Glossary Of Optical Terminology - FEA Industries, Inc.
Glossary ofOpticalTerminologywww.feaind.com800.327.2002
Glossary of Optical TerminologyAΛAllowance, vertex powerSymbol for prism.When following a number, it denotes the unitsknown as prism diopters.The amount by which the front surface curvature ofa lens must be flattened in order to compensate fora thickness-related gain in powerAngle, apicalAHorizontal dimension of the boxing systemrectangle that encloses a lens or lens opening.AcceleratorAn additive to a polishing slurry that is used toincrease the efficiency of the slurry.Accurate sag formulaSee Formula, accurate sag.Actual powerSee Power, actual.Adapter, offset blockA small metal device used on a lens that has beenblocked off center.It is used for the purpose of transferring pressureback to the center of the lens so that unwantedprism may be avoided.AddSee Addition, near.Addition, nearThe power that a lens segment has in addition tothat power already present in the main portion ofthe lens.Alignment, standardAn impersonal standard, independent of facialshape, for the alignment of spectacle frames.The angle formed by the junction of two nonparallelprism surfaces.Angle, effective diameterThe angle from the zero-degree side of the 180degree line to the axis of the effective diameter.The angle is referred to by the letter X and ismeasured using the right lens.Angle of deviationThe difference between the angle of incidence andthe angle of refraction.Angle , pantoscopic1. In standard alignment, that angle by which theframe front deviates from the vertical (lower rimsfarther inward than upper rims) when the spectaclesare held with the temples horizontal.2. In fitting, that angle which the frame front makeswith the frontal plane of the wearer’s face when thelower rims are closer to the face than the upper rims(opposite retroscopic angle).(Synonym: Pantoscopic tilt.)Angle, retroscopicThat angle which the frame font makes with thefrontal plane of the wearer’s face when the lowerrims are farther from the face than the upper rims(opposite pantoscopic angle).(Synonym: Retroscopic tilt.)www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 2
Glossary of Optical TerminologyAnisometropiaA condition in which one eye differs significantly inrefractive power from the other.Antireflection coatingSee Coating, antireflection.Antiscratch coatingSee Coating, antiscratch.ApertureAn opening or hole that admits only a portion oflight from a given source or sources.Aperture, lensThe portion of the spectacle frame thataccepts the lens(Synonym: Lens opening.)ApexThe junction point at which the two nonparallelsurfaces of a prism meet.AphakicA person whose crystalline lens has been removed.AsphericA nonspherical surface.An aspheric lens surface generally decreasesin power peripherally in order to correct foraberrations found in the periphery of a lens.Aspheric, full-fieldAn aspheric lens that begins its asphericity wherethe small central spheric region leaves off.It continues in its asphericity all the way to the edgeof the lens blank.Aspheric lenticularSee Lenticular, aspheric.AstigmatismThe presence of two different curves on a singlerefracting surface on or within the eye.This causes light to focus as two line images insteadof a single point.Axis, of a cylinderAn imaginary reference line used to specifycylinder or spherocylinder lens orientation andcorresponding to the meridian perpendicular tothat of maximum cylinder power.Axis, prismThe base direction of an ophthalmic prism,expressed in degrees.Axis, opticalThat line which passes through the center of a lenson which the radii of curvature of the front and backsurfaces fall.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 3
Glossary of Optical TerminologyBBBase upThe vertical dimension of the boxing systemrectangle that encloses a lens or lens opening.Back base curveSee Curve, back base.Back cutSee Cut, back.Back vertex powerSee Power, back vertex.Ball gaugeSee Gauge, ball.Bar gaugeSee Gauge, bar.BaseIn a prism, the edge of maximum surfaceseparation opposite the apex.Base curveSee Curve, base.Base downVertical placement of prism such that thebase is at 270 degrees on a degree scale.Base inHorizontal placement of prism such that thebase is toward the nose.Base outHorizontal placement of prism such that the base istoward the side of the head.Vertical placement of prism such that thebase is at 90 degrees on a degree scale.Baume degreesA system used to quantify the concentration of aslurry based on the specific gravity of the slurry; Be 145 - 145/SG, where SG specific gravity.BCDBoxing center distance.See Distance, boxing center. Be An abbreviation for degrees Baume .Bell gaugeSee Gauge, bell.BevelThe angled edge of the spectacle lens.Bevel, pinSynonym for safety bevel.Bevel, safety1. To remove the sharp interface between lens surfaceand the sharp point of the bevel apex.2. The smoothed interface between the lens surfaceand bevel surface and the smoothed lens bevelapex.Bevel, VA lens edge configuration having the form ofa V across the whole breadth of the lens edge.Bicentric grindSynonym for slab-off.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 4
Glossary of Optical TerminologyBifocalsBlank, roughLenses having two areas for viewing,each with its own focal power.A lens-shaped piece of glass with neither sidehaving the finished curvature.Usually the upper portion of the lens isfor distance vision, the lower for near vision.Both sides must yet be surfaced in order to bringthe lens to the desired power and thickness.Bifocal, blendedA bifocal lens constructed from one piece of lensmaterial and having the demarcation line smoothedout so as not to be visible to an observer.Bifocal, curved-topA bifocal lens having a segment that is roundin the lower portion and gently curved on thetop of the segment.Bifocal, ExecutiveAmerican Optical’s trade name for theFranklin-style bifocal.Bifocal, flat-topA bifocal with a segment that is round in the lowerhalf but flat on the top.Bifocal, FranklinA bifocal having a segment that extends the entirewidth of the lens blank.Bifocals, round segA bifocal with a segment that is perfectly round.The width of the segment is usually 22 mm, but maybe larger (usually 38 mm).Blank, finished lensA lens having both front and back surfacesground to the desired powers, but not yetedged to the shape of the frame.Blank geometric centerSee Center, blank geometric.Blank seg dropSee Drop, Blank seg.Blank seg insetSee Inset, blank seg.Blank, semifinished lensA lens with only one side having the desiredcurvature.The second side must yet be surfaced in order tobring the lens to its desired power and thickness.Blended myodiscSee Lens, blended myodiisc.BlockThat which is attached to the surface of a lensin order to hold it in place during the surfacingor edging process.Block markSee Mark, blockBlockerThe device used to place a block on the lensin order to hold the lens in place during thesurfacing or edging process.Blocking bodiesSurfacing blocks used in conjunction with pitch.Blocking, finishThe application of a holding block to an ophthalmiclens so that it may be edged to fit a frame.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 5
Glossary of Optical TerminologyBlocking, off-centerThe practice of placing the surfacing lens blockat the desired location for the major referencepoint of the lens.Off-center blocking does not grind prism fordecentration.Blocking, on-centerThe practice of placing the surfacing block at theblank geometric center of the lens and moving themajor reference point of the lens to its desiredlocation by grinding prism for decentration.Blocking, simplified on-centerThe practice of using sinesquared methods tomove the optical center horizontally.The surfacing block is centered horizontally,but positioned vertically above the seg top byan amount equal to the seg drop.Blocking, surfaceThe application of a holding block to an ophthalmiclens so that one side may be ground to the correctcurvature and polished.Blocks, glassA small, thick disk used to hold a glass lens in placeduring the surfacing process.Blocks, plasticA curved disk used to hold a plastic lens in placeduring the surfacing process.These blocks vary in diameter, but are normallylarger than glass blocks to prevent lens flex duringprocessing.Box-o-GraphA flat device containing grids and slides used in themeasurement of pattern and edged lens size.Boxing centerSee Center, boxing.Boxing center distanceSee Distance, boxing center.Boxing systemSee System, boxing.BridgeThe area of the frame front between the lenses.Burn, polishingA lens surface defect that looks like a blister ora small group of blisters on the lens surface.Polishing burn may be the result of a polishing padthat has not been sufficiently wetted with polish.These are generally 43 mm in diameter and may beadapted for use in blocking plastic lenses.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 6
Glossary of Optical TerminologyCCCenter., readingThe horizontal width of a lens at a level halfwaybetween the two horizontal tangents of the top andbottom of the lens shape.(Synonym: Datum length.)CarrierThe optically unusable outer portion of alenticular lens that “carries” the opticallyusable central portion.CataractA loss in clarity of the crystalline lens of the eye,which results in reduced vision or loss of vision.Center, boxingThe midpoint of the rectangle that encloses a lensin the boxing system.Center, Blank geometricThe physical center of a semi-finished lens blankor an uncut finished lens blank.The blank geometric center is the center of thesmallest square or rectangle that completelyencloses the lens blank.Center, datumThe midpoint of the datum length (C dimension) of alens along the datum line.Center, geometric1. The boxing center.2. The middle point on an uncut lens blank.Center, opticalThat point on an ophthalmic prescription lensthrough which no prismatic effect is manifested.That point on a lens at the reading level thatcorresponds to the near PD.Center, seg opticalThat location on the segment of a bifocal lens thatshows zero prismatic effect when there is norefractive power in the distance portion of the lens.CentersThree short, replaceable, cylinder-shaped piecesthat fit into a surfacing block and serve as pivotpoints during fining and polishing.Centrad ( )A unit of measurement of the displacement oflight by a prism.One centrad is the prism power required todisplace a ray of light 1 cm from the position itwould otherwise strike on the area of a circlehaving a 1 m radius.CentrationThe act of positioning a lens for edging such that itwill conform optically to prescription specifications.Chart, prismA circular chart used to find the sum of twoprismatic effects graphically.This sum is expressed with the amount givenin prism diopters and the base direction givenin degrees.Charts, surfacingSynonym for surfacing tables.ChillerA refrigerator unit used for cooling generatorcoolant, fining or polishing slurry, or blockingalloy in the ophthalmic surfacing laboratory.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 7
Glossary of Optical TerminologyChordA straight line intersecting two points of an arc.Chord diameterSee Diameter, chord.Clock, segDesigned like a conventional lens measure exceptthat the three points of contact are closely spaced.Coating, antiscratchA thin, hard coating applied to plastic lens surfacesin order to make them more resistant to scratching.Coating, antireflectionA thin layer or series of layers of material appliedto the surface of a lens for the purpose of reducingunwanted reflections from the lens surface and thusincreasing the amount of light that passes throughto the eye.Collector, mistA vacuum system installed above the generatorgrinding chamber, which removed droplets ofgenerator coolant caused by the rapidly spinninggrinding wheel that would otherwise form a mist.ColmascopeAn instrument that utilizes polarized light to showstrain patterns in glass or plastic.Compensated lap toolSee Tool, compensated lap.Compensated padSee Pad, tool compensated.Compounding (of prism)The process of combining two or more prisms toobtain the equivalent prismatic effect expressedas a single prism.ConcaveAn inward-curved surface.Convergence1. An inward turning of the eyes, as whenlooking at a near object.2. The action of light rays traveling toward aspecific real image point.ConvexAn outward-curved surface.CoolantA recirculating liquid used to cool and lubricate thelens/grinding wheel interface during the grindingprocess.Coolant manifoldSee Manifold, coolant.Countersink curveSee Curve, countersink.Cover lensSee Lens, cover.CR-39A registered trademark of Pittsburgh Plate Glass Co.for an optical plastic known as Columbia Resin 39.It is the standard material from which conventionalplastic lenses are made.CribbingThe process of reducing a semifinished lensblank to a smaller size in order to speed thesurfacing process or reduce the probability ofdifficulty in surfacing.Cross curveSee Curve. cross.Cross fittingA reference point 2 to 4 mm above the majorreference point on progressiveaddition lenses.The fitting cross is positioned in front of the pupil.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 8
Glossary of Optical TerminologyCross powerA schematic representation on which the two majormeridians of a lens or lens surface are depicted.CurvatureThe reciprocal of the radius of curvature of a curvedsurface, quantified in m-1, abbreviated by R.Curve, back baseThe weaker back-surface curve of a minuscylinder-form lens.When the lens is a minus cylinder-form lens,the back base curve and the toric base curveare the same.Curve, baseThe surface curve of a lens that becomes thebasis from which the other remaining curvesare calculated.Curve, countersinkFor the manufacture of semifinished bi- and trifocallenses, the countersink curve is that curve whichis ground into the main lens in the area where thesegment is to be placed.The countersink curve matches the back curve ofthe bi-or trifocal segment.When the segment is placed on the countersinkcurve of the main lens, the two may then be fusedtogether.Curve, toolThe 1.53-index-referenced surface power of a laptool used in the fining and polishing of ophthalmiclenses.Curve, true baseSynonym for true power.Cut, backIn generating, the practice of beginning with theback edge of the lens and making only a partialgrinding cut across the surface.This prepares the lens for a deep, full sweep acrossthe surface of the lens from the front edge andavoids tearing a chunk off the back edge of the lens.Cutter, lapA machine used to cut a lap tool to its correctcurvature.Cutter, templateA lap cutter that uses individual templates to guidethe accurate cutting of the lap tool.Template cutters will cut laps with especially lowbase curves and saddleback tools, which manystandard lap cutters are not able to do.CylinderA lens having a refractive power in one meridianonly and used in the correction of astigmatism.Curve, nominal baseA 1.53-index-referenced number assigned to thebase curve of a semifinished lens.For moderately powered crown-glass lenses theneeded backsurface tool curve may be found bysubtracting the nominal base curve from theprescribed back vertex power.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 9
Glossary of Optical TerminologyDDDegrees Baume An abbreviation for diopter of refractive power.See also Diopter, lens.Datum centerSee Center, Datum.Datum center distanceSee Distance, datum center.Datum lineSee Line, datum.Datum system.See System, datum.DBCSee Distance between center.DBLDistance between lenses.DCDDatum center distance.Decentration1. The displacement of the lens optical center ormajor reference point away from the boxing ordatum center of the frame’s lens aperture.2. The displacement of a lens optical center away fromthe wearer’s line of sight for the purpose of creatinga prismatic effect.Decentration, blockThe distance a lens surfacing block is moved fromthe blank geometric center of a lens.See Baume degrees.Depth, mid-datumThe depth of the lens measured through thedatum center.Depth, readingThe vertical position in the lens through which thewearer’s line of sight passes when reading.Depth, segThe longest vertical dimension of the lens segment.Depth, sagittal (sag)The height or depth of a given segment of a circle.Diameter, chordThe diameter of a lens used for calculating lensthickness.Chord diameter ED (A DBL - PD).Diameter, effectiveTwice the longest radius of a frame’s lens apertureas measured from the boxing center.Abbreviated ED.Difference, frameIn the boxing system, the difference between frameA and frame B dimensions, expressed in millimeters.Diopter, lens (D) Unit of lens refractive power, equalto the reciprocal of the lens focal length in meters.Diopter, prism (Λ)The unit of measurement that quantifies prismdeviating power, one prism diopter ( 1Λ) is thepower required to deviate a ray of light 1 cm fromthe position it would otherwise strike at a point1 m away from the prism.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 10
Glossary of Optical TerminologyDisk, trueingAn abrasive disk that may be selectively spunacross certain areas of a spinning spherical laptool for the purpose of bringing that tool backto its proper curvature.Dissimilar segsSee Segs, dissimilar.Distance between centersIn a frame or finished pair of glasses, the distancebetween the boxing (geometric) centers.Distance between lenses1. In the boxing system, the distance between the twoboxed lenses as positioned in the frame.[Synonym: datum minimum between lenses (MBL)]2. In the datum system, the distance betweenlenses in the frame as measured at the levelof the datum line.Distance, boxing centerSynonym for distance between centers.Distance, frame centerSynonym for distance between centers.Distance, geometric centerDistance, near centrationThe distance between the geometric centersof the near segments.Distance, vertexThe distance from the back surface of the lensto the front of the eye.DivergenceThe action of light rays going out from apoint source.DressTo resharpen the cutting surface of agrinding wheel.Drop, blank segThe vertical distance from the blank geometriccenter to the top of the multifocal segment.Drop, seg 1.The vertical distance from the major referencepoint (MRP) to the top of the seg when the segtop is lower than the MRP. 2.The vertical distance from the datum line to thetop of the seg when the seg top is lower than thedatum line (laboratory usage).(Antonym: seg raise.)The distance between the boxing (geometric)centers of a frame.Distance, interpupillary (PD)The distance from the center of one pupil to thecenter of the other when either an infinitely distantobject is being viewed (distance PD) or a nearobject is being viewed (near PD).www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 11
Glossary of Optical TerminologyEEDEffective diameter.EdgerThe piece of machinery used to physically grindthe uncut lens blank to fit the shape of the frame.Edger, handA grinding wheel made especially for grindinglenses by hand.Effective diameterSee Diameter, effective.Effective diameter angleSee Angle, effective diameter.Effective powerSee Power, effective.Elliptical errorSee Error, elliptical.EmeryAn impure form of aluminum oxide containingcertain iron oxides.Emery, roughA commonly used term for a coarse abrasive usedto obtain a curve on a glass lens that is beyond therange of the generator.The term is a misnomer, because the abrasive isreally carborundum, not emery.EquithinA term used by the Varilux Corporation whenreferring to the use of yoked prism for thicknessreduction on a pair of Varilux progressive-additionlenses.See also Prism, yoked.Equivalent, sphericalThe sum of the spherical component and one-halfof the cylinder component of an ophthalmic lensprescription.Error, ellipticalThe slight deviation of a lens surface away from asphere and toward an ellipse that occurs during lensgenerating when using a cup-shapedgenerator wheel.Executive bifocalSee Bifocal, Executive.Executive water ringSee Ring, Executive water.ExtenderAn additive to a polishing slurry, used to reduce theamount of cerium needed in the slurry.Eyesize1. In the boxing system, the dimension.2. In the datum system, the datum length.EyewireThe rim of the frame that goes around the lenses.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 12
Glossary of Optical TerminologyFFFocal powerOften used in equations to denote lens refractivepower in diopters.Alternate symbol for F is D.FEAFEA Industries, an optical lab located outsidePhiladelphia known for its fast, efficient, accurateservice, quality and price.Fiber ringSee Ring, fiber.FiningIn surfacing, the process of bringing a generatedlens surface to the smoothness needed so that itwill be capable of being polished.FinishingThe process in the production of spectacles thatbegins with a pair of uncut lenses of the correctrefractive power and ends with a completed pairof spectacles.Finished lap toolSee Tool, finished lap.Finished lensSee Lens, finished.Fitting crossSee Cross, fitting.Flat-top bifocalSee Bifocal, flat-top.Focal pointSee Point, focal.See Power, focal.Form, minus cylinderThe form a prescription takes when the value ofthe cylinder is expressed as a negative number.Form, plus cylinderThe form a prescription takes when the value ofthe cylinder is expressed as a positive number.Formula, accurate sags r - (r 2 - y 2 ).,where r is the radius of curvature of the surfaceand y is the semidiameter of the chord.Formula, lensmaker’sA formula used to find the dioptric power of asurface from radius of curvature or vice versa,which states that D (n1 - n)/r, where D lensrefractive power in diopters, n1 is the refractiveindex of the lens, and n is the refractive indexof the media surrounding the lens.Now, since there is only one letter n in the equation,n is used to denote the refractive index of the lensinstead of n1.For a lens surface in air this becomes D (n - 1) rFormula, sine-squaredA formula used to obtain the “power” of anoblique cylinder in the 180-degree meridian.Frame center distanceSee Distance, frame center.Frame differenceSee Difference, frame.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 13
Glossary of Optical TerminologyFrame, combinationFranklin bifocal1. A frame having a metal chassis with plastic top rimsand temples.See Bifocal, Franklin.Front vertex power2. A frame having some major parts of plastic construction and some of metal.See Power, front vertex.Full-field asphericFrame PDSee Aspheric, full-field.See PD, frame.GGauge, ballA zero-in gauge consisting of a small metal ballof known diameter, mounted on a handle.Gauge, barA type of sag gauge that has three contact pointson the end of three “legs,” which pass through abar and contact the lap or lens surface.Gauge, bellA type of sag gauge used to measure sphericalsurfaces and having a domelike outer rim and amovable center pin.The circular rim contacts the spherical surface andthe pin measures how steep or flat that surface is.Gauge, center thicknessA device used to measure the thickness of asemifinished lens while it is mounted on a lenssurfacing block.Gauge, lapOne of a set of flat pieces of metal, usually brass,having one edge that is precision-cut to a knownradius of curvature.The lap gauge is used as a standard and is placedagainst a surface in order to determine whether ornot the surface in question is true to the standard.Gauge, prismA device used to measure thickness differences attwo opposite points on a lens blocked for surfacing.Using thickness differences, the amount of prismpresent in the lens is determined.Gauge, saddleA type of zero-in gauge that uses a cylindrical pin ofknown length and is steadied on the generator ramwith a saddlelike mounting.Gauge, zero-inAn object of known thickness, such as a ball or pin,which is used to assure an exact distance betweengenerator wheel and lens center.The zero-in gauge is needed when using a manualor semi-automatic lens generator.Common types are a saddle gauge and a ballgauge.Geometric centerSee Center, geometric.GCDGeometric center distance.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 14
Glossary of Optical TerminologyGeneratingGlazingThe process of rapidly cutting the desired surfacecurvature onto a semifinished lens blank.1. The insertion of lenses into a spectacle frame.2. The clogging of empty spaces between theexposed abrasive particles of an abrasive wheel,resulting in reduced grinding ability.Generator marksSee Marks, generator.GraynessGenerator ringA lens surface defect caused by incompletepolishing.See Ring, generator.Generator wheelGrind, bicentricSee Wheel, generator.Synonym for slab-off.Geometric center distanceGripper padSee Distance, geometric center.See Pad, gripper.HHand edgerSee Edger, hand.Hand stoneSee Stone, hand.Height, segThe vertically measured distance from the lowestpoint on the lens or lens opening to the level of thetop of the seg.Hollow toolSee Tool, hollow.HydrometerAn instrument used to measure the specific gravityof a liquid. In surfacing, the specific gravity of anabrasive slurry is measured in degrees Baume .HyperopiaFarsightedness.Hide-a-BevelTradename for an edge-grinding system thatproduces a shelf effect behind the bevel onthick-edged lenses.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 15
Glossary of Optical TerminologyIImbalance, verticalA differential vertical prismatic effect betweenthe two eyes.At near this can be induced by right and left lensesof unequal powers when the wearer drops his or hereyes below the optical center of the lenses.Implant, intraocular lensA plastic lens placed inside the eye as areplacement for a crystalline lens that lost its clarity.Index, refractiveThe ratio of the speed of light in a medium(such as air) to the speed of light in anothermedium (such as glass).InsetThe amount of lens decentration nasally fromthe boxing (or datum) center of the frame’s lensaperture.(Antonym: outset.)Inset, net segThe amount of additional seg inset (or outset)required to produce a desired amount of horizontalprismatic effect at near, added to the normal seginset required by the near PD Inset, segThe lateral distance from the major referencepoint to the geometric center of the segment.Inset, totalThe amount the near segment must move from theboxing (or datum) center to place it at the near PD(near centration distance).IntermediateThe area of a trifocal lens between the distanceviewing portion and the near portion.Interpupillary distanceSee Distance interpupillary.Intraocular lens implantInset, blank segThe horizontal distance from the blank geometriccenter to the center of the multifocal segment.See Implant, intraocular lens.Iseikonic lensesSee Lenses, iseikonic.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and published by Butterworth-Heinemann.page 16
Glossary of Optical TerminologyKKnife-edgeA plus lens ground to an absolute minimumthickness such that the edge of the lens is sothin that it has a knifelike sharpness to it, i.e.,an edge thickness of zero.LLaminated lensSee Lens, laminated.LapA tool having a curvature matching that of thecurvature desired for a lens surface.The lens surface is rubbed across the face of thetool and, with the aid of pads, abrasives, & polishes,the lens surface is brought to optical quality.Lap cutterSee Cutter, lap.Lap gaugeSee Gauge, lap.Lap toolSee Tool, lap.LayoutThe process of preparing a lens for blockingand edging or surfacing.Length, datumThe horizontal width of a lens or lens openingas measured along the datum line.Lens, blended myodiscA minus lens, lenticular in design, with the edgesof the bowl blended so as to improve the cosmeticaspect of the lens.Lens center locatorSee Locator, lens center.Lens, coverA thin lens that is temporarily glued to the surface ofa semifinished blank in order to protect the surfaceof the lens and facilitate accurate grinding, as in thecase of a slab-off grind on a glass lens.Lens, finishedA spectacle lens that has been surfaced on bothfront and back to the needed power and thickness.A finished lens has not been edged for a spectacleframe, but is still in uncut form.Layout markerSee Marker.www.feaind.com800.327.2002Glossary was taken from “Understanding Lens Surfacing,”written by Clifford W. Brooks, O.D., and pub
Glossary of Optical Terminology Anisometropia A condition in which one eye differs significantly in refractive power from the other. Antireflection coating See Coating, antireflection. Antiscratch coating See Coating, antiscratch. Aperture An opening or hole that admits only a portio
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