Glossary

A number that rates how much a material spreads white light into colours. The higher the Abbe value, the less chromatic aberration the wearer sees.

Any way a lens fails to bring light from one object point to one sharp image point, so the image is blurred, coloured or bent out of shape.

The eye’s ability to focus up close. The ciliary muscle contracts, the zonules relax and the crystalline lens bulges, adding plus power.

Describes a lens or lens system that brings the colours of light to the same focus, so it shows no colour fringes.

The extra plus power a multifocal gives for near, on top of the distance Rx.

The structures around the eye that support it: lids, lashes, tear glands and ducts.

Corneal astigmatism in which the horizontal meridian is steeper, for example K readings of 44.00 @ 180 and 42.25 @ 090. Minus cylinder axis near 090.

An inherited lack of pigment, often with reduced acuity, nystagmus and strong light sensitivity. Tinted or opaque-iris lenses can help.

Reduced vision in an otherwise healthy eye that lenses cannot bring to 20/20, because the eye was not used well in childhood. Patching can help when started young.

Any refractive error, meaning distant light does not focus on the retina with accommodation relaxed: myopia, hyperopia or astigmatism.

Being born without an iris (or with very little iris).

The two eyes see images of different size or shape, which can make fusion uncomfortable or impossible. Often follows large anisometropia or antimetropia.

A large difference in refractive error between the two eyes, often taken as 2.00 D or more. Contact lenses avoid much of the image-size difference glasses would cause.

The fluid-filled space between the cornea and the iris, filled with aqueous humor.

One eye is myopic and the other hyperopic.

No crystalline lens in the eye, usually after cataract surgery without an implant. With an intraocular lens implant the eye is pseudophakic.

The watery fluid, index 1.33, that fills the front of the eye, feeds the cornea and lens, and sets eye pressure. It drains through the trabecular meshwork into the canal of Schlemm.

A grey-white ring of lipid deposit near the edge of the cornea, common with age.

Drops that add moisture to a dry eye or a lens. Use drops labelled safe for the lens type.

A lens with a surface that gradually changes curvature toward the edge instead of keeping one radius. It lets lenses be flatter and thinner with less off-axis blur.

A refractive error in which the eye has different power in different meridians, so light forms two focal lines instead of one point.

The meridian of a cylinder lens that has no cylinder power, written from 001 to 180 in three digits.

A soft lens worn to protect a healing or painful cornea rather than to correct vision.

The base is the thickest edge of a prism and the apex is the thinnest. Light bends toward the base; the image moves toward the apex.

For a spectacle lens, the front surface curve that a lens series is built on. For a contact lens, the curvature of the back optic zone that sits against the cornea.

The radius of the back optic zone of a contact lens in millimeters. A smaller radius is steeper. Convert with BC (mm) = 337.5 ÷ K (D).

Binocular means using both eyes together; monocular means one eye.

Inflammation of the lid margins, often with crusting and blocked oil glands, which makes lens wear less comfortable.

A tough, thin layer just under the epithelium. Damage to it can leave a permanent scar.

Severe corneal swelling with painful fluid blisters on the surface, from endothelial failure. A bandage soft lens can ease the pain.

A hand lamp with blue (ultraviolet) light and a magnifier, used to view fluorescein under a rigid lens.

The circular channel in the angle where the cornea meets the iris, through which aqueous humor leaves the eye. Poor drainage raises eye pressure.

The corner where the upper and lower lids meet: inner (nasal) or outer (temporal).

Any clouding of the crystalline lens. Most are age related; others are congenital or follow injury or medication. Treated by removing the lens, usually with an implant.

The thickness of a contact lens at its center, in millimeters.

A painless lump in the lid from a blocked, inflamed Meibomian gland.

Swelling of the conjunctiva with fluid.

The straight-line width of a contact lens from edge to edge.

The layer of blood vessels between the sclera and the retina that nourishes the outer retina.

Colour fringing caused by a lens bending each wavelength by a different amount. Lower-Abbe materials show more of it, especially when the wearer looks away from the optical center.

The ring of muscle and tissue behind the iris. Its muscle controls accommodation, and it makes the aqueous humor.

Removing deposits, debris and some germs from the lens surface, usually by rubbing. Cleaning alone does not disinfect.

Both focal lines fall behind the retina, as in +1.50 +1.00 × 020 (or +2.50 −1.00 × 110).

Both focal lines fall in front of the retina, as in −2.00 −0.75 × 170.

A solution that stores, disinfects and wets rigid lenses between wears.

Retinal receptors for colour and fine detail in good light, packed most densely in the fovea.

A thin, clear membrane covering the white of the eye (bulbar conjunctiva) and lining the inside of the lids (palpebral or tarsal conjunctiva).

Inflammation of the conjunctiva from bacteria, viruses, allergy or lens wear. Treat any red eye as possibly contagious and disinfect equipment afterwards.

Converging rays come together toward a point, as after a plus lens; diverging rays spread apart from a point, as after a minus lens.

The clear dome at the front of the eye and its strongest refracting surface, about 40 to 46 D, index 1.37. It has five layers and no blood vessels, and it is where a contact lens rests.

A scrape of the corneal epithelium. It stains with fluorescein and is painful but usually heals within a few days.

The central zone of the cornea, where curvature is close to constant. The periphery flattens toward the limbus.

The relatively dehydrated state the healthy cornea keeps, mainly through the endothelial pump, which is what keeps it transparent.

Swelling of the cornea, most often from too little oxygen under a lens. It can cause haze, halos and spectacle blur.

An instrument that maps the whole corneal surface by projecting rings on it (the idea behind the Placido disc) and measuring their reflection.

A sore where corneal tissue is lost, from infection, hypoxia or injury. A lens wearer with a painful red eye stops wear and sees the doctor the same day.

A change in corneal shape caused by a lens pressing on it, or a permanent bend in a rigid lens itself.

The standard glass lens material, with an index of 1.523. Clear and scratch resistant but heavy.

The clear, flexible, biconvex lens behind the pupil, index about 1.42. It changes shape to focus at near.

Temporary paralysis of the ciliary muscle with eye drops, so the doctor can measure refractive error with accommodation relaxed.

A lens with power in one meridian only, used to correct astigmatism. Combined with a sphere it makes a sphero-cylinder.

A detergent (surfactant) rubbed on the lens to loosen deposits and debris. It cleans but does not disinfect, so it is rinsed off and followed by a disinfecting step.

How far the optical center is moved away from the geometric center of the lens shape, in millimeters, to line it up with the pupil.

A shallow, dried-out dip in the corneal surface beside a raised area, because the tear film cannot wet it.

An elastic layer between the stroma and the endothelium.

A set of trial lenses in a range of base curves and powers, used to judge the fit on the eye before ordering.

The scattering of light as it passes through or reflects from a rough surface.

The unit of lens power: one over the focal length in meters. A +2.00 D lens focuses parallel light 0.5 m (50 cm) behind it.

Seeing one object as two. Sudden double vision in an adult needs prompt referral to the doctor.

Killing the germs likely to cause eye infections. It does not have to kill every organism.

A lens defect that makes straight lines look curved because magnification changes from the center of the lens to the edge.

How freely oxygen moves through a lens material: D is diffusion and k is solubility. It belongs to the material, whatever the thickness.

Oxygen transmissibility of an actual lens: the material’s Dk divided by its thickness. Thicker lenses (high plus, toric ballast) pass less oxygen.

Thinned zones at the top and bottom of a toric soft lens that the lids squeeze to hold the lens at the right axis.

The power a lens actually delivers at a new position. Moving a plus lens away from the eye makes it act stronger; moving a minus lens away makes it act weaker.

No refractive error: with accommodation relaxed, distant objects focus on the retina.

A serious inflammation or infection inside the whole eye.

The single inner layer of corneal cells that pumps water out of the stroma to keep it clear. Its cells do not regrow, so losing them can leave the cornea swollen.

Entropion is a lid margin turned inward, so lashes rub the eye; ectropion is a lid margin turned outward, away from the eye.

A liquid or tablet that breaks down the protein deposits tears leave on a lens, used daily or weekly as the eye care provider directs.

The outermost layer of the cornea, a few cells thick. It heals quickly (usually within days) and is the layer most often scratched by a lens.

A device with a fine adjustable nylon thread used to test corneal sensitivity.

Sleeping in lenses. It lowers oxygen to the cornea and raises infection risk, so only lenses approved for it are worn that way.

Awaiting expert review: this definition goes beyond the course handouts.

Folds of skin and muscle that protect the eye, spread the tear film with each blink and limit light. “Palpebral” means to do with the lids.

The farthest point the eye sees sharply with accommodation relaxed. For a myope it lies in front of the eye at 1 ÷ the Rx power.

Streaks or glare around lights when the edge of the optic zone falls across the pupil, from a small optic zone, a decentered lens or a loose fit.

A cornea with K readings below about 41.00 D. Above about 45.00 D is called steep.

A lead-containing glass with a high index (about 1.65 or more) and more chromatic aberration than crown glass.

An orange dye that glows green under cobalt blue light. It shows the tear layer under a rigid lens and stains damaged corneal epithelium.

The distance from a lens to the point where it brings parallel light to a focus (plus lens) or from which that light appears to spread (minus lens). It equals 1 ÷ power in diopters, in meters.

The point where light rays meet after a plus lens (a real focus) or appear to come from after a minus lens (a virtual focus).

The small pit at the center of the macula, made almost entirely of cones, where vision is sharpest.

An inherited loss of endothelial cells that leaves the cornea swollen and hazy, often worst on waking.

The brain combining the images from the two eyes into one. Breaking fusion (covering an eye) reveals a phoria.

The center of the boxed lens shape, where the diagonals of the box cross.

Empty corneal vessels left behind once the cause of neovascularization is removed.

Large bumps on the conjunctiva under the upper lid, usually a reaction to deposits on lenses. Patients report itching, mucus and a lens that rides up.

Damage to the optic nerve, usually with raised eye pressure from poor aqueous drainage. It steals side vision first and often has no symptoms, so screening matters.

A soft lens material that holds water. Oxygen travels through the water, so more water content generally means more oxygen.

Awaiting expert review: this definition goes beyond the course handouts.

A preservative-free disinfecting system. The peroxide must be neutralized to saline (in the special case, over the labelled time) before lenses go in, or it burns.

Distant light focuses behind the retina, usually because the eye is short for its power. Corrected with plus lenses.

Too little oxygen reaching the cornea, usually from low-Dk lenses, overwear or sleeping in lenses. Leads to edema and, over time, new vessels.

The speed of light in air divided by its speed in a material. A higher index bends light more, so the same power can be made with flatter, thinner surfaces.

How far the near segment is moved in toward the nose from the distance optical center, so the reading area follows the eyes as they converge.

The coloured muscular ring behind the cornea that changes the size of the pupil to control how much light enters.

Astigmatism from an uneven corneal surface (keratoconus, scarring) that cylinders cannot correct. A rigid contact lens masks it with a tear lens.

A spectacle lens designed to change image size, used to equalize the images of the two eyes in aniseikonia.

Inflammation of the cornea. In a lens wearer, a painful red eye with a white spot on the cornea may be an infection (microbial keratitis) and needs same-day care.

Awaiting expert review: this definition goes beyond the course handouts.

Dryness of the cornea and conjunctiva from too few or poor-quality tears; a common reason lens wear becomes uncomfortable.

A progressive thinning of the cornea that bulges it into a cone, causing irregular astigmatism. Usually fitted with rigid or specialty lenses.

An instrument that measures the curvature of the central cornea in its two principal meridians (K readings).

Corneal curvature readings written as flat K and steep K with their meridians, for example 42.50 @ 180 / 44.00 @ 090. The difference is the corneal astigmatism.

The gland above and to the outer side of the eye that makes the watery (aqueous) part of the tears. “Lacrimal” means to do with tears.

Left Add, Right Subtract: the axis correction for a rotated toric soft lens. As you face the patient, if the rotation mark has turned to your left (clockwise), add the rotation to the axis; if it turned to your right, subtract it.

A three-pin gauge that reads the surface power of a lens, calibrated for crown glass (1.53).

An instrument that reads a lens’s sphere, cylinder, axis, add and prism, and marks its optical center.

Astigmatism that comes from the crystalline lens rather than the cornea.

A lens made with a small central optic and a thinner surrounding carrier, to cut the weight and thickness of high powers.

The ring where the clear cornea meets the white sclera. It holds the stem cells that renew the corneal epithelium and is a handy landmark for measuring PD and lens diameter.

A lens that moves too much: too flat, too small or too thick. Signs are decentering, edge awareness and vision that blurs right after a blink.

The small central area of the retina responsible for detailed central and colour vision.

The point on the lens blank that the edger holds and turns the lens around.

Oil glands in the lids that supply the outer lipid layer of the tear film, which slows evaporation.

A lens with a convex front and a concave back. Almost every spectacle lens is a meniscus, because the shape reduces off-axis blur.

A line through the center of a lens or the cornea at a given angle. Cylinder power is greatest in the meridian 90° from the axis.

An edge design that is thicker at the periphery, like a minus lens, so the upper lid can grip and lift a heavy high-plus lens.

One focal line falls in front of the retina and the other behind it, as in +0.50 −1.75 × 090.

Correcting one eye for distance and the other for near, usually the dominant eye for distance, so a presbyope can manage without readers.

An all-in-one soft lens solution that cleans, rinses, disinfects and stores. The most used system; still rub and rinse, and never top off the case.

Mydriasis is a dilated pupil (a mydriatic drug causes it); miosis is a constricted pupil (a miotic drug causes it).

Distant light focuses in front of the retina, usually because the eye is long for its power. Corrected with minus lenses.

New blood vessels growing into the normally clear cornea, often a sign of long-term hypoxia. Pannus is a band of vessels and tissue creeping in from the limbus.

Finding a lens’s power by cancelling it: with a trial lens of opposite power until there is no movement (hand neutralization), or with a lensometer.

Turning hydrogen peroxide into harmless water and oxygen, usually with the catalytic disc in the special case, before the lens can be worn.

Awaiting expert review: this definition goes beyond the course handouts.

A ready-made table that suggests lens parameters from measurements such as K readings and refraction.

Involuntary, rhythmic movement of the eyes. Contact lenses help because they move with the eye, keeping the optical center in front of the pupil.

Regular astigmatism whose principal meridians are well away from 90 and 180.

A rigid lens base curve that matches the flattest K reading.

Does not let light through.

Where the optic nerve leaves the eye. It has no rods or cones, so it forms the natural blind spot.

The nerve that carries visual signals from the retina to the brain.

The central part of a contact lens that carries the prescription. If it is too small or decenters, the patient sees flare.

The point on a lens where light passes straight through without being deviated, so there is no prism there.

A sketch of a lens showing the power in its two principal meridians, used to transpose, find prism and check toric lenses.

A distance far enough that light from an object arrives practically parallel. In refraction, 20 feet (6 m) is treated as infinity.

The bony socket in the skull that holds the eye.

Flattening the central cornea on purpose with special rigid lenses, usually worn overnight, to reduce myopia temporarily.

Eye exercises that train the two eyes to work together.

Acute corneal hypoxia from wearing lenses too long: severe pain begins a few hours after the lenses come out. More common with rigid lenses.

An instrument that measures corneal thickness.

The almond-shaped opening between the upper and lower lids. Its height and lid position affect how a lens centers and moves.

Flatter curves outside the optic zone of a rigid lens that follow the flattening cornea and let tears flow under the edge.

A hidden tendency of an eye to drift when fusion is broken; the eyes stay aligned while both are open. Esophoria drifts in, exophoria drifts out.

Discomfort or pain from light.

A yellowish raised patch on the conjunctiva beside the cornea, linked to sun, wind and dust. It stays off the cornea.

Zero power, written “pl”.

The original hard lens plastic. It lets essentially no oxygen through, so the cornea relies on tear exchange with each blink.

Prism induced by looking through a point away from the optical center: prism (Δ) = decentration in cm × lens power in diopters.

The age-related loss of accommodation as the crystalline lens stiffens, noticed as trouble reading up close in the early to mid 40s. Corrected with an add.

A chemical that keeps a solution free of germs once opened. Some patients react to preservatives (older ones such as thimerosal were well known for it), and some bind to soft lenses.

A wedge of transparent material with two non-parallel faces. It bends light toward its base, so an object seen through it appears shifted toward the apex.

Base-down prism (about 1 to 1.5 Δ) built into a toric or bifocal lens so it settles with the thick edge down and stops rotating.

The unit of prism power. One prism diopter deviates light by 1 cm at a distance of 1 m.

A wing-shaped growth of conjunctival tissue that creeps onto the cornea, linked to sun and wind exposure.

A drooping upper eyelid.

A small wire attached to the frame that props up a drooping upper eyelid.

The opening in the center of the iris. It widens in dim light and narrows in bright light and for near work.

The distance between the centers of the pupils in millimeters, for distance or near. A monocular PD measures from the center of the bridge to each pupil.

An instrument that measures the base curve radius of a rigid lens by focusing on its surface.

Awaiting expert review: this definition goes beyond the course handouts.

A ray is the path of light from one point; a pencil is a bundle of rays from one point; a beam is a bundle of pencils, such as the light from a whole object.

The bending of light when it passes at an angle from one medium into another of different index. In the exam room the word also means measuring a patient’s refractive error.

Astigmatism whose strongest and weakest meridians are 90° apart, so a cylinder lens can correct it.

How often a lens is thrown away and replaced: daily, every two weeks, monthly and so on. Wearing schedule is a separate idea: daily wear means lenses come out before sleep.

Awaiting expert review: this definition goes beyond the course handouts.

The astigmatism left over when a spherical rigid lens neutralizes the corneal astigmatism, usually because the crystalline lens has astigmatism too. Found as refractive cylinder minus corneal cylinder.

The light-sensing inner lining at the back of the eye. Its rods and cones turn light into nerve signals.

A hand-held light the examiner sweeps across the pupil to measure refractive error objectively.

A slit lamp technique that bounces light off a deeper structure (iris or retina) to show the cornea or lens from behind.

A firm lens that lets oxygen pass through. It gives crisp vision and masks corneal astigmatism, but needs an adaptation period.

Retinal receptors for dim light and side vision. They are most plentiful outside the macula and do not sense colour.

How deep the back of a lens is over a given diameter. Increasing sag (steeper BC or larger diameter) tightens a fit.

Awaiting expert review: this definition goes beyond the course handouts.

Sterile salt solution for rinsing. It does not disinfect, so it never replaces the disinfecting step.

Steeper Add Minus, Flatter Add Plus: when you steepen a rigid lens’s base curve you add minus to its power, and when you flatten it you add plus, to cancel the change in the tear lens.

A strip of filter paper hooked over the lower lid for 5 minutes to measure tear volume. Less than 10 mm of wetting suggests dry eye.

The tough white outer coat of the eye. With the cornea it forms the eye’s protective outer wall.

A blind or dim spot in the field of vision.

A soft lens material with silicone added, which carries far more oxygen than a traditional hydrogel.

Awaiting expert review: this definition goes beyond the course handouts.

One focal line on the retina and the other behind it, as in plano +0.75 × 045.

One focal line on the retina and the other in front of it, as in plano −1.25 × 090.

A multifocal lens whose distance and near zones sit in front of the pupil together, in rings or a gradual aspheric change; the brain picks the clear image.

A microscope with a bright, adjustable slit of light for examining the lids, cornea and lens fit at roughly 10× to 40× magnification.

Blurred vision with glasses for a while after removing contact lenses, from corneal swelling or reshaping. Long-lasting spectacle blur points to edema or warpage.

A slit lamp technique that uses the mirror-like reflection from a smooth surface, used to view the endothelium.

A lens with the same power in every meridian.

The sphere power that places the circle of least confusion on the retina: sphere + half the cylinder. Used to pick a spherical contact lens for a small cylinder.

Killing every microorganism, including spores. Lens care systems disinfect; they do not sterilize.

The two eyes do not point at the same object because of muscle or refractive imbalance. Managed with prism, vision therapy or surgery.

The thick middle layer of the cornea, made of regularly spaced collagen fibers that stay clear only while the cornea is kept dehydrated.

Tiny dots of fluorescein staining scattered on the corneal surface, from dryness, solution sensitivity or lens rubbing.

Seconds from a blink until the first dry spot shows in the fluorescein-stained tear film. Under 10 seconds suggests an unstable tear film.

The thin fluid layer over the cornea: an outer oil (lipid) layer, a middle watery (aqueous) layer and an inner mucin layer that helps tears cling to the eye (LAM, outside in).

The layer of tears trapped between a rigid lens and the cornea, which acts as a lens of its own. Steeper than K adds plus; flatter than K adds minus (SAM-FAP).

Disinfecting soft lenses with heat, an older method now rarely used.

A dial or digital caliper that measures lens center or edge thickness in fractions of a millimeter.

Drying and staining of the cornea at the 3 and 9 o’clock positions, typical with rigid lenses whose edges keep the lids from wetting that area.

A lens that moves too little: too steep or too large. Signs are little or no movement, vision that clears just after a blink, and redness or an imprint at the edge.

Adding fresh solution to old solution in the case instead of emptying it. It weakens disinfection and is a known infection risk.

A lens with cylinder power to correct astigmatism, kept from rotating by ballast, truncation or thin zones. Rotation marks show how far it has turned.

A rigid bifocal that rests on the lower lid so the eye looks through the near zone when it turns down to read.

Awaiting expert review: this definition goes beyond the course handouts.

Rewriting a sphero-cylinder in the opposite cylinder form without changing its power: add sphere and cylinder for the new sphere, flip the cylinder sign, and change the axis by 90°.

A constant, visible turn of one eye that fusion cannot overcome. Esotropia turns in, exotropia turns out.

Cutting a straight edge along the bottom of a lens so it rests on the lower lid and resists rotation.

A tapered channel with a millimeter scale; a rigid lens slid down it stops at its diameter.

How much light is converging (plus) or diverging (minus) at a given point, in diopters. Adding a lens’s power to the incoming vergence gives the outgoing vergence.

The distance from the back of a spectacle lens to the front of the cornea. Moving a strong lens closer or farther changes its effective power, which is why contact lens powers differ from spectacle powers.

Lens power measured from the back surface to the focal point. It is what a lensometer reads and what the Rx specifies.

The width of the visible iris (limbus to limbus), usually measured horizontally with a ruler, and used as a stand-in for corneal diameter when choosing lens diameter. Most adults measure about 10.5 to 12.5 mm.

Sharpness of vision, recorded as the smallest detail seen at a set distance (20/20 at distance, J1 at near).

The clear gel, index 1.33, filling the large chamber behind the lens. It helps hold the eye’s shape.

The angle a drop makes on the lens surface. A smaller angle means the tears spread better and the lens is more wettable.

A solution for rigid lenses that helps tears spread over the lens surface and cushions the lens when it goes in.

Corneal astigmatism in which the vertical meridian is steeper, for example K readings of 42.00 @ 180 and 44.50 @ 090. Minus cylinder axis near 180.

Fine fibers that hold the crystalline lens to the ciliary body. When the ciliary muscle contracts, they slacken and the lens rounds up.