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Optical Glossary & Dictionary

Helpful Eyewear, Vision and Optician Terms and Definitions

20/20 Vision – The clarity or sharpness of vision measured at a distance of 20 feet.

Abbe Value – A way to measure chromatic aberration in lenses.

Anti-Reflective coating – Anti-reflective coatings improve the quality and the value of your lenses. An anti-reflective coating reduces disturbing reflections. It makes your lenses more attractive. And you will experience the most precise, crisp and clear natural vision in a brilliance that is not possible with uncoated lenses.

ANSI Z87.1 Standards – Safety standards that must be met for glasses to be considered safety glasses. Watch the Oakley High Velocity Impact video to see the results!

Astigmatism – A misshape of the eye, where vision is blurred by an irregularly shaped cornea. The cornea, instead of being shaped like a sphere, is ellipsoidal (like an egg) and reduces the cornea's ability to focus light. Toric lenses make excellent contact lenses for astigmatism.

Axis – Indicates the placement in degrees of the astigmatic lens.

Backside CoatingPolycarbonate lenses and high-index plastic lenses are thinner and have flatter prescription curves and so require a hard backside coating to protect them from scratching.

Ballistic – Involves high velocity projectiles. Ballistic eyewear is shatter-resistant, rugged and exceeds ANSI Z87.1 safety and military MIL-STD-662 standards.

Base curve – The curve on the back surface of the contact lens.

Bifocal – Corrective eyewear lenses containing regions with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia, hypermetropia, and/or astigmatism.

Bridge size – The width between the lenses on a eyeglass/sunglass frame. Manufacturers typically measure this width at the widest point between the two lenses.

Blue Blocker – Lenses that block blue light.

Blue Blur – A condition of unclear vision due to the blue light waves being short and scattering easily in the visible light spectrum. A blue blocker lens is recommended to remedy this aversion in visual acuity.

Cataracts – A clouding of the lens of the eye that can lead to blindness.

Clip-on – A small device with sunglass lenses that hooks onto your prescription eyeglasses. Clip-ons are handy, convenient and easy to use.

Color-Coated Lenses – Lenses that have a color coating applied to the outside of the lens.

Contrast – The difference in brightness between the light and dark parts of an image. A higher contrast lens provides greater visual acuity.

Cornea – The transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eye's optical power

CR-39 – A lightweight plastic lens material that is easily tinted to just about any color. Standard Plastic CR-39 Lenses are much lighter and less breakable than glass lenses. Traditional plastic lenses are made from a hard resin that is cast or molded in the wet state into lens blanks. These plastic blanks can be ground into specific shapes to fit any lens frame. Plastic lenses are great for prescription sunglasses due to their tintability factor.

Cylinder – An indicator of astigmatism on your prescription. If there is no cylinder value on your prescription, you do not have astigmatism.

Diopter – Unit of lens refractive power, equal to the reciprocal of the lens focus length in meters.

DriveWear – A type of lens that transitions behind the windshield of a car. This type of lens is specifically for driving. The lens shade will vary depending on exterior lighting conditions, but never turn completely clear.

EN1836:2005 – A standard for health and safety applied by European Union Countries to check that sunglasses conform to the health and safety requirements laid down in The European Directive 89/686/EEC.

Eye size – The horizontal measurement of the lens on any frame at its widest part. This measurement is measured in millimeters.

Frame measurements – Most prescription frames will have your exact measurements engraved on the temples (arm pieces) or behind the nose bridge area as shown in the image below (taken from our Buying Frames Online page):

The last three sets of numbers will be your frame measurements, which, in this case, translates to an eye size of 52, bridge size of 18 and temple length of 139.

For more information on sizing frames for eyewear, see our related page: What's My Eyeglass Frame Size?.

G 15 – A green-gray lens that is a popular general purpose lens.

Glass – Glass lenses are scratch resistant and easily tinted, but are double the weight of plastic lenses. Glass lenses have excellent optical qualities and can have a refractive index as high as 1.90. Glass lenses need to be thicker than newer lens materials like high index plastic.

Grilamid nylon frames – A rugged, resilient, strong, flexible, lightweight material used in frames that retains its shape and withstands extreme temperatures.

High Index 1.56 – 1.56 high index lenses are the only lenses that combine thin, flat surface cosmetics and lightweight comfort with the optical performance of plastic. 1.56 lenses are up to 25% thinner than plastic and comparable to other high index lenses, giving the wearers slimmer glasses and a wider choice of frames. A flattened, aspheric design makes 1.56 hi index lenses up to 50% flatter than regular plastic and 35% flatter than spherical high index lenses, so lenses bulge less from the frame and maintain clear optics.

High index 1.67 – Using a higher index of refraction than plastic or polycarbonate, high prescriptions no longer mean thick chunky lenses that can only be glazed into heavy frames. Thinner and lighter in weight, these lenses also come with a built in UV protection.

High Index 1.70 – The next step in lightweight, our Hi-Index 1.70 lenses have built-in UV protection and automatically anti-reflective coated. Available in single vision spherical and single vision aspherical, the lens design offers wearers a most stable and natural viewing experience. The ultimate high index material offers patients with higher prescriptions and uncompromised visual clarity in the thinnest, lightest and most fashionable lenses available anywhere in the world. The 1.70 refractive index combined with an unparalleled Abbe Value of 36 transforms even plastic lenses into an optically superior lens option.

High Index 1.74Hi-Index 1.74 lenses are the thinnest, flattest, and most cosmetically appealing lens ever developed and deserves the best of all technologies including the most advanced anti-reflective (AR) lens treatment.

Hi Index Glass Lenses – Available in a wide range of refractive indexes to provide the ultimate in thin, optically superior glass lenses. 1.6 high index lenses can be finished to a 1.5mm center in minus prescriptions and still meet United States FDA impact resistance standards. These high quality lenses are also available in 1.7, 1.8 and 1.9 indexes outside the United States.

Hyperopia – Farsightedness – inability to focus on near objects

Infrared (IR) Radiation – Radiant energy not normally considered harmful but that can cause a burning of the unprotected eyes, especially for contact lens wearers. Be sure your sunglasses stop IR radiation.

Lens materials – The right lens is just as important as the frame shape, color or size. The lens choice will determine the thickness and visual accuracy of your prescription. FramesDirect.com recommends polycarbonate lenses for active people, children under the age of 18, and for eye-threatening working situations.

Lens color & tints – Fashion tints or tints to protect light-sensitive eyes are also available in a wide range or colors, including gray, rose, yellow, G 15, brown, blue, purple and gradient tint options.

Lens rating or lens usage – The Lens Rating for sunglasses refers to the following:


0 – Dimmed Brightness

1 – High Contrast Brightness

2 – Medium Brightness

3 – High Brightness

4 – Exceptional brightness (not suitable for driving)

N = Normal lens material

P = Polarizing

F = Photochromic

So, for instance, 3P would be a polarized lens that can be used in High Brightness conditions.

Macular degeneration – A degenerative disease that causes deterioration of the central portion of the retina known as the macula and leads to blindness. Macular degeneration can result in loss of central vision, which entails inability to see fine details, to read or to recognize faces.

Melanin polarized lenses – Protect against UV radiation, blue light and glare, are impact protective and are well suited to outdoor enthusiasts.

Mirrored coating – A surface coating applied to the outside of a lens that absorbs 10 to 60 percent more light than uncoated lenses. The reflective property of the lens means it will appear darker and add additional glare protection.

Myopia – Nearsightedness – Those with myopia see nearby objects clearly but distant objects appear blurred.

Nose pad – The pads mounted to eyewear on either side of the nose that help support the frames.

Oakley Plutonite – Oakley's patented XYZ Optics maintains visual clarity at all angles of view, even at the edge of raked-back lens contours that maximize peripheral vision and protection. Oakley premium eyewear surpasses the protection requirements for high-mass impact as well as high-velocity impact, as defined by The American National Standards Institute (ANSI).

Ocular – Anything related to the eye

O. D. & O. S. – O.D. is the right eye and O.S. is the left eye.

Pentax – Pentax produces lenses that are thinner and lighter than they have ever been before, by using the highest index of refraction at 1.67, most advanced material. High prescriptions no longer mean thick, chunky lenses that can only be glazed into heavy frames. 1.67 lenses are up to 60% thinner than regular plastic lenses!

Photochromic – These lenses automatically turn dark in bright light and lighten indoors. The lenses are activated by ultraviolet light and will not darken behind the protection of your windshield. Lenses such as Transitions Lenses don't get quite as dark as normal sunglasses nor do they get perfectly clear when they lighten.

Polarized lensesPolarized lenses possess a filter that reduces the amount of reflected light that enters the eye. This filter reduces reflected glare which is most noticeable on water, snow, or concrete and asphalt surfaces.

Polycarbonate – Polycarbonate lenses are the most impact resistant material. They are lightweight, have built in UV protection, and recommended for children, sports, as well as rimless frames. Originally used primarily for industrial safety glasses, they are now used for children, sports wearers, or anyone requesting greater impact resistance in their lenses.

Presbyopia – Inability to accommodate focus difference between distance vision and near (reading)

Prism – A wedge-shaped lens which is thicker on one edge than the other. This lens bends light. Prisms can be used to measure an eye misalignment and/or treat a binocular dysfunction. A prism is sometimes added to glasses to help improve eyesight due to an eye misalignment or visual field loss

Progressive lenses – These are no-line bifocals. There is no discernible line between the regions of optical power on the corrective lens with progressive lenses.

Pupillary Distance (PD) or Pupil Distance – The distance (measured in mm) between the center of your pupils of your eyes when looking far away in the distance. If the prescription lenses are not set at the same distance as the distance between your eyes, then an unwanted prism is induced which may cause eyestrain. In lower prescriptions, the amount of prism induced will be of no consequence and will not cause eyestrain. DO NOT measure your own PD. A PD measurement MUST BE measured by an authorized optical professional only.

Round-Seg Bifocal – Uses a round reading section for the bifocal lens that is put on the back of the lens rather than the front, putting the lens closer to the eye for an expanded reading area.

RX – Short for prescription. So, RXable would mean that the eyeglass or sunglass frame can hold prescription lenses.

Scratch resistant coating – A coating that makes lenses less prone to scratching.

Sphere – A part of your prescription. The sphere number denotes the strength of the lens in diopters.

Temple length – The length of the arm of the frame running from the hinge to the end that wraps behind your ear in Millimeters.

Transitions Lenses – Photochromic lenses that transition from clear to dark in the presence of ultraviolet light and block 100% of harmful UV rays. Innovative photochromic technologies have produced unparalleled lens performance in nearly every lens design and material, including shatter-resistant lenses, bifocals, trifocals, progressives, and standard and high index materials.

Trifocal – Corrective eyewear lenses containing regions with three distinct optical powers. The three standard regions are distance at the top, intermediate in the middle and reading towards the bottom of the lens.

Trivex lenses -Trivex lens material lets you prescribe a single, thin lens with the qualities of many. This revolutionary material combines the key lens attributes while offering superior optics. Only the finest lenses provide such tri-performance — superior optics, impact resistance and ultra light weight.

Ultraviolet (UV) radiation – Invisible to the eye, prolonged exposure to solar UV radiation may result in acute and chronic health effects to the eyes. UVC rays are the highest energy, most dangerous type of ultraviolet light and your eyewear should offer UV protection.

UV Filter – A lens coating, either on or embedded in the lens, that filters UV radiation.

Visible Light – The part of the light spectrum that the eye recognizes as color. The eye can be protected from excessive amounts of visible light through protective eyewear.

Wrap Frame – A frame that wraps around your face. Wrap around sun glasses are popular.

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