If you’re wondering what PD, ANSI, or photochromic means, you’ve come to the right place—we’ve created a glossary of the most common optical terms and abbreviations to help demystify eye health and eyewear language. If you need more help, feel free to one of our expert opticians.
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 eyeglasses lenses.
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, 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 Coating – Polycarbonate lenses and high-index plastic lenses are thinner and possess flatter prescription curves and require a hard backside coating to protect them from scratching.
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.
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.
Bridge Size – The width between the lenses on an eyeglass or sunglass frame. Manufacturers typically measure this width at the widest point between the two lenses.
Blue Blocker – Lenses that block blue light.
Cataracts – A clouding of the lens of the eye that can lead to blindness.
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.
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 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.
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 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.
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.
Glass – Glass lenses are scratch-resistant and easily tinted, but they’re 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 – Glass lenses available in a wide range of refractive indexes to provide the ultimate in thin lenses with superior optics. High index lens types include.
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 from—common options are polycarbonate, high index, or 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.com 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.
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 the inability to see enough fine detail to read or recognize faces.
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 percent 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.
Nose Pad – The pads 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.
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.
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.
Prism – A wedge-shaped lens which is thicker on one edge than the other. 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 – 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.
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.
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.
Transitions® Lenses – A popular brand of photochromic lenses that gradually darken in response to ultraviolet light. 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 Lenses – A revolutionary lens material that offers superior optics, impact resistance, and an ultra light weight.
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 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.
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.