
The horizontal measurement in millimeters between two lenses in a pair of glasses.
The act of moving an ophthalmic lens from it"s centered position.
Measurement of the transparency of a medium.
Foreign matter, protein, mucus, lipids or other substances built up onto the surface of lens material.
Depth perception:
Impression of differing distances of objects from the viewer.
Direction of a ray of light changing from its expected path; Misalignment of one or both eyes due to muscular imbalance.
The spreading of light by irregular reflection at a surface or within an optical medium.
Optical unit of measurement used to represent the strength of a lens; the reciprocal of the secondary focal length in meters.
Disinfecting solution:
Substance that kills bacteria on the surface of contact lenses.
The splitting of white light into the colors that make it up.
Disposable contact lenses:
Contact lens that is used for one day to one week and then discarded.
Aberration that causes straight lines to be perceived as curved.
Movement of eyes away from each other, causing the lines of sight to meet behind the eyes; Light rays fanning out from a common source.
Edged Lenses:
Lenses that have been shaped around their periphery to fit a specific frame.
"Normal Vision;" Primary focal point of eye is in the retinal plane.
Equithin (cosmetic prism)::
Usually helpful on high add and hyperopia prescriptions that are a one piece multifocal lenses (ex/ e-style and progressives). This process creates an equal amount of base down prism to create a matching thickness on top and bottom of the lenses.
Extended wear contact lenses:
Contact lenses made to be kept in the patient's eyes twenty-four hours a day for periods of one to seven days.
Far Point:
The point in space where the retina, at a relaxed state, is focused on a figure.
See Hyperopia
Figure 8 Lining:
Lining used on top portion of a semi rimless frame (rimblon frame). It is used as an interlining that fits within the groove of the lens. Called figure 8 for its profile appearance with top side of the 8 fitting within the frame and the bottom portion of the 8 fitting into the lens itself.
A precise place at which light rays converge or diverge when entering or exiting an optic system; To change the components of an optical system to gain clear, focused imagery.
Frame Elbow:
Joint of frame that brings together the temple and the frame itself.
Frequent & planned replacement contact lenses:
Contact lens routines in which lenses are replaced on a predetermined schedule.
Geometric Center:
Intersecting points of diagonals unifying opposite corners of a box and/or the vertical and horizontal lens bisectors within the boxing system, which distinguish a lens shape.
Glass Lenses:
Excellent optics and high scratch-resistance, but heavy in weight and high risk of breaking and possibly hurting the eye.
A group of eye diseases that cause unusually high pressure within the eye and consequently, damage the optic nerve; Loss of eye sight can occur.
High Index Lenses (1.60, 1.66 and 1.67):
Excellent for strong prescriptions because thinner and lighter than standard lenses; the higher the index of refraction, the thinner the lens and denser the material.
Mechanism that connects eyeglass front and temple, allowing swinging motion between them; Built of interlocking barrels.
"Farsightedness;" Defect in the eye causing rays of light from a distant source to hit the retina before coming into focus.
Index Of Refraction:
The ratio of the velocity of light in air to the velocity in a given medium; capacity of a lens material to refract a ray of light of a specified wavelength; higher index equals more refractive power.
In optical science, the term "infinity" is used to denote a distance long enough that rays of light from it appear to be parallel.
Distant Intraocular Distance in millimeters minus Near Intraocular Distance in millimeters and then divide the result by two.
Intraocular lens (IOL):
Manufactured plastic lens that is inserted behind the cornea or iris in place of the crystalline lens in cataract surgery.
Circular, pigmented membrane that surrounds the eye"s pupil; Muscles that change pupil size to control the quantity of light entering the eye.
Keratoconus is a greek based word that translates as "Kerato" = cornea and "Conus" = cone, hence a cone shaped cornea. It is clinically defined as a noninflammatory ocular condition causing a conical protrusion of the center of the cornea. This condition usually affects both eyes.
Tool used to make spectacle and contact lenses; Various bases to grind or polish lenses on, to create refractive or reflective surfaces.
Laser-assisted in situ keratomileusis; Type of laser eye surgery that changes the shape of the cornea to improve clarity of sight.
Lenticular Lens:
Also referred to as the fried egg lens; this lens consist of a small round convex lens that is fused on to a carrier lens that fills the eyeglass frame, hence giving a fried egg resemblance. The lenticular lens is used to help correct severe hyperopia patients by condensing the prescription, usually 12 diopters or higher, into a small convex lens ranging between 20mm to 40mm which is then fused onto a carrier lens. However, with technology evolving there are other lenses available like the aspheric lens to accommodate a more ascetically pleasing lens appearance.
Often used when lens is a fraction small for a frame. A lining is applied serving the purpose of a cushion/filler between the lens and the frame creating a snug fit.
Low Vision:
Ranges from slightly blurred vision to blindness; Cannot be fixed by eyeglasses, contact lenses, surgery or drugs.
Macular Degeneration:
Poor vision and eventually blindness resulting from the degeneration of the cells of the macula lutea; usually affects both eyes; More common in the elderly; build up of yellow deposits in, thinning of and scarring of the macula lutea.
Mechanical Center:
The calculated center on a spectacle lens that is to be cut/edged.
An imaginary line(s) on a sphere like surface that intersect a perpendicular axis point. Meridians may be used to help graph the anatomy of a lens, the eye or any other spherical body.
Made up of one color of light.
Pertaining to vision with one eye.
A corrective lens composed of a concave lens fused on to the outer surface of a carrier lens also referenced as a minus lenticular lens. This lens is usually used in the correction of high myopia low vision patients of 12 diopters or higher to avoid aggressively thick lenses. The prescriptions are usually condensed to a 20mm to 30mm in diameter concave lens, but have been known to be customized in some instances, and then fused on to a much thinner carrier lens that fits the shape of the eyeglass frame. These lenses have also been offered as a blended myodisc, which blends the fusion from the concave lens to the carrier lens creating a smoother transition and cleaner looking lens. However, these lenses may create a visual effect commonly referred to as scotoma, which is a ring of distorted visual acuity where the edge of the concave lens and the carrier lens meet, hence limiting the clear visual acuity to a smaller portion in the center of the concave lens.
"Nearsightedness;" Problem in patients who can see things close-up, but not far away; Light from objects being viewed does not focus on the retina, but in front of it.
The combining of plus with minus or minus with plus lenses so as to produce a combination without power, thus determining the power of the examined lens.
Nose Bridge:
The center piece of the frame which connects the right and left sides of the frame. Often holds nose pad arms, saddle bridge, other buffers, or are void leaving the nose bridge itself as the buffer.
Nose Pad:
A pad made of acetate, silicon, titanium metal, or other materials used as buffer between frame and the nose. Some styles include primaddona, snap-in, screw-in, saddle bridge, strap bridge amongst others.
Nose Pad Arm:
A small arm stemming from the frame used as crutch to hold nose pads. Some styles include primaddona, snap-in, screw-in, strap bridge amongst others.
Ophthalmic terminology. The direction of any meridian other than 90 degrees (vertical) and 180 degrees (horizontal).
Oculus Dexter, Latin for right eye.
Impermeable to light; Not transparent or translucent
Pertaining to the eye.
Medical doctor concentrating on the medical, optical and surgical care of the eye.
Optical Center:
Point on the optical axis where the lens does not have prism power; An intersection point on the optical axis of a lens; Usually the thinnest point on a minus lens and the thickest point on a plus lens.
A paramedical professional who reads eyeglass prescriptions, orders lenses and sells eyeglasses and contact lenses to consumers.
Optometrist (OD):
State-licensed doctor who diagnoses and treats eye problems; Prescribes glasses, contact lenses and medicines for eye health.
A cast-molded epoxy plastic frequently used by high-end designer frame lines. This material is sensitive to heat, but has a memory that allows it to reconfigure to its original shape if overheated. Considered a very durable and anti-allergic material. Ideal for patients sensitive skin reactions.
Ocular sinister, Latin for left eye.
A lens that requires a lens blank larger than 70mm, which is usually considered a standard lens blank diameter.
Oxygen permeability:
The quantity of oxygen diffusing through a specified amount of lens material in a specified quantity of time, under given testing conditions.
Progressive Addition Lens; Lens in which the prescription strength changes continuously, providing correction for more than one seeing range. see progressive lenses.
The seeming movement of an object when the viewing eye is moved or when the viewer looks alternately with one eye and then the other.
Lens that changes color depending on the quantity of light; More light makes the lens darker and less light makes the lens lighter Also known as transitions.
Photorefractive Keratotomy (PRK):
Changing the shape of the cornea with a computer powered laser to improve eyesight; flattens cornea to fix myopia and astigmatism.
Surface where a straight line joining any two points on the surface lies entirely on the surface; Flat.
Plastic or CR-39 Lenses:
More impact resistant than glass and approximately half the weight; easier to scratch than glass; can be tinted virtually any color.
Lens with no refractive power; The two lens surfaces are flat or uniformly meniscus in shape.
Lens with a coating of parallel-oriented material that sends light waves pulsating in one direction; Filter out reflected light, eliminating glare; Eliminate UV light
Polycarbonate Lenses:
Due to high index of refraction, these lenses are thinner, weigh less and are much more impact resistant than normal plastic lenses.
Vision problems because of old age and hardening of the eye"s lens; May include, decreasing ability to accommodate, less contrast sensitivity, requirement of more light, inability to cope with glare, etc.
A wedge-shaped, clear piece of glass or plastic that deviates light to its base, making the apparent image displaced in the direction of its apex.
Prism Diopter:
Optical unit of measurement; The angle of deviation of a ray of light via a prism or lens.
Progressive Flat top:
Lenses that change power gradually; one line divides two different prescriptions.
Progressive no lines:
Lenses that change power gradually; more aesthetically pleasing, because no visible lines.
Pupillary Distance (PD):
Interocular distance; horizontal linear measurement in millimeters between the centers of the pupils.
Radial Keratotomy (RK):
Surgery to correct nearsightedness; spoke-like incisions made on the eye surface to flatten the cornea; most precarious type of eye surgery.
Change in direction of light when going through one medium into a different medium of varying density; Act of deciding focal condition of eye and how to correct it.
Innermost layer of the eye; Light sensitive membrane that obtains images from the lens and delivers them to the brain via the optic nerve.
Retinoscopic Unit:
A unit used to determine and analyze the refractive properties of the eye.
Saddle Bridge
(Unifit Bridge):
This bridge was derived to imitate a standard plastic frame without nosepad arms. It distributes the weight evenly around the nose hence the name unifit. Originally derived of hard acetate the saddle bridge is now offered in soft silicone.
Saline solution:
A sterile salt solution used in cleaning, rinsing, and sometimes storing of contact lenses.
An area of distorted visual acuity usually surrounding a normal visual field.
Scratch resistant coating:
A strong transparent compound applied to plastic and light weight lenses in an effort to minimize scratches to the lens surface. However there is no lens material that is scratch-proof. Most lightweight lenses (high-index, polycarbonate etc.) are made of a dense materials (the higher the index the denser the material), which requires an additional coating to improve its resistance to scratches. The denser the material the harder the coat is required. One thing to keep in mind is that the harder the coating the less the lens will accept tints.
(laboratory ground)
Slab off is usually used to treat Antimetropia, a form of anisometropia, which creates a vertical disparity of at least two or more diopters between each eyes. This condition usually creates a plus sphere for one eye and a minus sphere the other. Considering this condition a prism is placed on the spectacle lens of the weakest plus or highest minus to help counterbalance any disparity, most notable on multifocal lenses.
Slab-Off (reverse):
Same benefits as laboratory ground slab off. The difference being it is made at the lab on the opposite lens. The weakest minus or strongest plus lens.
Snellen Chart:
A standardized test chart introduced in 1862 by Dutch ophthalmologist Hermann Snellen to measure visual acuity. Visual acuity is expressed as a fraction (e.g. 20/20) in which the numerator denotes the testing distance and the denominator indicates the distance at which a person with normal eyesight can read the letters on the chart. For example, if the smallest letters that the person being tested can see are on the "20/40" line, it means a person with normal eyesight can see these same letters at a testing distance of 40 feet.
A circular object in which the surface is equidistant from the center core.
Spherical Aberration:
A defect where rays of light from various distances cannot converge to a common focus due to a defect of the spherical surface of the lens.
Spring Hinge:
A mechanism inset within the frame or temple allowing the hinge to hyperextend. Normally located where the frame and temple come together. This feature allows for a comfortable fit as it self adjusts to the contour of the face.
Stock Lens:
Single vision finished lenses cast by the lens manufacturer. Available in a limited power range and lens materials.
An imbalance of the eyeball muscles which can prevent the the eyes from obtaining binocular vision with both eyes usually leaving one eye astray. Also called heterotropia, squint, tropia.
Single Vision (SV):
Single vision is a mono visual lens yielding a single field of vision; whether there is no prescription or a single RX prescription. The three forms of single vision lenses are plano, concave (most commonly referred to clinically as a minus prescription used to treat Myopia) and convex (most commonly referred to clinically as a plus prescription used to treat Hyperopia). There are several different materials to accommodate different needs, lifestyles and prescriptions.
The part of the frame that stems from the frame and rests over the ear.
Shades of color applied to lenses. Plastic, polycarbonate, trivex and high index lenses are dipped in dyes with temperatures reaching over 200"F. These tints can be lightened by a bleaching process. Glass lenses are heat treated with oxides. Most plano sunglass lenses are cast with colored materials, so the lens itself is consistently tinted throughout. These types of lenses do not fade. Lenses treated with a chemical dye made fade over time. Different types of tints include solid, gradient, double gradient.
A light, strong, lustrous, corrosion-resistant transition metal with a white-silvery-metallic color. Titanium is as strong as steel, but about 43% lighter, and about 60% heavier than aluminum, but twice as strong.
A standard eye test that determines the fluid pressure inside the eye. Elevated pressure is a possible sign of glaucoma.
Toric lenses:
Contact lenses designed to correct astigmatism by bearing two different optical powers at right angles to each other.
Transmittance (Transmission)::
The percentile of light relating to one part of the spectrum from transmitted light to incidental light.
A visual RX prescription may have a (-) minus or (+) plus cylinder. Both yield the same visual effect for a patient and technically both are the same prescription, although they look completely different. There is a formula to transpose (switch) a prescription from a (-)minus to a (+)plus cylinder and vice versa, hence the word "transposition". See our Prescription Lenses page for the transposition formula.
Trifocal lenses are part of the multi-focal lens family most commonly used to accommodate presbyopia patients. The trifocal lens offers three fields of vision, which include distance, intermediate and near. The higher the ADD power on a RX prescription, the more notable the intermediate range is for the viewer. As the successor of the bifocal the trifocal consist of an added segment for a near field of vision like the bifocal, but also has a segment directly above to address an intermediate field of view. The trifocal lens may be an alternative to the progressive lens. Unlike the progressive lens the trifocal has a clear peripheral view with most materials, and also offers an intermediate visual range. The trifocal is offered in different segment sizes, which include 7X25, 7X28, 7X35, 8X35 and an executive style.
Trivex was invented for the military as a polymer that is clearer and lighter than poly. The name Trivex was derived to reflect the combination of the three distinctive characteristics: excellent optics, impact resistance and lightweight nature. However, Trivex is considerably thicker than poly with a 1.53 index vs 1.59 (poly). First introduced to the public in 2003 this lens medium is an ideal material for patients that want rimless, safety, and/or sports eyewear and cannot adapt to poly and/or are looking for the clearest optics/impact resistance.
Ultraviolet protection (UV) Coating:
Besides glass, most lens materials used on eyewear have a natural composition that filters out most UV light. Added protection may be applied to achieve 100% protection from UVA and UVB light. UV coating can be applied to clear lenses, providing 100% UV coating without any color applied to the lens. Added UV protection may be added with a chemically baked on technique. However, with technological advances UV protection is most commonly applied by submerging the lens into a clear chemical agent heated to approximately 200˚ Fahrenheit for a set time, usually only a few minutes at most. (See our Ultra Violet (UV) Coatings page for more information).
Laboratory surfaced lenses. Available in all lens styles, powers and materials. Not edged.
The point where the optical axis of a lens coincides with the surface of the eye.
Vertex Power:
The refractive power of lens measured from its vertex to its principal focus. Vertex power is the significant factor in determining the power of a corrective lens.
Visual acuity:
Expressed as a fraction (e.g. 20/20) in which the numerator denotes the testing distance and the denominator indicates the distance at which a person with normal eyesight can resolve the letters on the chart.
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