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A . A Measurement:
Refers to the horizontal length of a lens shape’s box measurement. (See box measurement)


Abbe Value:
The Abbe value is the reciprocal of the V value or disperse power. Generally ranges between 20 & 60. Higher Abbe lenses have less chromatic aberration or distortion. High-index materials usually have lower Abbe values.


An optical defect created by the lens design or inherent defects in the lens material. Generally results in a failure of the lens to clearly focus light rays. (See Abbe value & chromatic aberration.)


Lenses with built-in filters or tints will absorb a certain amount of transmitted light. Absorptive tints are used to reduce glare.


Sharpness of vision.


Unit used for measuring wavelengths of light.


Acronym for the American National Standards Institute, the organization making recommendations for industry standards.


Anti-Reflective Coating:
A vacuum-deposited coating applied to lenses to increase their light transmission. Also reduces the amount of light reflected from lens surfaces & eliminates ghost images. Patients benefit by seeing better & looking better.


The thinnest edge of a prism.


Condition of the eye with crystalline lens removed. Also refers to post-cataract patients who have had cataract surgery.


AR Coating:
See anti-reflective coating.


Front surface of lens is not spherical but has curves that gradually flatten or steepen as they move away from the optical center. Aspheric lenses are flatter than conventional lenses. Patients generally see better with aspheric lenses & the flatter profile makes them more attractive.


See aspheric.


Astigmatism results when the correction needed in one meridian of the eye is different than the required power for the opposite meridian. Astigmatism prevents spherical correction from bringing the light rays to a clear image point. Cylinder correction is required for clear vision.


Automated Lensometer:
Electronic lensometer that automatically measures the power of a lens with no operator input. (See lensometer)


Usually refers to the direction of a cylinder lens in front of the eye. Governs how toric surface of the lens is oriented in the eyeglass frame.


B Measurement:
The vertical length of a lens shape’s box measurement. (See box measurement)


The thickest edge of a prism.


Base Curve:
Generally refers to the front curve of a lens. In most lenses this is a spherical (or aspheric) curve. Most cylinder lenses have the cylinder (toric) surface on the back side.


Best-Form Lens:
These lenses have specially computed front curves to reduce marginal astigmatism which can occur as the patient looks away from center of the lens. Most lenses today are considered best-form or corrected curve. (See also corrected curve.)


To hold a lens in a metal or plastic frame requires the edges of the lens to be formed into a V shape that fits in a grove in the frame rim.


Lens with both front & back concave surfaces. Only used for very high minus lenses.


Lens with two focal lengths, one for distance & one for near.


Binocular PD:
Determined by measuring from one side of one pupil to the same side of the other pupil. The resulting measurement will be something like 65 mm. Modern dispensers take monocular PD’s. (See monocular PD)


Blank Size:
The overall diameter of a lens as it comes from the manufacturer, before beveling.


Box Measurement:
When a rectangular frame is drawn around a lens shape, dimensions of that frame are called that shape’s box measurement. Three elements are “A”, “B” & “ED” measurements.


Cataract Lens:
Lens used following cataract extraction. Once widely used for post-cataract patients, IOL (intraocular) lenses are now the most commonly used replacement for the removed crystalline lens, eliminating the necessity for cataract lenses. (See aphakic & IOL)


Chem. – Tempered:
Glass lenses must be heat or chemically-treated to increase impact resistance. Chemical tempering (chem.-tempered) is the preferred method for maximum impact resistance. Chem.-tempering requires more time than heat-treating. (Usually 2 to 16 hours.)


Of or pertaining to color.


Chromatic Aberration:
Lens property that causes white light to be broken up into the colors of the spectrum, with each color focused at a different distance from the lens.


Coated Lenses:
Can refer to vacuum coatings applied to glass lenses. The most common vacuum coating is anti-reflective (AR) coating. Same process also used for applying mirror coatings & color coatings on glass lenses. Also can refer to scratch coatings. (See scratch coating.)


Color Coating:
Glass lenses can not be dyed. Clear glass lenses can have color added by applying color vacuum coating. (See coated lenses.)


Concave Surface (CC):
A minus surface which is depressed as compared to a flat surface. It is the opposite of a convex surface & is usually the back surface of the lens. (See convex)


When the eyes are turned toward one another when viewing a near point.


Convex (CX):
A plus surface which is raised as compared to a flat surface. It is the opposite of a concave surface & is usually the front surface of the lens. (See concave)


Corrected Curve:
See best form lenses.


The trade name for conventional hard resin lenses. The chemical name is allyl diglycol carbonate, a thermosetting plastic. Initials stand for Columbia Resin #39 because it was the 39th formula developed by Columbia Laboratories.


Crown Glass:
The conventional material used for glass spectacle lenses. Has a refraction index of 1.523.


Trade name for a series of glass lenses made by Corning and sold through Winchester Optical that filtering out specific bands of light for those suffering from certain medical conditions.


Cathode Ray Tube, the monitor or display screen of a computer. Special lens coatings are available to increase or enhance vision while viewing computer screens.


Same letters also apply to special Franklin/Exec type trifocals which has extra deep intermediate area for a wider deeper mid-range reading area. Intermediate depth is 14 mm. rather than the conventional 7 mm.
* Note – X-CEL OPTICAL no longer carries these lenses, your best choice would be to choose from any of our Acclaim lenses in CR 39.


Corneal Reflection Pupilometer, also known as an electronic or corneal pupilometer. (See pupilometer)


Curve Top:
Bifocal or trifocal with curved top line as opposed to flat top bifocal/trifocal.


Cut Out:
Knowing the patient’s PD & the PD & the size of the patient’s frame permits calculating the size of lens blank required to fit the frame.


Distance Between Lenses. The distance from the bottom of the right lens groove to the bottom of the left lens groove, measured across the bridge of the frame.


Most patient PD’s are not the same as the frame PD, so optical centers must be centered in front of the pupils. This often requires the optical centers to be move nasally or temporally.


The act of moving the optical center to one side or the other when edging the lenses to the frame.


Double D:

Usually prescribed for occupational use, this is a flat-top bifocal at the bottom for close work with a second flat-top bifocal at the top of the lens for close vision overhead.


Double Seg:
Double seg occupational lens. Can be ordered with round segments or flat-tops. (See double D)


Drop Ball Test:
To test lens-impact resistance, the FDA requires a steel ball of a given diameter to be dropped from a specific height onto the lens surface. Testing the lens impact resistance is the responsibility of the last person modifying the lens. Most plastic lenses carry a waiver that the lenses were batch-tested by the manufacturer. Waiver applies only when conventional finishing procedures are used.


ED Measurement:
The distance measured diagonally, from the mechanical center of the lens shape to the furthest point, & then doubled. ED determines blank size required for lenses cut to that frame. (See box measurement)


*ED Trifocal:
Useful occupational trifocal that combines a flat-top bifocal within a Franklin/Exec segment. Flat top is used for close work & wide Exec segment used for intermediate power. Also known as FD trifocal.
* Note – X-CEL OPTICAL no longer carries these lenses, your best choice would be to choose from any of our Acclaim lenses in CR 39.


Edge Coating:
Lens edges can be painted or color-treated. Sometimes done on high minus lenses to reduce or eliminate concentric rings often observed on higher corrections. Edge coating can be applied to disguise thick edges of higher minus lenses. Also done for cosmetic reasons to coordinate with frame color.


Turning inward of the eye.


Bifocal or trifocal that extends fully across the width of the lens. Offers advantage of wide field of vision in reading area. Disadvantage is the somewhat unattractive appearance of the ledge separating the distance portion from the reading area. Higher adds have wider ledges. (See franklin/split)


Eye Size:
Refers to width of the lens portion of the frame, measured horizontally from the depth of the groove on the left side to the depth of the groove on the right side. Measurement is accomplished at the midway point between the top & bottom of the lens area.


F. Face Form Bend:
Moderate bending of a frame that tends to wrap it around the facial contours. (See wrap around)


Decorative polished bevels on rimless lens edges, usually for cosmetic reasons.


Factory Coating (Scratch):
Plastic lenses are often ordered with a protective coating to guard against hairline scratches. Generally, coatings applied at the factory are assumed to be tougher & more protective. When the lens has been surfaced (multifocals & higher corrections), factory coating will be on the front surface only. Most labs apply a tough coating on the back side.


(See hyperopia)


FD Trifocal:
(See ED trifocal)


Finished Lenses:
Lenses that have been shaped & edged to fit a frame, as opposed to uncut lenses.


The act of sizing & beveling a lens to fit a frame. Also applies to sizing, beveling & grooving a lens to fit a rimless frame.


Flat Top:
Most common bifocal. Visualize a round segment with the top portion flattened. Also available with trifocal section for arm’s length vision.


Focal Length:
Reciprocal of dioptric power. Distance from the optical center of the back surface to the principle focus of the lens. (See vertex distance)


The point at which light rays through a lens form an image.


(See pattern)


Frame PD:
Determined by measuring the distance from the temporal side of one opening to the nasal side of the other lens opening.


(See executive)


Franklin Split:
Another name for Franklin/Exec bifocal or trifocal. (See executive)


(See flat top)


Merging of simultaneous retinal images into a single perception or image.


G. Geometric Center:
The point of a lens lying midway from all edges.


Elevated interocular pressure that can ultimately lead to blindness.


Gradient Tinting:
Usually applied for cosmetic purposes. Tinting is darker at top of the lens than in the middle. Tint is lightest at the bottom.


The most scratch-resistant lens material. Heavier than plastic lenses, glass comes in a wide selection of lens styles & can be ordered with absorptive tints & photochromic tints. (See crown glass)


Hard Resin:
Most non-glass lens materials other than polycarbonate are considered hard resin. (See CR-39)


Heat Treated:
Glass lenses are required by law to be heat-treated or chemically treated to increase impact resistance. Heat treating is accomplished by heating the lens in an oven & quenching with air. Process can be accomplished in less than an hour. Chemical tempering requires a much longer time but is the preferred method for maximum impact resistance. (See chem. tempered.)


High Index:
Any lens material with a higher refractive index than glass (1.523) or CR-39 (1.498) is considered high index. Plastic high-index lens materials range from 1.54 to 1.66. Glass high-index ranges from 1.60 to 1.80.


Defined as ‘water loving’. AR coatings tend to be hydrophilic in their normal state.


Defined as ‘water hating’. Water ‘beads up’ on hydrophobic surfaces. More advanced AR coatings now include final hydrophobic coating, making the lenses easier to keep clean.


Far-sightedness. When light rays from infinity focus behind the retina of an uncorrected eye. Plus power lenses are required. These people see better in distance than near.


Those with far-sighted corrections requiring plus corrections.


A turning downward of one eye.


Index Of Refraction:
When applied to lens materials, a number indicating the ability of that material to bend light rays. Index number equals the ratio of the speed of light in air to the speed of light in that medium. The higher the index the more light is bent.


Plural of index.


In optics, infinity refers to any distance great enough that light rays from that distance are considered parallel. Generally accepted to be 20 feet or beyond.


Infra-Red (IR):
The portion of the spectrum where wavelengths are too long for the human eye to see.


Because eyes converge when looking at anything up to close, the reading portion of multifocals must be positioned toward the nasal side of each lens. The amount segments are moved nasally from distance optical center is called “segment inset.”


Intermediate Zone:
Refers to the middle zone of sight, considered to be an arm’s length. Top trifocal segment corrects vision for this distance.


Interpupillary Distance:
Distance between patient’s pupils. Commonly referred to as PD.


Lens Clock:
Instrument for measuring lens surface at three points to determine the curve of the surface. Aspheric curves can not be measured with a lens clock.


Lens Power:
Measurement of how the lens bends light, expressed in units called diopters. One diopter lens focuses light at one meter. Two diopter lens focuses light at one half meter.


Instrument used to measure the power of a lens. (See automated lensometer.)


Lens design to reduce weight & thickness. Used primarily for post-cataract lenses.


Low Vision:
Lenses or optical devices used for those with little correctable sight. Aids include hand-held magnifiers & other high magnification devices.


Managed Eyecare:
Third-party insurance programs that provide eyecare for participants.


Minus power lens.


Mirror/Reflective Coating:
Mirrored coatings are applied by vacuum deposition, similar to the way AR coatings are applied. Mirrors can be applied to glass or plastic lenses.


MM or mm:
Common symbol for ‘millimeter.’


Bifocal with both distance & reading segment sharing a common optical center. Best example is Franklin/Exec bifocal.


Refers to one eye.


Monocular PD:
Individual PD measurements for each eye. Best way to do this is with a pupilometer. Measurements refers to the distance from pupil center to center of nose.


Multi-Layer AR:
Modern AR coatings apply a series of coatings to increase light transmission. State-of-the-art AR coatings have five layers of coatings. Hydrophobic coating adds a sixth layer.


Any lens having more than one power or correction. Includes bifocals, trifocals, progressives and quadrifocals.


Special grinding used for very high minus corrections. A secondary flat surface is applied to the outside rim of the lens to reduce edge thickness.


Near-sighted condition. Light rays are focused in front of the retina. People with myopia see better near than in distance.


Condition of near-sightedness. (See myopia.)


N: Nasal:
Inside edge of lens or side closest to the nose.


Near Point:
Denotes distance used in conventional reading, generally considered 14 inches.


O. D.:
Latin abbreviation for right eye.


O. S.:
Latin abbreviation for left eye.


O. U.:
Latin abbreviation for both eyes.


Slanting angle, ranging between horizontal line (180’) and vertical line (90’).


Occupational Lens:
Any lens prescribed primarily for a specific visual task at work, or for participation in a hobby, sport, or other leisure activity. Usually applied to special design multifocals, but any lens can serve as an occupational lens.


Blocks all light, as opposed to transparent or translucent.


Optical Axis:
(See Axis)


Optical Center:
Point where optical axis intersects the lens. In a plus lens, it is the thickest point. In minus, it is the thinnest point of the lens. In general, optical centers must be directly in front of the pupil center for proper vision.


Frames with lens size that will not cut out of conventional size blanks are considered oversized. Labs usually charge extra for oversized lenses.


Acronym for Progressive Addition lens, where power of the lens gradually changes from distance correction in the top half to reading correction in the bottom half with clear vision provided for intermediate zone. (See intermediate zone.)


Panasonic Tilt:
Proper frame-fitting requires the bottom of the frame to be closer to the face than the top. Normal pantoscopic angle is considered 15’from vertical.


Pattern, Frame:
Conventional edging requires use of plastic or metal pattern (former) that has the exact shape of the lens opening in the frame. Edgers reproduce pattern shape when the lens is edged. Many labs now trace the frame shape & use stored digital information to control egders, eliminating the use of patterns. (See former.)


Pattern, Lens:
(See frame pattern.)


P D:
(See interpupullary distance.)


Lenses that darken when activated by bright daylight & lighten when brought indoors. Originally only made of glass, they are now available in some forms of plastic lenses. Photosensitive lenses darken more quickly and are darker in cooler weather than in hot.


(See photochromic.)


Pilot Shape:
One of the most popular lens shapes (used for aviators’ goggles in World War II.) Shape features gradual nasal flare towards the bottom of the lens.


Lenses that reduce reflected glare & make excellent sunwear, particularly for water sports & driving. Polarized lenses are available in glass & plastic.


Polycarbonate (PC) is a clear, colorless polymer used extensively for engineering & optical applications. It is available commercially in both pellet & sheet form. Outstanding properties include impact strength & scratch resistance. The most serious deficiencies are poor weatherability & chemical resistance.


Press-On™ Adds:
Special plus power flexible lenses that can be shaped with scissors. They adhere to conventional lenses through static attraction. They provide temporary bifocals when there is a requirement to continually change the patient’s near correction over time.


Press-On Prisms:
Permits doctors to gradually change a prism correction for visual training or other special needs. (See also press-on ™adds.)


Progressive Corridor:
The narrow portion of a progressive addition lens (PAL) where the lens power changes from distance to reading. This contains the area used for arm’s length vision.


(See PALs)


As people age, they gradually lose the ability to focus (accommodate) close, as in reading. This loss of accommodation is called presbyopia & usually happens after age 40.Presbyopia is corrected by wearing glasses or bifocals that provide that added plus power.


Of, or having. (See presbyopia.)


Also called CRP (for Corneal Reflection Pupilometer) or electronic pupilometer, this device measures individual PD for each eye. (See monocular PD.)


Occupational lens featuring trifocal in the bottom half of the lens & bifocal at the top for seeing close when looking upwards.


Readables ™:
Trade name for flat-top bifocal featuring progressive addition power in the segment.


Refractive Index:
(See index of refraction.)


Round Seg:
Bifocals with a round segment containing the near power.


Router Edger:
Many labs edge polycarbonate, a soft material, on special edgers using a router blade to shape & bevel the lens rather than conventional diamond wheels. Router edgers can be used for edging any non-glass lens.


Scratch Coating:
Coating applied to plastic lenses to increase resistance to handling scratches. Front surface coating can be factory-applied. Back coating is usually applied by the laboratory.


Seg Height:
A measurement that determines where the reading segment is located in the frame. Commonly measured from the top of the reading segment to the lowest edge of the lens.


Semi-Finished Lenses:
Lens blanks that have front surface completed by the manufacturer, but are also thick, so labs can grind the back curve & create a wide range of corrections.


Silicon Coating:
A recent development that is applied over the top of antireflection coatings. Leaves a smooth hydrophobic surface that is easier to clean.


Single Vision:
Lenses with one power, as opposed to bifocals, trifocals, quadrifocals or multifocals.


Slab Off:
Special way of grinding two curves on a lens surface to produce base up or down prism in reading area to correct vertical imbalance. Prescribed when correction for one eye is considerably stronger than the other.


Solid Tints:
Color tints that have the same density throughout the lens.


Specific Gravity:
Numerical ratio comparing the weight of the lens material with that of water. The lower the specific gravity, the lighter a lens made of that material will be.


Split PDs:
(See monocular PDs.)


Stock Lenses:
More commonly used single-vision corrections are usually available in stock form, ready for edging. Stronger single-vision corrections & most multifocals are manufactured in semi-finished form & must be surfaced by the laboratory.


Surfaced Lens:
Any semi-finished lens that has had the back surface applied by a laboratory.


The act of grinding the back curve.


The side of a lens or frame furthest away from the nose.


Third Party Eyecare:
(See managed eyecare.)


Some non-glass lenses like hard resin plastic can be dyed to add color for cosmetic purposes or to reduce light transmission. Glass lenses must be made from colored glass or have color applied by vacuum coating.


Lens with three distinct powers for distance, arm’s length & near corrections.


The first new optical monometer of the 21st century, Trivex™ tri-performance .lens material from PPG® Industries, Inc. offers the unprecedented combination of excellent optics, impact resistance & light & thin qualities in a single material.


A one-piece, round bifocal, created by having two curves on one lens surface. Available with segment on the front surface or the back.


Uncoated Lens:
Plastic lenses without a factory or lab-applied scratch coating.


Lenses that have not been shaped or beveled.


UV Coating:
Lens treatment applied to absorb the harmful portion of UV light found in daylight. When applied to plastic lenses, it is actually a dye, not a coating.


UVA: (Ultra Violet A Band.)
The longer wavelength of ultraviolet light which is from 315 to 380 nm. Some believe UVA contributes to cataract formation.


UVB: (Ultra Violet B Band.)
Ultraviolet light ranging from 290 to 315 nm. UVB can cause sunburn & snow blindness.


Vertex Distance:
Distance measured from the front surface of the cornea to the back surface of the correcting lens.


Vertical Imbalance:
Created when the power of one lens is considerably stronger than the other & the patient looks downward when reading. (See slab off.)


Acronym for Vision Service Plan, one of the more popular managed eyecare programs.


Wrap Around:
Moderate bending of the frame that tends to wrap it around facial contours.


Commonly-used term referring to the ANSI Z80. 1-1987 recommendations for ophthalmic lenses issued by the American National Standards Institute. These are tolerance guidelines used by laboratories to control quality.




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