FreeOptical Glossary

Anti-reflection coating A thin layer or series of layers of material applied to the surface of a lens for the purpose of reducing unwanted reflections from the lens surface and thus increasing the amount of light that passes through to the eye.

Anti-scratch coating A thin, hard coating applied to plastic lens surfaces to make them more resistant to scratching.

Aspheric lens A lens whose surface gradually changes in power from the center to the edge for the purpose of optimizing the optical quality of the image or reducing the thickness of the lens.

Astigmatism The presence of two different curves on a single refracting surface on or within the eye. This causes light to focus as two line images instead of as a single point.

Bifocal A lens having two areas for viewing, each with its own focal power. usually the upper portion of the lens is for distance vision, the lower for near vision.

Bridge The area of the frame front between the lenses.

Colour coating/ Tinting A coating applied to the surface of a lens for the purpose of reducing light transmission.

CR-39 A registered trademark of Pittsburgh Plate Glass Co. for an optical plastic known as Columbia Resin 39. It has been the standard material from which conventional plastic lenses are made.

Distortion The lens aberration that results when image magnification changes from the center to the edge of the image resulting in either a barrellike minification or pincushion-like magnification when compared with the object.

Eyewear Eyewear designed for everyday use.

Glazed lens A lens that absorbs wavelengths toward the blue end of the spectrum in an attempt to reduce glare and increase contrast.

Gradient tinting The variation in light transmission of a lens from a low transmission (dark) to high transmission (light) from one area of the lens to another. Usually the lens is dark at the top and lightens at the bottom.

High-index lens A lens with an index of refraction that is at the upper end of the range of available indices of refraction for lenses, yielding a lens that is thinner than other lenses of the same size and power.

Memory plastic A plastic material that can be bent or twisted and still return to its original shape.

Multifocal A lens having a sector or sectors where the refractive power is different from the rest of the lens, such as bifocals or trifocals.

Nosepads Plastics pieces that rest on the nose to support the frame.

OD Latin, oculus dexter (right eye).

OS Latin,oculus sinister (left eye)

PD An abbreviation for interpupillary distance. The distance from the center of one pupil to the center of the other when either an infinitely distant object is being viewed (distance PD) or a near object is being viewed (near PD).

PD, Monocular PD The distance from the center of the frame bridge to the center of the wearer's pupil measured for each eye separately.

PD, Near PD The interpupillary distance as specified for a near viewing situation.

PD, Distance PD The wearer's interpupillary distance specified for a situation equivalent to when the wearer is viewing a distant object.

Polycarbonate A 1.586-index lens material known for its strength.

Prism Two nonparallel, transparent surfaces that cause entering light to change direction as it exits.

Photochromic (Transition) A lens that changes its transmission characteristics when exposed to light.

Polarized lens A lens that blocks light polarized in on e plane, such as light reflected from a smooth nondiffusing surface.

Progressive lens A lens having optics that vary in power, gradually increasing in plus(or decreasing in minus) power from the distance to near zones.

Refraction The bending of light by a lens or optical system.

Refractive index The ratio of the speed of light in a medium (such as air) to the speed of light in another medium (such as glass).

Refractive power The dioptric value that accurately describes the ability of a lens or lens surface to converge or diverge light. For a lens surface in air, the refractive power is expressed as F=n-1/r where n is the refractive index of the lens material and r is the radius of the surface expressed in meters.

Rimless Having to do with frames (mountings) that hold lenses in place by some method other than eyewires. Most rimless mountings have two points of attachment per lens.

Single-vision lens A lens with the same sphere and/or clyinder power throughout the whole lens, as distingusished from a multifocal lens.

Stock lens A lens that is premade, does not have to be custom surfaced, and is ready to edge.

Sphere Lens A lens having a single refractive power in all meridians.

Stock lens An inventory of lenses.

Temple The part of a pair of spectacles that attaches to the frame front and hooks over the ears to hold spectacles in place.

Transitions A trade name for a brand of plastic photochromic lenses. A lens that changes its transmission characteristics when exposed to light.

Tri-focals Lenses having three areas of viewing, each with its own focal power. Usually the upper portion is for distance viewing, the lower for near, and the middle or intermediate portion for distance in between.

Trivex The brand name for a PPG Industries plastic lens material known for its high impact resistance and ability to be processed in a amanner similar to that of other plastic lenses.

UV Rays having a wavelength somewhat shorter than those at the violet end of the visible spectrum.

FreeOptical Glossary

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