Q Academy


The Snellen fraction that indicates normal visual acuity. (See Snellen acuity, Snellen chart, and Snellen fraction.)

“A” measurement

The horizontal width of a lens shape’s box measurement. (See Box measurements.)

Abbe value

A relative measure of a lens material’s dispersive power. The Abbe value of commonly-used eyeglass lens materials generally ranges from 20 to 60. Lenses with higher Abbe values have less chromatic aberration.


An optical defect causing blurred or distorted vision. In eyeglass lenses, aberrations are caused by a defect in the lens design or fabrication, or by optical limitations of the lens material. (See Chromatic aberration.)


The process in which light energy is converted into a different energy form (usually heat). Sunglass lenses use absorptive tints and filters to
absorb excess light and reduce glare. (See polarizing filter.)


Sharpness of vision. Visual acuity is usually measured with a Snellen chart and recorded as a Snellen fraction (e.g. 20/20).

Add power

In a multifocal lens, the added magnification power prescribed for that portion of the lens to correct the wearer’s near vision.


Acronym for the American National Standards Institute, the organization that makes quality assurance recommendations to the eyewear industry.

Anti-reflective coating

A vacuum-deposited coating applied to lenses to decrease surface reflections and increase light transmittance. Anti-reflective (AR) coatings make eyeglass lenses more attractive and provide better night vision, compared to uncoated lenses.

AR coating

See Anti-reflective coating.


Not spherical. Refers to the front surface of an eyeglass lens. The front surface of an aspheric lens gradually flattens or steepens from the center of the lens to its edge. Aspheric lenses have a slimmer, more attractive profile and provide sharper peripheral vision than conventional spherical lenses.


A refractive error resulting from unequal curvature in different radial meridians of the eye. Astigmatism is usually due to unequal curvature in the cornea. For example, the 12-to-6 o’clock meridian of the cornea may be steeper or flatter than the 3-to-9 o’clock meridian. An analogy often used to describe astigmatism is that the eye is shaped like a football instead of like a baseball. Astigmatism affects vision at all distances, and is corrected by eyeglass lenses that have cylinder power.


A computerized screening device used in an eye exam to detect and measure refractive errors (i.e. nearsightedness, farsightedness and astigmatism).


Refers to the meridian of least power (on a 1-to-180 degree scale) in an eyeglass lens with cylinder power for the correction of astigmatism. The axis is the third number in an eyeglasses prescription, following the “x.” For example, in the prescription -3.00 -1.00 x 180, the lens power includes 1.00 diopter of cylinder power, with its axis at the 180 (horizontal) lens meridian.

“B” measurement

The vertical height of a lens shape’s box measurement. (See Box measurements.)

Base curve

Generally refers to the curvature of the front surface of a lens. A limited range of prescriptions can be created on a semi-finished lens of a given base curve. The base curve determines the profile (or “bulge”) of the front of the finished lens.


The V-shape cut into the edge of an eyeglass lens so the lens fits securely in the groove of the frame’s rim or eyewire.


A lens shape where both the front and back surfaces have a concave (or minus power) shape. Used only on very strong lenses for the correction of high degrees of myopia.


A lens with two distinct powers – one for distance vision and one for near vision. Bifocal lenses are generally used to correct presbyopia.

Binocular PD

The distance between the pupils. Generally measured in millimeters. (Also see Monocular PD.)

Blank size

The overall diameter of an eyeglass lens before it is cut to fit the size of the frame.

Box measurements

A standardized way to measure the size of an eyeglass lens. The box measurements of a lens or frame are the horizontal (“A” measurement), vertical (“B” measurement) and diagonal (“ED” measurement) of an imaginary rectangle drawn around the lens shape.


The part of the eyeglass frame that rests on the wearer’s nose.


Clouding of the lens inside the eye. Wearing sunglasses that block 100% of the sun’s UV rays may decrease the risk of cataract formation.

Chromatic / -chromic

Of or pertaining to color.

Chromatic aberration

An optical imperfection wherein white light is broken up into a number of colors, with each color being refracted differently by the lens, similar to the effect of a prism.

Color vision deficiency

The inability to distinguish certain colors, most commonly, shades of red and green. Often (incorrectly) called “color blindness.”


A shape where the center of the lens surface is depressed, compared to a flat surface. The opposite of a convex shape, where the center is elevated. In most eyeglass lenses, the front surface is convex and the back surface is concave. Also called a minus power (or “–”) surface.

Converge / Convergence

The inward movement of the eyes when we look at close objects. Accounts for why the distance PD measurement differs from the near PD measurement.


A shape where the center of the lens surface is elevated, compared to a flat surface. The opposite of a concave shape, where the center is depressed. In most eyeglass lenses, the front surface is convex and the back surface is concave. Also called a plus power (or “+”) surface.


The clear front surface of the eye that refracts (bends) light and allows it to enter the eye to create visual images.


The trade name for the original and most popular plastic material used for eyeglass lenses. The abbreviation stands for “Columbia Resin #39,” because it was the 39th formula of a thermosetting plastic developed by the Columbia Resins project of PPG Industries in 1940. The first commercial use of CR-39 monomer was to help create lighter, more durable fuel tanks for the B-17 bomber aircraft in World War II. After the War, the Armorlite Lens Company in California is credited with manufacturing the first CR-39 eyeglass lenses in 1947. CR-39 plastic has a refractive index of 1.498.

Crown glass

The type of glass used for eyeglass lenses. Because glass is much heavier and less impact-resistant than plastic and polycarbonate, it is no longer a popular material for eyeglass lenses. Crown glass has an index of refraction of 1.523.

Cylinder power

Power within an eyeglass lens to correct astigmatism. Characterized by unequal lens curvature in different meridians so the least powerful and most powerful meridians of the lens are at a right (90-degree) angle to each other.


Abbreviation for “distance between lenses.” The horizontal distance between the two lenses in an eyeglass frame, measured across the bridge of the frame. Typically, the DBL is measured in millimeters.

Demo lenses

Thin, non-prescription plastic or acrylic lenses inserted in eyeglass frames for display purposes only.

Dispersive power

The ability of a lens material to separate white light into its component colors. Lens materials with a low Abbe value have a higher dispersive power, resulting in chromatic aberrations.

Distance PD

The measurement of the distance between a person’s pupils when they are looking at a distant object.


A special type of occupational bifocal lens that has two near segments – one in the top half of the lens to see close objects above the wearer’s normal line of sight, and one in the standard position in the bottom half of the lens. Double-D bifocals are very helpful for auto mechanics and other workers who have to frequently see near objects above their head.

Diopter (D)

The standard unit used to measure the refractive power of a lens. Preceded by a “–” for minus power lenses that correct nearsightedness and nearsighted astigmatism. Preceded by a “+” for plus power lenses that correct farsightedness. Lens powers in eyeglass prescriptions are usually specified in quarter-diopter increments (e.g. -1.25 D, -1.50 D, -1.75 D, etc.).

Drop Ball Test

The FDA-required test to measure the impact resistance of eyeglass lenses. A steel ball of a specified size and weight is dropped from a specified height onto the lens surface. To pass the test, the lens cannot chip, crack or break. Lens manufacturers are allowed to test a sampling of their lenses rather than every lens. Safety glasses have higher impact resistance requirements than regular eyeglass lenses.

Duty to Warn

The responsibility of eyecare and optical professionals to fully inform consumers about the relative impact resistance of lens materials. Polycarbonate lenses provide the best impact resistance and should be recommended for children’s eyewear and for sport and safety eyewear.

“ED” measurement

The diagonal box measurement of a lens shape. The ED (“effective diameter”) measurement determines the required blank size of the lens in order for it to fit in the frame. (See Box measurements.)

Electromagnetic energy

The energy formed when a electrical field comes in contact with a magnetic field. Visible light is one form of electromagnetic energy. Others include radio waves, microwaves, ultraviolet (UV) radiation, and X-rays. Electromagnetic energy travels in radiating waves, and its strength is determined by its wavelength.


A bifocal or trifocal lens where the near segment (and intermediate segment, in the case of a trifocal) extend across the entire width of the lens.

Eye size

Refers to the width of the lens opening of an eyeglass frame, measured at the midway point between the top and bottom of the lens opening.

Facets / Faceting

Diagonal cutting of the lens edge to create reflective surfaces for cosmetic reasons.


The ability to see with greater ease far away than up close. The medical/optometric term for the condition is hyperopia. Farsightedness is a common refractive error and can be corrected with single vision eyeglass lenses. It is often confused with presbyopia.

Finished lens

A lens that has been surfaced to the specifications of a prescription. If the lens has also been shaped and edged to fit a frame, it is called a “cut” finished lens. If it has not yet been shaped and edged to fit in a frame, it’s called an “uncut” finished lens.

Fitting height

The proper placement of the seg height of a bifocal or trifocal lens so the near zone of the lens (and intermediate zone, in the case of a trifocal) can be comfortably accessed by the wearer without interfering with their distance vision.

Flat top (FT)

The most common type of bifocal and trifocal lenses. Called this because the near segment (and intermediate segment, in the case of a trifocal) is flat (horizontal) at the top. Also called a “Straight Top” (ST) or “D seg.”


See Executive.


Abbreviation for “finished single vision” lens. These are single vision lenses that have been surfaced to the specifications of an eyeglasses prescription. (See Finished lens.)


The most scratch-resistant lens material. But because glass lenses are significantly heavier and less impact-resistant than lenses made of other materials, glass has limited popularity as an eyeglass lens material. (See Crown glass.)

Gradient tint

A lens tint that is darkest at the top of the lens and gradually gets lighter from top to bottom.


Refers to the process of using automated surfacing tools to cut the proper curves on the back surface of a semi-finished lens blank to create the desired finished prescription lens.


See scratch-resistant coating.

Hard resin

The term generally used to describe conventional plastic lenses. (See CR-39®.)

High energy visible (HEV) light

The violet and blue portions of the visible light spectrum: high energy light with wavelengths from 380 to 500 nm. Some researchers feel long-term exposure to HEV light may contribute to macular degeneration. For this reason, many eyecare professionals recommend wearing sunglasses that block HEV light as well as UV radiation.

High index

Used to describe any lens material that has a higher index of refraction than crown glass (1.523) or CR-39 plastic (1.498). High index lenses are thinner than glass or plastic lenses of the same power. Generally, the higher the index, the thinner the lens. Popular high index plastic lens materials have indices of refraction ranging from 1.54 to 1.74.


Literally, “water hating.” Most anti-reflective (AR) coatings for eyeglass lenses include an outer hydrophobic layer that keeps the lenses clean longer and prevents water spots.


See Farsightedness.

Index of refraction

Also called refractive index. A relative measure of a lens material’s ability to refract (bend) light. The higher the index, the more efficiently the material is at bending light. Therefore, high index
eyeglass lenses are thinner than lenses with a lower refractive index. The refractive index of popular lens materials ranges from 1.50 (hard resin plastic) to 1.74 (ultra high index plastic).


The amount of inward displacement of the reading segment or near zone in multifocal eyeglass lenses so it is aligned with the eyes during reading, taking into account the normal convergence of the eyes when looking at close objects.

Intermediate zone

Refers to the middle zone of sight, at approximately arm’s length. Computer use is a good example of a visual activity in the intermediate zone of vision. If a person with presbyopia cannot see clearly in the intermediate zone with bifocal lenses, they could benefit from trifocals or progressive lenses.

Interpupillary distance (IPD)

The distance between a person’s pupils. More commonly called pupil distance (PD).


The part of the inside of the eye that gives our eyes their color. The iris surrounds and controls the size of the pupil.

Lens blank

A thick, semi-finished lens capable of having its back surface modified by an optical lab to create a finished prescription lens.

Lensometer / Lensmeter

A manual or automated device that measures the power of eyeglass lenses.

Macular degeneration

An age-related breakdown and loss of function of the tissues in the most sensitive part of the retina (the macula). Age-related macular degeneration (or AMD) causes a loss or distortion of central vision and can be very debilitating, making it difficult to read, recognize faces, or see road signs. AMD is the leading cause of blindness in Americans 65 years of age and older.


Any of a number or imaginary, radially-arranged lines superimposed on the eye (to localize astigmatism) or on an eyeglass lens (to localize cylinder power.) Meridians are numbered in degrees, as they are on a protractor scale, beginning with 001 on the right side of the lens and ending at 180 on the left side. The vertical meridian is the 90° meridian, and the horizontal meridian (to the left of center) is the 180° meridian.

Mirror coating

A highly-reflective coating applied to the front surface of sunglass lenses to reduce the transmittance of light through the lens. Mirror coatings are applied using a vacuum deposition process, similar to the process for applying anti-reflective (AR) coatings.


Refers to one eye.

Monocular PD

An individual pupil distance (PD) measurement for one eye. The measurement refers to the horizontal distance from the center of the pupil to the center of the bridge of the nose. Monocular PD measurements are required to properly fit progressive lenses.


Refers to a lens with more than one power. Bifocals, trifocals and progressive lenses are all examples of multifocal lenses.


See Nearsightedness.

Nanometer (nm)

The unit used to measure the wavelength of light and ultraviolet (UV) radiation. A nanometer (nm) is equal to 1 millionth of a millimeter (mm).


The side of a lens or frame that is closest to the wearer’s nose, as opposed to the temporal side, which is farthest from the nose.


The ability to see things close up but not far away. The medical/optometric term for the condition is myopia. Nearsightedness is a common refractive error and can be corrected with single vision
eyeglass lenses.


See Nanometer.


Latin abbreviation for “right eye.” O.D. is also the abbreviation for the professional title, Doctor of Optometry.


Latin abbreviation for “left eye.”


Latin abbreviation for “both eyes.”

Occupational lens

Any lens designed or prescribed for a specific visual task or work activity. Single vision “computer lenses” and special design multifocals (e.g. “Double-D” bifocals) are examples of occupational lenses.


Pertaining to the eye. Eyeglass lenses are also called “ophthalmic lenses” because they are worn in front of the eyes.


A medical doctor (M.D.) specializing in eye care. Ophthalmologists can prescribe eyewear and treat eye health problems with medications and surgery.

Optical center (OC)

The refractive center of the lens, where there is no prism power present. In a plus lens, the optical center is located at the thickest point on the lens. In a minus lens, the optical center is at the thinnest point of the lens. In general, eyeglass lenses should be positioned so the optical center of the lens is directly in front of the center of the wearer’s pupil.


An optical professional trained to fit and adjust eyewear. Training requirements for opticians vary from state to state. In some states, opticians are also allowed to fit contact lenses.


A Doctor of Optometry (O.D.). Optometrists are not licensed to perform eye surgery, but they can prescribe eyewear and treat a variety of eye problems with medications.


Abbreviation/acronym for “progressive addition lens.” (See Progressive lenses.)

Pantoscopic tilt

The vertical angle that exists in a properly-fit eyeglass frame so the top of the frame is slightly farther from the face than the bottom. A normal pantoscopic tilt is 10 to 15 degrees from vertical.


Abbreviation for “pupil distance.” (See Binocular PD and Monocular PD.)


Literally, “light coloring.” Refers to lenses that automatically darken in response to sunlight, then return to a clear state indoors. The #1 manufacturer of photochromic technology for plastic and high index eyeglass lenses is Transitions Optical. For this reason, photochromic lenses are sometimes called “Transitions lenses.”


See Photochromic.


The altering of light after it strikes a reflective surface so that light waves travel in a less random fashion. The result is an extremely bright refection of light that causes significant glare.


Refers to sunglasses that contain a special filter to reduce glare from reflected light. Polarized lenses are especially useful when performing activities in high-glare environments, such as boating, fishing, driving, skiing, or relaxing at the beach. Also, light reflecting from water, snow, sand and other surfaces is called polarized light.

Polarizing filter

The thin filter within the lenses of polarized sunglasses that selectively absorbs light rays of a specific orientation. This filter reduces glare from reflected light significantly better than regular sunglasses.

Polycarbonate (PC)

A lightweight and extremely impact-resistant high index eyeglass lens material. Polycarbonate lenses are up to 10 times more impact resistant than glass or plastic lenses, making them the preferred choice for safety glasses, sports glasses, children’s eyewear, and for anyone who has an active lifestyle. Polycarbonate lenses are also lighter and less expensive than lenses made of other high index lens materials, making them an excellent value for anyone wanting thin, light and affordable eyeglass lenses. Polycarbonate has a refractive index of 1.59.


The normal age-related loss of near focusing ability in the eye that becomes apparent to most people sometime after age 40. Vision problems due to presbyopia can be corrected with multifocal lenses or reading glasses.

Prism power

An angular deviation of light as it passes through a lens. Sometimes, an eye doctor may specifically add prism power to an eyeglass prescription to treat an unusual vision problem. Prism power can also occur if the optical center of the lens is not properly aligned in front of the wearer’s pupil. Unwanted prism power can cause eyestrain and headaches.

Progressive addition lens

See Progressive lenses.

Progressive corridor

The part of a progressive addition lens (PAL) that runs down the center of the lens and contains the power changes necessary for clear vision at intermediate distances (i.e. arm’s length) and up close.

Progressive lenses

Multifocal lenses that gradually change in power from the top of the lens (which contains the power for distance vision) to the bottom of the lens (which contains the add power for reading and other close-up tasks) with no visible lines in the lens separating the powers. Progressive lenses are also commonly called progressive addition lenses (PALs) and “invisible bifocals.” Although the latter is a misnomer because a progressive lens has many lens powers, whereas a bifocal lens has only two powers – one for distance and one for near.


The circular opening in the eye that is surrounded by the iris. The size of the pupil determines how much light enters the back of the eye to be focused on the retina.

Pupil distance (PD)

The distance between a person’s pupils (binocular PD) or the distance from the center of pupil to the center of the bridge of the nose (monocular PD).


A manual or electronic device used to measure a person’s binocular or monocular pupil distance (PD).


Slang for reading glasses.

Reading glasses

Glasses prescribed or made specifically for reading and other near vision tasks. Most reading glasses are single vision lenses.

Refractive error

Any of the three common vision problems of nearsightedness, farsightedness and astigmatism.

Refractive index

See Index of refraction.


The inner lining of the back of the eye where light energy is transformed into electrical impulses that the brain then interprets to create visual images.


Conventional symbol to indicate a doctor’s prescription. Eyeglasses and contact lenses must be prescribed by a licensed eye doctor.

Scratch-resistant coating (SRC)

A microscopic coating applied to eyeglass lenses to increase the hardness of the front and back surface. Lenses with a scratch-resistant coating are more durable and resist surface scratches significantly better than uncoated lenses. Glass lenses are the only lenses that don’t benefit from a scratch resistant coating.

Seg / Segment

The specific part of a bifocal or trifocal lens that contains the lens power for near vision (and intermediate vision, in the case of a trifocal).

Seg height

The position of the top border of a bifocal or trifocal segment within an eyeglass frame. Commonly measured from the top of the segment to the lowest point on the finished lens.

Semi-finished lenses

Lens blanks that have the front surface of the lens completed by the manufacturer, but the back surface is left unfinished so the optical lab can grind the curves on this surface to create the desired finished prescription lens. Because the final prescription has not yet been ground on the back surface, semi-finished blanks are much thicker than finished prescription lenses. Semi-finished lenses can be either single vision or multifocal lenses.


Abbreviation for “semi-finished single vision” lens. These are single vision lenses that the front surface of the lens has been completed by the manufacturer, but the back surface is left unfinished so the optical lab can grind the curves on this surface to create the desired finished prescription lens.

Single vision lenses

Lenses that have the same power throughout the entire lens, as opposed to multifocal lenses that contain two or more powers. Single vision lenses are typically used to correct the common refractive errors of nearsightedness, farsightedness and astigmatism, or to create reading glasses for the correction of presbyopia.

Snellen acuity

The most common measurement of clarity of eyesight (visual acuity). The measurement is accomplished by use of a wall chart with lines of letters in successively smaller sizes (the Snellen chart). The measurement is named after Hermann Snellen, a Dutch ophthalmologist who is credited with developing the technique in 1862.

Snellen chart

The common wall chart of letters used to measure visual acuity. The letters on the chart get progressively smaller from top to bottom. The top of the chart usually has the large single letter “E,” which corresponds to a Snellen fraction of 20/400. The small letters on the bottom of the chart correspond to visual acuity of 20/20 or better.

Snellen fraction

The visual acuity measurement attained by using a Snellen chart, expressed in the form of a fraction, such as “20/20.” In the Snellen fraction, the top number (typically “20”) refers to the testing distance (usually 20 feet). The bottom number is the distance at which a person with normal vision can still see the letter on that line of the chart. For example, if the smallest letters you can see on the chart are on the 20/40 line, your vision is worse than normal, because someone with normal vision could be 40 feet away from the chart and still see letters of that size.

Conversely, if you can see letters on the bottom of the chart that are on the 20/15 line, your vision is better than normal, because a person with normal vision can only see those letters when they are closer to the chart – at a maximum distance of 15 feet.

Solid tint

A tint that has the same density (darkness) throughout the entire lens.


The act of grinding the back curve on a semi-finished lens blank to create a finished prescription lens.


The side of a lens or frame that is farthest from the wearer’s nose, as opposed to the nasal side, which is closest to the nose.


A colored dye applied to an eyeglass lens, either for fashion purposes (cosmetic tint) or for protection from sunlight (sunglass tint).


A very lightweight, strong and hypoallergenic metal that has become a popular material for eyeglass frames.


A lens with three distinct lens powers – one for distance vision, one for intermediate vision (e.g. computer use), and one for reading and other close-up tasks.

Ultraviolet radiation / UV rays

Invisible electromagnetic radiation that has higher energy than visible light. UV rays can cause sunburn and have been associated with cataracts and other eye health problems.

UV protection

The degree to which a lens protects the wearer’s eyes from the sun’s harmful ultraviolet (UV) rays.


Lower energy ultraviolet radiation that tans the skin. UVA rays have wavelengths in the range of 320 to 380 nm. (Some sources place the upper limit at 400 nm). Although they have less energy than UVB radiation, UVA rays can penetrate deeper into the eye and may contribute to the development of cataracts and macular degeneration. Good quality sunglasses should block 100% of UVA rays.


Higher energy UV rays that cause sunburn and can cause premature aging of the skin, photokeratitis (“snow blindness”) and other eye health problems. UVB rays have wavelengths in the range of 290 to 320 nm. Good quality sunglasses should block 100% of UVB rays.

Vertex distance

The distance between the front surface of the cornea and the back surface of an eyeglass lens.

Visible light

Electromagnetic energy that is visible to the human eye. Visible light has wavelengths ranging from approximately 380 to 700 nm.

Visual acuity

Sharpness of vision. Usually measured with a Snellen chart and recorded as a Snellen fraction (e.g. 20/20).


The distance between two identical points on adjacent electromagnetic waves. The shorter the wavelength, the higher the energy (frequency) of the wave – and the more potential it has to cause harm to our eyes and bodies. Visible light has wavelengths ranging from approximately 380 to 700 nm. The wavelengths of high energy visible (HEV) light range from about 380 to 500 nm. Ultraviolet (UV) radiation from the sun that reaches the Earth’s surface has wavelengths ranging from 290 to 380 nm. Some sources place the break between UV radiation and visible light at 400 nm.