If you’re wondering what glasses terms, like PD, ANSI, or photochromic mean, you’ve come to the right place — we’ve created a glossary of the most common optical terminology and abbreviations to help demystify eye health and eyewear language. If you need more help, feel free to contact us.
20/20 vision – This is considered standard or “normal” vision. In other words, at 20 feet your vision looks how it should from 20 feet away. The first number will always be 20, while the second number varies depending on your vision. The smaller the second number, the sharper your vision is, and the higher the number, the blurrier your vision is.
Abbe value – A way to measure chromatic aberration in eyeglasses lenses.
Acetate – A hypoallergenic material made from cellulose derived from wood pulp or cottonseed fibers. Acetate is known for its long-lasting luster and durability.
Add power glasses (ADD) – Abbreviation for “Addition,” which indicates a need for near vision correction. This abbreviation is used in bifocal, or multifocal, prescriptions to communicate that reading correction is needed in addition to distance correction.
Anti-reflective coating – Anti-reflective coatings improve the quality and the value of your lenses. An anti-reflective coating reduces distracting reflections to achieve the most precise, crisp, and clear natural vision. It also reduces reflections from lenses in photographs.
ANSI Z87.1 standards – The safety standards as established by the American National Standards Institute (ANSI) that glasses frames and lenses must meet to be considered safety glasses.
Astigmatism – A misshape of the eye in which 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 are excellent contact lenses for astigmatism.
Axis – Indicates the placement in degrees of the astigmatic lens.
Backside coating – Lens coating or treatment that is applied to the backside of the lens rather than (or in addition to) the front side of the lens. This is commonly done with anti-reflective coating, scratch-resistant coating, or UV-protective coating. Some proprietary lenses, including options from Wiley X and Oakley, include backside coatings.
Ballistic eyewear – Ballistic eyewear is shatter-resistant, rugged, and exceeds ANSI Z87.1 safety and military MIL-STD-662 standards for impact from high-velocity projectiles.
Base curve – The curve on the back surface of a contact lens. It also refers to the front curve of an eyeglass or sunglass lens. The base curve is used to determine how “wrapped” the lens is.
Bifocal – Corrective eyewear lenses containing regions with two distinct optical powers. Bifocals are commonly prescribed for people with presbyopia who also require a correction for myopia, hypermetropia, and/or astigmatism.
Blue filtering lenses – Lenses that filter blue-violet light to help remediate eye strain.
Bridge size – The width between the lenses on an eyeglass or sunglass frame. Manufacturers typically measure this width at the narrowest point between the two lenses.
Carbon fiber frames – Eyeglass frame material made from carbon fiber, which is a hypoallergenic, lightweight, and ultra-durable substance.
Cataracts – A clouding of the lens of the eye that can lead to loss of vision.
Clip-on – A small pair of sunglass lenses that clip onto your prescription eyeglasses, often with magnets or hooks. Clip-ons are convenient and easy to use.
Color-coated lenses – Lenses with a color coating applied on the outside, rather than a tint mixed into the lens material prior to molding.
Computer glasses – Glasses specifically designed for viewing digital devices, such as computers, smartphones, and tablets. The lenses are magnified for close-up tasks and may relieve eye strain caused by focusing for long periods of time.
Contrast – The difference in brightness between the light and dark parts of an image. A higher contrast lens provides greater visual acuity.
Core wire – Part of eyeglasses frames that connect the arms of the glasses to the lenses and ensure the arms keep their proper shape. They may or may not be visible, depending on the color and style of the frame material.
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 weight 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 make a great option 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 (used to measure the correction), equal to the reciprocal of the lens focal length in meters.
Driving glasses – Prescription or non-prescription glasses that should have an anti-glare coating on the lenses, which may provide more comfortable vision when on the road.
EN1836:2005 – A standard European Union countries apply to check that sunglasses conform to the health and safety requirements as explained in The European Directive 89/686/EEC.
Eye size – The horizontal measurement of the lens at the frame’s widest part. This measurement is listed in millimeters.
Frame measurements – Most prescription frames will include your exact frame measurements. The measurements are either stamped or engraved on the temples (arm pieces) or behind the nose bridge area. For more information on sizing frames for eyewear, see our Frame Sizing Guide.
Frame shape – The silhouette or rim constitutes a pair of frames’ shape. Some of the most popular frame shapes include rectangular, round, oval, and aviator.
G15 – A green-gray tinted sunglass lens with a 15% light transmission rate. The G15 is a popular general-purpose lens and the classic Ray-Ban lens.
Gaming glasses – Prescription or non-prescription glasses that have blue light filtering or yellow-tinted lenses, which may help reduce eye strain associated with extended screen use. The glasses have become popular among those who play video games and often engage in a lot of screen time.
Glass – Glass lenses are scratch-resistant and easily tinted. They’re also double the weight of plastic lenses, as they are thicker than high-index plastic and other lightweight lenses. Glass lenses boast excellent optical qualities and their refractive index can be as high as 1.90.
GRILAMID® TR-90 nylon – A rugged, resilient, strong, flexible, lightweight frames material that retains its shape and withstands extreme temperatures.
High-index lenses – Eyeglass lenses that offer a higher index of refraction than standard lenses and can accommodate stronger prescription strengths. The higher the number, the thinner the lens. High-index lens types include:
Hinge – A small mechanism generally used to attach frame fronts to temples. Hinges come in several types, including spring, conventional, and proprietary designs.
Hydrophobic glasses – Glasses with lenses that have been coated with a water-repellent treatment. The treatment also helps prevent dirt and debris from sticking to the lens.
Hyperopia – Also called “farsightedness,” hyperopia is the term that describes an inability to focus on near objects.
Lens materials – The lens material is what your lenses are made of. Common options are polycarbonate, high index, and plastic. Finding the best lens material is as important as finding the right frame shape, color, or size. The thickness and comfort of lenses depend on the material you choose. FramesDirect recommends polycarbonate lenses for active people, for children under the age of 18, and for safety in eye-threatening work environments.
Lens color & tints – Sunglasses are available in a range of lens colors or tints to protect light-sensitive eyes. Tints include gray, rose, yellow, G15, brown, blue, and purple. Mirrored and gradient tints are also available.
Lens rating or lens usage – The lens rating for sunglasses refers to the use scale of sunglasses tints and available lens features. The number scale designates the tint’s ideal use, from dim to very bright conditions. The lens type specifies any lens treatments or features.
The lens rating for sunglasses refers to the following VLT (visible light transmission) percentages:
LENS RATING/USAGE
0 – Very low VLT (<10%); suitable for extremely bright conditions, like snow or desert environments
1 – Low VLT (10-20%); suitable for bright conditions, like sunny days
2 – Medium VLT (20-57%); suitable for average daylight conditions
3 – High VLT (57-82%); suitable for low-light conditions, like overcast days
4 – Very high VLT (over 82%); suitable for extremely low-light conditions, like early morning or late evening
LENS TYPES
N = Normal (non-polarized)
P = Polarized; reduces glare and improves visual clarity in bright conditions
F = Photochromic; a type of lens that darkens in response to UV radiation and lightens in low-light conditions
Low bridge glasses – Eyewear frames that are designed to flatter and rest comfortably on features like high cheekbones, a wide face, and a low nose bridge.
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 the inability to see enough fine detail to read or recognize faces.
Matte frames – Eyewear frames that have a soft or muted surface rather than a shiny or glossy one.
Mineral glass lenses – Lens material made from hybrid glass, which uses sand, soda ash, and other natural elements to create distortion-free, scratch-resistant lenses.
Melanin polarized lenses – Lenses that include synthetic melanin to protect against HEV blue light and offer UV protection. This lens type is well suited to outdoor enthusiasts.
Mirrored coating – Also called “flash coating,” this surface coating is applied to the outside of a lens to block 10% to 60% more light than uncoated lenses. A reflective lens appears darker and boasts added glare protection.
Myopia – Also called “nearsightedness,” myopia describes an eye condition where a person can see nearby objects clearly, but distant objects appear blurred.
Night vision glasses – Prescription or non-prescription glasses that may help improve visual clarity when driving at night. The lenses often have a yellow tint and anti-reflective coating.
Nose bridge – The part of eyeglasses frames that arches across the nose and connects the right and left lenses.
Nose pad – The pads that are mounted to eyewear on either side of the nose to help support the frames, distribute the weight of the glasses, and provide a comfortable fit.
Nylon fiber frames – Eyewear frames made from nylon, which is often blended with other materials for added durability. Nylon-blend frames are common in athletic performance because of their lightweight flexibility.
Oakley Plutonite® – Oakley's patented lens material that promises 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 and high-velocity impact, as defined by The American National Standards Institute (ANSI).
Ocular – Related to the eye.
Ophthalmologist – A medical doctor (MD or DO – osteopathic doctor) who specializes in vision care, and is licensed to perform surgery for eye and vision conditions.
Optical glasses – Eyeglasses with a prescription added to the lenses. Used to improve vision and allow the wearer to see clearly.
Optician – An eye care technician who is trained to fit, adjust and verify prescription and non-prescription eyeglasses and contact lenses.
Optometrist – A doctor of optometry (OD) who can provide eye exams, prescribe corrective lenses, and diagnose and treat certain eye conditions. Unlike an ophthalmologist, optometrists are not surgically trained.
O. D. – Prescription shorthand for Oculus Dexter, meaning the right eye.
O. S. – Prescription shorthand for Oculus Sinister, meaning the left eye.
Photochromic – Known by the popular brand name Transitions® Lenses, these lenses automatically darken in bright light and become clear again indoors. The lenses are activated by ultraviolet light.
Polarized lenses – Polarized lenses include a filter to reduce the amount of reflected light that enters the eye, which in turn reduces the glare that can bounce back from water, snow, or concrete and asphalt surfaces.
Polycarbonate – Polycarbonate lenses are the most impact-resistant lens material. Originally used for industrial safety glasses, they are lightweight, boast built-in UV protection, and they’re now recommended for children, athletes, and for use in rimless frames.
Presbyopia – Age-related loss of ability to focus one’s near vision (as for reading), usually beginning in the mid-40s.
Prescription sunglasses – Sunglasses with a prescription added to the lenses.
Prism lenses – Lenses used to correct diplopia (double vision) or eye strain. A prism is sometimes added to glasses to help improve eyesight due to an eye misalignment or visual field loss.
Progressive Lenses – Also called “no-line bifocals,” progressive lenses offer multiple regions of optical power with no discernible line between the regions.
Pupillary distance or pupil distance (PD) – The distance (measured in mm) between the center of the pupils of your eyes. A PD measurement should be obtained by an authorized optical professional only. An incorrect PD can cause eyestrain. If your prescription does not include pupil distance, FramesDirect can measure PD from a photo.
Reading glasses – Lenses that have varying levels of magnification to help correct blurry vision when performing near tasks, like reading. Reading glasses can be purchased over the counter, but you should see an eye doctor if you also experience blurry distance vision.
Round-seg bifocal – This type of lens includes a round reading section on the back of the lens rather than on the front, putting the lens closer to the eye for an expanded reading area.
Rx – Short for “prescription.” “Rxable” means the eyeglass or sunglass frame can accommodate prescription lenses.
Scratch-resistant coating – A coating that makes lenses less prone to scratching. We provide a scratch-resistant coating with every prescription at no charge.
Segment height – A measurement that only applies to bifocal or progressive lenses. Uses millimeters to measure the distance from the bottom of the lens to the top of a bifocal line, or the start of progressive power. Also called “seg height” or “SH.”
Single vision lenses – Prescription lenses that only correct vision at one distance (either near or far).
Snellen chart – A chart used during eye exams to help measure visual acuity. The chart presents letters that gradually get smaller with each line. The lower you can read on the Snellen chart from 20 feet away, the sharper your visual acuity is.
Sphere – A number that denotes the strength of the lens in diopters; the sphere is part of your prescription.
Temple length – The length (in millimeters) of the arm of the frame, as measured from the hinge to the tip of the temple. This number may be obtained by first measuring the temple from the hinge to the place where it begins to curve behind the ear, and from the curve to the tip, and then adding those two numbers.
Titanium – A featherweight metal alloy known for its high tensile strength and durability. Titanium is often hypoallergenic and can be used to make Memory Titanium, a material that will “remember” or return to its original shape after twisting and bending.
Transitions® lenses – A popular brand of photochromic lenses that are clear indoors, then gradually darken outdoors. Transitions lenses block 100% of harmful UV rays. A photochromic treatment can be added to nearly every lens type and material, including shatter-resistant lenses, bifocals, trifocals, progressives, and standard and high-index lenses.
Trifocal – Corrective eyewear with distinct optical powers in three regions of the lenses. The top region of the lens typically corrects distance vision, the middle intermediate vision, and the bottom reading vision.
Trivex – This revolutionary lens material offers superior optics, impact resistance, and an ultra lightweight lens.
Ultraviolet (UV) radiation – Radiation (emission of energy) from the sun and certain artificial sources, including tanning beds. There are three types of UV radiation – UVA, UVB, and UVC. Prolonged exposure to solar UV radiation may result in acute or chronic damage to the eyes. Eyewear lenses (especially sunglasses) should offer UV protection.
UV filter – A coating added to or embedded within the lens to filter out UV radiation. A UV coating comes standard on all polycarbonate and high-index lenses from FramesDirect.com. Not all sunglasses offer UV protection. Check to ensure the fashion sunglasses you order come with 100% UV protection.
Visible light – The part of the light spectrum the eye recognizes as color. Protective eyewear can safeguard your eyes from excessive amounts of visible light, such as blue light.
Vision insurance – A benefit that covers a percentage of eye care costs, such as eye exams and corrective lenses. Insurance coverage varies and should be verified by your insurance carrier.
Visual acuity – How clear a person’s vision is, which is determined by measuring their ability to read from a Snellen chart, then comparing it to the standard.
Wrap Frame – An eyewear frame that wraps around your face. Wraparound sunglasses are popular for sports and active lifestyles for the increased field of vision, slip-free fit, and extra protection they offer.
Reviewed by Brian Boxer Wachler, MD on May 23, 2023