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Reading Glasses Definitions

Add power
The “add power” is the part of your prescription that tells the lens laboratory how to make the reading (near vision) field of your multifocal lenses. Multifocal lenses cannot be made without the add power value.

Anti-reflective coating
Anti-reflective coating goes on both sides of an eyeglass lens, and on the backside of a sunglass lens, and it allows light to pass more freely through the lens. AR lenses help to reduce eye fatigue in many situations, particularly while viewing computer screens and driving at night. Normally, approximately 8% to 10% of light is reflected away from the lens, "robbing" you of valuable detail for the eye. Also, as this light is reflected off the lens surface, it creates more glare that further impairs your vision. Anti-reflective coating allows approximately 99% of available light to pass through the lens without interruption, thereby reducing glare considerably.

Actors and newscasters like AR coating because it greatly reduces the glare reflected from camera lighting and flashes. AR coating makes it easier for other people to see the wearer’s eyes, and it makes it easier for the wearer to see through the lens. However, AR coating is more difficult to clean, and more difficult to keep clean.

When anti-reflective coating was first available, the new and imperfect technology caused the coating to flake off, and AR coating developed a bad reputation. Now, however, application methods are far more sophisticated and AR rarely flakes off. However, when exposed to extreme heat (for example, if the glasses are left on the dashboard of a car in hot sunlight), the AR coating can “craze”, developing unsightly cracks. If this occurs, the coating may need to be reapplied.

Aspheric
The term aspheric means “not spherical”. A spheric reading glass lens surface is regular (like the surface of a ball), with one radius of curvature. An aspheric lens surface changes shape: It does not have the same radius of curvature over the whole surface. An aspheric lens allows a lens to be made flatter, thereby reducing magnification and making it more attractive. An aspheric lens is also thinner and lighter weight than spheric lenses, because there is less lens material present.

Aviator style
This reading glass frame shape is named after WWII aviators who wore eyeglasses frame with a distinctive shape. Each eye has a tear drop shape, with a diagonal cut from the nose down to the cheeks.

“B” measurement
The “B” measurement refers to the vertical height of an eyeglasses lens.

Bifocal
A bifocal is a single lens that is divided by a visible line into two viewing areas, each with a different prescription. The lower section is for reading and for viewing objects up-close, and the upper section is for viewing objects in the distance.

Binocular PD
The pupillary distance (PD) can be measured from one pupil to the other (called a binocular PD), or from the centerline of the nose to each pupil (called the monocular PD). In cases where the pupils are not equidistant from the nose, a monocular PD is required on the prescription in order to ensure that the lenses will work properly.

Bridge
The bridge of an eyeglasses frame refers to the portion of the frame that connect the two eyepieces and rests on your nose.

Bridge measurement
This is the distance in millimeters between the two lenses. It is one of the three basic measurements which define a frame’s size.

Cable temples
Sometimes referred to as “curly temples”, cable temple curve at the end to securely hold the eyeglasses around the ears.

Clip-on sunglasses
Clip-on sunglass lenses are available for some eyeglasses frames. The clip-ons match the eyeglasses frame in shape and color, and attach either by clips or with magnets. Regular clip-ons require two hands to add and remove the clips, but magnetic clip-ons (hold to the frame with magnets instead of clips) can be added or removed with just one hand.

Computer glasses
The phrase “computer glasses” refers to any pair of eyeglasses frames with lenses that have a focal length set at arm’s length, and with anti-reflective (AR) coating. The wearer needs to measure the average distance from his/her face to the computer screen, and give that measurement to the eye doctor. The eye doctor will then prescribe an intermediate (arms-length) prescription power to be used in the lenses. The anti-reflective coating helps to reduce the glare that may come off of the computer screen.

Correction
A person with perfect eyesight does not need vision “correction”. If an eye doctor issues a prescription for eyeglasses, the prescription defines the amount of “correction” that is required in order for the person to have perfect vision (with the aide of eyeglass lenses).

Corridor
Progressive lenses do have a restricted viewing area - restricted to an hourglass shaped area in the middle of the lens. The bulge areas at the bottom and top of the lens are for near and distance vision, and the thinner “corridor” connecting them is for intermediate distances.

Double bridge
A double bridge is a frame bridge with two connection bars rather than the more common single bar connecting the two eyepieces.

Double gradient tint
A “double gradient tint” describes a lens with a full tint at the top and bottom of the lens, and a medium tint in the center of the lens. Double gradient tints are popular with skiers, because the lens blocks glare coming from above (sun) and below (snow), while allowing for a clearer viewing area in the middle (see the Tint and Gradient tint sections for more information).

Earpiece This is the section of the temple that goes behind the person’s ear, the “bent-down” portion.

Edging
Once a semi-finished blank has been surfaced, it is called a “finished blank”. The finished blank is then “edged”.  It is inserted into an edging machine along with the frame. The edging machine analyzes the frame, and the cuts the finished blank to the exact shape of the reading glass lens.

Endpiece
The endpiece is the portion of the frame front that connects the eyepiece to the temple, including the area onto which is screwed onto the hinge.

Eyepiece
The eyepiece is the part of the eyeglasses frame front that hold the lens. Each frame has two eyepieces.

Eye size
The eye size refers to the horizontal width in millimeters of an eyeglasses frame. The measurement is taken from the inside of one side of the frame to the inside of the other side of the frame.

Eyewire
On semi-rimless frames, the lens in held in place by a nylon cord called the “eyewire” that attaches to the frame in two places.

Far PD
The “far” pupillary distance is also know as the distance PD (DPD), or sometimes as the interpupillary distance (IPD). It is the pupillary distance measurement for lenses that are to be used for distance vision—seeing objects in the distance. The “near” pupillary distance (or NPD) is the pupillary distance measurement for lenses that are to be used for reading vision—seeing objects up-close.

Finished blank
Once a semi-finished blank has been surfaced, it is called a “finished blank”. The finished blank is then “edged”: it is inserted into an edging machine along with the frame.

Fit-over sunglasses
Fit-over sunglasses are useful for eyeglasses that do not have matching clip-ons, and they also serve to block out light from entering around the sides of your glasses. Side-glare is extremely annoying, especially near the water, so fit-overs may be a better solution that sunglass clip-ons.

Flash Mirror
A flash mirror coating applied to the outside of a lens helps to deflect reflected light from entering the eye. However, the flash mirror does not reflect all light the way that a pure mirror does, so in certain lighting conditions other people can see your eyes. It is important to know that the wearer does not see the flash mirror coating from the inside, but only sees the lens tint.

Flexible Material
Flexible eyeglasses frame material is available in many compositions, but the goal is always the same. This kind of material is used in eyeglasses frames in order to reduce breakage. It is normally found in the shaft of the temple and in the bridge, and it allows these areas to endure tremendous twisting without breakage or permanent distortion of the frame’s shape.

Gradient Tint
A “gradient tint” describes a lens with a full tint at the top, fading gradually to no tint at all at the bottom of the lens (see the Tint and Double gradient tint sections for more information).

Half Eye
This term refers to reading glass frames that are designed to sit lower on the wearer’s nose, allowing the wearer to look over the top of the lens easily. Half eyes are normally used as reading glasses.

Hinge
Hinges are used to connect the temple to the front of a reading glass frame, while allowing the temple to fold flat against the frame.

Hinge, External Spring
An external spring hinge is a hinge that is mounted on the inside of the temple shaft, and allows for spring action. Spring action hinges can bend beyond the limit of normal hinges, which can limit breakage of the frame under stress.

Hinge, Internal Spring
An internal spring hinge is a hinge that is buried inside the temple shaft, and also allows for spring action. Spring action hinges can bend beyond the limit of normal hinges, which can limit breakage of the frame under stress.

Hypo-Allergenic
Some people have an allergic reaction to certain metals, especially nickel and certain plastics, which are common component materials used to make eyewear. Hypo-allergenic frames, such as titanium frames, do not contain such materials.

Internal Spring Hinge
An internal spring hinge is a hinge that is buried inside the temple shaft, and also allows for spring action. Spring action hinges can bend beyond the limit of normal hinges, which can limit breakage of the frame under stress.

Lens Coatings
Eyeglasses lenses and sunglasses lenses can be coated with a variety of compounds that have beneficial qualities. Coatings can be applied to the exterior of the lens, or absorbed by the lens.

L.O.
A licensed optician or L.O. can fill a doctor’s prescription, expertly fit eyewear, and in some states, fit and fill a prescription for contact lenses as well. Because there is no national consensus as to the need for licensed opticians, Opticianry boards are regulated differently by each state. Currently, there are 24 U.S. states that have a Board of Opticianry which grant licenses: AK, AZ, AR, CA, CT, FL, GA, HI, KY, MA, MD, NV, NH, NJ, NY, NC, OH, RI, SC, TN, TX, VT, VA, WA. However, there are 28 U.S. states (and protectorates) that do not have opticianry boards, and licenses are not required to perform the functions of an optician in these states: AL, CO, DE, DC, ID, IL, IN, IA, KS, LA, ME, MN, MS, MO, MT, NB, NM, ND, OK, OR, PA, PR, SD, UT, VI, WV, WI, WY.

Library temple
A library or “straight” temple does not have a bend in it as seen in a skull temple.

Monocular PD
The pupillary distance can be measured from one pupil to the other (called a binocular PD), or from the centerline of the nose to each pupil (called the monocular PD). In cases where the pupils are not equidistant from the nose, a monocular PD is required on the prescription in order to ensure that the lenses will work properly.

Mirror Lenses
A pure mirror coating applied to the outside of a lens helps to deflect reflected light from entering the eye. The outside of the lens looks just like a mirror. It is important to know that the wearer does not see the mirror coating from the inside, but only sees the lens tint.

Multifocal
Multifocal lenses have multiple uses, so that you can see objects at varying distances using different lens corrections. A bifocal lens offers two different viewing fields (near and far). Progressive and trifocal lenses offer three different viewing fields (near, intermediate, and far).

Near PD
The “near” pupillary distance (NPD) is the pupillary distance measurement for lenses that are to be used for reading vision—seeing objects up-close. The “far” pupillary distance is the pupillary distance measurement for lenses that are to be used for distance vision—seeing objects in the distance.

Nosepads
Many eyeglasses frames, most commonly metal frames, have nosepads attached to the inside of the bridge. Nosepads are normally made of a plastic or silicone materials, and aid in the comfortable resting of the frame on the wearer’s nose. Nosepads can be adjusted to accommodate an individual’s nasal structure, as well as the proper positioning of the lenses in front of the wearer’s nose.

O.D.
An O.D. or optometrist (Doctors of Optometry) specializes in vision examinations and recommends lens options. He/she can also test for eye diseases, fit contact lenses, and in many states, diagnose and treat certain eye conditions with medication.

Ophthalmologist
An ophthalmologist medical doctor who specializes in vision care. They can perform vision examinations and recommend lens options, fit contact lenses, prescribe medications, test for and treat eye diseases, treat eye injuries, and perform eye surgery.

Optician
A licensed optician or L.O. can fill a doctor’s prescription, expertly fit eyewear, and in some states, fit and fill a prescription for contact lenses as well. Because there is no national consensus as to the need for licensed opticians, Opticianry boards are regulated differently by each state. Currently, there are 24 U.S. states that have a Board of Opticianry which grant licenses: AK, AZ, AR, CA, CT, FL, GA, HI, KY, MA, MD, NV, NH, NJ, NY, NC, OH, RI, SC, TN, TX, VT, VA, WA. However, there are 28 U.S. states (and protectorates) that do not have opticianry boards, and licenses are not required to perform the functions of an optician in these states: AL, CO, DE, DC, ID, IL, IN, IA, KS, LA, ME, MN, MS, MO, MT, NB, NM, ND, OK, OR, PA, PR, SD, UT, VI, WV, WI, WY.

Optometrist
An O.D. or optometrist (Doctors of Optometry) specializes in vision examinations and recommends lens options. He/she can also test for eye diseases, fit contact lenses, and in many states, diagnose and treat certain eye conditions with medication.

O.U.
This is a Latin abbreviation for Oculus Unicus, and it means “both eyes”.

PD
Your pupillary distance measurement (PD) is the distance in millimeters between your pupils (the centers of your eyes). Your adult PD never changes, and it averages 60 millimeters (mm) for women, and 64 mm for men. Sometimes your eye doctor will write your PD for each eye (for example “33/34”, called a monocular pd). Or, the eye doctor may write the PD as “67/64”. This means that your PD for distance vision is 67, and for near vision (for reading eyeglasses or multifocal lenses) it is 64. Your near vision PD is almost always 3 mm smaller than your distance vision PD.

Photochromic
Sun-sensitive lenses are also called “photochromic” and “transition”, because they transition from light to dark, and dark to light. 

Plano
Sometimes written as “PL”, this terms refers to eyeglasses lenses that have no prescription in them.

Prescription
A prescription for eyewear is issued by an ophthalmologist (MD) or optometrist (OD), and it specifies the appropriate correction that is required by your eyes. Eyeglasses lenses alter the light entering your eyes to correct for the eyes’ imbalance.

Progressive
A progressive lens is a trifocal lens without visible lines separating the three visual areas. Instead of a line, there is a progressive change in the lens from one corrective area to the next.

Pupillary Distance Measurement
Your pupillary distance measurement (PD) is the distance in millimeters between your pupils (the centers of your eyes). Your adult PD never changes, and it averages 60 millimeters (mm) for women, and 64 mm for men. Sometimes your eye doctor will write your PD for each eye (for example “33/34”, called a monocular PD). Or, the eye doctor may write the PD as “67/64”. This means that your PD for distance vision (DPD) is 67, and for near vision (or NPD, for reading eyeglasses or multifocal lenses) it is 64. Your near vision PD is almost always 3 mm smaller than your distance vision PD.

Reading glasses
This is a generic term that refers to eyeglasses frames that hold lenses that are used only for reading (near vision). A reading lens simply magnifies objects that are close to us, like the letters in a book. Your eye doctor can determine the magnification “power” that is best for you, or you can figure it out by yourself through trial and error. A doctor’s prescription is not required in order to select a reading lens. Drugstore “reading glasses” are simply eyeglasses frames with lenses “pre-loaded” in a range of magnification powers. It is usually less expensive to purchase reading eyeglasses this way, but stock lens powers are rarely provide a perfect vision correction. For best results, reading lenses with magnification customized to your needs can be installed in any pair of eyeglasses frames.

Rim
The rim of a pair of eyeglasses is the part of the eyeglasses frame’s eye that holds the lens in place. The rim can completely encircle a lens in the case of full frames, or partially hold the frame in the case of semi-rimless frames, or there can be no rim at all in the case of rimless frames.

Rimless frame
A rimless frame has no rim. The bridge and temples attached directly to the lens usually with nuts and bolts, and the lens itself provides part of the structure of the eyeglasses. Rimless frames should only use polycarbonate lens material, because other lens materials tend to crack under the strain.

Rxable Sunglasses
You can build your own sunglasses by first selecting a pair of eyeglasses or “rxable” sunglasses. “Rxable” sunglasses are sunglass frames that can accept a prescription lens. Sunglass frames with an excessive wrap design or a shield design can only accommodate a prescription up to +/-5.00 sphere.

Saddle bridge
This term refers to a style of bridge in which the arched portion of the bridge lies directly on the bridge of the wearer’s nose. Saddle bridges are more common with plastic frames than with metal frames. Metal frames with saddle bridges do not have nosepads.

Scratch-resistant coating
The material in plastic lenses is relatively soft and is easily scratched if it is not coated with scratch-resistant material.. This material is a much harder plastic compound that is applied to the lens, but it too can be scratched. Even with scratch-resistant coating, your lenses will become scratched through normal use over time. Once scratched, a lens cannot be repaired or “buffed” free of the scratch. No scratch coatings are “scratch-proof’”and most are not guaranteed; the only guaranteed coatings are provided by Zeiss and Essilor, and only on their own lenses.

Seg Height
The seg height is the vertical distance from the bottom of the lens up to a point on the lens which is even with the wearer’s lower eye lid (for bifocals) or the wearer’s pupil (for progressives and trifocals). On a new pair of glasses, the optician will use a felt-tipped black pen to mark the spot on the frame’s demo lenses that is even with the wearer’s lower eyelid or pupil. He then measures the distance in millimeters, and writes the distance down on the order form to be sent to the lens laboratory. Then he installs the lenses using the unique combination of those frames, your seg height, and your prescription.

Semi-finished blank
Every lens starts out the same way—as a solid block of material (called a “semi-finished blank”). Sophisticated machines then carve and polish the front and back surface of the semi-finished blank, and the resulting shape determines the lens’s prescription.

Semi-rimless frame
A semi-rimless eyeglasses frame design has a rim on part of the frame’s eye holding a lens, and on the other part a nylon cord secures the lens in place. The rim on a semi-rimless frame is most commonly on the top of the lens, although occasionally it is located on the bottom of the lens.

Shaft
The middle section of a temple is known as the shaft. It is the straight part that extends from the connection point at the eyeglasses frame front to the bend in the temple.

Shield
Recently popular, “shield” design sunglasses have one large sunglass lens that is mounted on the front of the frame and extends from one side of the wearer’s face to the other. Shields cannot be made with a prescription, and they cannot be custom made. Shields are only available with stock sunglasses.

Single bridge
A frame with a single bridge design, the most common design, has only one piece connecting the two frame eyes.

Single vision lens
The term “single vision lenses” refers to a lens that has only one use, either to see objects in the distance, or for reading (reading glasses). Multifocal lenses have multiple uses, so that you can see objects at varying distances using different lens corrections.

Sizes, how to measure
Every frame size has three components which are always measured in millimeters, and are always in the same order: eye size, bridge size, and temple size (e.g., 43-18-145). Frames are only available in the sizes made by the manufacturer, are not available in custom sizes, and it is not possible to mix and match the three size components among different frame sizes. The eye is measured horizontally from one side of the lens to the other at their furthest points. The bridge is measured horizontally from one lens to the other at their closest points. The temple is measured from one end to the other including the bend.

Skull temples
A skull temples is a “normal” temple, the style most commonly seen. The temple shaft goes straight back to a 45° turn down at the top of the ear.

Spring Hinge
A spring hinge is a hinge that is mounted on the temple shaft and allows for spring action. Spring action hinges can bend beyond the limit of normal hinges, which can limit breakage of the frame under stress. Spring action hinges also help to keep frames properly aligned.

Stock lens
Lens manufacturers can sell semi-finished blanks and finished blanks. A lens laboratory can purchase semi-finished blanks and do the surfacing themselves. A lens laboratory can also purchase finished blanks, and only do the edging and mounting. If a finished blank is purchased from a lens manufacturer, it is called a “stock lens”.

Straight temple
A straight or “library” temple does not have a bend in it as seen in a skull temple.

Temples
The temples of an eyewear frame connect and hold the frame’s front to the wearer’s head behind and below the ears.

Tint
Lenses bathed in tint colors can assume just about any color shade or color density. The lens actually absorbs the tint color into the lens material. Tint density is defined as a percentage, where 0% is completely clear, and 100% permits no light to pass through (solid). A 10% to 20% tint is used for a “fashion” tint, and a 50% to 80% tint is used for outdoor protection from the sun. Read more information about the uses for different lens colors in the custom sunglasses section. (see the Gradient tint and Double gradient tint sections for more information).

Titanium
Titanium is a more expensive metal composite used in eyewear known for its beneficial qualities of strength, lightness, and flexibility. Titanium is also a hypo-allergenic material that will not cause an allergic reaction in wearers with sensitivity to certain metals like nickel, stainless steel, and plastics.

Transition
Sun-sensitive lenses are also called “photochromic” and “transition”, because they transition from light to dark, and dark to light.

Trifocal
A trifocal lens has two lines, and three different lens sections for different viewing distances. The top part of the lens is for distance vision (over 40”), the middle part of the lens is for intermediate (arm’s length 30” to 36”) vision, and the bottom part is for near vision (reading, 14” to 16”).

Ultraviolet (UV) Protection
The sun emits ultraviolet radiation which is damaging to the eyes. Lenses with ultraviolet protection prevent that harmful radiation from touching your eyes. Polycarbonate, high index, polarized, and sun-sensitive lenses all automatically contain UV protection. Hard resin lenses need an additional coating to add UV protection. According to United States Federal law, all sunglasses sold in the United States must have UV protection. However, many low-priced sunglasses for sale through street vendors do not comply with this rule. Using tinted sunglass lenses without UV protection is extremely damaging to your eyes, because the dark tint causes your pupils to dilate and increases the surface area of your retina that can be damaged by the ultraviolet radiation.

Wrap design
Some sunglass frames are designed so that the lens curves around the side of the wearer’s head in order to protect the wearer from sun and glare entering into the side of the eye. Wrap design sunglasses can only be made with plano or prescription lenses where the prescription does not exceed +/-5.00. However, unlike the “shield” design sunglasses, wrap sunglass lenses can be customized in other ways (tint, AR coating, flash mirror, pure mirror).

 

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