Phoenixville Eye Care Optical Shop

Glossary of Optical Terminology

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.

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

Backside Coating - Polycarbonate 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.

Base curve - The curve on the back surface of the contact lens, or the curve on the front of spec eyeglass lenses.

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.

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.

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 Polarized Transition 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.

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.

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.

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.

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.

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.

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

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.

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

O. D. - O.D. is the right eye.

O. S. - O.S. is the left eye.

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 lenses - Polarized lenses possess a filter that reduces the amount of reflected light that enters the eye. This filter reduces reflected glare which is most noticable on snow, or concrete asphalt surfaces and inside your windshield.

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.

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 discernable 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 the 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.

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.

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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